THE FULL REPORT
This is a summary of the full research report, How Trees Affect Soils (CSL-3A, 98/16) which is available from RIRDC on 02 6272 4819. The principal investigators were Andrew Noble of CSIRO Land and Water, phone: (07) 4753 8555, fax: (07) 4753 8650, and Peter Randall of CSIRO Plant Industry, phone: (02) 6246 5119, fax: (02) 6246 5000.
 
 Trees can affect soil properties in many ways. Such effects can be either positive or negative, but trees can help improve soil health and productivity if used wisely and in the right situations.

This was the conclusion reached by CSIRO scientists Andrew Noble and Peter Randall after an exhaustive literature review conducted on behalf of the RIRDC/LWRRDC/FWPRDC Joint Venture Agroforestry Program.

The Joint Venture Agroforestry Program is funded jointly by the Rural Industries, Land and Water Resources and Forest and Wood Products Research and Development Corporations (RIRDC, LWRRDC and FWPRDC).

The DPIE Farm Forestry Program, the Grains R&D Corporation, the Cotton R&D Corporation and the Murray Darling Basin Commission also contribute financially to the program.

Such a conclusion may not be a surprise to many: it has long been thought that trees can improve soils and add to productivity.

But as Noble and Randall discovered as they reviewed the literature, broad statements about the benefits of trees to soils are of little practical use. Choosing the right tree species for the right situation is critical if trees are to assist and not hinder in the management and rehabilitation of agricultural soils. And for that, we need more information.

Fertile grounds

One of the reasons for an increased interest in agroforestry is the belief that trees can help improve soil fertility. One possible way they might do this is by increasing soil organic matter.

Organic matter

Soil organic matter can be thought of as the debris created by living organisms. Its maintenance is essential to efficient nutrient cycling and the sustainability of agroforestry production systems. Trees can contribute to soil organic matter through the supply of litter and root residues; in some agroforestry systems, tree prunings are added to soils as organic mulch.

Noble and Randall found that plant residue, particularly of legume species, does act as a source of nitrogen for subsequent crops: in one study, between 4 and 20% of nitrogen applied to the soil surface as prunings can be recovered in the immediate succeeding crop.

While this may be lower than recoveries from the application of inorganic nitrogen, organic materials have a greater residual effect on soil fertility due to their slow-release characteristics.

However, the authors report that for many agroforestry systems with a cropping component, particularly those on soils with a low inherent fertility, nutrients added in organic mulches are often inadequate to meet all the nutrient demands of a "moderate" crop. Additional fertilisers will often be required.
 

Beneath a shady tree

The presence of trees may affect soil fertility in rather indirect ways. For example, Noble and Randall note a link between the positive growth responses of the pasture species Paspalum notatum under the tree canopy of rose gum (Eucalyptus grandis) in northern Australia and the effect of shade on the availability of soil nitrogen. Under some leguminous trees, there may also be an additional benefit to soil nitrogen from accumulated leaf drop.
 

Animal droppings and nutrient pumps

Trees growing at low densities in arid and semi-arid pastoral ecosystems have often been found to improve their understorey environment. In southern Spain, the term ædehesa is used to describe a centuries-old land-use system in rangelands with scattered oak trees (Quercus rotundifolia, Q. suber and Q.áfaginea).

Scientists have compared soil chemical properties under the canopies of these trees with those in the open. They found that soil organic matter content, total exchange capacity, potassium, phosphorous, total nitrogen and calcium contents were twice those outside the tree canopy (Table 1).

Table 1. The main characteristics of soils collected beneath the oak tree canopy and in the adjacent grassland from a dehesa system in El Pedrosa, southern Spain.
 
                                                   Oak canopy                                       Grassland 
 
0-5 cm
5-20 cm
0-5 cm
5-20 cm
Organic matter (%)
7
5.2
4
3.2
CEC (cmolc kg -1
14
9
8
6.5
Exch. K (mg kg -1)
0.75
0.50
0.34
0.32
Exch. Ca (mg kg -1 )
10
6
5.1
3.8
Exch. Mg (mg kg -1)
2
1.2
1.3
1.0
Organic C (%)
42
24
24
1.5
Total N (%)
3
1.65
1.8
1.1
Adapted from Jaffre, R. Vacher, J. de Los Llanos, C & Long, G. 1988.The dehesa: an agrosilvopastoral system of the Mediterranean 
region with special eference to the Sierra Morena area of Spain. Agroforestry Systems 6:71-96.
 
