5. INTERACTION BETWEEN TREES AND CROPS

5.2 How trees improve soils

The processes through which trees improve soils can be grouped into four different categories:

• Increasing inputs (organic matter, nitrogen fixation, nutrient uptake)

• Reducing losses (organic matter, nutrients) by promoting recycling and checking erosion

• Improving soil physical properties, including water-holding capacity

• Beneficial effects on soil biological processes.

The processes by which trees maintain or improve soils are listed in Table 5.1 and illustrated in Figure 5.1. Some of these processes will be discussed in more detail.

Table 5.1 Processes by which trees maintain or improve soils (not all of the listed effects are proven — see text)

Figure 5.1 Processes by which trees maintain or improve soil (after Young, 1989)

Litter and mulch

During the lifetime of a tree, leaves, twigs and branches die and fall to the ground as litter. In agroforestry, trees are often managed and the biomass from the tree may be cut and used as mulch. The roles played by mulch and litter are similar.

In general, tree canopies only reduce the erosive effect of rainfall by about 10%, and in certain situations the canopy may make the rainfall even more erosive than if there were no tree. If the soil is covered with litter or mulch, on the other hand, erosion will often be reduced to low levels. Another characteristic of litter is its contribution to the organic-matter content of the soil after it decomposes. A soil that is rich in organic matter has a better capacity to absorb and retain water, and thus is also more resistant to erosion. A good cover of litter or mulch can also be very effective in suppressing weeds.

In general, trees do not necessarily lead to control of erosion. What matters is their spatial arrangement and the way they are managed.

Nitrogen fixation

Many leguminous trees and a few non-leguminous ones have the ability to fix atmospheric nitrogen through symbiosis with bacteria or fungi in root nodules. The fixation of nitrogen has been proven and found to be a significant factor in soil fertility. Tree species that have the ability to fix nitrogen may not always be efficient in doing so, however. One of the pre-conditions for efficient fixation of nitrogen is a minimum level of phosphorus in the soil. In exhausted soils which are low in phosphorus, therefore, nitrogen fixation may be insignificant even if nitrogen-fixing species are planted.

The roots of a plant constitute 20-30 % of its biomass. But the roots often contribute more to net primary production than the above-ground biomass. This is because the fine roots only have a short lifespan and therefore there is a rapid turnover of them. Hence, root decay is a significant process, and it is probable that cutting back the biomass above ground (coppicing, pollarding, etc.) results in a temporary increase in root die back. When the roots die, nitrogen is released and can be used by other plants, e.g. crops. So far, these processes of release of nitrogen from roots are not very well understood but they are being studied. Besides release of nitrogen, the death of the fine roots also contributes to organic-matter build-up.

A major part of the nitrogen which is fixed by the roots is used by the nitrogen-fixing plant for its own growth. The litter from nitrogen-fixing plants is often rich in nitrogen which is added to the soil when the litter or mulch decomposes.

Figure 5.2 The nitrogen cycle under agroforestry simplified to show major stores and flows (after Young,1989)

Nutrient uptake

Tree roots normally penetrate deeper into the soil than the roots of crops. It has, therefore, been assumed that trees are more efficient than crops in taking up nutrients released by weathering deep in the soil. Potassium, phosphorus and micronutrients are essential for plant growth and these elements are often released through such weathering. The nutrient uptake from deep layers of the soil, sometimes called nutrient pumping, has still not been experimentally verified.

Figure 5.3 The roots of trees and crops penetrate the soil at different levels

Nutrients from the atmosphere

The presence of a tree reduces wind speed and creates good conditions for the deposition of dust. Nutrients in the atmosphere are conveyed to the soil when they are dissolved in rain or settle with dust. Rain water dripping from leaves and flowing along the branches carries the nutrients to the ground, together with those released from the tree itself and associated plants growing on it. It is known that the amounts of nutrients reaching the ground in this way are substantial.

Protection from erosion

Soil erosion can be controlled by checking the flow of water down a slope by establishing runoff barriers—the barrier approach—or through maintaining a cover of living plants and litter on the ground—the cover approach. A cover of mulch or litter on the soil reduces the impact of rain drops and provides dispersed micro-barriers to runoff (Young, 1989). Soil erosion involves loss of topsoil, including loss of both organic matter and nutrients. Trees play an important role in erosion control both through the barrier and cover effects. Formerly the barrier approach was thought to be the most important, but in recent years the cover approach has received increasing attention.

In the barrier approach, trees and shrubs play a direct role in reducing erosion if they are grown in hedgerows on contours. Another effective control method is to combine trees and grass in strips along contours. Combining trees and grass is a means of making optimal use of the strips of land which are taken out of direct crop production. If productive use is made of the strips, the soil-conservation measures are more likely to be regarded as beneficial by the farmer and therefore to be adopted on a permanent basis.

Figure 5.4 Examples of the barrier approach

1. Barrier hedges of double rows of Leucaena with maize developing naturally into terraces, Philippines

2. Leucaena barrier hedges planted at 90-cm spacing in furrows between rows of maize developing into terracettes, Malawi

3. Trees on conservation works, Malawi: fruit trees on grass strips and Leucaena on marker ridges (ridges laid out along contours to guide cultivation ridges below)

4. Alternative arrangements for trees on conservation structures, Cameroon.

5. Alternative positions for trees on fanya juu structures, Kenya, Fanya juu structures are bunds in which the bank is above the ditch, promoting natural terrace formation.

If physical soil-conservation structures are constructed, trees or shrubs that are planted on or near them can help to strengthen and stabilize the structures. Another example of the use of trees in the barrier approach is the growing of trees as a windbreak.

The benefits of trees in the cover approach have been discussed earlier in connection with litter and organic-matter maintenance.

Reduction of the rate of organic matter decomposition by shading

Trees influence the microclimate (Section 4.10). The shade resulting from tree canopies and litter reduces temperatures during the heat of the day, and this cooling effect slows down the decomposition of organic matter.

Further information on how trees may improve soil can be found in Young, 1989.

Figure 5.5 Leaf mulch is important in preventing splash erosion and promoting infiltration