Farmers turn to 'fertiliser tree' to boost crop production

A tree that is almost perfectly complementary to crop growing, doubles and even triples crop yields, and thrives in dry areas, is being rolled out across Kenya in a drive to use the ‘fertiliser tree’ to regenerate soils so poor they are damaging agricultural output and human health.

In Zambia, where over 160,000 farmers are using the Faidherbia Albida tree over an area of 300,000 hectares, unfertilized maize yields are averaging 4.1 tonnes per hectare when planted near the tree, compared to 1.3 tonnes per hectare for those grown away from the tree’s canopy.

Now, some 2200 farmers from Eastern and Western Kenya are piloting the planting of the tree, which also conserves soil moisture and provides fodder feed for livestock.

The tree’s natural habitat is dry tropical Africa, yet it remains rare in Kenya’s arid North Eastern region. However, it is now the central platform of a new wave of farming across Africa called Evergreen Agriculture, which is combining fertiliser trees with minimal ploughing and permanent soil cover to raise soil fertility.

According to a 2006 report by the International Fertilizer Development Centre (IFDC), three-quarters of African land is now so degraded that is is causing low, stagnant yields, deficient of vital minerals and nutrients such as zinc, iron and manganese in the food crops. These deficiencies are having a knock-on impact on human health, weakening immune systems and contributing to lifestyle diseases.

At the same time, the low soil quality is causing grain yields of an average one tonne per hectare in Africa, compared with three tonnes per hectare in the rest of the world.

However, the fertiliser trees, which include Sesbania, Tephrosia, Gliricidia and Faidherbia Albida, all draw nitrogen from the air and transfer it to soils through their roots or through the leaves they drop.

“Combining trees and agriculture….is a pathway to a reinvention of agriculture”, said Dennis Garrity, the Director General of the World Agro-Forestry Centre Gigiri, speaking at a May 23rd conference on how ‘Trees and Landscape Restoration can help deliver a triple win’.

Garrity laid out a vision of a future where food crops will all be grown under a canopy of trees. Of the fertilizer trees, Faidherbia Albida is delivering the best results with crops like maize and legumes due to its unique seasonal
cycle. Unlike most trees, it sheds leaves in the rainy seasons, just as farmers are planting, in a phenomenon known as reverse phenology. The fallen leaves form mulch soil cover that later conserves soil moisture in dry spells.

Faidherbia’s leaf canopy is not dense, so even as crops grow they are able to tap as much sunlight as they need. But as crops continue growing and intense sunlight begins to be a problem for them, the tree’s leaves grow back again and provide much needed shading.

At the same time, the tree’s deep roots bring nutrients up to the soil surface where crops whose roots are short can reach them. Where ploughing has created a hardpan soil – through the creation of hardened surface by the plough blades - the Faidherbia also helps loosen the soils.

Together, these features mean the tree never competes with growing crops for light, water or nutrients. Moreover, while Faidherbia Albida takes some 12 to 15 years to mature, its effects on soil after planting are immediate, like “adding nutrients into the soil”, said Jonathan Muriuki, a forester with the World Agro-Forestry Centre Muriuki.

At the World Agro Forestry Centre Gigiri, where a mature Faidherbia Albida provides a canopy, Napier grass under the tree is healthier and higher in yields than Napier plants just a short distance away.

On land in current use, 100 Faidherbia Albida trees per hectare, spaced at 20 by 20 metres, are ideal to complement crops. On land that is being regenerated for future farming, the centre recommends some 400 trees per hectare. “Thinning can be done later to remove the weak ones,” said Muriuki.

In Malawi, maize yields have increased by 280 per cent when grown under a canopy of Faidherbia Albida tree. But the popularity of the tree is greatest in Niger, where there are now more than 4.8 million hectares of the trees intercropped with millet and sorghum crops.

By James Karuga

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