Predicting phosphorus requirements of young Pinus radiata using sequential Bray soil extraction

Abstract

We used data from 33 field experiments with 4 year old Pinus radiata in which response to phosphate fertilisers was measured. We divided the data into two categories: one in which there was cultivation and weed control, and one in which there was neither because interactions between effects of fertilisation and those of weed control are common. We used the ratio of stem volume index between unfertilized and fertilized trees as a measure of the response. We measured Bray P as an indicator of inorganic phosphorus (P) availability in the soils, and continued the extraction for a total of ten times in order to reflect the ability of the soil to continue to supply P. Responses of trees to phosphate fertilisation were related to Bray P by a Mitscherlich equation. The equation differed between the two categories. The soils differed in the extent to which they were able to maintain supply to the repeated extractions. With some soils, the amounts extracted declined rapidly with successive extractions; with others they declined less rapidly and in some soils the second extraction removed more phosphate than the first extraction. The cumulated amount of extracted P (y) was described as a function of the number of extractions (N) using the equation, y = αNb where b = βN γ. In this equation, the β parameter was a measure of the ability of the soils to maintain phosphorus supply under repeated extraction. When this parameter was added to a modified Mitscherlich equation, r-squared increased from 0.495 to 0.644 for the data in which there was weed control or soil cultivation. Response to fertilisation was significantly (P = 0.001) better predicted by the repeated Bray P extraction modelling procedure than it was by a single Bray P extraction for each soil among data from plots with either weed control or cultivation. In soils that lacked both weed control and cultivation interactions between effects of weed competition and fertilisation very likely contributed to the lack of any clear relationships between repeated Bray P extractions and response to fertilisation. Further validation of the results reported here may lead to better predictions of effects of fertilisation on crop growth from soil samples.