Phosphorus Buffering Capacity Indices as Related to Soil Properties and Plant Uptake

ABSTRACT

Evaluation of the plant-available phosphorus (P) in calcareous soils is commonly performed by removing a portion of solid phase P using chemical extractants. Critical soil test values, however, may be affected by variation in sorption and buffering behavior of different soils. The objective of this study was to evaluate the importance of buffering capacity indices to predict P uptake by wheat (Triticum aestivum). Eleven surface soil samples were assayed for a number of P intensity (CaCl₂-P) and quantity (Olsen-P, Colwell-P, and Resin-P) factors. Some phosphorus buffering indices were obtained from P sorption equations. A single-point index of buffering was also determined experimentally. In a greenhouse experiment, wheat was grown for 35 and 70 days on the same soils and P uptake was determined. Nonlinear and linear equations described the P sorption data (P < 0.001). Buffering indices derived from these equations were highly correlated with single-point index of capacity. Clay content was the most important soil property affecting the buffering capacity factor. The phosphorus intensity index (CaCl₂-P) was weakly related to P uptake (P < 0.05). Among the quantity factors only Resin-P was significantly correlated with P uptake. Buffering indices showed significant but inverse relationships with P uptake only at 70 days harvest (r = −0.69 to −0.71; P < 0.05). Combination of intensity or quantity factors with buffering capacity indices, such as intensity/capacity or quantity/capacity indicators, improved considerably the ability to account for variations in P uptake by wheat.

Keyword:

• phosphorus
• sorption
• buffering capacity
• calcareous soil
• plant-availability
• wheat

Notes

^a In 1:2.5 soil: water suspensions.

^b Organic carbon.

^c Calcium carbonate equivalent.

^a All the values of determination coefficients are significant at P < 0.001.

^a μ g g^{− 1} soil.

^b Maximum buffering capacity, the slope of the tangent at the origin of the Langmuir equation.

^c Buffering index, the slope of the tangent of the Freundlich equation at I = 1 μg mL⁻¹.

^d Phosphorus buffering capacity, the slope of the linear equation.

^e Phosphate sorption index, the single-point index calculated from single addition of P to soil.

***: Significant at P < 0.001.

*Significant at P < 0.05.

^ns: Nonsignificant.

^a Calcium carbonate equivalent.

^b Organic carbon.

^c Cation exchange capacity.

***: Significant at P < 0.001.

^a Organic carbon.

^b Calcium carbonate equivalent.

^a For each soil, P uptake value is the mean of four replicates.