An evaluation of water extraction as a soil-testing procedure for phosphorus II. Factors affecting the amounts of water-extractable phosphorus in field soils

Abstract

The effects of seasonal variation, sampling depth, and fertilizer P addition on water-extractable P values were investigated in two field experiments, involving soils of contrasting P retention capacity (Ramiha and Tokomaru) under permanent pasture over 12 months. The effects of the same parameters on Olsen-extractable P were also evaluated. The amounts of water-extractable P in soil were always lower than those of Olsen-extractable P. Over the 12-month period, the average value of water-extractable P in the unfertilized Ramiha soil (0–7.5 cm depth) was 1.8µg g⁻¹ soil compared to an Olsen-extractable P value of 12.6µg g⁻¹. The variability associated with water-extractable P at each sampling time was comparable with that for Olsen-extractable P. However, the relative seasonal variation over 12 months was larger for water-extractable P than for Olsen-extractable P. The results obtained with both extractants showed a seasonal fluctuation which was closely related to the pattern of pasture P uptake. The amounts of water- and Olsen-extractable P were higher in samples taken from the 0–4.0 cm than the 0–7.5 cm sampling depth. Fertilizer P addition resulted in larger increases in water-extractable P in the 0–4.0 cm sampling depth than in the 0–7.5 cm depth. The relative increase in water-extractable P following fertilizer P addition was larger than that of Olsen-extractable P. Seasonal changes in the soil microbial biomass P were not related to changes in either water-extractable P or plant uptake of P. Microbial biomass P may be a less sensitive index of soil P availability than is commonly thought.