Masters Degrees (Soil, Crop and Climate Sciences)

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Open Access
Sampling and extraction methods for soil inorganic N determination to calibrate the EM38 in irrigated fields
(University of the Free State, 2019-06) Steenekamp, Diandra; Van Rensburg, L. D.; Du Preez, C. C.; Barnard, J. H.
Precise management of N variability in crop fields are required to increase yields and ensure sustainable and economic crop production, whilst not having a negative impact on the environment. A popular type of sensor for characterizing soil variability is the EM38-MK2 that measures apparent electrical conductivity (ECa), operating on the principle of electromagnetic induction (EMI). After analysis, inorganic N results can be calibrated to ECa measurements. It has been established that NH4+-N and NO3ˉ-N can be predicted from ECa. This study presented three main research aims to: i) compare single and composite samples for the determination of NH4ˉ-N, NO3ˉ-N, and total inorganic N (TIN), ii) determine if the saturated paste extract (SATe) could replace the standard 2.0 M KCl extraction for determination of NH4ˉ-N and NO3ˉ-N, and iii) determine what combination of single or composite ECa measurements and inorganic N at different sampling depths would produce the most statistically significant inorganic N prediction model. EMI surveys were conducted on four study sites under centre pivots, located on commercial irrigation farms in the districts of Douglas, Luckhoff, Hofmeyr and Empangeni. Using ECa data with the “Electrical Conductivity Sampling Assessment and Prediction” (ESAP) software and it‟s featured “Response Surface Sampling Design” (RSSD) sampling methodology, soil sampling points were identified based on the degree of ECa variability. Before sample collection, additional ECa readings were taken at each sampling point, one in the centre and one on each corner of a 1 m2 area. Afterwards soil samples were collected in the same manner in 300 mm depth increments up to 1500 mm. Samples collected in the centre were considered single, while those from the corners were composited. Concentrations of NH4+-N and NO3ˉ-N in KCl and SATe soil extracts were simultaneously determined colorimetrically. For the first aim, inorganic N concentrations in KCl extracts was loge transformed and pooled to compare sampling methods irrespective of study site, sampling point, and depth. For the second aim, data of inorganic N concentrations determined in KCl and SATe extracts were transformed and pooled for comparison, irrespective of site, sampling point, sampling method, and depth. The third aim was divided into three parts, determining agreement between single and composite ECa measurements, determining what inorganic N values to use, i.e. what sampling method and extract, and finally model calibration. Statistical analysis focused on assessing agreement using the Bland-Altman method for assessing agreement on a 95% confidence interval and multiple-linear regression calibration models were developed in Microsoft Excel. Results revealed poor agreement between single and composite samples for NO3ˉ-N and TIN. A composite sample taken in a 1 m2 area was more suitable when investigating NO3ˉ-N or TIN. Good agreement was found for NH4ˉ-N and a single sample proved sufficient. Agreement between the two extracts was poor for both NH4+-N and NO3ˉ-N and it was concluded that SATe cannot replace KCl for inorganic N determination. Agreement between single and composite ECa measurements was good and one ECa measurement was sufficient per 1 m2 sampling location. Based on the conclusions from the previous research questions, inorganic N results used for model development were those from composite samples extracted using KCl and an average between the single and composite ECa measurements was used. Values of inorganic N, ECa and elevation were loge transformed. Results showed that the majority of the calibration models were statistically insignificant except for one sampling depth (900 to 1200 mm) at Douglas (ECa 0 to 750 mm; R2=0.54 and ECa 0 to 1500 mm; R2=0.57). It was concluded that for the sites investigated, inorganic N was not the dominant soil property influencing ECa.
Open Access
Besproeibaarheid van gestruktureerde gronde
(University of the Free State, 1991-11) Nell, Johannes Petrus; Bennie, A. T. P.
Afrikaans: Data van 333 profiele se fisiese (breukmodulus, infiltrasie, hidrouliese geleivermoë, lugwaterdeurlatendheidverhouding, waterretensie en Atterberggrense), chemiese (KUK, UNP, NAV, pH, elektriese weerstand, elektriese geleivermoë, Fe-SBD, Al-SBD en organiese koolstof) , morfologiese (grondkleur, struktuur, kleiverhoudings en slik tot kleiverhoudings) en kleimineralogiese eienskappe was gebruik om neokutanies, pedokutanies, prismakutanies, rooi gestruktuurde en vertiese gronde te karakteriseer. Gronde van 15 dreineringstreke is in die ondersoek gebruik. Klem word egter op kalkryke gronde in die Sondagsrivier-, Klein-Visrivier- en Groot-Visrivierdreinerings- gebied geplaas, aangesien heelwat van die gronde deur verdere ontwikkeling van die Oranjerivierprojek geraak word. Daar bestaan regverdiging om verskille tussen diagnostiese horisonte as besproeiingsnorm te gebruik, aangesien duidelike fisiese en chemiese verskille tussen grondvorms, binne dreineringsgebiede voorkom. Daar is 'n afname in die geskiktheid van gronde vir besproeiing met 'n toename in graad van struktuurontwikkeling. Dit kan hoofsaaklik toegeskryf word aan 'n toename in opbou van skadelike soute en 'n verskuiwing van 'n oorwegend illietiese kleimineraalsamestelling na 'n smektietiese samestelling wat tot swelling en/of dispersie aanleiding gee. Laer infiltrasie en hidrouliese geleivermoë waardes, met 'n toename in struktuurontwikkeling, kan grootliks hieraan gewei word. Alhoewel die Na-inhoud ook 'n rol speel by die strukturele eienskappe van vertiese en rooi gestruktuurde gronde, het die tipe en hoeveelheid kleiminerale en seskwioksiedes 'n duideliker invloed by dié gronde se struktuurstabiliteit. Daar bestaan egter nie regverdiging om kleur volgens seriedifferensiasie as besproeiingsnorm by pedokutaniese gronde te gebruik nie. Die stabiliserende effek van die ysteroksiedes wat vir rooi grondkleur verantwoordelik is, word egter in sommige dreinerings- gebiede oorbeklemtoon. Die soort en hoeveelheid soute tesame met die tipe kleiminerale het 'n meer dominante invloed op hierdie gronde se struktuurstabiliteit. Die groot variasie in hidrouliese eienskappe van gestruktuurde gronde kan moontlik beter verstaan word, indien die beskrywing van die graad, tipe en grootte van die struktuureenhede met spesifieke parameters gekwantifiseer kan word. Die oorgang tussen horisonte is ook van belang. By dupleksgronde kom daar in die algemeen 'n drastiese afname in infiltrasie van die A- na die B-horisonte voor. Die lae infiltrasievermoë van die B- horisonte veroorsaak dat horisontale infiltrasie tot 'n groter mate as vertikale infiltrasie plaasvind. Aansienlike interkorrelasie tussen fisiese, chemiese en kleimineralogiese eienskappe kom voor wat tot lae verwantskappe aanleiding gee. Indien gronde egter in pedologiese eenhede gegroepeer word en die korrelasies vergelyk word, kan sommige daarvan verhoog of beter verstaan word.
Open Access
Biokatalitiese komponente in sade van enkele spesies uit die families Fabaceae en Caryophyllaceae
(University of the Free State, 2002-11) Du Plessis, Helena Aletta; Pretorius, J. C.; Potgieter, G. P.
