The influence of different P, NH⁺ ₄ and NO⁻ ₃ supply levels on the yield and P and Ca relationships in wheat

Abstract

In a water culture experiment with wheat, the influence on growth and nutrient uptake of varying the P supply from 0,02 to 5 mmol H₂PO₄ ⁻ dm⁻³ and the NH₄ ⁺:NO₃ ⁻ ratio from 0:15 to 12:3, with a Hoagland nutrient solution (No 2) as control, was studied. Poor growth due to low P supply was characterized by a higher root:top growth ratio than plants which received adequate P, indicating that under these conditions root development of wheat takes preference over top growth. Root P was also higher than top growth P with low P supply, indicating that wheat requires a certain level of P in the roots before P can be transported freely to the tops. It was postulated that NO₃ ⁻ reduced the detrimental effect on growth of high P supply by causing P to be. precipitated as Ca₃(PO₄)₂ in the roots, while with high P and N supply, more Ca²⁺ is required for balanced nutrition. Increased NH₄ ⁺ supply increased the P demand with added P apparently reducing the antagonistic effect of NH₄ ⁺ on Ca²⁺ uptake. This is especially important in highly leached acid soil where the Ca²⁺ content tends to be low and fertilization early in the season should be carefully planned.