Agronomic evaluation of unacidulated and partially acidulated Minjingu andChilembwe phosphate rocks for clover production in Ethiopia

Most soils in sub-Saharan Africa are P deficient, but the high cost of wate r-soluble P fertilizers limits the use of P fertilizers by resource-poor fa rmers. A low-cost alternative is to apply phosphate rocks. We evaluated the effectiveness of unacidulated (PR), 25% partially acidulated (PAPR25), and 50% partially acidulated (PAPR50) Minjingu and Chilembwe phosphate rocks r elative to triple superphosphate (TSP) on Trifolium quartinianum grown on a Vertisol in the Ethiopian highlands. The fertilizers were applied once at 0-80 kg P ha(-1) and their effects were followed in four consecutive clover crops. In the Minjingu phosphate rock experiment, clover DM yields without applied P were below 1 t DM ha(-1) in all except the first of four consecu tive crops. With P application, herbage yields reached as high as 14 t ha(- 1). Significant (P<0.05) effects of P on clover DM and P uptake were observ ed at least up to crop 3. Over all the four crops, PR was 114%, PAPR25 was 113% and PAPR50 was 107% as effective as TSP in increasing clover herbage y ields. The corresponding relative responses in P uptake were: 128, 121 and 112% for PR, PAPR25 and PAPR50, respectively. The substitution rates in her bage yields were: 130% for PR, 128% for PAPR25 and 114% for PAPR50 while th ose for P uptake were 164% for PIP, 146% for PAPR25 and 125% for PAPR50. In the Chilembwe phosphate rock experiment, clover DM yields without applied P were below 1.3 t DM ha(-1) in the five years of the study. With P applica tion, herbage yields reached as high as 7 t ha(-1). Significant effects of P on clover DM and P uptake were observed only in the first two crops. Over all the five crops, PR was 27%, PAPR25 was 57% and PAPR50 was 73% as effec tive as TSP in increasing clover herbage yields. The corresponding relative responses in P uptake were 18, 49 and 59% for PR, PAPR25 and PAPR50, respe ctively. The substitution rates in herbage yields were 7% for PR, 32% for P APR25 and 53% for PAPR50 while those for P uptake were 3% for PR, 24% for P APR25 and 35% for PAPR50. It was concluded that raw Minjingu phosphate rock is highly effective on clover in these Vertisols and partial acidulation i s not necessary, but raw Chilembwe phosphate rock is ineffective and 50% pa rtial acidulation is recommended. (C) 1999 Elsevier Science B.V. All rights reserved.