English: In the south of Germany it was observed that species of the families Fabaceae and Caryophyllaceae survived an unknown environmental catastrophy while this was not the case with plant species from other families (Huster, Personal observation). Seed suspensions of some species belonging to these two families were applied to agricultural crops as foliar sprays, improving both the vegetative growth and yield of the crops. This supplied the rationale to test the seeds of South African species from these two families for its biocatalytic properties. Seeds of two species from each of the mentioned families namely Acacia karroo and Acacia erioloba from the family Fabaceae as well as Pollichia campestris and Dianthus basuticus from the family Caryophyllaceae, were subsequently screened for similar biocatalytic properties. Not much information on the chemical composition of these four plant species and nothing at all about their biocatalytic activity could be found in literature. In the study that followed biocatalytic activity was confirmed in a seed suspension of Acacia erioloba (Fabaceae) as it increased the respiration rate of a mono culture yeast cells as well as root growth of Cress-seedlings markedly. Activity directed liquid-liquid extraction of the A. erioloba seed suspension revealed that most of the biocatalytic activity was present in the ethyl acetate fraction. Further column chromatographic fractionation of the ethyl acetate fraction produced 13 combined column fractions of which two were active. Activity directed preparative thinlayer chromatographic purification of compounds in one of these fractions produced one active compound that was identified as 2-ethylhexylphtalate by means of nuclear magnetic resonans (NMR) spectroscopy. From the second column chromatography fraction six compounds were isolated but in such small amounts that NMR analysis was not possible. By means of colour reagents it was, however, established that three of the six compounds were alkaloids, two were terpenoides and one a coumarin. It is recommended that active components in this group should be identified in a follow-up study by initially extracting much more seed material.
Open Access
The effect of nitrogen fertilisation on the growth, yield and quality of swiss chard (Beta vulgaris var. cicla)
(University of the Free State, 2008-05) Motseki, Pontso Christina; Engelbrecht, G. M.; Ceronio, G. M.
English: The sustainability of subsistence farming associated with the health of rural communities’ necessitated research on Swiss chard as it forms an integral part of food consumed by the poor in developing countries. Two separate pot experiments were carried out during the 2005/06 and 2006/07 seasons in the glasshouse of the Department of Soil, Crop and Climate Sciences at the University of the Free State. The objective of this study was to determine the effect of nitrogen fertiliser on growth, yield and quality of Swiss chard. The first pot trial was conducted to evaluate the effect of five nitrogen levels (0, 50, 100, 200, 400 kg N ha-1) and four application times on the growth, yield and quality of two Swiss chard cultivars (‘Fordhook Giant’ and ‘Rhubarb’). Two Swiss chard seedlings were planted per pot, filled with topsoil of the fine sandy loam Bainsvlei form. Two weeks after planting plants were thinned to one seedling per pot. Different nitrogen levels were applied to the relevant pots as follows: once every second, fourth, sixth or eighth week. It was only the total dry mass per plant and total nitrogen content per leaf of ‘Rhubarb’ plants that was significant higher than that of ‘Fordhook Giant’. The other parameters measured for the two cultivars did not differ significantly from each other. Nitrogen levels positively influenced the early growth, yield and quality parameters measured. It was the highest nitrogen level (400 kg N ha-1) that resulted in the highest number of leaves harvested, leaf fresh and dry mass, leaf area and leaf nitrogen content. Nitrogen application times significantly influenced only the early growth of Swiss chard plants and the nitrogen content of leaves. Early plant growth reacted better where nitrogen was split into three equal applications (T4). The total nitrogen content of Swiss chard leaves was significantly higher where nitrogen was split into five equal (T2) or three equal (T4) applications. In the second pot trial the effect of different nitrogen sources applied at different levels on the growth, yield and quality of Swiss chard cultivars was determined. The response of Swiss chard plants to nine nitrogen levels (0, 100, 200, 300, 400, 500, 600, 700, 800 kg N ha-1) from six different nitrogen sources (ammonium nitrate, potassium nitrate, calcium nitrate, ammonium sulphate, urea ammonium nitrate and urea) were studied. Based on the findings of this study, nitrogen significantly influenced growth, number of leaves harvested, leaf fresh and dry mass, leaf area and leaf nitrate content of ‘Fordhook Giant’ plants with best results obtained at 800 kg N ha-1. Nitrogen sources did not influence either the total number of leaves harvested nor the fresh mass of harvested Swiss chard leaves. In both cases, ammonium nitrate gave the best results and calcium nitrate the poorest. Urea influenced the leaf area positively followed by urea ammonium nitrate, with calcium nitrate resulting in the smallest leaf area per plant. Dry mass of Swiss chard leaves was also significantly higher where urea was used as nitrogen source compared to where calcium nitrate was used. No significant differences amongst the other nitrogen sources. Ammonium nitrate and potassium nitrate significantly stimulated the accumulation of nitrate in Swiss chard leaves, whereas the other nitrogen sources did not play any role in nitrate accumulation in the leaves of Swiss chard.
Open Access
Organic matter restoration by conversion of cultivated land to perennial pasture on three agro-ecosystems in the Free State
(University of the Free State, 2002-01) Birru, Tilahun Chibsa; Du Preez, C. C.; Hensley, M.
English: Understanding the process of organic matter degradation and restoration is important with regard to sustainable agricultural production on any agro-ecosystem, and of particular importance where degradation is relatively rapid, such as in the coarse textured savannah soils of the South African highveld. Organic matter degradation studies on such soils in three agro-ecosystems, Harrismith, Tweespruit and Kroonstad, have been undertaken by Du Toit et al. (1994), and Lobe et al. (2001). This study is concerned with organic matter restoration on the same agro-ecosystems, and is therefore complementary to the two earlier studies. The objective was to investigate organic matter restoration at three depths, 0-50, 50-100 and 100- 200 mm, on perennial pastures of different ages that had been established on lands which had been cultivated continuously for more than 20 years. Representative C and N values for degraded lands and virgin grasslands for the three agro-ecosystems were obtained from the studies of Du Toit et al. (1994) and Lobe et al. (2001), and used as reference values. To reduce within-site error samples were collected at six places, separated from each other by a few meters, at each site. At each of these places six subsamples of each layer were taken to make up the final sample. There were therefore 18 soil samples per site. A total of 28 sites, ranging in ages from 4 to 25 years, were identified and sampled on the three agro-ecosystems, All the samples were analyzed for C and N, and selected samples were analyzed to characterize the soil fertility levels and particle size distribution at each site. Results showed a wide variation in the rate of organic matter restoration between sites in each of the agro-eco systems , due mainly to differences in natural resource factors and management techniques. Most important of the latter was the application of N fertilizer. Where this was inadequate or absent, very low organic matter restoration rates were generally measured. An approximate threshold value of available N below which organic matter restoration is severely impaired appears to be about 15 mg kg". On pastures up to the age of25 years most of the C and N storage has been in the 0-50 mm layer, a little in the 50-100 mm layer, and very little in the 100-200 mm layer. This observation accentuates the importance of the sampling depth in such studies. These results are in accordance with those of Potter ef al. (1999). The mean C gains over all the sites in the three agroecosystems, excluding those with a Nfertility level considered too low to initiate efficient C sequestration, is 0.56 Mg ha-I yr' as compared to 0.8 Mg ha" yr" suggested by Bruce el al. (1999) for the United States of America and Canada. The relatively coarse texture of the Free State soils, and the lower aridity indices, may account for the difference. An attempt was made by pooling the data for the three agro-ecosystems, and adopting a normalization procedure, to identify common C and Nrestoration curves with time. Although a definite upward trend is visible, large inter-site variation and the shortage of data points above 20 years results in relatively low correlation coefficients and the curves being unreliable at their top end. Further research to obtain data from very old pastures is recommended, as well as ecotope specific research on benchmark ecotopes to define in a reliable way the shape of the organic matter restoration curve.