 
 
The increase in soil organic matter is attributable to litter fall and to the droppings deposited by animals attracted to the shade cast by the trees. In addition, it may be that oak trees have the ability to extract nutrients from deep soil layers and concentrate them in the surface horizon, an ability known as nutrient pumping. Noble and Randall noted that there is only a limited amount of evidence from agroforestry research to suggest that such pumping can contribute significantly to soil fertility.
 

The acid test

Soil acidification may be thought of as the progressive lowering of soil pH due to land management practices and natural processes. In Australia, the phenomenon is estimated to affect 17 million hectares of pasture and crop/pasture rotation farming systems, due largely to the cultivation of pasture and grain legumes. Severe soil acidification can cause several nutritional problems for plants, including phosphorous deficiency, molybdenum deficiency, and aluminium and manganese phytotoxicity.

There are at least two ways in which trees may help reverse soil acidification. First, trees may act as a pump, taking up calcium and magnesium from deep in the soil profile and returning them to the soil surface as leaf litter. This would have a similar effect to liming the surface, although an increase in pH at the soil surface would be at the expense of acidification lower in the soil profile.

According to Noble and Randall, there is evidence that some tree species, such as Gmelina arborea, are able to accumulate nutrients such as calcium and magnesium in its litter. However, direct evidence of trees increasing soil pH remains scarce.
 

Let's get physical

Trees can play an important role in determining soil physical properties. For example, they can slow down the rate of soil loss through wind and water erosion by providing protection to the soil through their canopies and litter. Tree roots can also promote water infiltration and help to bind the soil. In addition, the accumulation of organic matter that occurs under most tree canopies can increase the water retention capacity of the soil and influence soil fauna such as earthworms that improve soil permeability.

The authors report a study of changes in soil physical parameters when a natural forest was converted to an agroforestry and a non-agroforestry system. The physical structure of the soils deteriorated in both systems when compared to an uncleared control, but less so in the agroforestry system.
 

Using water from the depths

A study reported by the authors investigated water use by 3 and 9-year-old Eucalyptus grandis plantations during a simulated drought. The plantations showed considerably more drought resistance than expected, a result explained by the capacity of the trees to use water from deep in the soil profile. In the 3-year-old stand, only about 14% of the sap flow was obtained from above 8 metres, pointing to a heavy reliance on deeper soil water reserves.

Trees in the 9-year-old stand appeared to survive almost entirely on water obtained from below 8ámetres. According to Noble and Randall, this is clear evidence of that trees are able to exploit soil profiles to depth.

The authors also noted a study investigating the ability of Eucalyptus camaldulensis and Chamaecytisus proliferous to reduce rising saline water tables. It reported higher evapotranspiration and water depletion under the tree species when compared to the grass pasture due to greater extraction from depth under the trees.

Agroforestry trials involving eucalypts planted in recharge areas have resulted in a lowering of the saline watertable, and there is evidence from Western Australia that intensive tree plantations are lowering the water table substantially there.
 

The roots of the matter

Noble and Randall noted some evidence of competition for resources between the roots of trees and those of annual crops.

A study investigating the influence of boundary trees (Acacia nilotica) on the growth and yield of adjacent wheat crops under irrigated conditions in Punjab, Pakistan, showed that wheat yields were reduced up to a distance of 8.5 m from the base of the tree.

Studies in the tropics of the fine root distribution of Cassia siamea, Eucalyptus camaldulensis, E.átereticornis, Leucaena leucocephala and Prosopis chilensis suggested that the vertical root distribution of these species was similar to maize (Zea mays) and thus may lead to competition.

Yet the root architecture of many agroforestry tree species is not well known. This is surprising, given that root competition could be of major significance in the eventual viability of agroforestry systems. Noble and Randall point out that agroforesters should look for trees with deep tap roots capable of pumping nutrients and water from deep in the soil profile and that have fewer roots that extend laterally.
 

Summing up

One of the difficulties in drawing conclusions about the role of trees in soil improvement is a lack of rigorous scientific experimentation.

Well-designed field experiments in agroforestry are expensive and time-consuming; consequently, they are rare. In addition, drawing general conclusions from a localised experiment or set of observations may be misleading.

There is a clear need for continued research to build on the current knowledge base. More research must be done on the influence of agroforestry systems on soil properties: the current evidence is patchy and often unreliable.

And we need to know more about the best species to plant to solve particular land degradation problems while also minimising competition with other crops. Until we do, much agroforestry will continue to be based on trial and error.