2. THE SOIL AND ITS ENVIRONMENT ... 59 2.1 Why is knowledge of soil important? ... 59 2.2 Origin of soils and their distribution ... 59 2.2.1 Soil composition and functions ... 59 2.2.3 Soil development ... 61 2.3 Recognizing important soil properties ... 64 2.4 Impact of clay content on agronomic management ... 69 2.5 Classifying Soils ... 71 2.6 Use of soil-specific management guidelines ... 78 2.7 The role of soil mapping ... 79 2.7.1 Choosing an appropriate classification system ... 79 2.7.2 Potential benefits of soil survey maps ... 80 2.7.3 Range of soil mapping options ... 81 2.8 Soil health issues ... 81 2.8.1 The new emerging view of soil health ... 81 2.8.2 Yield plateau assessments ... 81 2.8.3 Paired site outcomes ... 82 2.8.4 Loss of organic matter ... 83 2.8.5 Acidification ... 84 2.9 Management strategies for improving soil health ... 87 2.9.1 What constitutes a healthy soil? ... 87 2.9.2 Good management practices to improve soil health ... 88 2.9.3 Green manuring ... 88 2.9.4 Bioremedial amelioration with organic amendments ... 91 2.9.5 Minimum or reduced tillage ... 94 2.9.6 Managing soil acidification ... 94 2.9.7 Trashing ... 95 2.9.8 Managing soil compaction ... 98 2.9.9 Financial, social and environmental costs of improving soil health ... 99 2.10 Conclusions ... 100 2.11 References ... 101
2. THE SOIL AND ITS ENVIRONMENT
In this chapter the reader is provided with the necessary tools to know and understand his or her soils better; what they look like, how they were formed and where they may be found in the landscape. In time the successful implementation of the ‘Triple Bottom Line’ (TBL) approach will be affected by how well a manager knows his soils and it is therefore important to consider what soil types there are, where they occur and what their main physical and chemical limitations are. Selected examples are provided, illustrating how soil specific guidelines can be used for managing a range of agricultural operations and inputs on a particular sugarcane estate or outgrower farm.
2.1 Why is knowledge of soil important?
Many of the decisions a manager makes are affected by the soils that occur on the farm or estate. Good management practices must be economically and environmentally sustainable, and include systems of land preparation for re-establishing cane, cultivar selection, nutrition, weed control, the use of
agricultural chemicals, irrigation planning and control, the optimum time to harvest, and trash management. A key philosophy for all to follow to ensure long term sustainability is:
Man is dependent on good soils and good soils are in turn dependent on man and the use he makes of them.
While the global sugar industry is characterized by a wide range of climates, the soils in the industry show even greater variation, differing widely in their physical and chemical properties, their ability to produce crops and their management requirements. It is also not uncommon to find a number of soils differing in these characteristics on the same estate.
2.2 Origin of soils and their distribution
2.2.1 Soil composition and functions
Soil is composed of a mixture of mineral particles of varying sizes which are derived from rock fragments (< 2 mm in cross-section), finely weathered minerals (particularly clay), organic matter, humus, soluble nutrients, live bacteria, fungi, algae, air spaces and water. Soil forms part of the thin porous mantel covering most of the terrestrial parts of the globe. Main functions of soil:
Acts as a medium for plant and root growth by providing stored moisture and nutrients, and physical support for the plant to remain upright.
Deeper and lighter textured soils tend to support deeper root systems. In general the amount of roots below ground may be as large as the amount of biomass above the ground.
Soil modifies the effect of climate by influencing the amount of runoff and the portion of rainfall retained in the root zone and released to the plant.
A principal factor in the hydrologic cycle system.
Provides a habitat for organisms.
Soils also determine the rates of fertilizer application due to their different nutrient supplying and immobilizing characteristics.
2.2.2 Weathering agents contributing to soil formation
Weathering refers to the physical and chemical disintegration and decomposition of rocks, aided by biological agents, under varying temperature, pressure and moisture conditions on the earth's surface. Initially, weathering precedes soil formation, particularly in fresh rocks creating the parent material from which soil formation takes place. Physical agents tend to be more important than chemical agents in dry and/or very cold climates.
Box 2.1 Principal agents of physical weathering
Temperature changes causing differential expansion and contraction resulting in exfoliation or ‘onion skin’ weathering of rocks.
