Establishing sorption isotherm to meet phosphorus requirement for tomato seedling growth1

Abstract

Abstract An Indiana silt loam soil was equilibrated with various amounts of Ca(H2PO4)2 H2O and a 0.01 M CaCl2 solution to construct its phosphorus sorption isotherms. Using the isotherms, the P buffering capacity of the soil was calculated and amounts of fertilizer P necessary to give several levels of P in the soil solution, for experiments conducted over a 2 year period, were determined. Twenty‐four day old tomato seedlings were grown and measured for leaf area, root length, dry weights and P concentrations in leaf, petiole, stem and root. Phosphorus concentration in soil solution increased slowly with the first increment of P added to the soil. Subsequent P additions increased the P concentrations in solution exponentially. The maximum P absorption by the soil was 324 μg P/g soil and the constant related to P binding energy was 1.37. In addition, the soil buffering capacity decreased with an increase in the amount of P in the soil solution. Plant shoot dry weight increased linearly with P increase in the concentration range 0.65 to 6.5 μM P in soil solution. However, beyond this level the response was low. The leaf area rate of increase in the 0.65 to 6.5 μM P solution concentration range was 75 times that in the 6.5 to 84 μM P. The root length: shoot dry weight ratio decreased with increasing P supply in the soil solution. P uptake by the plants increased with increased P concentration in soil solution. At soil solution concentrations above 6.5 μMP the rate of P uptake in the shoot was 20 times less than the rate for concentrations below 6.5 μM P. Of the P taken up by tomato seedlings about 65% was in the leaf, 13% in the stem, 13% in the petiole and 9% in the root.