Aggregate Stability and Soil Degradation in the Tropics
Joe S.C. Mbagwu1
Department of Soil Science, University of Nigeria Nsukka, Nigeria
Lecture given at the College on Soil Physics Trieste, 3-21 March 2003
LNS0418022
Aggregate Stability and Soil Degradation in the Tropics 247
INTRODUCTION
Aggregate stability is a measure of the structural stability of soils. Factors that influence aggregate stability are important in evaluating the ease with which soils erode by water and/or wind, the potential of soils to crust and/or seal, soil permeability, quasi-steady state infiltration rates and seedling emergence and in predicting the capacity of soils to sustain long-term crop production. Aggregate stability of soils can be measured by the wet-sieving or raindrop techniques. A reduction in soil aggregate stability implies an increase in soil degradation. Hence aggregate stability and soil degradation are interwoven.
EXTENT OF STUDIES COVERED
The factors that influence aggregate stability of tropical soils depend on the soil environment, the type and dominance of the stabilizing substances and land use. On some tropical soils in Nigeria decreased aggregate stability was observed in unmulched than mulched plots. Also it was shown that the more frequent the irrigation, the more was the disintegration of water-stable aggregates on the unmulched than mulched plots. The type of surface active agents (surfactants) which enter into the soil is important in maintaining aggregate stability, with the anionic surfactants degrading and the nonionic surfactants aggrading aggregate stability. Using simple step-wise regression analysis, the relative importance of organic carbon, iron and aluminum oxides on aggregate stability of tropical soils in Indonesia was compared. It was, found using the raindrop technique for determining aggregate stability, that the oxides of Fe and Al and CaCO3 were more important stabilizing substances than organic carbon. In subtropical soils in China it was observed that the mechanisms of aggregate breakdown were slaking > mechanical breakdown > micro- cracking. The normalized mean-weight diameter, which is a measure of aggregate stability, correlated with soil properties than clay and soil organic carbon. In Nigeria and Ethiopia it was observed that as aggregate stability of soils increased with time, soil carbohydrates decreased which implies that aggregate stability did not depend on the content of carbohydrates in the soils. Some authors who reviewed the physical, chemical and mineralogical soil properties that influence aggregate stability in alluvial and upland soils in Nigeria pointed out that organic carbon was less important as an aggregating agent than other intrinsic stabilizing substances. They also showed the importance of some aggregate stability indices in predicting the potential of soils to erode and the possibility of using humic substances to improve the stability of degraded tropical soils.
SOIL DEGRADATION
Soil degradation is the temporary or permanent lowering of the productive capacity of soil caused by overgrazing, deforestation, inappropriate agricultural practices, over exploitation of fuel wood leading to desertification and other man-induced activities. 248 J.S.C. Mbagwu
All processes of soil degradation are grouped into six classes: water erosion, wind erosion, soil fertility decline, salinization, water logging and lowering of the water table. (i) Soil erosion by water includes inter-rill and rill erosion, gullying, and land sliding caused by clearing of vegetation and road construction. Soil erosion by wind produces sand dunes. (ii) Soil fertility decline refers to deterioration in soil physical, chemical and biological properties caused by (a) reduction in soil organic matter status, leading to decline in soil biological activity; (b) degradation in soil physical properties (structure, aeration, water holding capacity) caused by reduced OM; (c) adverse changes in soil nutrient status, including reduction in availability of the major nutrients (N, P, K), initiation of micronutrient deficiencies and development of nutrient imbalances; and (iv) build up of toxicities (by heavy metals, xenobiotics, and acidification through incorrect use of fertilizers). (iii) Water logging is caused by over irrigation, and restricted infiltration of water into the soil. This lowers land productivity through rise in ground water close to the soil surface. (iv) Salinization refers to all types of land degradation brought about by increased concentration of salts in the soil. It occurs by planning mistakes and mismanagement of irrigation schemes (salinization in the strictest sense), and sodification (also called alkalinization), which refers to the dominance of the exchange complex by Na+. (v) Lowering of the water table is brought about by pumping of ground water for irrigation which exceeds the natural recharge capacity. Pumping of