Raindrop splash can be very destructive on an unprotected soil surface, causing dispersion and capping.
Water from heavy rains and flowing waters rupture weakened rocks, exposing the inner portion to weathering as well as transporting materials.
Wind exerts abrasive action, and acts as a carrying agent.
Alternate wetting and drying results in downward leaching or an upward movement of soluble salts due to capillary water movement and evaporation.
Lichens and mosses growing on bare rocks are capable of gradually disintegrating rock material. Grasses, shrubs and trees growing in rock crevices extend the cracks by the growth of their roots.
Chemical weathering is more important in hot and wet climates. Where this takes place in the soil profile the weathering is termed pedochemical, whereas weathering in the deeper underlying rocks may be termed geochemical. The chemical decomposition of rocks is brought about by solution, hydration, hydrolysis, carbonation, oxidation and reduction reactions.
Box 2.2 Principal processes of chemical weathering
1. SOLUTION: Through the solvent action of water dissolving soluble salts, accelerated by carbon dioxide and organic acids released during the decomposition of organic matter.
2. HYDRATION: Hydration involves the slow conversion of certain compounds by water molecules to more easily decomposable compounds. For example, yellow ochre has more water attached to the molecule than red ochre and is less stable.
3. HYDROLYSIS: This is the most important process of chemical weathering, whereby weatherable minerals are converted into hydroxides of potassium, iron, magnesium or calcium in the presence of water and elevated temperatures.
4. CARBONATION: The hydroxides produced during hydrolysis react with the dissolved carbon dioxide to form carbonates which may either leach out or accumulate according to drainage or weather conditions.
5. OXIDATION: Oxidation is an important reaction in well-aerated rocks and soil material where oxygen supply is high and biological demand is low, and it is the dominant process for soil groups such as Nitosols and Ferralsols to have red or chromic colors due to the oxidation of ferrous to ferric iron.
6. REDUCTION: During reduction, particularly under prolonged waterlogging, the reverse of oxidation occurs. When biological oxygen demand is high and the supply of oxygen is low, the ferric ion is reduced to the ferrous ion, causing the red color to disappear.
2.2.3 Soil development
Soils form part of a landscape which itself is the result of the operation of present and past geomorphic regimes, dominated by the weathering processes just described. These processes are largely driven by climate, which is one of the major soil forming factors, along with parent material, topography, biological conditions and geological time. Throughout geologic time, wide variations in climatic conditions, and thus geomorphic regimes, have affected regions to different extents worldwide, resulting in a diverse range of landscapes and even more diverse range of soils that can often result in up to three or more different soil types occurring in a field.
The character of the soil profile itself, in terms of the individual layers called horizons, is fashioned by a number of soil development factors that include:
Climate
Parent material
Topography or drainage
Biological conditions
Geological time.
Climate
Mainly through temperature and moisture acting over thousands of years, climate has had a major effect on soil development and distribution, and the sequence of soils on a regional scale. The following are some examples of climatic influence on soil development.
Tropical moist climates speed up the rate of chemical weathering and cause the development of deep, porous, leached, acid soils. In general, the rate of chemical reactions described earlier doubles for every 10 °C rise in temperature.
Dry climates, resulting in evaporation in excess of precipitation, cause soluble salts and nutrients to accumulate in surface soils, often to the extent of causing saline sodic conditions (see Chapter 7).
Chemical weathering is slow in deserts but physical weathering, particularly exfoliation and sand blasting or wind erosion, is common. Soils are less weathered and often contain a higher proportion of 2:1 lattice clay minerals that can provide an excellent source of potassium as well as other cations.
Cold climates slow down the rate of chemical weathering, restricting soil development and causing shallower soils, but generally with above average cation and organic matter contents.
Climate and soil age are therefore often associated, as well as climate and natural vegetation, and these resulted in the zonal concept of soil classification (White 1979).
Parent material
This is essentially the underlying geological material from which the soil is derived. Geological materials can range from common igneous rocks such as high silica or quartz content granites, granodiorites and rhyolites to low silica rocks such as dolerite, diabase and basalt, to sedimentary rocks such as sandstone, shale, conglomerate, siltstone and limestone to metamorphic rocks such as gneiss, schist, amphibolite and marble. In general the high silica containing rocks such as granite, gneiss and sandstone containing minerals such as quartz, mica and feldspars weather to form mainly light textured, grey sandy to sandy loam soils that have coarse texture, are friable with a low base status, are acid and are poorly supplied with nutrients. In contrast, the low silica parent materials such as dolerite and basalt that contain some feldspar clay forming minerals and iron containing
minerals such as Hornblend, Augite and Olivine, tend to produce heavier sandy clay loam to clay soils, often red or black in color, and generally well supplied with nutrients, except phosphorus.
