L.J. AYUKO *
(*) Head of Range Management Divisions Ministry of Agriculture, P.O. Box 30028, Nairobi, Kenya (East Africa).
Assessment of the potential of a range site compares the value of present vegetation to that which might grow in its place. This capability of a range site to grow more valuable and more abundant vegetation is the foundation for range improvement, and it is only possible if the current management is directed towards range improvement (also referred to as range trend). In order to determine trend, soil and plant indicators must be used; and once an acceptable trend in range condition is achieved, it should be checked at various stages.
Potential denotes some standard of excellence (capability) that characterises the use to which rangeland can be put, all other factors being equal. Several degrees of potentiality exist for any site. It is, therefore, necessary to have one of these degrees in mind so that the potentials for improvement are geared towards the selected goal. The objective for most areas will be for a sustained higher production of suitable forage, so that the rangeland supports more livestock per unit area over a given period. In evaluating the site potential we need d to consider:
a) The natural succession that would take place if the resource were no longer managed by man. This will indicate the effort that is required on man's part.
b) How far the influence will be felt. In other words, how far can the developer get with the limited resources. Is it all worthwhile?
c) The time required before the developer attains his goals. If the expenses are large, he may feel it is not worthwhile to develop the site.
Factors affecting range improvement potential
The potential for range improvement will depend on the following factors: environmental; edaphic; vegetation composition; location of the site; water availability for livestock; and socio-economic.
These include the following:
a) Topography: It affects the rainfall disposition and soil formation. Increase in gradient results in increased runoff, soil erosion, and lower productivity.
Some slope may, however, be desirable where water is likely to accumulate; but usually the main problem is water shortage and not water accumulation. In effect, therefore, gradient affects the extent to which development can be realized.
b) Climate: The extent to which and ease with which improvement can be carried out is very much dependent on the climate. Precipitation in terms of total quantities and distribution is especially important. Indirect variables such as depth, exchange capacity and organic matter of the soil are highly related and functionally dependent on precipitation.
Temperature is important in that it influences evapotranspiration, which determines the proportion of the precipitation that is retained in the soil.
Where precipitation is low and evapotranspiration is high, moisture deficit is likely to occur. In such places only the hardy plants will survive. Attempts to make such rangeland more suitable for the less hardy and usually more palatable species will be difficult.
c) Parent material: Parent material influences site quality and consequently the potential for improvement. Acid soils, for example, will yield infertile, coarse, and erosive soils; while basic rocks generally produce fertile, dark, fine soils rich in calcium and phosphorus.
Parent material may not have immediate effects, but its influence will be felt as the material wears down.
Both the physical and chemical properties of the soil are important in the soil quality determination. They indicate the quantity and type of soil nutrient available. Soil property closely related to soil moisture or aeration may be expected to be correlated with site quality. This is particularly true in rangelands where even the least quantity of water from precipitation is important. Range site evaluation indirectly estimates the soil moisture regime and specifically the water available for plant growth.
The following properties should be considered:
a) Depth of horizon A has been found to be closely correlate with the site potential and thus is the best indicator of growth potential. Total soil depth is used in measuring the effective rooting zone or the capacity of the site to furnish water.
b) Soil texture and stone content have been used as indicators of relative fertility, as well as giving us the percent of clay silt and sand composition. Texture is, therefore, second to depth in the determination of land potential. It is usually expressed by textural class (i.e. sandy, loam, clay, etc.). It can be easily estimated in the field by the feel technique.
c) Bulk density (or weight of soil per given volume) is useful in determining the depth of inches available to the plants. The moisture and nutrient data are more useful when expressed on a volume basis. Using the bulk density method and knowledge on plant requirements and losses due to evaporation, etc., we can decide how long the moisture will last. If we want, for example, to introduce better grass species, and if we know the water requirements of the grass, we can determine whether soil water will be sufficient or not.
d) Drainage, aeration and permeability affect the diffusion of gases and water movement. Deficiency of oxygen, compact horizons, or high water table may restrict the root growth. Restricted drainage further contributes to salinisation.
The extent to which drainage, aeration, and permeability are restricted indicates the potential for improvement. The ease with which the improvements can be made should also be considered because, while improvement methods might be available, the cost might be prohibitively high.
e) The most important soil components to be considered include nitrogen content, organic carbon (organic matter), phosphorus, sulphur, exchange capacity, and other properties associated with salt-affected soils. N. S. and organic carbon are particularly vulnerable to losses in the deteriorating ranges.
In arid ranges, the concentration of soluble salts and exchangeable sodium may reach levels detrimental to plant growth. For site potential studies we can use such measurements as electrical conductivity of the saturation extract, or exchangeable sodium, etc., to indicate the extent to which the soil is affected, and decide on what corrective measures to apply. In some cases the soils may be so badly affected that no practical way to improve the range is available.
Vegetation production and composition
a) Production can be used as a single factor in site evaluation. It provides effective approaches to relative dominance, species diversity, and other aspects. It also establishes a base for evaluating the grazing capacity for range units.
Production can be determined, using various estimation methods or clipping techniques. Increases of production in terms of kg/lb per hectare/acre can then be determined. Estimation methods include estimation at the site in the field, cutting and weighing samples from known areas at appropriate intervals; and using the grazing animal to measure either annual production in terms of weight increase or amount of herbage consumed and digested.
Production is directly proportional to available water in the soil and inversely proportional to the evaporating power of the air. By studying the production data and the species on the site, we can learn something about the water retention capacity of the soils, evaporation, and the suitability of the site in general. Production should not, however, be used on its own in drawing conclusions. Other factors used should be considered. In addition, total production and not only herbs and shrubs must be considered.
b) Vegetation composition refers to the relative amounts of a particular species as a percentage of the total number of species in a community. Several qualitative and quantitative methods can be used to determine this, but we can evaluate the site and decide on its potential if we know the ecology and physiology of the existing plants. Composition can indicate the kind of community that might develop under given management. Alternatively, we can identify the management practices that will help increase the percentage of a particular species. Like production, however, composition should not be used on its own; other factors need to be considered.
Location of the site
Accessibility is important if the site is to be of any value. It must be accessible to the developers as well as to the animals that will use it, or its potential is reduced.
In most cases rangelands are improved so as to be used by livestock. They need to have water on them to be of any use.
Rangelands serve people as well as animals: if any improvement is to be made, it must be with their co-operation, especially through involvement in the development. They should also be instructed as to the social and economic benefits that will accrue to them.