Assessment of soil N, P, and K status of selected paddy growing areas of Tanzania

Abstract

Suitable diagnostic criteria to assess the N, P and K status of rice growing areas particularly in developing countries like Tanzania are required. The objective of this study was to develop such criteria in selected rice growing areas of Tanzania and relate nutrient levels to rice response to fertilizer application. Soil samples from 10 paddy growing areas of Tanzania were tested for N, P and K using selected indices. Nitrogen supply was assessed by organic carbon (OC), total Nand alkaline-KMn0_4-N while P was assessed by Olsen, Bray-1 and the iron oxide coated-filter paper strip (Pi) methods. Potassium was extracted by neutral 1 N NH₄0Ac. Extractable nutrients were correlated with DM Yields and nutrient uptake from a glasshouse experiment which was also used to assess response of rice to these nutrients. Of the three N indices evaluated, only OC correlated significantly (r = 0.73, P < 0.05) with rice DM yield thus serving as a good index of N supply. As for P indices, Olsen P gave a highly significant correlation with rice DM yield (r = 0.85, P < 0.01) while the relationships with the other two methods gave lower correlation coefficients. All the soils tested were deficient in N and thus no critical level for N could be established while six soils out of 10 were deficient in P. The critical soil P level for Olsen method was estimated to be 20 mg/kg. The extracted K values were all found to be higher than critical values established elsewhere. Therefore, N and P are important constraints to rice production in Tanzania and appropriate recommendations should be drawn and demonstrated to farmers.

Keywords: Suitable diagnostic, criteria, indices, correlation

Tanzania J. Agric. Sc. (2000) Vol. 3 No. 1, 1 - 10