agricultural production guidelines
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Veld in KwaZulu-Natal
Co-ordinated Extension KwaZulu-Natal Veld 5.1 1999
INFLUENCE OF ENVIRONMENTAL FACTORS ON VELD
C N MacVicar
KwaZulu-Natal Department of Agriculture
Towards
Understanding the Nature of Veld
Summary of
Effects of Environmental Factors on Veld
Environmental Factors
Major Lithological Classes
Veld Ecotopes
TOWARDS UNDERSTANDING THE NATURE OF VELD
Environmental influences on veld are complex. The factors outlined below, when integrated, produce an array of environments which vary in areal extent from less than a hectare to areas measurable in square kilometres. These environments, in turn, provide a complex array of growing conditions for veld. Not surprisingly, therefore, veld tends to vary greatly, often over relatively short distances, in its production and species composition, depending on the balance between the various environmental factors which affect plant growth. It is suggested that the following steps be taken in order to arrive at an understanding of the nature of the veld in a landscape.
- Establish the mean annual rainfall and its distribution through the year.
- Establish the month-by-month temperature regime.
- Determine which lithological classes occur in the landscape (see Major Lithological Classes below), and where they occur.
- Determine the kind of upland soils that occur on each lithological class and the kind of soils that occur in the wetlands and in well-drained alluvium (see Soil Classification - A Taxonomic System for South Africa, 1991, and any available soil maps).
- Determine which veld ecotopes are present, and the recommended management for them (see Veld Ecotopes below).
- Try to obtain information on how the vegetation of the landscape has been managed in the past.
- Examine the vegetation in the wetlands, and on the main soil-geology units, paying attention to aspect (i.e. examine the vegetation on the veld ecotopes).
Taking the foregoing into account, as well as special factors that might affect vegetation, such as a high soil soluble salt content, wind, cloud cover and topography, the investigator ought to be able to reach a good understanding of the vegetation of the landscape.
SUMMARY OF EFFECTS OF ENVIRONMENTAL FACTORS ON VELD
Factors favouring vegetative growth
- Increasing soil water until the evapo-transpirative demand of the desired veld species has been met. This usually excludes prolonged anaerobic conditions (waterlogging), as these tend to favour less desirable species. See Environmental Factors, Water below.
- Increasing radiant energy up to a point. See Environmental Factors, Radiant Solar Energy below.
- Increasing temperature up to a point. See Environmental Factors, Temperature below.
- Windless conditions or conditions of light winds. See Environmental Factors, Wind below.
- Adequate nutrients in the soil and a soil medium which permits these nutrients to be taken up by the roots of desired species. No significant amounts of plant-available elements toxic to desired plant species. See Environmental Factors, Plant Nutrients in Soil below.
- No significant soluble salt content. See Environmental Factors, Soluble Salts in Soil below.
Factors favouring sourveld
A low soil pH and plant nutrient status.Note: Dystrophic (highly leached) soils invariably support a sour veld. The boundary from sourveld to mixed veld probably occurs in the higher base status section of the mesotrophic (moderately leached) range.
Factors favouring sweetveld
A soil pH value of pH 7 or higher and a high plant nutrient status of the soil.Note: Soils with fully base-saturated or calcareous topsoils normally support a sweetveld. However, it is possible that sands with a very low cation exchange capacity might not support a sweetveld, even though base-saturated.
Factors favouring mixedveld
Conditions between those favouring sourveld and those favouring sweetveld are associated with what is generally regarded as mixed veld.
Water
The amount of water made available to roots from day to day through the year and the form in which it is made available (aerobically, anaerobically and gradations between the two) is influenced by several factors, the more important of which are described below.
- Mean total annual rainfall.
- Seasonal distribution of rainfall.
- The frequency of years with a very low and a very high rainfall.
- Intensity of rainfall - the higher the intensity, the greater the likelihood of loss because of runoff.
- Slope - the steeper the slope, the greater the runoff on the one hand, and soil waterlogging and anaerobic conditions in concave bottomlands on the other, with gradations between.
- Internal soil drainage - the presence or absence of layers which restrict water movement in soils (e.g. Kroonstad soil form).
- Soil infiltration capacity - the lower the infiltration capacity, the greater the runoff;
- Soil water holding capacity - the lower the water holding capacity, the greater the runoff or loss through deep percolation. Water holding capacity decreases as soil depth and clay content decrease.
- The manner in which soil releases water. In sands, nearly all the water is easily available for rapid uptake by plants, after which little water is available for uptake. As soil clay content increases, more water is held under increasing tension - initial uptake is easy and rapid but this is soon followed by a longer period of relatively slow uptake of water.
- Aspect - the more exposed soil is to the sun (e.g. northerly aspect), the greater is evaporation from the soil surface and the less water is available to the plant roots.
- Vegetal cover physically reduces runoff and decreases evaporation from the soil surface.
Radiant solar energy
The amount of radiant energy received during the year by plants is affected by, inter alia, the following, the influence of which is self-evident:
- latitude;
- aspect;
- cloud cover;
- elevation.
Temperature
Some of the more important factors influencing temperature are:
- latitude;
- elevation;
- aspect;
- topography, as it affects the distribution of cold and warm air (e.g. frost may occur in the valley bottoms of an otherwise frost-free landscape);
- oceanic and continental influences;
- cloud cover.
Wind
The following two factors are relevant.
- Wind increases evaporation of water from the soil surface, reducing the amount of water available to the plant roots.
- Wind, up to a point, increases transpiration of water from plant leaves by increasing atmospheric demand. At higher wind velocities this trend may in fact be reversed due to partial or total stomatal closure resulting from reduced humidity at the leaf surface and possible internal leaf water deficits. With stomatal closure, photosynthesis is also reduced, resulting in reduced growth.
