Home cure for soil sickness?
Climate change and rising temperature combined with declining soil fertility
bode ill for agriculture and food production in many parts of the tropics,
not least in Sub Saharan Africa. The prevalence of hunger in many parts of
Africa is directly attributable to depletion of soil nutrients over decades,
if not centuries. If predicted climate changes make dry regions drier and
wet regions wetter, there will be additional moisture stress in arid areas,
water logging, erosion and disease stresses in others, that will reduce yield
potential further.
Additionally, it has been found that thermal stress reduces cereal grain pollination;
for each 1° C maximum temperature increase, there is 16% decrease in fertility
in cereals.
Yet, the prospect of such dire projections should be seen in the context of research that has demonstrated how small scale farmers in the sub-humid tropics can halt the decline in the fertility of their soils and even enhance it. Fifty thousand farmers in Eastern and Southern Africa have implemented low cost practices and used locally available materials to achieve between three- and five-fold increases in maize yields. Some have gone further, exploiting the enhanced soil fertility of their land to switch from subsistence cereal production to higher value dairying and vegetable production, raising their family incomes from $1 to $10 per day.
The main requirements for low fertile soils in Africa are nitrogen, phosphorus and organic matter. With imported fertilizers too costly to purchase and transport (1 mt of urea costs $90 in Europe but $400 in Western Kenya and $770 in Malawi), and high solar radiation oxidising meagre soil organic matter, the only practical option is a home cure for soil sickness. This consists of capturing nitrogen from the air via the biological nitrogen fixation of leguminous shrubs and trees grown as fallow crops, utilizing phosphate rocks, indigenous to the region, and concentrating additional nutrients and biomass at the soil surface through nutrient-accumulating shrubs.
N + P + B(iomass)
Leguminous trees and shrubs, including Sesbania, Tephrosia, Crotalaria, Gliricidia and Cajanus (pigeonpea), are interplanted with young maize and are allowed to grow on after the maize harvest in the dry season. In 6-24 months they can accumulate 100-200kg/ha of nitrogen, comparable to nitrogen fertilizer rates used on maize in Europe. This increases maize yields by a factor of 2-4 as N deficiency is overcome, and farmers are now establishing tree fallow crop rotations in bimodal rainfall areas of East Africa of one year of trees followed by one year of maize, and two years of trees followed by 2-3 maize crops in the unimodal areas of southern Africa.
Phosphorus deficiency is also widespread in East Africa and the Sahel. Utilizing indigenous phosphate rock deposits provides the partial or complete alternative to imported superphosphate. (see News, Improving utilization of phosphate rock) The mild acidity of most of these soils helps dissolve high quality rock phosphates and, under such conditions, direct application of rock phosphate can double or triple maize yields, giving results equivalent to 90% of what might be expected from application of superphosphate.
Finally, biomass transfer and additional nutrients can be provided by spreading leaves of nutrient-accumulating shrubs such as Tithonia diversifolia, gathered from roadsides and from deliberately planted hedges, onto cropped fields. Tithonia also contributes potash and micronutients, as well as providing further organic carbon to the soil. Farmers and communities that have applied these techniques report that hunger periods have been eliminated and that there has been decreased susceptibility to diseases. They have also benefited from the supply of fuelwood, produced by the tree fallows, reducing encroachment on nearby forest land, and, in addition, the diversity of plant species grown has mitigated the effects of pest attacks and market price fluctuations.
It has to be said that this agroforestry approach is limited to the subhumid regions of Africa, since that is where the trees and shrubs grow fastest, where Tithonia grows naturally, and where suitable rock phosphate deposits are to be found. But fortunately, it is the subhumid tropics where the majority of rural Africans live.