Open Access
The effect of crop residue cover and soil texture on crusting
(University of the Free State, 2002-10) Massingue, Felicidade Isabel; Bennie, A. T. P.
Large areas of cultivated soils throughout the world develop rainfall-induced soil crusts. The soil crusts are usually the cause of reduced seedling emergence. To have quantitative information on the factors influencing the development of surface crusts and on the influence of ameliorating treatments on crust strength is valuable. The objectives of this study were firstly, to determine the influence of soil texture on the susceptibility of different soils for crusting; secondly, to quantify the effect of soil crusts on the emergence of wheat, sorghum, soybean and sunflower; and thirdly to determine the optimum level of crop residues that can be used as a mulch to mitigate the effect of soil crust strength. Five soils ranging in texture from sand to loam were sampled from the surface (0 - 200 mm). The soil samples were used in four greenhouse pot experiments that were conducted to examine the effect of crust strength on seedling emergence. Separate pot experiments in the greenhouse were conducted to determine how particle size distribution was related to soil crust strength. Regression analyses showed that silt, silt plus clay and clay contents were related to crust strength as indicated by modulus of rupture, penetration resistance and emergence force. All the relationships were of third order polynomial nature. The crust strength increased initially with increasing silt plus clay contents up to about 35 to 40 %, or clay contents up to about 25 %, then declined as a result of cracking that occurred upon drying. The emergence of wheat, soybean and sunflower was little affected at crust strengths less than 0.7 Mpa or 500 gf when measured as penetration resistance and emergence force respectively. Above these values seedling emergence decreased linearly with increasing crust strength. Large areas of cultivated soils throughout the world develop rainfall-induced soil crusts. The soil crusts are usually the cause of reduced seedling emergence. To have quantitative information on the factors influencing the development of surface crusts and on the influence of ameliorating treatments on crust strength is valuable. The objectives of this study were firstly, to determine the influence of soil texture on the susceptibility of different soils for crusting; secondly, to quantify the effect of soil crusts on the emergence of wheat, sorghum, soybean and sunflower; and thirdly to determine the optimum level of crop residues that can be used as a mulch to mitigate the effect of soil crust strength. Five soils ranging in texture from sand to loam were sampled from the surface (0 - 200 mm). The soil samples were used in four greenhouse pot experiments that were conducted to examine the effect of crust strength on seedling emergence. Separate pot experiments in the greenhouse were conducted to determine how particle size distribution was related to soil crust strength. Regression analyses showed that silt, silt plus clay and clay contents were related to crust strength as indicated by modulus of rupture, penetration resistance and emergence force. All the relationships were of third order polynomial nature. The crust strength increased initially with increasing silt plus clay contents up to about 35 to 40 %, or clay contents up to about 25 %, then declined as a result of cracking that occurred upon drying. The emergence of wheat, soybean and sunflower was little affected at crust strengths less than 0.7 Mpa or 500 gf when measured as penetration resistance and emergence force respectively. Above these values seedling emergence decreased linearly with increasing crust strength.
Open Access
Studies oor die toeganklike fosforstatus van sekere Vaalhartsgronde
(University of the Free State, 1971-12) Eloff, Jacobus Frederick; Laker, M. C.
Afrikaans: Op die Vaalhartsbesproeiingskema is 30 veldproewe uit gelê om die toeganklike fosforstatus van die verteenwoordigende gronde te bepaal. Sitroensuur-P, Bray-P, Olsen-P, CAL-P en hars-P uit die grond geëkstraheer, is gekorreleer met plantmassas van Zambesi-koring op pypstadium, fosforopname op pyp en met oesopbrengs ten einde die geskikste metode vir die voorspelling van "plantbeskikbare" fosfor vir hierdie gronde te vind. Ander plantvoedingstofkonsentrasies (K, Ca, Mg en Zn) in die plante op die pypstadium is bepaal, terwyl die P-, K-, Ca-, Mg- en Zn-konsentrasies in die graan en die hoeveelhede verwyder deur die oes ook bereken is. Al die ekstraksieprosedures het hoogs betekenisvol (P=0,01) met mekaar gekorreleer, alhoewel die metode van Olsen deurgaans 'n laer korrelasiekoëffisiënt met al vier die ander metodes vertoon het. Hoewel alle metodes beteknisvol gekorreleer het met plantmassas op pypstadium, fosforopname en oesmassas het die Olsenekstraheerbare-P in alle gevalle, maar veral met oesmassas, die beste korrelasies gelewer. Hierdie metode word dan ook aanbeveel vir die bepaling van plantbeskikbare fosfor in die gronde onder bespreking, waarin die Manganoserie oorheersend is. Dit is bevind dat die fosforstatus van 64% van die bogronde van die bewerkte grond reeds voldoende opgebou is en dat slegs onderhoudsbemesting daar nodig is. Die ondergronde het in alle gevalle prakties gesproke geen "plantbeskikbare" fosfor bevat nie. Dit is baie duidelik dat fosforbeweging as gevolg van loging nie plaasvind in die gronde onder bespreking nie. 'n Insiggewende verwantskap is aangetoon tussen die KAV's van die kleifraksies van die bogronde en die kalum-, kalsium- en sinkkonsentrasies en K:Mg verhouding in plante op pypstadium. Hierdie verwantskap is ook bevestig deur die K-konsentrasies in die graan.
Open Access
Leaching of excess salts from the root zone of apedal soils
(University of the Free State, 2006-05) Barnard, Johannes Hendrikus; Van Rensburg, L. D.; Bennie, A. T. P.
English: In South Africa a huge amount of energy was spend on irrigation research over the past two decades, mainly to optimise water application in order to prevent crop water stress. In the quest to conserve water for transpiration, researchers tended to neglect the importance of drainage or percolation, which eventually results in the accumulation of salts in the root zone. Salts also accumulate in the root zone where shallow water tables are present. Farmers along the Lower Vaal River expressed their concern about yield losses induced by build-up of salts in the root zone. The detrimental affect of salinity on field crops are extensively reported in the literature and the only way to address the problem is through leaching. Sustainable utilization of these saline or potential saline soils depends on adequate natural drainage or artificial drainage systems, which ensures a net downward flux of water and salts below the root zone for optimum development and functioning of roots. This dissertation focuses mainly on the management of salts in the root zone of apedal soils. The research was conducted on two soil types (Clovelly and Bainsvlei) reconstructed in 5000 litre lysimeters on the experimental farm, near Bloemfontein, of the Department of Soil, Crop and Climate Sciences (University of the Free State). A total of 30 lysimeters, 15 per soil type arranged in two parallel rows under a moveable rain shelter were used. It was assumed that the artificially prepared soil profiles are stable because more than 10 cropping cycles were completed before the commencement of this experiment. The first aim of Chapter 3 was to address the effect of irrigation water salinity on the accumulation of salt in the root zone under shallow water table conditions. A total of 612 mm was irrigated with irrigation water salinity treatments that varied between 15 and 600 mS m-1. Results showed that in the absence of drainage, salts will accumulate in the root zone at an alarming rate. In fact, salinity of the soil water almost doubled with respect to that of the irrigation water during only one growing season. These various saline profiles were used to characterise the impact of soil water salinity on the hydraulic characteristics of the two soils under investigation. After saturation of the profiles, drainage curves were in situ determined by allowing water to drain freely from the profiles for approximately a month. These drainage curves revealed that the initial soil water salinity did not significantly influence the hydraulic characteristics of both soils. It was possible to quantify the amount of salt removed during a drainage cycle. Although both soils are apedal, the two soils differed markedly in their discharge rates and amounts. Chapter 4 had focused on quantifying the pore volume of water required to leach excess salts from the profiles. It was found that piston flow can describe the leaching process, because one pore volume of drainage was sufficient to remove 100% of the excess salts, irrespective irrigation water salinity or soil water salinity. The results also showed that it is more efficient to remove 80% of excess salts in stead of 100%. On freely drained soils it is therefore possible to effectively and efficiently manage the salinity level of the root zone through controlled irrigation in excess of crop water demand, when necessary. Complex dynamic models are helpful in understanding the nature and complexity of solute movement in soils, but unfortunately they are not widely used by irrigators and managers. The final objective (Chapter 5) was to derive a simple model capable of estimating the depth of water required to remove excess salts from the root zone. The non-linear exponential association (y = a {1- exp –b x}) of the in situ determined leaching curves provided the best mathematical description of the fraction of excess salts removed in relation to the depth of leaching water required per unit depth of soil. Verification of the proposed model showed that it is possible to accurately estimate the leaching requirement for effective and efficient management of root zone salinity in apedal soils. It was recommended that the proposed model should be expanded to include more soil types.
