Overstory #42 - Improved Fallow

Improved Fallow and Land Rehabilitation

Given enough time, natural processes will restore productivity to degraded or damaged land. Traditionally, farmers have used the practice of "fallow" to allow crop land to rest without crops and be rejuvenated naturally. When the fallow is enriched with fast-growing trees, shrubs or vines, the practice is called "improved fallow." Improved fallow is an agroforestry practice that has its origins in slash-and-burn agriculture. Farmers use improved fallow to accelerate the process of rehabilitation and thereby shorten the length of their fallow periods. The technology can be applied to any agricultural land that is not under cultivation in order to accelerate recovery, increase nutrient reserves, and improve the potential for future productivity on the site.

To create an improved fallow system, farmers scatter seeds or plant seedlings of fast-growing plants after harvest of the crops from the site. Normally, nitrogen-fixing plants are used, because they are vigorous, deep-rooted, tolerant of drought, and have the ability to accumulate atmospheric nitrogen (see Overstory #4 on Nitrogen Fixing Trees ). The trees and shrubs are left to occupy the site for several months or years. During the fallow period, the plants accumulate nitrogen from the air and from deep layers of the soil, and drop their leaf litter to enrich the soil and conserve moisture. When the trees are removed at the end of the fallow period, their roots remain in the soil to decompose gradually, releasing additional nutrients to the subsequent crops.

The trees and shrubs in the fallow also provide another important service to the farmer: they fill the space and impede the establishment of undesirable weeds. Many kinds of invasive and problematic weeds thrive in open, sunny conditions on vacant land, but do not spread into areas that are cooler and shadier. The plants that are part of the improved fallow create conditions that are unfavorable to most problematic weeds, making the subsequent establishment of crops easier than if the area had to be cleared of undesirable weeds.

Benefits of improved fallow:

• Improve soil fertility
• Accumulate nutrients
• Add organic matter
• Keep down undesirable weeds while land is not under cultivation
• Break up hard soil
• Regulate temperatures (less extremes of hot/cold)
• Provide shade
• Protect from winds
• Reduce erosion
• Encourage or sustain populations of beneficial soil microorganisms
• Break up physical barriers to root growth (rock and hard pan)

When the trees are removed at the end of the fallow period, they can also yield products such as firewood or poles for sale or farm use.

Improved Fallow Species

The effectiveness of the fallow in improving the subsequent productivity of the land depends on many factors. It is important that the land is kept in fallow long enough for the conditions to improve. The condition of the land will in part dictate the necessary length of the fallow, as severely degraded land will need more time than healthier land. The effectiveness of the species used to regenerate the land is also a key factor.

The optimal species to use for improved fallow are fast-growing, deep-rooted, tolerant of drought, easy to establish, and preferably nitrogen-fixing. There are many species that fit this description. However, in most cases it is also important that the species be easily removed or short-lived so as not to interfere with future productivity. This narrows the list of appropriate species considerably.

Characteristics of species used:

• nitrogen-fixing and/or produce large amounts of organic matter
• hardy; tolerant of drought and neglect
• easy to establish
• removable or short-lived; will not resprout continually if cut down
• not weedy; will not spread to neighboring crop areas.
• deep rooted
• able to produce useful or marketable by-products such as firewood, poles, edible seeds, etc.

Example species used for this purpose:

• Inga edulis (Inga, or ice cream bean)
• Cajanus cajan (pigeon pea)
• Crotalaria sp. (sunn hemp)
• Sesbania sesban (sesban)
• Samanea saman (monkeypod)
• Gliricidia sepium (madre de cacao, rata maton)
• Erythrina sp. (E. poeppigiana, E. fusca)
• Senna siamea (pheasantwood)

The links below include photos or descriptions of various improved fallow systems from around the world. Some examples include:

• Sesbania sesban and Tephrosia vogelii improved fallow land between maize production in Zambia
• Acacia mangium improved fallow/land rehabilitation in Costa Rica
• Gliricidia sepium improved fallow in Southeast Asia
• Mixed species improved fallow in Amazonia

References

P. Ramachandran Nair, An Introduction to Agroforestry. 1993. Kluwer Academic Publisher.

Related Editions to The Overstory

• The Overstory #70--Rhizosphere
• The Overstory #66--Carbon Sequestration: Storing Carbon in Soils and Vegetation
• The Overstory #61--Effects of Trees on Soils
• The Overstory #28--Microlife
• The Overstory #22--Pioneering Difficult Sites
• The Overstory #20--Five Fertility Principles
• The Overstory #8--Mycorrhizae
• The Overstory #4--Nitrogen Fixing Trees -- A Brief Introduction