Fertilizer Research 30: 47-53, 1991. © I991 KluwerAcademic Publishers. Printedin the Netherlands. 47 Assessment of plant-available phosphate in limed, acid soils using several soil-testing procedures R. Naidu 1, J.K. Syers 2, R.W. Tillman & J.H. Kirkman Department of Soil Science, Massey University, Palmerston North, New Zealand. ~Present address: CSIRO Division of Soils, Private Bag No. 2. Glen Osmond, South Australia 5064; 2Present address: Department of Agricultural and Environmental Science, University of Newcastle upon Tyne, NE1 7RU, UK Accepted 3 July 1991 Key words: Soil P test, plant available P, P-buffering capacity, limed acid soils 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. Introduction Phosphate (P) deficiency and aluminium (A1) toxicity problems in acid soils are often over- come by liming. Liming acid soils can reduce P sorption and A1 toxicity by decreasing the con- centrations of soluble A1 which could otherwise interact with fertilizer P, thereby reducing the availability of P to plants. However, there is considerable controversy in the literature regard- ing the effects of liming on the amounts of P extracted by various soil-testing procedures. For instance, it has been reported that liming highly- weathered, acid soils can increase [10], decrease [23], or not affect [17] the amount of P that can be extracted from acid soils. Although some of these conflicting reports have been explained by careful consideration of the soil-testing proce- dures used [23, 30], little work has been done to assess the ability of the procedures to predict plant-available P in acid soils which have been limed. In this study, a range of soil-testing procedures was used to assess the plant-available P status of acid soils which had been treated with lime and added P. Materials and methods Soils The soils used in this study and their preparation have been described in detail in an earlier paper [23]. All the soils were initially strongly acid with pH (M KC1) values varying in the relatively nar- row range of 3.9 to 4.9. The amounts of AI extracted by M KC1 varied considerably between the soils, ranging from 0.3mmol kg -t in the Batiri soil to 35.5 mmol kg -~ in the Nadroloulou soil. The amounts of P extracted by the Olsen reagent were low (0.01-0.1mmol kg -1) and total P was less than 0.5 mmol kg -1 in all except Seqaqa soil, in which it was 23.0 mmol kg -~ soil.