Open Access
The effect of water quality on the growth and yield of irrigated crops
(University of the Free State, 2006-05) Dikgwatlhe, Shadrack Batsile; Ceronio, G. M.; Van Rensburg, L. D.
English: Salinity is a major limitation and threat sustainable crop production in South Africa and elsewhere in the world. An intensive study by Du Preez et al. (2000) on the water quality of the lower Vaal River system using international salinity indicators were conducted and it was concluded that unacceptable damage to crops would be experienced if the salinity of the irrigation water continued to increase at the projected rates. Following that, intensive germination and glasshouse pot experiments were conducted based on the projected longterm salt accumulation on irrigated soils. Little or no quantitative information on the subject was available for South African conditions. This study was also part of a WRC project titled: The effect of irrigation water salinity on the growth and water use of selected crops. The objective of this study was to quantify the effect of different saline irrigation water levels (electrical conductivity, ECi ) on the germination, growth and yield of selected crops, viz. wheat (Triticum aestivum L. - SST 806.), maize (Zea mays L. - PNR 6335), peas (Pisum sativum L. - Solara) and beans (Phaseolus vulgaris L. - Teebus) at different growth stages. These crops were subjected to five different ECi levels (15 - control, 150, 300, 450, 600 for wheat and maize, and additionally 1200 mSm-1 for wheat, and 15 - control, 75, 150, 225, 300 mS m-1 for peas and beans). Three stages were used for each crop, viz. tillering, flag leaf and maturity (wheat), 2, 4 and 6 wae - weeks after emergence (maize) and 5 wae - five weeks after emergence, flowering and maturity (peas and beans). Various morphological indicators presented on a relative scale were measured to quantify the impact of ECi levels on both below and above ground growth. When subjecting the crops to the different ECi levels, the salt tolerance of these plants was taken into consideration. These crops were affected at varying degrees, depending on the salt sensitivity of the crop involved. Interesting results were obtained for all the crops and based on the findings the study agreed well with the international accepted salinity classification system where wheat is classified as moderately tolerant and peas and beans as sensitive. Maize proved to be sensitive in this study, but it has to be noted that it was only at a very early growth stage. The water use of the pot experiment plants was found to be very high and this was attributed to the smaller volume of soil relative to the canopy. All plant growth indicators proved to be negatively affected over the selected ECi range. The reduction in growth for all crops followed a similar trend with increasing ECi levels. The EC of the saturation soil extract (ECe) was also measured in order to determine the rate of soil salinisation and was found to be 2 to 3 times that of ECi. The study also compared the responses of these crops by using primary growth indicators (leaf area, root mass, biomass and seed yield) and also attempted to determine the salt tolerance values using the regression coefficients. Therefore, all the objectives set for the study were achieved. The effect of salinity on a variety of other crops under South African conditions should be thoroughly and comprehensively investigated in future.
Open Access
Effect of liquid Maxiflo (Azospirillum SPP) and Trykoside (Trichoderma SPP) cultures on the growth and yield of selected crops
(University of the Free State, 2006) Mashamba, Nwagu Rodney; Pretorius, J. C.; Engelbrecht, G. M.
English: The challenge for science is to address the need for adequate food provision and a sustainable future for agriculture. The solution for increasing food production can probably only be obtained through expansion of arable land, by increasing irrigation practices or by increasing harvestable yields on available land through the improvement of agricultural technology. With regard to the latter approach, field experiments were conducted at the experimental farm of the University of the Free State, Bloemfontein, during the 2003 and 2004 growing seasons, to determine the effect of Maxiflo and Trykoside (liquid formulations of Azospirillium and Trichoderma based products, respectively) on vegetative growth and yield of two leafy vegetable crops (cabbage and lettuce), a cereal crop (wheat), a fruit crop (tomato), a tuber crop (potato) and a legume crop (peas). A randomized complete block design with six treatments (Control, Maxiflo, Trykoside, Maxiflo + Trykoside, ComCat® and Ke/pak®) was applied in all cases. Maxiflo and Trykoside were applied either separately or together. Two commercially available natural bio stimulants, ComCat® and Ke/pal<®, served as positive controls. Different growth and yield parameters were used to quantify the effect of the test products in the above six economically important crops. In cabbage and lettuce vegetative growth (plant height, plant diameter and stem thickness) were not affected but a significant increase in head mass was observed. Peas were most responsive to treatment with the bio-products in terms of the increase in yield obtained. Treatment with Maxiflo and Trykoside in combination increased the medium/large and large size fruit yield in tomatoes while exactly the same was observed for tuber size in potatoes. However, in both crops the total yield was not significantly affected. In wheat root growth was stimulated significantly by treatment with Trykoside but no significant yield increase was observed at the 5% probability level (P<0.05).
Open Access
The influence of land use on humic substances in three semi-arid agro-ecosystems in the Free State
(University of the Free State, 2009-05) Akhosi-Setaka, Makuena Cynthia; Du Preez, C. C.; Kotze, E.
