Soil solution composition in association with the toxicity of banded di-ammonium phosphate to wheat and amelioration by CaCO3

Abstract

Our previous publications showed that gradients of pH, electrical conductivity, ammonium, phosphorus, and calcium were formed between di-ammonium or mono-ammonium phosphate bands and roots. These gradients shifted and diminished with time. Roots suffered from ammonia toxicity near the band, but soil liming before banding ameliorated the toxicity. In the present study, DAP was banded 1 cm away from wheat (Triticum aestivum) seeds sown in slightly acidic sandy Lancelin soil that was either limed (CaCO₃) or not. After 35 days, the pH and concentration of 9 ions were measured in soil solution extracted from soil obtained at different distances between the fertiliser band and seed.

Toxicity symptoms were noted on 7-day-old plants grown in the non-limed treatment; in contrast, plants grown in the CaCO₃ treatment did not show these symptoms during the whole growth period. In comparison with the non-limed treatment, CaCO₃ addition markedly lowered the ammonium and P concentration in soil solution extracted from soil between the fertiliser band and the seed. Although a lower Ca concentration was measured in the vicinity of the DAP band in the non-limed than in the limed treatment, Ca in non-limed soil was still sufficiently high to prevent deficiency in plants, implying that there might be no ground for the association of an injurious effect of DAP and Ca deficiency as suggested in other studies. Around 2.8 mg Al/L soil solution was detected in the non-limed treatment, but liming with CaCO₃ reduced Al concentration in all soil sections, especially those with the high rooting density. Therefore, a possibility that Al toxicity was related to the DAP toxicity in non-limed soils cannot be excluded, considering that even higher Al would have existed in the soil solution in the vicinity of the fertiliser band during the first couple of days. In conclusion, the causal factors associated with DAP toxicity might be high concentration of ammonium and free ammonia resulting from hydrolysis of DAP, and high P and possibly high Al concentrations.