Improved Fallows

PRINZ (1986, 1987) defines an improved fallow as a targeted use of planted species in order to achieve one or more of the aims of natural fallow within a short time or in a smaller area. Traditionally fallows take several years to restore fertility. Natural vegetation is slow in reaching the peak of biological productivity. By contrast, fast growing trees - if correctly identified and selected, planted and managed in fallows - can grow and mature within a short time. These tree species can enhance soil fertility by bringing up nutrients from lower soil layers, litter fall and atmospheric nitrogen fixation.

‘Fertiliser trees’. Tephrosia (front) and Sesbania (background).

At the end of the fallow period the trees are harvested and the biomass that is not useful as fuelwood is returned to the soil. The development of short-duration ‘fertiliser trees’ is necessitated by the fact that long-term fallowing (20 to 30 years) is no longer feasible and medium-term natural fallows (5 to 10 years) do not adequately replenish soil fertility.

Over the years the World Agroforestry Centre (ICRAF) and its partners have been involved in identification, research and development of short-term (1 to 3 years) improved or managed fallows to allow for rapid replenishment of soil fertility. A range of leguminous trees and shrubs was screened to identify promising species that would add high amounts of nitrogen and organic matter into the soil, while also producing fuel wood. Sesbania sesban, Tephrosia vogelii, Gliricidia sepium and Leucaena leucocephala were found to be the most promising N-fixing trees identified for soil fertility replenishment.The techniques for integrating these species as short-duration planted fallows, in rotation with crops to build up N-capital in farmers’ fields, are now in place. Several thousands of farmers in the region have collaborated in developing these technologies by testing them on their farms.

Maize following 2 years of Sesbania improved fallow

Photo taken in eastern Zambia

Improved fallows benefit farmers in the form of increased food crop yields and returns to land and labour. For example, in experiments conducted over the last 15 years in Chipata, Zambia, maize yields in three normal rainfall years after two years of Sesbania fallow averaged 5.6 t ha–1 compared to 2.0 t ha–1 in unfertilized continuous maize and 4.1 t ha–1 when maize was fertilised with 112 kg N ha–1.Over a six-year period, from 1988 to 1993, the Sesbania improved fallows required less than half the amount of labour needed for one hectare of continuously cropped maize. Furthermore, two-year Sesbania fallows produced 15 t ha–1 of fuelwood. Current evidence suggests that high maize yields following such fallows are primarily due to improved nitrogen and organic matter input.

A farmer in Mozambique tending a Sesbania nursery

A newly adopted provenance of Tephrosia candida (Madagascar) produces high biomass and is suitable for a two-year fallow. It does not produce seeds in the first year. Sesbania fallows were also found to reduce or completely eliminate ‘witch weed’ (Striga asiatica) populations in colonised fields from 1532 counts in the continuously cultivated unfertilised plot to zero in a plot after Sesbania fallow.