English: This study was initiated to complement earlier investigations into soil organic matter degradation and restoration on account of agricultural land use in the Free State Province of South Africa. In these studies no attention was given to the response of humic substances which represent the most active fraction of organic matter. The aim with this study was therefore to quantify the influence of agricultural land use on humic substances in soils of semi-arid regions. Topsoil (0-200 mm) samples from distinctive agro-ecosystems at Harrismith (Mean annual rainfall, MAR = 624 mm and Mean annual temperature, Ta = 13.8°C), Tweespruit (MAR = 544 mm and Ta = 14.8°C) and Kroonstad (MAR = 566 mm and Ta = 16.6°C) were selected for use in this study. An agroecosystem implies a region where the three environmental factors affecting yield, namely climate, slope and soil are for practical purposes homogeneous. The selected samples represent a virgin (grassland soil never cultivated before), cultivated (formerly grassland soil cultivated for at least 20 years) and restored (formerly cultivated soil converted to perennial pasture for at least 15 years) Plinthustalfs (10.6 to 13.5% clay) at every agro-ecosystem. Parameters quantified comprise crude soil, extractable soil, humic acid and fulvic acid C contents, N contents and C/N ratios. Concerning these parameters, cultivated soil was compared with virgin soil and restored soil with cultivated soil. The crude soil C content of the virgin soils varied from 7.3 g C kg-1 soil in the warmer, drier Kroonstad agro-ecosystem to 21.6 g C kg-1 soil in the cooler, wetter Harrismith agro-ecosystem. Across agro-ecosystems the contribution of extractable to crude soil C was almost constant, namely 47.1 to 48.4%. The contribution of humic acid C to extractable soil C decreased and that of fulvic acid C to extractable soil C increased from the Kroonstad to Harrismith agroecosystem. Cultivation reduced crude soil C in the three agro-ecosystems with 50.2 to 51.8%. This is equivalent to absolute losses of 3.8, 8.2 and 10.8 g C kg-1 soil at Kroonstad, Tweespruit and Harrismith agro-ecosystems respectively. Loss of extractable soil C was more variable ranging from 36.7% or 1.3 g C kg-1 soil in the warmer, drier Kroonstad agro-ecosystem to 48.2% or 5.1 g C kg-1 soil in the cooler, wetter Harrismith agro-ecosystem. Trends of this nature were nonexistent for either humic or fulvic acid C losses. Gains of crude soil C ranged from 5.4 g C kg-1 soil in the warmer, drier Kroonstad agro-ecosystem to 8.0 g C kg-1 soil in the cooler, wetter Harrismith agroecosystem. This trend manifested also in extractable soil C gains which were lowest at Kroonstad (1.5 g C kg-1 soil) and highest at Harrismith (2.8 g C kg-1 soil). Neither humic acid C nor fulvic acid C showed trends of this nature. The N contents although more variable than the C contents are to a large extent supportive concerning organic matter in the virgin, cultivated and restored soils of the three agro-ecosystems. Further elaboration on the N contents is therefore not justified here. Based on both C and N indices, it can be stated that humic substances did not show explicit trends on account of land use as was the case with organic matter per se. This phenomenon warrants further investigation since humic substances are regarded as the most reactive fraction of organic matter.
Open Access
The role of ferrolysis in the genesis of selected soils of the Eastern Free State
(University of the Free State, 2007-11) Macheli, Malerata Suzan; Le Roux, P. A. L.; Du Preez, C. C.
English: Plinthic and duplex soil forms make up a substantial part of the soils under cultivation in South Africa. The tendency of these soils to occur either in isolation, or in association with one another, leaves the impression that ideal conditions for the formation of each occur independently but are closely related. This implies that ideal conditions for the development of each vary over short distances. Little research has been done on the duplex-plinthic soil association but a relationship between the two soil groups is implied in literature. A proper understanding of soil genesis may therefore contribute to the better classification, interpretation and evaluation of these soils for sustainable land-use purposes. The hypothesis is that the redistribution of Fe-Mn and degradation of silicate clays are important processes involved in the formation of soils with either a duplex or plinthic character. The objective of the study was to establish the role of ferrolysis and redistribution of Fe-Mn in the genesis of the duplex-plinthic soil association. The catena concept; which describes a sequence of soils of about the same age, occurring under similar macroclimatic conditions and derived from the same parent material; but with different characteristics due to variation in topography and drainage; was adopted. A toposequence of 10 representative profiles was selected in the Eastern Free State. The soils were described, sampled and photographed. Representative composite and undisturbed samples were analysed for several chemical, physical and morphological soil properties. The selected toposequence commences at the crest with a profile of the Hutton soil form. Soils of the Westleigh, Longlands, Avalon (3), Kroonstad (3) and Estcourt forms follow down slope. The characteristic red colour grades to yellow-brown in the soft plinthic soils on the mid slope and grey duplex soils in the valley bottom. The change in colour dominates morphology in the midslope but changes to be dominated by texture differentiation in the valley bottom. Signs of redox activity prevail across the toposequence and its prominence increases drastically with depth in the profiles and down slope in the catena. Subsoil acidification indicates the presence of an early stage of redox activity. Fe-Mn redistribution, present as mottles and concretions, and colour changes are indications of intermediate redox activity. The texture differentiation present indicates an environment supporting ferrolysis to the full.
Open Access
Yield and fruit quality assessment of cactus pear (Opuntia ficus-indica (L.) Mill.) treated with natural bio-stimulants
(University of the Free State, 2004-11) Buitendag, R. A.; Pretorius, J. C.; Van Rensburg, L. D.
English: In arid and semi-arid regions of South Africa, both subsistence and commercial farmers experience economic pressure due to varying prices of staple crops such as maize and wheat as well as weather uncertainties. The prickly pear, Opuntia ficus-indica, is an alternative crop that can partially offer a solution to these problems as an extra income. Further, it is capable to produce both fruit and young cladodes under rather extreme weather conditions, including severe drought. Further, its economic potential per hectare far exceeds that of maize and wheat. This supplied a rationale for investigating new ways for manipulating the crop with the aim to increase either fruit yield and quality or new cladode production or both, by applying natural bio-stimulants as foliar sprays to adult plants while standard management practices were adhered to. ComCat® (CC), a commercially available natural bio-stimulant known for its potential to stimulate yield, growth and development in some crop plants, along with SS, a prototype natural bio-stimulant still in the developmental phase, was used in this study in an attempt to reach the set objectives. Kelpak®, a commercially available bio-stimulant also known for its growth stimulating properties, was used as a positive control. Although, in the two trials conducted, the results were not consistent as far as all measured parameters are concerned and were not statistically significant in all cases, ComCat® (CC) consistently contributed to a increase in the total fruit yield expressed in ton ha-1 while the SS treatment consistently contributed to elevated new cladode production in both trials. When fruit and new cladode yield data was expressed per old cladode, a different picture arose. This prompted the need to correlate the relationship between morphological and yield data. Subsequently, multiple regression and correlation analyses were performed using morphological parameters such as fruit-, peel- and pulp mass as well as fruit diameter and length to predict fruit yield and quality. Although the linear equation models resulting from this statistical calculation did not consistently show significant prediction accuracy at the 95% probability level, it was a worthwhile exercise as definite correlations were observed at least at the 90% probability level. Prediction models resulting from multiple regression and correlation analyses of morphological, yield and quality data strongly indicated that this approach might become a handy tool for farmers assisting them in managing their orchards not only in a specific growing season but especially for the following season. More research is necessary to pursue this aspect. Finally, manipulation of fruit yield in O. ficus-indica by foliar sprays of adult plants with ComCat® and manipulation of new cladode production by treatment with SS, can become additional techniques to optimize the productivity of this crop plant.
Open Access
Evaporation from aeolian soils with shallow water tables
(University of the Free State, 2016-02) Mengistu, Achamyeleh Girma; Van Rensburg, L. D.; Mavimbela, S.
