Improving soil fertility

Research has shown that below a yield of 15 kg per hectare, cassava removes much less nitrogen, phosphorus and potassium than most other crops. Each ton of fresh roots produced per hectare removes about 3 kg of nitrogen, 1 kg phosphorus and 7 kg potassium per hectare from the soil. This is because most nutrients, except K, are mainly present in cassava leaves and stems. Therefore, if leaves and stems are returned to the soil, nutrient removal will be minimal. However, in areas where leaves and stems are also utilized and removed from the field, nutrient removal will increase. In this case, nutrient depletion can become a serious concern if soil fertility is not maintained properly. Returning leaves and stems to the soil is an essential first step in preventing nutrient depletion and maintaining soil fertility. It is, therefore, important to avoid burning of cassava and other crop residues. The practice of burning crop residues is not allowed in organic agriculture.

Animal manure contains plant nutrients and improves the physical condition of the soil. The integration of farm animals can, therefore, contribute to better recycling of nutrients within the farm, if the animals are fed on farm-own forage legumes and the manure is returned to the field. Cassava plants will benefit from the valuable manure the cattle, sheep, goats, pigs or chicken offer. In order to minimize nutrient losses from manures, special attention must be paid to their collection, storage and application.

An interesting example of animal manure application is the ’Parcagem’ system in Brazil, which is also applicable in Africa.

Organic wastes such as kitchen wastes, animal manure and plant materials can also be collected, composted and applied to the cassava field to replenish the nutrients taken away through harvest.  These materials improve the amount of nutrients as well as the soil organic matter content, and thus the soil’s physical conditions and water-holding capacity.

It is advisable to avoid using weed residues containing seeds, rhizomes, stolons, or tubers as mulch because these can increase weed problems on the farm. Also, straws of maize and guinea grass are bad mulch materials, as they take too long to rot and temporally reduce soil nitrogen before they are decomposed. In addition, organic farmers can also grow ’live mulches’. For example, egusi melon (a food crop), planted at very close spacing on cassava seedbeds is a good live mulch.

Planting of green manures and their subsequent mulching or incorporation into the soil before planting cassava is a traditional way to improve soil fertility. Leguminous crops such as cowpea (Vigna unguiculata), jack beans (Carnavalia ensiformis) or Crotalaria (C. ochroleuca G. Don.) make good green manure and have a significant beneficial effect on the subsequent yield of cassava. However, in many systems, cowpea has proved to be more promising. It can provide 80 kg of nitrogen per hectare to a subsequent crop in 8 to 10 weeks of growth. It is tolerant of drought and well-adapted to sandy and poor soils.