INDUCED PHOSPHORUS FIXATION AND THE EFFECTIVENESS OF PHOSPHATE FERTILIZERS DERIVED FROM SUKULU HILLS PHOSPHATE ROCK

A greenhouse study was conducted with two surface, acidic soils (a Hiw assee loam and a Marvyn loamy sand) to measure the effect of increasin g P-fixation capacity, on the relative agronomic effectiveness (RAE) o f phosphate fertilizers derived from Sukulu Hills phosphate rock (PR) from Uganda. Prior to fertilizer application, Fe-gel was added to incr ease P-fixation capacity from 4.4 to 14.3% for the Marvyn soil and fro m 37.0 to 61.5% for the Hiwassee soil. Phosphate materials included co mpacted Sukulu Hills concentrate PR + Triple superphosphate (CTSP) at a total P ratio of PR:TSP = 50:50; 50% partially acidulated PR (CPAPR) from Sukulu Hills concentrate PR made with H2SO4; and Sukulu Hills co ncentrate PR (PRC) made by magnetically removing iron oxide from raw P R ore. Triple superphosphate (TSP) was used as a reference fertilizer. After adjusting soil pH to approximately 6, P sources were applied at rates of 0, 50, 150, and 300 mg total P kg(-1) soil. Two successive c rops of 5 week old corn seedlings (Zea mays L.) were grown. The result s show that the RAE of the phosphate materials measured using dry-matt er yield or P uptake generally decreased as P-fixation capacity was in creased for both soils. CTSP was more effective in increasing dry-matt er yield and P uptake than CPAPR. PRC alone was an ineffective P sourc e. Soil chemical analysis showed that Pray 1 and Mehlich 1 extractants were ineffective on the high P-fixation capacity Fe-gel amended Hiwas see soil. Mehlich 1 was unsuitable for soils treated with PRC since it apparently solubilizes unreactive PR. When all of the soils and P sou rces were considered together, Pi paper was the most reliable test for estimating plant available P.