Soil water evaporation under water table conditions was identified as a very important water loss in the soil water balance in aeolian soils. Globally it is estimated that 7.2 × 1013 m3 of water lost through evapo-transpiration per year. In South Africa, an average of 65% of the precipitation will be lost through evapotranspiration. The main aim of this study was to determine the effect of water table depth on daily evaporation (Es) for two aeolian soils that cover large areas in Africa below the equator. The study was divided in to three content chapters: The first chapter concentrated on characterization of the basic soil physical properties, the second content chapter investigated the effect of soil water content and temperature on the thermal properties, while the last content chapter described the effect of soil type and water table depth on the daily rates of evaporation as well as the temperature distribution in the profiles. In the first content chapter, soil physical properties were determined with application of laboratory, field or indirect estimation methods and summarized as follows. (i) Particle size analysis was determined in laboratory by the pipette method. The two soils had similar textural class (loamy-sand) in the Ap horizon, but Clovelly categorized as sandy and Hutton as sandy-loam in the B-horizon. (ii) Bulk density was determined by the core sampling method and varied between 1.3 to 1.7 g cm-3 with Clovelly the lowest. (iii) Soil water retention curves (SWRCs) were determined in laboratory by the Hanging Water Column and the Pressure Plate Apparatus (assisted by the RETC computer program and the van Genuchten model). The water content at lower suctions was higher in Clovelly than Hutton and the opposite was true for higher suctions to the dry end. Hence, Clovelly had a well-developed S-shape SWRC than Hutton. (iv) Saturated hydraulic conductivity (Ks) was determined in situ by the constant head permeameter and was higher in the Clovelly soil. (v) Unsaturated hydraulic conductivity (KL) and hydraulic diffusivity (Df) were estimated from the SWRCs by the Mualem-van Genuchten conductivity model. The Clovelly soil horizons had higher KL and Df. (vi) The pore volume-pore size response curve was inferred from the SWRC by applying the capillary theory. The classification of the soil pore by their function proposed in the literature was also modified based on the concepts of field water capacity of soils. In the second content chapter, the influence of soil water content and temperature on thermal properties (Kt, C and D) was analysed in laboratory using the KD2 Pro Thermal Analyser. Five water and temperature levels were investigated in a two factor factorial experiment. The results showed an interaction effect between water and temperature on all thermal properties. From the analysis, three important water content and temperature combinations were identified, i.e. wetting of a dry soil with a rising temperature (up to 60oC), the effect of freezing (0oC) and thawing (10oC) with increasing water content, and the excessive wetting of soils beyond 0.16 mm3 mm-3 with increasing temperature. The relationship between thermal properties with the combined effect of soil water content and temperature was non-linear. A mathematical model was also developed with an average R2 of 0.8 that enables to estimate the three thermal properties by using the soil water content and temperature data. The validation procedure showed that the model can predict thermal properties within the temperature range of 0-60oC. In the last content chapter, soil water evaporation and temperature distribution on the different water table depths was characterised. Time series soil water and temperature data was collected by using DFM probes. The soil water evaporation was determined by the water balance technique. Whereas the effect of water table depth on the diurnal temperature distribution was analysed by using the daily experimental mean, water table mean and the daytime amplitude temperatures as indicators of temperature variation. The study showed that the daily rate of evaporation (Es) and the cumulative evaporation (ΣEs) was highly influenced by potential evaporation and the soil’s hydraulic properties. In shallow water table depths of 0-500 mm, Es was shown to be dependent on the potential evaporation. However, as the water table depth increased beyond 500 mm, the soil hydraulic properties were the determinant factors. The results showed that as the water table depth increased, daily and cumulative evaporation also increased linearly. The relative loss of water by Es was compared among the water table depths and the average contributions from two measurement cycles varied from a minimum of 5.55% from the No WT treatment to a maximum of 100% from the 0 mm depth. The influence of soil type was also significant with higher Es in Hutton soil.The distribution of temperature was affected by the water table depth. As the water table depth increased, the temperature of the profiles increased and vice versa. But there was no significant difference in temperature distribution between the two soils. A significant amount of water is lost through Es from shallow water tables especially from 0-500 mm depth. This unproductive water loss can be converted into transpiration if water table depth under irrigated crop fields are maintained within 500 to 1000 mm depth. In addition, integration of conservation tillage practices such as mulches and zero tillage could reduce evaporation. A further study on the contributions of shallow water table depths on evaporation under different conservation tillage are recommended.
Open Access
Downscaling of global circulation model predictions to daily rainfall over the upper Olifants River catchment
(University of the Free State, 2008) Steyn, Abraham Stephanus; Walker, Sue; Engelbrecht, Francois A.
English: Climate change could have far reaching consequences for all spheres of life. Continued greenhouse gas (GHG) emissions at or above current rates will cause further warming and induce further changes in the global climate system. This is particularly true for southern Africa where an ever-increasing population is already causing an increase in the demand for fresh water and much of the agricultural food production depends on rain. Global Circulation Models (GCMs) are the main source of climate projections under varying GHG emission scenarios. The spatial resolution of GCMs is too coarse to resolve sub-grid processes such as convection and precipitation. However, agrohydrological application models often require information at a network of point locations, implying the need to downscale the GCM output. Downscaling approaches have subsequently emerged as a means of employing large-scale atmospheric predictor variables (such as the 500 hPa meridional velocity) to develop station-scale meteorological series. Variables such as daily rainfall, which are not always accurately represented by the GCMs, can be derived using statistical approaches to build relationships between the required forecast parameter and variables that are simulated more accurately. Previous investigators have used the statistical downscaling model (SDSM) to downscale climate projections of daily rainfall over North America and Europe. A similar methodology was adopted to downscale daily rainfall projections under the A2 and B2 emission scenarios at five selected quaternary catchments (QCs) within the Upper Olifants River catchment. The downscaling was performed for the summer months of December, January and February (DJF). The set of generic predictors which were identified across all five QCs included surface airflow strength, vorticity, divergence and specific humidity, 850 hPa wind direction and relative humidity as well as 500 hPa relative humidity and meridional wind velocity. Generally, all the predictors exhibited a reasonably low explanatory power. The considerable variation in the resultant correlations between the large-scale predictors and the observed daily precipitation at the selected QCs may very well have stemmed from the convective nature of the rainfall patterns, being irregularly distributed in space and time. Generally, the downscaling model results were not very encouraging as the model failed to produce satisfactory results for four of the five QCs. For one of the QCs, namely Groblersdal, the projected changes for the future climate were assessed by calculating several delta-statistics. Only a few of the indices revealed a clear change, while most indices exhibited inconsistent changes for DJF across three future periods centred on the 2020s, 2050s and 2080s. Similar changes in the characteristics of the daily rainfall series are projected for the early and mid 21st century under the A2 and B2 scenarios. Differences in the expected GHG forcing under the B2 scenario does not seem to affect any of the rainfall indices differently from the A2 scenario until the late 21st century. It should however be noted that the projected changes are often smaller than the model errors which implies that the downscaling model is simply not sensitive enough for the projected changes to be taken at face value. Therefore the results should only be used with caution. The fact that the downscaling procedure provides similar results for the A2 and B2 scenarios suggests that it is at least to some extent robust and stable.
Open Access
The compaction susceptibility of soils in the Free State
(University of the Free State, 1998-12) Nhantumbo, Alfredo Bernardino Julio Da Costa; Bennie, A. T. P.