These soils tend to be neutral to basic in reaction at low elevations, but at high elevations the red soils are older and more highly leached, often containing toxic levels of aluminum in the profile.
Topography and aspect
Topography or relief has a dramatic impact on soil development and in determining the pattern or toposequence of soils in a particular landscape. This pattern of soils is closely linked to the water balance at every site, and drainage can play an enormous role in changing the nature or characteristics of soil over relatively short distances. An example of this sort of change is given in Fig. 2.1 for a
toposequence on granite in Mpumalanga, South Africa.
Figure 2.1. Cross-section of soils along a toposequence on granite.
Soils on the crest position have the highest rate of soil loss through erosion, resulting in a very shallow soil which progressively becomes deeper with descent , reaching a maximum depth in the bottomland position with the accumulation of eroded material. Soil in and around the valley floor not only receives moisture from rainfall, but also from deep percolation and seepage from the well drained soils above.
The term ‘hydromorphic’ (water loving) is used to describe this bottomland soil, also known as a
‘Gleysol’ in terms of the World Record Base (WRB) of soil classification, as it is frequently waterlogged in the wet season, with a mottled, clay subsoil (G-horizon). The footslope soil just above the gleysol is associated with an eluviated (bleached or washed out) layer, called an ‘E’ or ‘Albic’ horizon overlying a gleyed subsoil. This is usually caused by water moving laterally through the soil above the impervious subsurface layer. This horizon usually contains very little clay, organic matter and nutrients and is not a good environment for plant roots. The soil profile is classified as a ‘Planosol’ in terms of the WRB soil classification. This horizon disappears further upslope into a better drained but much shallower profile comprising a shallow grey loamy sand topsoil merging smoothly into a denser subsoil with prominent clay tongues penetrating the soft granite saprolite. In the above example the soil qualifies as a
‘Cambisol’ in terms of the WRB.
The soils in a large part of the Herbert Valley sugarcane areas in Queensland, Australia, have been mapped in this way and at least seven toposequences have been identified, covering 24 soil types (Wood et al. 2003).
Aspect is also an important factor in soil development. In the southern hemisphere, northerly and westerly aspects are hotter than southerly and easterly aspects, and northerly and westerly aspects are often in a rain shadow, compared to south-easterly slopes that face the cool rain bearing winds.
In South Africa, the northerly aspects tend to develop shallower soils, black or red in color, and have greater structural development, whereas southerly slopes are often deeper soils, brown or yellow in color with apedal structure on the same parent material.
Time
This soil forming factor has been referred to as the ‘age effect’ or the ‘geological erosion effect’ (Maud 1965). Throughout geologic time wide variations in climatic conditions, and thus geomorphic regimes, have affected regions to different extents worldwide. Generally warm prevailing climatic conditions have been operative only for the past 10 000 years or so, known as the geological period of the Holocene (or Recent). Prior to this, during what is known as the ‘Last Glacial’ period, which began about 120 000 years ago but which had its maximum effect only about 18 000 years ago, climatic conditions were generally significantly cooler and drier than those of the present, with consequent changes in the prevailing geomorphic regime that prevailed at that time. Of significance in the presently warmer and moister sugarcane growing regions of the world, the ‘Last Glacial’
period was one of enhanced erosion and deposition. As a result, most of the present-day soils in these regions are ‘young’ and have formed during the Holocene. However, this is not to say that some older soils, or remnants of older soils, have not survived in some situations conducive to their preservation (Dr R.Maud personal communication 2011). Examples of this are the survival of older elevated alluvial river terraces, but more notably the survival of remnants of ancient petro-plinthite (laterite) profiles which had their origin in the geological Cretaceous period, more than 60 million years ago, and which occur widely in the warmer portions of the former Gondwana remnant continents of Africa, South America, Australia and India (Fig. 2.2).
Old African surface Inanda plateau
Highly mature strongly
acid soils Young low lying
Quartenary land surface, Valley Thousand Hills, Immature soils less acidic soils