| Abstract:
| Soil acidity is one of the major biophysical constraints to crop production in the communal areas (CAs) of Zimbabwe. The Diagnosis and Recommendation Integrated System (DRIS) progranune has established that many crops in fields with considerable P-fertility build-up respond poorly to fertilizer applications when the soil pH is not corrected. Upon correction of soil pH through Urning there is often a substantial increase in NP fertilizer use e.fficiency as reflected by increased stover and grain yields and nutrient uptake. Maize grain yield increases, ranging from 0.6 t/ha to 2.64 t/ha. were realized on limed plots when compared with un-limed plots. Percent increase in gross margin per hectare ranged from 6 to 68 over the un-limed control In Chinarnhora CA (a high potential area), 43% of the soils in 1992-94 had pH values (in O.OlM CaCW of 4.0 to 4.5 (very strongly acid) compared with just 1996 in 1982- 84. In 1992-94, 34% of the soil samples were strongly acidic (4.6 to 5.0) compared with 22% in 1982-84. During the same period about 12% of the soils had become strongly acid. Soils with pH values above 5.0, which is the favorable range for most arable crops, had decreased by about 35%. The results indicated potential problems with crop production which include low fertilizer effectiveness in 77% of the soils with pH of 5.0 or less. and Al toxicity and P deficiency in 43% of the soils with a pH of 4.5 or less. For maize production, the granitic sandveld soils of Zimbabwe need to be limed to a pH value of about 4. 7 (in O.OlM CaCb). Based on that criterion, 56% of the soils analyzed in 1992-94 needed to be limed compared with 26% in 1982-84. The soil pH decline over a 10-year period is likely to become a major soil fertility constraint to crop production in CAs in the (near) future. Cattle manure can also raise the pH of sands. Rates of 10, 20, 40 and 80 t/ha cattle manure progressively increased the soil pH, with a high N quality manure (1.2996 NJ being more effective than a low N (0.65% N) type. |
