A relook at the fertilizer and plant population recommendations for grain sorghum in the communal areas of Zimbabwe

Sorghum [Sorghum bicolor (L.) Moench] is the third most widely grown summer cereal crop in Zimbabwe after maize (Zea mays L.) and pearl millet [Pennisetum glaucum (L.) R. Br.] with more than 165 000 ha estimated to be planted annually. Subsistence communal farmers in the low rainfall (450 to 650mm) region's III, IV and V account for about 92 of this area with the rest coming from large-scale commercial, small-scale commercial and resettlement farmers. However, grain yields of sorghum in the communal areas are generally low (about 456kg/ha) We hypothesized that these low yields were partly caused by inadequate soil fertility, in particular nitrogen and phosphorus, the most commonly limiting nutrients, and partly by planting sorghum at excessively high plant populations which accelerated soil water depletion before the crop reached physiological maturity. General extension recommendations on fertilizer and plant population are available from the Department of Agricultural and Extension Services (AGRITEX), but communal farmers rarely apply them. This led us to think that these recommendations may be out of date and, thus, need revision. Two on-farm experiments: a) nitrogen x phosphorus trial 1988-1994, and b) plant population trial, 1987-1994, were, therefore, initiated in order to determine optimal fertilizer (nitrogen and phosphorus) and plant populations for grain sorghum at different communal area locations in natural regions III and IV of Zimbabwe. Nitrogen increased sorghum grain yield at all locations. Phosphorus slightly increased yields at three out of five locations depending on the season. When averaged across locations in regions III and IV, fertilizer rates of 64kg/ha (range: 42-100kg N/ha) and 23kg Phosphorus/ha (range: 3-32 Phosphorus/ha) were optimal for grain sorghum. Our results suggested that the current AGRITEX fertilizer recommendations (46kg N/ha and 21kg Phosphorus/ha) underestimated the nitrogen requirement for grain sorghum but generally agreed with our estimate for phosphorus. On the other hand, the Chemistry and Soils Research Institute (CSRI) fertilizer recommendations (60kg N/ha and 52kg Phosphorus/ha) for grain sorghum agreed with our nitrogen estimate but overestimated the phosphorus requirement. Grain yields of sorghum increased with each increase in plant population for on-farm locations in regions III and IV of Zimbabwe was 102 746 plnats/ha (rdange 64 278-172 110 plants/ha) The optimal population for grain sorghum was about half the general AGRITEX recommendation of 200 000 plants per hactare, a recommendation we thinis too high for the low rainfall regions III and IV of Zimbabwe