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| hi Jared,
you are asking a great question... there is lots of evidence that applying lime to acid soils (for example raising soil pH from 5 - 6) increases plant uptake of P without any addition of P. If soil test procedures worked perfectly they would reflect this increase in availability.
Unfortunately, research suggests that the impact of liming on soil test P (i.e., the level of P extracted by various soil testing procedures measured) is inconsistent due to the variable effects of pH and the presence of free lime on the efficiency of different soil test extraction procedures.
My guess is that the most serious problems occur when soil samples are collected fairly soon (e.g., within a few months) after liming and that soil test results are more related to plant uptake the longer one waits (e.g., a year) after liming.
I will look into this more and post additional thoughts later... perhaps Keith in Ontario and others have some thoughts to share.
Joel
WIU Agriculture
BTW Here are the abstracts for 2 articles that discuss this issue:
Effect of liming on phosphate extracted by two soil-testing procedures
R. Naidu, R. W. Tillman, J. K. Syers and J. H. Kirkman
Journal Nutrient Cycling in Agroecosystems
Issue Volume 14, Number 2 / June, 1987
Abstract
Lime and phosphate (P) additions had a variable effect on Olsen- and Mehlich-extractable P in 4 acid soils from Fiji. Olsen-extractable P was at a minimum between pH values of 5.5–6.0, on either side of which it increased, particularly in soils which received large amounts of added P. The initial decrease in Olsen-extractable P is attributed to the removal of P from solution by precipitation during the Olsen extraction. The increase at higher pH values is thought to be due to the slow release of P from precipitated Ca-P compounds. There was a consistent decrease in Mehlich-extractable P with increasing soil pH. When the pH of the Mehlich reagent was kept constant, using an autotitrator, there was no decrease in Mehlich-extractable P, suggesting that in the absence of pH control the decrease in extractable P was largely due to the neutralizing effect of lime on the Mehlich reagent.
Assessment of plant-available phosphate in limed, acid soils using several soil-testing procedures
R. Naidu, J. K. Syers, R. W. Tillman and J. H. Kirkman
Journal Nutrient Cycling in Agroecosystems
Issue Volume 30, Number 1 / October, 1991
Abstract
A range of soil-testing procedures was used in a factorial glasshouse study to assess the plant-available phosphate (P) status of soils which had been treated with lime and added P. A close 1:1 relationship (r = 0.90***) was obtained between plant P uptake and resin-extractable soil P. In contrast, Olsen-, Colwell-, Bray (I) and (II)-, and Mehlich-extractable P were only weakly correlated with P uptake. Inclusion of 4 different indices of P-buffer capacity did not improve the relationship between plant P uptake, and extractable P. The difficulty in relating plant P uptake data to extractable-soil P levels is attributed to the problems associated with extracting P from limed soils. There was no useful relationship between plant P uptake and isotopically-exchangeable P in the soils.
Edited by jbgruver 4/5/2010 12:01
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presentation on soil acidity
