Gradients of pH, ammonium, and phosphorus between the fertiliser band and wheat roots

Abstract

Di-ammonium phosphate (DAP) band application generally improves phosphorus (P) fertiliser use efficiency but can cause ammonia toxicity to plants. We used specially constructed pots to study P and ammonium (NH₄⁺) gradients between the DAP band and plant roots grown in 2 soils. The pots were with or without a 30-mm nylon mesh between the roots and the fertiliser band, and had a movable side to enable sampling of soil layers at various distances from the fertiliser band and/or plant roots. After 28 days, ammonia toxicity symptoms occurred in plants grown in Lancelin soil in both mesh and no-mesh pots with the band placed 1 cm from the seed. Smaller root and shoot dry weights and a shorter root length were measured in both soils in no-mesh pots when the banding distance from the seed was 1 cm compared with 2.5 cm. In contrast, no differences in root and shoot dry weight occurred when direct contact between the roots and the fertiliser band was prevented in mesh pots; however, roots were shorter in mesh pots when fertiliser was banded 1 cm, compared with 2.5 cm, from the seed. The highest values for pH, NH₄⁺ concentration, and bicarbonate-extractable P level occurred 4–5 cm from the seed in mesh pots, and in the layer between the fertiliser band and the seed in no-mesh pots. Phosphorus concentration in shoots was greater when plants were grown in Lancelin than in New Norcia soil, and when grown in no-mesh compared with mesh pots. In conclusion, large gradients of pH, ammonium, and bicarbonate-extractable P existed around plant roots when DAP and ammonium sulfate were banded in the vicinity of the seed. The frequently employed practice of growing plants in mesh pots to characterise nutrient gradients in soil close to roots, including the rhizosphere soil, results in plants with altered growth and nutrient accumulation characteristics in comparison to plants with no physical restriction for root growth within the pot.