Gypsum amendment and exchangeable calcium and magnesium related to water quality and plant nutrition

The beneficial effects of gypsum and Ca on erosion control by improving soil structure and increasing water infiltration has been reported. Phosphorus loss in agricultural systems is strongly associated with runoff and erosion, while N loss is more related to subsurface drainage. Changes in soil solution nutrient concentrations may influence not only the availability for transport from soil to water but also the availability for plants with consequences on nutrient absorption and growth. Rainfall, column and greenhouse experiments were conducted to evaluate the effect of gypsum and exchangeable Ca and Mg on loss of N and P in runoff and leaching, and on nutrient absorption and plant growth. The treatments were: control; gypsum (CaSO4·2H 2O at 5000 kg ha−1); and five different soil exchangeable Ca/Mg ratios. Calcium and Mg on the exchange complex were modified by saturation with CaCl2/MgCl2 solutions according to the target ratio followed by washing with CaSO4/MgSO4 solution to adjust the electrolyte concentration similar to the control. Treatment with gypsum significantly decreased concentration and loss of dissolved reactive P, total P, soluble NH4-N, and total N in runoff when compared with control. Total recovery by leaching of all P forms and soluble NH4-N was decreased and increased, respectively, by gypsum application. Nitrate N in runoff as well as in leachates was not affected by gypsum amendment. Exchangeable Ca/Mg ratios did not affect the transport of nutrients, except in the column experiment, where the transport of sediment bound-P was decreased by Ca-treated soil compared with the Mg-treated soil, as a result of a decrease in clay dispersion. The great effect of electrolyte concentration on decreasing surface transport of N and P was verified. Results from all three experiments demonstrated the importance of Ca in the soil solution on decreasing P and increasing NH 4-N water solubility. Exchangeable Ca/Mg ratios affected Ca and Mg but did not consistently affect K and N uptake or root and shoot growth. Compared with the control, gypsum increased K and Ca, decreased P, but did not interfere with Mg and N uptake or with root and shoot growth