Leaching of excess salts from the root zone of apedal soils

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.