Water Management in Organic Agriculture
SUMMARY
Scarcity of water for agriculture is a common phenomenon in many countries. In some regions it is almost impossible to grow crops without irrigation. Even in areas with large amounts of rainfall in the rainy season, crops may get short of water during dry periods.
Organic farming aims at optimising the use of on-farm resources and at a sustainable use of natural resources. Active water retention, water harvesting and storing of water are important practices, especially for organic farmers. Organic farmers know that it is more important to first improve the water retention and the infiltration of water into the soil.
A.How to Keep the Water in the Soil? (FIGURE 5-1)
Keep soil moisture: During dry periods, some soils are more and some are less in a position to supply crops with water. The ability of a soil to absorb and store water largely depends on the soil composition and on the content of organic matter. Soils rich in clay can store up to three times more water than sandy soils. Soil organic matter acts as storage of water, just like a sponge. Therefore, crop residue or a cover crop protects the soil, prevents crusting on the surface, and slows runoff. Roots, earthworms and other soil life maintain cracks and pores in the soil. Less water runs off, and more sinks into the soil.
Reduce evaporation: A thin layer of mulch can considerably reduce the evaporation of water from the soil. It shades the soil from direct sunlight and prevents the soil from getting too warm. Shallow digging of the dry top soil can help to reduce the drying up of the soil layers beneath (it breaks the capillary vessels). A better retention of water within the soil saves costs on irrigation.
Better use of season’s rainfall: Ripping during the dry season allows farmers to plant earlier – right at the start of the rains.
ATTENTION: A green manure or cover crop is not always a suitable way of reducing evaporation from the soil, due that they also use water. In dry areas, you should consider using other types of mulch, such as crop residues or plant remains brought in from outside the field. That will help conserve moisture in the soil where it can be used by the crop.
B. Harvesting Water
1.INCREASING INFILTRATION
During strong rains, only a part of the water infiltrates into the soil. A considerable part flows away as surface runoff, thus being lost for the crop. In order to get as much of the available rainwater into the soil, the infiltration of rainwater needs to be increased (Figure 5-2).
The most important for achieving a high infiltration is to maintain a topsoil with a good soil structure containing many cavities and pores (e.g. from earthworms). Cover crops and mulch application are suitable to create such a favourable top soil structure. Further, they help to slow down the flow of water, thus allowing more time for the infiltration.
Some techniques to harvest water include:
Planting Pits
Planting pits (known as zai in Burkina Faso and tassa in Niger, Figure 5-3) are hand-dug circular holes which collect water and store it for use by the crop. Each pit is about 20 cm across and 20 cm deep. After planting, the holes are left partly open so they collect water. Planting pits take a lot of work to dig when the soil is dry. But they produce good yields in areas where otherwise crops might die because of a lack of water. Once made, the pits can be used again, season after season. Leave the soil covered, and add compost or fertilizer to the pits to increase their fertility.
Contour Bunds and Catchment Strips
In areas with low rainfall, there may not be enough water to grow a crop over the whole area. On gentle slopes (less than 3%), one possibility is to use contour bunds and catchment strips. Catchment strips are areas where no crops are planted. When rain falls on this ground, it runs downslope and is trapped by the contour bund. Plant rows of crops behind the bund to use this water. This can produce a good yield even with very little rain. Mulch the cultivated areas with crop residues to prevent erosion, help water sink in, and slow evaporation.
The picture below (Figure 5-4) shows an example of a farmer in Botswana, who makes his cropped strips 0,8–1 m wide a 3,3 m apart. He subsoils these strips using a tractor-powered subsoiler to a depth of 0,7 m. He shapes the land between the strips so it slopes towards the cropped strips, so rainwater will flow towards the crop. He plants two rows of maize in each strip, and sows a cover crop such as cowpea in between the strips. The strips are permanent: they can be used to grow crops season after season. The soil in the strips gradually improves in fertility as crop residues accumulate there. Rotating maize with a legume crop will improve the soil fertility further. The farmer has been able to grow up to 6 t/ha of maize with less than 400 mm/season of rain.
Road Catchments
Water from roads – and from other unproductive areas such as paths and homestead compounds – can be channelled onto fields. It may be possible to divert water from structures that already exist, such as the ditches below fanya juu terraces. Or special bunds can be built around fields close to the road. Another possibility is to direct the water into a pond, which can be used to irrigate crops. (Figure 5-5 & 5-6).
Half-Moon Microcatchments
Half-moon microcatchments are small, semicircular earth bunds. They are quite common on the desert margins of the Sahel, where they are called “demilunes.” The half-moons catch water flowing down a slope. Crops such as sorghum, millet and cowpeas can be planted in the lower portion of the halfmoons. Half-moons are helpful to rehabilitate degraded land. (Figure 5-7).
2. Water Storage
Excess water in the rainy season may be made use of during dry periods. There are many possibilities of storing rainwater for irrigation, but most of them are labouring intensive or costly. Storing water in ponds has the advantage that fish may be grown, but water is likely to be lost through infiltration and evaporation. The construction of water tanks may avoid these losses, but needs appropriate construction materials. To decide whether or not to build water storage infrastructure, the benefits should be weighed against the costs, including the loss of arable land.
C. Drip Irrigation Systems
The major factors that determine the necessity of irrigation are the selection of crops and an appropriate cropping system. Obviously, not all crops (and not even all varieties of the same crop) require the same amount of water, and not all need water over the same period of time.
Some crops are very resistant to drought while others are highly susceptible. Deep rooting crops can extract water from deeper layers of soil and hence they are less sensitive to temporary droughts.
With the help of irrigation, many crops can nowadays be grown outside their typical agro-climatic region. This may cause not only the above mentioned negative impacts, but also some advantages. It may make it possible to cultivate land which would otherwise be unsuitable for agriculture without irrigation. Or the cultivation of sensitive crops can be shifted into areas with less pest or disease pressure.
There are irrigation systems of higher or lower efficiency and with more or less negative impact. If irrigation is necessary, organic farmers should carefully select a system, which is does not overexploit the water source, does not harm the soil and has no negative impact on plant health.
One promising option are drip irrigation systems (Figure 5-8). From a central tank, water is distributed through thin perforated pipes directly to the single crop plants. There is a continuous but very light flow of water, thus allowing sufficient time to infiltrate in the root zone of the crops. In this way, a minimum of water is lost and the soil is not negatively affected.
The establishment of drip irrigation systems can be quite costly. However, some farmers have developed low cost drip irrigation systems from locally available materials. Whatever irrigation system the farmer chooses, he will reach higher efficiency if it is combined with accompanying measures for improving the soil structure and the water retention of the soil, as described above.
REFERENCES
IFOAM. 2003. Training Manual for Organic Agriculture in the Tropics. Edited by Frank Eyhorn, Marlene Heeb, Gilles Weidmann, p 100-107, http://www.ifoam.bio/
IIRR and ACT. 2005. Conservation agriculture: A manual for farmers and extension workers in Africa. International Institute of Rural Reconstruction, Nairobi; African Conservation Tillage Network, Harare.
SOURCES
NRC / IIRR
ON TECA
Water Management in Organic Agriculture: http://teca.fao.org/read/8366