Soybean Root Growth and P Uptake as Affected by the Joint Application of P and Ammonium

Soil phosphorus is transported to plant roots by diffusion, which is very slow. Therefore, the position of the applied fertilizer and the plant root density will affect P uptake and utilization efficiency. It has been shown in neutral/alkaline soils that the joint application of P and N-NH⁺­₄ increases maize root proliferation and P acquisition, but this has not been shown in tropical acidic soils. An experiment was conducted in glass-walled pots, were soybean was grown for 25 days. Treatments were a control without P, monoammoium phosphate (MAP) and simple super phosphate (SSP) applied either on the soil surface or localized, close to the seed furrow. Root growth was assessed every two days. Fertilizer application increased soil P irrespective of P source and localization. Rizosphere pH was decreased by P fertilization whilst soil bulk pH was not affected. Simple superphosphate increased root length by 55 % and MAP by 76 % over the control, on average, and the number of root tips was increased by 21% on average with SSP, 58% by MAP applied on the soil surface and 76% by MAP localized. Soybean dry matter yields, N and P uptake, as well as P use efficiency were increased by P fertilization, mainly when MAP was localized. The effect of the joint application of ammonium and P on root proliferation has been attributed to a decrease in rizosphere pH and soil P release in alkaline/neutral soils. However, in this experiment rhizosphere pH was decreased by both P sources, but MAP resulted in more roots, with more ramifications and higher P uptake, showing that the effect of the joint application of P and N-NH⁺­₄ on the root system is independent of the rhizosphere pH.