Plant nutrients in soil
The materials and processes which give rise to soils cause each soil to contain a particular assemblage of plant nutrients, and, in certain cases, also elements toxic to some plants (e.g. boron, exchangeable aluminum), and, very occasionally, toxic to animals (e.g. copper). In addition, however, each soil has its own particular soil solution and colloidal (i.e. clay particles, humus) fabric which influence which elements in the soil are taken up by plant roots (soil reaction, i.e. the pH, is particularly important in this regard). The following are some of the more important factors which determine soil nutrient content and the chemical environment to which roots are exposed.
- Parent material. Some parent materials (e.g. basalt, dolerite, amphibolite, Karoo shales) are rich in plant nutrients, others (e.g. Karoo sandstones, Dwyka tillite) are less rich, and others (e.g. granite, Table Mountain sandstone) are relatively poor. There are certain elements which, when taken up in sufficient quantities by plants, are toxic to animals which graze those plants; such toxic elements are rare in nature. There are other elements (e.g. iodine) which are taken up by plants in trace quantities and which are essential to the diet of grazing animals. The occurrence of these elements in soil is usually related to the parent material.
- Rainfall. The higher the rainfall, the greater is the removal, by leaching, of plant nutrients from the soil and the more acid the soil becomes.
- Landscape age. The older the landscape, the longer the period over which leaching will have taken place.
Therefore, for example, the soils of the very old Nottingham Road landscape (mean annual rainfall 875 mm) are extremely leached and the young soils of Verulam (mean annual rainfall 1 000 mm) are weakly leached. Dystrophic members of Inanda, Kranskop, Magwa and Nomanci soil forms, and, to a lesser extent, Hutton, Griffin, Clovelly and Mispah forms, dominate the very old, extremely leached landscapes of KwaZulu-Natal.
- Soil infiltration by water. The less the runoff, the more water passes into the soil to cause leaching and removal of nutrients.
- Soil water holding capacity. The less water a soil can hold, the more rain water will move through the soil to cause leaching and removal of nutrients. Water holding capacity decreases as soil depth and clay content decrease.
- Evaporation from the surface. The greater the evaporation, the less water is available for leaching.
- Fixation or unavailability of nutrients. In certain soils, conditions are such that, although a nutrient element is present, it cannot be taken up in adequate amounts by some plants, which then suffer a reduction in the quantity and/or quality of vegetal material produced.
Soluble salts in soil
Certain soils accumulate water soluble salts, sodium chloride in particular. These are rare in KwaZulu-Natal. The quality of the grazing on such soils is normally poor. The vegetation of the acid, sulphidic, organic soils of some KwaZulu-Natal estuarine swamps is generally unsuitable for grazing.
Particular soils are associated with a lithological class. Because soil properties (e.g. nutrient status, water infiltration, water holding capacity) influence the nature of veld, and because a lithological class often occupies large areas, a knowledge of the lithological class (or classes) on a farm provides immediate insight into environmental factors important to veld. The lithological classes listed below include examples of their occurrence in KwaZulu-Natal.
- Calcarenite. Coast aeolianites cover large areas near the coast from the Makatini Flats through Kwambonambi and Durban Berea to Port Edward.
- Quartzite/quartzitic sandstone (usually containing more than 90% quartz). Table Mountain Subgroup sandstones are common on the coast and in the coast hinterland; Karoo Sequence sandstones are found inland, north of Colenso and in parts of the Little Berg.
- Dwyka tillite. Common on the coast and in the coast hinterland of KwaZulu-Natal.
- Rhyolite/felsite. Lebombo rhyolite is an important part of the Lebombo Mountain Range in KwaZulu-Natal.
- Granite/gneiss. Granite, gneiss, etc., of the Basement Complex of Southern Africa are common on the coast and in the coast hinterland of KwaZulu-Natal.
- Dolerite/basalt/gabbro/diorite/diabase. Karoo dolerite, Drakensberg basalt lavas, Basement Complex are common in KwaZulu-Natal (e.g. Golela to Mtubatuba, Little Berg and High Berg, dolerite outcrops throughout KwaZulu-Natal).
- Feldspathic sandstone. Sandstones of the Karoo Sequence, some sandstones of the Table Mountain Subgroup are common throughout inland KwaZulu-Natal below the Drakensberg basalt.
- Shale/mudstone. Karoo Sequence is frequent south of the Tugela River, and less frequent to the north.
- Undifferentiated, unconsolidated sediments. Alluvium and colluvium are common in stream and river valleys.
- Amphibolite/amphibole schist. Occur occasionally as small to largish areas in the coast hinterland and near the KwaZulu-Natal coast.
- Aeolian sands
. Occur intermittently as dunes adjacent to and within 0.5 km of the coastline.
- Mica, chlorite and talc schists/phyllite. Occur occasionally as small to largish areas in the coast hinterland and near the KwaZulu-Natal coast.
Note: In his book "The Natal Monocline : Explaining the Origin and Scenery of Natal, South Africa" (2nd revised edition published by the University of Natal Press, 1982), Lester King presents route guides along major and minor roads which indicate where examples of some of the lithological classes can be seen to good advantage.
A veld ecotope is a class of land (it may occur in many places) in which the variation of natural resources is insufficient to influence significantly the quality and quantity of veld that such land will produce under defined management. It is the basic resource unit for the proper planning of veld management on a farm. However, because ecotopes may often be small in area and irregular in shape, sensible combinations of adjoining whole ecotopes, or portions of ecotopes, into "Veld Type Units" are usually needed in order to demarcate camps of a suitable size for veld management and livestock production.