Uptake and partitioning of salt by wheat and maize under irrigation in a semi-arid climate

Abstract

Irrigated soil is affected by salinity on a global scale, resulting in impaired crop development and reduced yields. Mitigation measures exist to decrease root zone salinity through leaching practices but this has detrimental offsite impacts. With increasing temperatures inducing water limitations, additional tactics are required to decrease root zone salinity. Crop salt removal is a tactic often overlooked due to the generally assumed small quantity of salts removed when crops are harvested but can aid as a viable measurement when quantified, especially for precision agriculture. This study therefore presented three research aims with focus on the salinity inducing cations; potassium, calcium, magnesium and sodium. The first research aim was to quantify cation uptake and partitioning of wheat (Triticum aestivum L.), maize (Zea mays L.) and popcorn (Zea mays everta), while the second research aim quantified the relationship between soil cations and cation uptake by these crops. The third research aim tried to spatially characterise soil cations and thus cation uptake using apparent soil electrical conductivity (ECa) measurements. Electromagnetic induction (EMI) surveys were conducted on three fields under centre pivots, which were located on commercial irrigation farms in the districts of Douglas and Luckhoff. These surveys were conducted at the beginning of the wheat (June 2016) and beginning of the maize and popcorn (December 2016) season. Soil and plant sampling points were identified based on the degree of soil apparent electrical conductivity (ECa) variability determined with the “Electrical Conductivity Sampling Assessment and Prediction” (ESAP) software and its featured “Response Surface Sampling Design” (RSSD) sampling methodology. The topsoil (0-300 mm) was sampled at the beginning of each crops’ season before planting, while the crops were sampled past physiological maturity (at harvest). Standard laboratory procedures were used to analyse the soil properties; bulk density, clay, gravimetric water, pH(water) and electrical conductivity of the saturation extract (ECe). Soil plant available ions were extracted with the Ambic-2 extraction method, while the closed-tube wet decomposition with acid technique was used to obtain the plant analytes. The Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), which is an electro-spectroscopy technique, was used to analyse the cations present in the wheat, maize and popcorn plant material, as well as in the soil. For the first aim, the cation concentrations and weighted mean of the components were determined and compared with the sufficient and critical ranges of established literature, while the biomass and yield were compared with that of commercial cereal crops. Cation uptake was then calculated by multiplying the components weight with the concentration, from which biomass uptake can then be determined. The significant partitioning of cations by crop components was determined with mean separation at the 95% confidence interval. The stem-leaf of wheat, maize and popcorn was the dominant components to remove cations, (P ≤ 0.001). For the second aim, linear regression analysis was performed with Microsoft Excel to obtain a relationship between soil properties (ECe, SAR and clay) and cation uptake but only the SAR had a relationship and correlated with cation uptake. A linear relationship (R2 = 0.7) was then only found between SAR and Na+ in the soil for wheat. For the third research aim, the ESAP software was used to find ECa correlations between the afore mentioned soil properties, yield, biomass and soil cations but the results were unfavourable. Correlations for both aims were done at the 95% confidence interval. The results revealed that the mean cation uptake for the wheat, maize and popcorn biomass was 384.2 kg ha-1, 508.9 kg ha-1 and 359.8 kg ha-1, respectively. From this biomass uptake, it can be estimated that the wheat, maize and popcorn seeds partitioned a mean 11.5%, 12.7% and 8.5% of cations, respectively which is used to suit the need of the commercial market. However, crop residue is commonly left behind as a soil cover and can induce soil salinity when re-introduced into the soil medium. The wheat, maize and popcorn stem-leaf were therefore estimated to partition a mean 80.8%, 81% and 86.8% of cations, respectively. Crop salt removal can serve as an important mitigation measurement in minimising fertiliser applications and excessive irrigation practices when using crop components to enhance soil conditions such as organic matter, water retention and structure.