Dynamics of forms of inorganic phosphorus during wheat growth in a continuous maize-wheat cropping system

The transformation of added phosphorus (P) greatly depends on the physical and chemical environment of the soil. The effect of continuous application of nitrogen (N), P and potassium (K) to a maizewheat annual sequence on the changes in Olsen- P, inorganic forms of P (saloid-P, Al-P, Fe-P and Ca-P) and total P at tillering, ear initiation and wheat harvest is reported. The application of P (17.5 or 35 kg P ha⁻¹) increased all the P forms, irrespective of the growth stage whereas, N (120 and 180 kg N ha⁻¹) and K (0 and 33.2 kg K ha⁻¹) application caused decrease in the P fractions. Olsen-P in the control plot (N₀P₀K₀) decreased from its initial status of 6.50 to 5.75 mg kg ⁻¹ after 22 cycles of maize-wheat sequence. The relative abundance of inorganic P fractions was in the order of saloid-P < Fe-P < Al-P < Ca-P. The various inorganic P fractions tended to decline with the crop age. The inorganic P forms (except saloid P) decreased after wheat harvest in comparison to their respective initial status. The increasing rate of NPK addition resulted in a significant increase in P content of plant/plant parts at all the growth stages. The P uptake by wheat significantly increased with fertilizer addition under all the treatments. The P concentration of wheat exhibited a decline whereas, the total P uptake continued to increase with the plant age. The P uptake in the partitioned plant parts at earing stage was in the order of stem > ear > leaves. Olsen-P showed a significant relationship with all the inorganic P forms at tillering stage but it was significantly related only with Al-P and Ca-P at the ear initiation and saloid-P and Fe-P at the harvesting stage. The correlation coefficient values between inorganic forms of P at various growth stages and P concentration in/uptake by different plant parts at the respective/subsequent stage were worked out. The saloid-P served as a good index of P availability to wheat at tillering. A sizeable amount of Al-P and Ca-P was utilized between the first (tillering) and second stages (earing) of sampling. However at the harvesting stage, saloid-P mainly contributed towards P nutrition of wheat (the contribution from other P forms at the same time cannot be ignored). Based on the physico-chemical approach to the problem of P availability, the energy required for the absorption of P by wheat from saloid-P and Fe-P is less than that required for Ca-P by 389 and 733 joules, respectively. There is a refixation of the released P on Al-P, Fe-P and again on Al-P at tillering, ear initiation and harvesting stages of wheat, respectively.