Crop production in the Free State is mainly performed on sandy soils. The compaction of these apedal soils can present a management problem which affects crop yields and financial returns. In addition, environmental problems such as soil erosion and siltation of rivers may result. The main objective of the study was to see whether the easily measured soil properties, texture and organic matter, could be used to classify the compact ion degree of soils and to predict their compaction susceptibility. Additional objectives were to define critical densities beyond which crop growth would be impeded, to test the applicability of the relative bulk density concept proposed by Bennie & Van Antwerpen (1988) and to propose a procedure for the prediction and classification of the compactibility of the soils in the Free State. Finally, the results were compared to the data from forestry soils (Smith, 1995) for more humid conditions. Twenty two (22) samples of selected soils covering a large range of soil texture variation were submitted to a determination of the maximum bulk density (using the Proctor test), minimum bulk density, uniaxial compression test and penetrometer resistance measurements. A procedure for the determination of minimum bulk density was developed. Good quadratic relationships were obtained between the maximum bulk density and silt plus clay or organic matter content as loss on ignition. The minimum bulk density can be predicted from the silt plus clay percentage or loss on ignition. When the actual bulk density and the silt plus clay or loss on ignition percentages are available, the relationships allow the estimation of maximum and minimum bulk densities. The measured bulk density can then be used to calculate the relative bulk density of a soil. Threshold relative bulk density classes (5) were derived which can be used to evaluate the degree of denseness or looseness of the soil, and its suitability for different applications. It is proposed that compactibility be regarded as the difference between the maximum and minimum bulk densities, instead of simply the maximum bulk density. Different classes of compactibility were derived. The compression index of all the soils at the critical water content showed good relationships with the silt plus clay, clay or organic matter contents of the soils. Compressibility classes were also established. A combination of the compactibility and compressibility of soils was used to determine its susceptibility to compaction. Compaction susceptibility was classified into five groups varying from very low to very high degrees of susceptibility for apedal soils. The data set from this study when combined with the data reported from the study of forestry soils by Smith (1995), made it possible to derive equations with a more universal applicability for the prediction of the maximum and minimum bulk densities, critical water content as well as compression index. The results from this investigation, when properly transferred in popular terminology set guidelines for the evaluation of the degree of compactness and compaction susceptibility of soils. These guidelines will allow persons responsible for soil evaluation to draw more meaningful conclusions from bulk density measurements.
Open Access
Grondversuring en bekalking van Oos-Vrystaatse gronde onder droëlandkoring
(University of the Free State, 2003-04) Van Zyl, Hendrik Johannnes Jakobus; Du Preez, C. C.; Kilian, W. H.
English: The value of liming of agricultural land has been demonstrated all over the world during the past century. Therefore liming as an agro-economical practice has been adopted widely by farmers with a beneficial impact on sustainable crop production for future generations. However, soil acidity is still a common yield limiting factor for a variety of crops in the Free State, Mpumalanga, KwaZulu-Natal and parts of the Western Cape. Due to above mentioned reasons incubation and field experiments were conducted over several years to quantify the following after liming of various Eastern Free State soils. Firstly, the changes in soil acidity parameters and secondly, the growth and development of wheat cultivars that differ in tolerance to aluminium (AI). Over time liming increased the pH(KCI) and decreased the percentage acid saturation (%AS) significantly in the top- (0-200mm) and subsoil (200-400mm) of all the localities. In the topsoils a good correlation was obtained between pH(KCI) and %AS. An increase in pH(KCI) of 0.1 will more or less be concomitant with a decrease of 5.5 in %AS. The amount of lime needed to raise the pH(KCI) or to lower the %AS up to a specific level, differed between the localities because of buffer capacity. Since pH(KCI) showed a better correlation with lime than %AS, it was decide to use pH(KCI) as parameter to develop a model with which the lime requirement of a soil can be calculated: Y=8.3240X1-O.0459X2-1.0370, where Y=lime requirement (ton ha"), X1=~pH(KCI and X2=clay content (%) At all localities the good AI-tolerant cultivar gave significant higher yields than the medium and poor AI-tolerant cultivars. When no lime was applied on the unlimed soils the good AI-tolerant cultivar produced economical yields, which was not the case with the medium and paar AI-tolerant cultivars. The medium and poor Al-tolerant cultivars responded therefore very well to liming with yield increases up to 90%. The good, medium and poor Al-tolerant cultivars showed a yield decrease of 15% at an %AS of 30, 10 and 8, respectively. Considering that %AS correlated better with yield than pH(KCI), liming to the required %AS of a cultivar is a possibility. Differences in AI-tolerance between cultivars resulted in that the optimum economic lime levels differ between cultivars. The optimum lime level of the cultivar with good AI-tolerance was the lowest, followed by the medium and paar AI-tolerant cultivars. Liming of acid soil has become an expensive practice for farmers, which led to the adoption of planting AI-tolerant cultivars to bridge the soil acidity problem. During this process soils has slowly started to deteriorate with an accompanying lost of yield and income per hectare. The results of this study clearly indicate the economical value of liming acid soils for wheat production.
Open Access
Quantifying spatio-temporal soil water content using electromagnetic induction
(University of the Free State, 2017) Edeh, Judith Amarachukwu; Barnard, J. H.; Van Rensburg, L. D.
Water scarcity is still a global concern, and the fact that water is not evenly distributed within the soil remains a case study in agriculture. Apparent electrical conductivity (ECₐ) measured with the EM38 devices have been consequently used to distinguished areas of water management in precision agriculture, before irrigation planning. However, to efficiently use EM38 and its newer model “EM38-MK2” required site specific calibration. This involves collecting soil samples for volumetric water content the same time the device is used. Repeated soil sampling over time series have been reportedly stated to be time consuming and destructive. Therefore, this thesis proposed to use DFM capacitance probes that only need to be installed once in the soil to continuously record water content. The study presented three main objectives to: (i) examine the influence of DFM probes, and other possible obstructions including neutron water meter galvanized-steel access tubes and profile pits on ECₐ measurements with the EM38-MK2, (ii) calibrate the EM38-MK2 using DFM probes installed in the field, and (iii) spatially characterize soil water content estimated from multiple EM38-MK2 surveys. On relative homogenous soils of Kenilworth Experimental Farm and with DFM probes, steel access tubes and profile pits consecutively inserted into the soil, EM38-MK2 was moved towards these interferences, over it and away from it without zeroing the EM-device. Results showed that while trenches had no effect, both DFM and steel tubes influenced ECₐ readings when the EM-device was closer than 1 m to these instruments. This effect was inconsistent with large values that were either negative or positive. After encountering the interferences and without EM zeroing, ECₐ readings were either less stable (only at vertical mode for the DFM) or reduced. Although the instability was statistically significant, the mean ECₐ before and after the probe-interference were not significantly different. The decreases in mean ECₐ values at horizontal mode for DFM and at both modes for steel tubes were all relatively small (< 2 mS m-1). This study concluded that the EM38-MK2 can be used together with DFM probes, but keeping the EM-device at least 1 m away from the probes. On a practical level, there should be no need to re-zero the EM38-MK2 after an encounter with such metal-containing interferences. Rather, re-zeroing is advised after extended use in the field as suggested by other researchers. On the heterogeneous soils of Paradys Experimental Farm comprising of four diverse soil forms, field calibration of DFM probes and EM38-MK2 were performed under both dry and wet conditions. The calibrated capacitance probes accurately predicted water content that spatially explained on average, up to 96% of the observed water content. The DFM estimated soil water values on individual plots were consistent and were used for site-specific calibration of EM38-MK2. ECₐ-based estimated water content for individual plot models explained on average, 97% and 90% of variation in soil water content, at 0.38 m and 0.75 m depth, respectively. With the general models ECₐ values could predict 74% and 69% of the volumetric soil water content at 0.38 m and 0.75 m, respectively. This was regarded as satisfactory, especially considering the heterogeneity of the soils on the experimental site. Therefore, the models developed in this study, performed well both at individual plot and over spatial scales. When the general models were applied on spatial scale, ECₐ-based estimated water content was temporally stable. The spatio-temporal soil water maps produced an accurate representation of topographical effects on soil water distribution over the area. Therefore, the proposed use of the DFM capacitance probe method for site specific-calibration of EM38-MK2 was successful and could be adopted for future research.
Open Access
Soil quality of kikuyu, ryegrass and clover pasture mixtures in the Tsitsikamma
(University of the Free State, 2016) Phohlo, Motsedisi Portia; Kotze, E.; Du Preez, C. C.
