Improving soil fertility and fertilizing the crop

Most farmers in sub-Saharan Africa grow sorghum without any added fertilisers. Sorghum is much more tolerant to low soil phosphorus levels than most cereals. It can also tolerate some degree of salinity and poor drainage. However, in order to achieve full yield potential the crop requires appropriate nutrition.

a) Soil conservation

Soil conservation implies putting appropriate measures in place in order to protect the soil from being eroded by water and wind or being overheated by the sun. Most of the regions of Africa where sorghum is grown have sandy soils and are prone to both runoff and wind erosion. It is therefore necessary to ensure that the soil is protected from water and wind erosion. This can be done in a number of ways:

First, traditional methods such as ‘slash and burn’ and over-exposure of harvested lands to grazing animals should be discouraged to prevent exposure of the soil to erosion and loss of organic matter.

Secondly, in case of sloping land, there may be a need to construct terraces followed by planting across the slope.

Other practices like mulching of the soil with crop residues as well as intercropping with legumes like beans reduce exposure of the soil. Cover crops can also be used to cover the soil and smother weeds between sorghum growing seasons.

b) Introduction of leguminous crops to the sorghum cropping system

Sorghum grows better when planted with or after a legume crop like common beans, cowpea, pigeon pea, green gram and soybean. Therefore, planned rotation and intercropping of sorghum with legumes is essential for crop growth. Besides fixing nitrogen most legumes also hinder the development of pest populations, diseases and weeds. Leguminous shrubs can act as windbreaks, help in recycling leached nutrients from the deeper layers of the soil, and provide mulching material and organic material to maintain soil fertility. Green manure crops like jack beans, perennial peanut or mucuna, besides fixing nitrogen, produce large quantities of biomass that can be used to feed the soil and increase availability of nitrogen and other nutrients.

Sorghum does not take as much water out of the soil as maize, sunflower or soybeans do, but it exhausts soil nutrients. To prevent depletion of the soil and prevent development of pests, diseases and Striga, sorghum should only be grown every 2 to 3 years in the same field. Furthermore, other plants in the grass family should not be planted in rotation with sorghum.

Recommended rotation partners are legumes, cotton, leafy vegetables or cassava. Recommended rotations may be groundnut – sorghum – pigeon pea/cowpea; or field bean and amaranth –sorghum.

Intercropping of sorghum with cowpea, common bean, groundnut or Bambara groundnut is quite common. So, too, is intercropping with other grasses like maize or millet. Intercropping with sweet potatoes or cassava is also practised. Leguminous intercrops contribute more to improvement of soil fertility than other species. When grown for forage, sorghum can also be intercropped with legumes such as cowpea, as this improves the nutritional value of the forage.

When intercropped with pigeon pea, both crops are planted at the beginning of the rainy season. Sorghum is harvested after 100 days, while pigeon pea is left to use the remaining soil moisture and nutrients until harvesting after 160 days.

c) Addition of organic fertilisers

The nutrient requirements of the crop are highest during rapid growth and early bloom. Requirements of the main nutrients nitrogen, phosphorus and potassium for an average yield of 7,500 kg per hectare are 185 kg N, 80 kg P2O5 and 285 kg K2O. Without any provision of nutrients production depends on soil-stored nutrients only, which in poor soils will give low yields. Grain sorghum responds well to a balanced supply of plant nutrients.

Application of farmyard manure or ash is quite common. Application of compost to the field increases the water storage capacity of the soil and provides balanced nutrients to the soil. Animal manure provides large amounts of nitrogen, phosphorus and potassium. Better availability of water and nutrients results in higher grain yields of sorghum.

Animal manures and compost are best applied before land preparation by spreading them in the field and incorporating them into the topsoil before planting. If the land is prepared using a ripper only, which leaves the soil surface largely untouched, organic manures are best applied to the furrow and mixed with the soil into which sorghum will be planted. The standard farm wheelbarrow holds approximately 25 kg of dry manure or compost. For a low rate of fertilisation two wheelbarrows are enough for an area of 10 metres by 10 metres, making 200 wheelbarrows or 5 tons of manure or compost per hectare. For a high rate 400 wheelbarrows or 10 tons are to be applied per hectare.

Smallholder farmers in Africa rarely apply commercial fertilisers because of high costs. Based on the IFOAM Norms for Organic Production and Processing application of commercial fertilisers (including lime) is allowed in certified organic agriculture with some restrictions. While synthetic fertilizers such as urea are not permitted, use of commercial fertilizers should be justified by recommendations from soil or plant analysis. They must only be applied in their naturally occurring form and used in combination with other techniques such as addition of suitable organic matter, green manures, crop rotations and nitrogen fixing plants.

The East African Organic Product Standards permits the following fertilizers of mineral origin for East African organic farmers: basic slag, calcareous and magnesium amendments, limestone, gypsum, marl, maerl, chalk, sugar beet lime, calcium chloride, magnesium rock, kieserite and Epsom salt (magnesium sulphate), mineral potassium (e.g., sulphate of potash, muriate of potash, kainite, sylvanite, patentkali), natural phosphates, pulverized rock, stone meal, clay (e.g., bentonite, perlite, vermiculite, zeolite), sodium chloride, and sulphur. This list excludes Chilean nitrate for organic farming.

For further information on the qualities and use of organic and mineral fertilisers please refer to module M02 of this manual.