Role of soil pH, Ca supply, and banded P fertilisers in modulating ammonia toxicity to wheat

Abstract

Di-ammonium phosphate (DAP) and mono-ammonium phosphate (MAP) banded close to seed may cause ammonia (NH ₃ ) toxicity by inhibiting seed germination and early seedling growth. A pH increase around the fertiliser band and/or limited Ca supply may exaggerate ammonia toxicity. In the current study, wheat growth and gradients of pH, electrical conductivity (EC), ammonium (NH ₄ ⁺ ), and P concentration in soil were studied by comparing banded DAP + urea or MAP + urea fertilisers in control or gypsum- or CaCO ₃ -treated Lancelin soil. After 21 days of wheat growth, visual symptoms of ammonia toxicity appeared in the gypsum-treated and control plants, but not in the CaCO ₃ -treated plants. The symptoms were more severe in the MAP treatments than in DAP treatments. An addition of CaCO ₃ increased soil pH CaCl2 from 5.1 to 7.3, and the ammonium concentration was lower than that in the gypsum-treated and control soils. In the gypsum-treated and control soils, pH increased by nearly 1 unit in the vicinity of the DAP or MAP band. In contrast, soil pH decreased in the location of the DAP or MAP band in the CaCO ₃ -treated soil. Banding of MAP or DAP in the gypsum-treated soil caused an increase in soil EC. The good plant growth on the soil amended with CaCO ₃ might have been related to the low ammonium concentration in the soil and the high concentration of Ca. Gypsum and CaCO₃ decreased the availability of P supplied in the MAP or DAP band. It was concluded that increased pH and higher Ca content in sandy soil may alleviate ammonia toxicity to wheat.