FertiLlizer Research 14:143 152 (1987) © Martinus Nijhoff Publishers, Dordrecht - Printed in the Netherlands Effect of liming on phosphate extracted by two soil-testing procedures R. NAIDU, 1 R.W. TILLMAN, J.K. SYERS & J.H. KIRKMAN 2 Department of Soil Science, Massey University, Palmerston North, New Zealand IPresent address." School of Pure and Applied Science, University of the South Pacific, Suva, Fiji: 2Present address." Department of Soil Science, The University, Newcastle upon Tyne, NE1 7RU, UK Accepted 18 May 1987 Key words: lime, soil testing, Olsen P, Mehlich P, coprecipitation, constant pH 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. Introduction It has generally been observed that liming highly weathered, acid soils increases plant growth. This has been attributed to an alleviation of A1 toxicity and/or an increase in the availability of P and other nutrients [14]. There have been a number of reports however, (e.g., [17]), based on plant P data, where liming acid soils to near neutrality has decreased P availability after an initial increase in P availability up to soil pH values of approximate- ly 6. In contrast to these observations, considerable controversy exists in the literature regarding the effects of liming on the amount of P extracted by various soil-testing procedures. For example, whereas Rhue and Hensel [12] reported an increase in Mehlich-extractable P with increasing lime rates,