South African soils have long been classified as being severely degraded. The state of the soils is even more pronounced in sandy soils that are managed for pasture production in the Tsitsikamma region. This is mainly due to the fact that these soils have poor soil organic matter (SOM) content and poor soil fertility. The result of this is nutrient leaching which leads to contamination of ground water; water loss through deep percolation resulting in wasteful irrigation; poor pasture yields which have a direct influence on farm efficiency and profitability. Such occurrences are more detrimental in the dairy farming industry because the quality of soil and quantity of pasture produced has an overriding influence on the main farm produce, which is milk. A system of continuous supply of nutrients and irrigation is not a sustainable system for dairy farmers as it results in enormous financial pressure. Better strategies that ensure effective use of resources need to be developed and implemented and must compliment sustainable farming. Assessment of soil quality is one of the fundamental methods that have long been identified as tools, which farmers can use in order to improve farm efficiency. Soil quality as defined by Karlen et al. (1996) is the capacity of a specific kind of soil to function within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation. Managing and understanding soil quality means evaluating and managing soil so that it functions optimally now, and in the future. Land managers should be monitoring changes in soil quality on a regular basis, and using this to adopt sustainable practices, which aims to improve the productivity of soil (Doran, 2000). Soil quality can only be measured by assessment of its indicators which vary according to cropping systems. The general consensus amongst researchers is ensuring that indicators of soil quality should reflect the soils’ chemical, physical and biological status. In this study selected indicators of soil chemistry (extractable P, exchangeable Ca, K, Mg and Na, pH (KCl)); soil physics (bulk density (BD)) and soil biology (total carbon (C), active C, total nitrogen (N), C/N ratio, PMN rate and inorganic N) were measured. The selection thereof was based on their ease and reliability of measurement, the sensitivity of the measurement to changes in soil management as well as the skills essential for interpreting the results. The study was carried out in the Tsitsikamma region of the Eastern Cape where farms distributed in the upper and lower Tsitsikamma region were selected as study sites. The farms were selected based on the criteria that they were all irrigated farms with kikuyu, ryegrass and clover pasture mixtures, they had adopted minimum tillage or no tillage practices, had pastures established for at least 6 years, and lastly had accurate records of management practices that had been implemented, especially those relating to fertiliser application. An average of 5675 soil samples were analysed across the farms. These samples were taken at increments of 0-15, 15-30, 30-45 and 45-60 cm, respectively. The samples were analysed using the Veris spectrophotometer probe otherwise known as the Veris P4000. Calibration soil samples were also taken and analysed by a commercial laboratory (BemLab, De Beers RD, Somerset West, South Africa) in order to standardize the soil samples analysed with the Veris P4000. Based on the selected indicators, the objectives of the study were structured to answer 3 principal research questions, namely: Firstly, do the farms in the Tsitsikamma differ significantly within soil depth? The soil depth comparison was done at increments of 0-15, 15-30, 30-45 and 45-60 cm respectively, while farm comparisons were done at a 0-30 cm increment; Secondly, are management practices responsible for variations observed in the Tsitsikamma region?; And lastly, which soil quality indicators play the most significant role in the variations observed? Data used for this study was presented in concentration (% or mg/kg) and in stock (kg/ha) values. The analysis of variance was measured at 99% confidence level. Values that had significant differences had p values < 0.001, whereas those that showed no significant difference had p values > 0.001. Correlations between soil quality indicators were analysed using two-tailed Pearson correlation tests at 1% and 5% level. Principal component analysis (PCA) was computed using SPSS statistical program. The findings showed that both the upper and lower Tsitsikamma followed the same trend in terms of nutrient movement through soil depth. It was observed that the most significant differences occurred within the 0-30 cm depth for all indicators except for pH (KCl) and C/N ratio. The two former indicators showed statistical significances in all depth layers with a very gradual decline with depth in both regions of the Tsitsikamma. The results further showed that farms differed significantly within this region and that management practices had a significant influence in the differences observed. This was clearly illustrated in the PCA conducted, which grouped the farms according to similar management practices with those farms that had a more biological approach falling in the same category. Incidentally, the farms that had been more chemical dependent fell into their own category. Furthermore, farms that exhibited better SOM indicators generally held more nutrients, even though no heavy applications of those nutrients were done in the sampling year. Farms that were observed to also have more concentration of nutrients in the soil, even with poor SOM content, had applied those nutrients in chemical fertilisers during the sampling year. This therefore justified as to why those farms also had more nutrients in the soil. The PCA conducted also showed that 54% of the variations observed in the Tsitsikamma region could be explained in the following order by these indicators: total N, pH, exchangeable Ca, exchangeable Mg, total C, active C, exchangeable K and BD. These findings emphasised the need for farmers to not only focus on replenishing or managing N in the soil, but also to pay careful attention to pH, exchangeable Ca, exchangeable Mg, total C, exchangeable K, and BD in order to improve soil quality. The findings also highlighted the urgent need for farmers to change their line of thinking and abandon soil management practices that enhance soil degradation, a problem that is very common in South Africa. Proper management of soil quality is vital in ensuring sustainable soil management and food security; therefore researchers along with governments need to build a better transfer of knowledge to farmers in order to ensure the former.
Open Access
Evaluation of salinity and irrigation guidelines for lucerne
(University of the Free State, 2017-02) Fourie, Kevin Louis; Van Rensburg, L. D.; Barnard, J.
Evaluation of salinity and irrigation guidelines for lucerne remains important to improve current management practices under irrigation. Internationally, well-established yield response curves, set over 30 years ago, serves as a general guide for salinity management. However, more specific guidelines for lucerne production under South African conditions are needed. The aim of this study was to determine the effect of increasing irrigation and soil water salinity on the water uptake and yield of lucerne and evaluate simulations of these results, with the model SWAMP, under osmotic stress conditions. An experiment was conducted in a non-weighing lysimeter facility. Lucerne (cv. SA Standard) was grown under controlled conditions using irrigation water with salinities that ranged from a control treatment up to 1200 mS m-1. Irrigation water of the different treatments consisted of various amounts of salts to achieve the desired concentrations. The soil water balance was used to reflect on water gains and losses during the growing season. The mean daily transpiration rate as well as the seasonal transpiration of the cuttings decreased with increasing irrigation water salinity. Similarly, water table depletion and yield decreased with increasing water and soil salinity. The relationship between relative mean above-ground biomass and water salinity was curve linear, which differs from the well-established relationship reported in literature. A calculated critical level divided water and soil salinity into two management classes each with different rates of a reduction in yield. A linear decrease in the crop productivity with an increase in water salinity was obtained. The cultivar SA Standard is more salt tolerant than those used in literature. Results from the lysimeter trail was used to validate water uptake and yield simulation under osmotic stress conditions with SWAMP. Most of the soil parameters e.g. evaporation, transpiration, root density, infiltration and redistribution of rainfall and/or irrigation water, drainage and water table uptake have been calibrated for the two soils. Data from the control treatment was used to calibrate the parameters used in simulating the transpiration requirement. Default values were used for the remaining parameters. Various indices and test statistics were aggregated into a single indicator module (ISWAMP when 0 = good and when 1 = poor) with a fuzzy-logic based expert system, which represent the model’s aggregated accuracy, correlation and pattern performance. SWAMP was able to reasonably simulate a yield decline due to an in increase in water salinity (ISWAMP = 0.0903), which was also true for seasonal transpiration (ISWAMP = 0.0305). Weekly simulations of transpiration were not good. A high pattern value indicated the presence of some macro-patterns. This was attributed to the fact that the residuals were not evenly distributed during the growing season, which was not the case with an increase in water salinity. Hence, the crop growth algorithm for simulating the daily transpiration requirement needs to be improved.