water for urban and industrial use also causes this form of land degradation. Other types of land degradation include (i) deforestation, (ii) forest degradation (reduction in biotic resources and lowering of the productive capacity of forests), (iii) range land degradation (lowering of the productive capacity of range lands), (iv) acid sulphate formation, (v) soil pollution, (vi) soil destruction through mining and quarrying activities, (vii) urban and industrial encroachment on to agricultural land, (viii) destruction of irrigation schemes, and (ix) potential effects of global climatic change (including global warming which may lead to modifications in the general atmospheric circulation, causing changes in rainfall pattern) In Nigeria, some workers summarized the causes of as (i) fire and burning of vegetation, (ii) deforestation, (iii) increasing intensity of farming and cultivation (including irrigation practices in the north) and tillage-related practices, (iv) low input agriculture, (v) accelerated erosion by water and wind, and (vi) road building and other construction works. About 85% of the causes of land degradation worldwide are due to soil erosion by wind and water. Hence soil and land degradation are often used interchangeably. In Fig. 1 we show the casual nexus between land, population, poverty and degradation. In this figure it is argued that increase in rural population and decrease in usable land resources enhance land degradation. Aggregate Stability and Soil Degradation in the Tropics 249
INCREASE IN RURAL POPULATION LAND SHORTAGE LIMITED LAND RESOURCES
LAND DEGRADATION POVERTY
NON-SUSTAINABLE LAND MANAGEMENT PRACTICES
Figure 1. Casual nexus between land, population, poverty and degradation
Processes, Factors and Causes of Soil Degradation
As already defined, soil degradation is the loss in the productive potential of soil induced by human activities. The processes of soil degradation are the mechanisms responsible for the decline in soil quality (Fig. 2) and they are grouped into three types: physical, chemical and biological types, each of which has different processes affecting it (Fig. 3). In Fig. 4 it is shown that soil degradation is governed by environmental agents and catalysts which propel their actions. 250 J.S.C. Mbagwu
SOIL DEGRADATION
INDUSTRIAL AGRICULTURAL URBAN CONTAMINATION DEGRADATION POLLUTION
INDUSTRIAL MILITARY ACID RAIN URBAN SEWAGE BY-PRODUCTS BY-PRODUCTS WASTE WASTE
PHYSICAL CHEMICAL - Structural decline BIOLOGICAL - Acidification - Compaction - Loss of soil diversity - Nutrient depletion - Crusting - Soil organic C decline - Salinization - Erosion
Figure 2. Principal types of soil degradation mechanisms
SOIL DEGRADATION
HUMAN-INDUCED NATURAL (SOIL FORMATION)
URBAN LAND INDUSTRIAL LAND PHYSICAL CHEMICAL BIOLOGICAL - Pollution - Soil Compaction AGRICULTURAL - Pan formation - Laterization - Decline in soil - Compaction - Soil Contamination LAND - Hard-setting - Calcification diversity - Erosion -AcidRain - Leaching/ Illuviation
PHYSICAL CHEMICAL BIOLOGICAL - Compaction - Acidification - Decline in soil organic C - Crusting - Nutrient depletion - Soil biodiversity reduced - Water imbalance - Leaching - Decrease in biomass C - Impeded erosion - Nutrient imbalance - Runoff - Salinization/alkalization
Figure 3. Principal types of soil degradation: (i) natural and (ii) anthropogenic Aggregate Stability and Soil Degradation in the Tropics 251
Measures Used to Prevent or Combat Soil Degradation
The measures used can either be preventive or remedial. Preventive practices minimize the chances of soil degradation occurring or the magnitude or severity of the damage when the degradation manifests. These include in Nigeria, (i) manuring and mulching, (ii) planted fallows and cover crops, (iii) sustainable farming systems, (iv) adequate rotations, (v) home gardens or compound farms, (vi) alley cropping and related agro forestry systems, and (vii) chemical fertilizers which are mainly remedial measures. Because of alterations in soil properties that affect particular land uses, soils may degrade for one crop (maize rather sorghum). As long as some land use is possible soil degradation is not always an absolute concept. Decline in agricultural productivity should be evaluated in terms of inputs such as fertilizer use, water management and tillage methods. We can alleviate some types of soil degradation by use of micronutrients, inorganic fertilizers and organic residues. Soil that responds to management practices cannot be said to be degraded. Since crop growth depends on weather, degraded soils may be more sensitive to harsh weather (e.g. drought, temperature) than undegraded soils. A soil is degraded if its productivity falls below the economic threshold even under favourable weather conditions or with judicious inputs. All human-induced changes in soil properties are not always degrading to all soil’s functions. Therefore, soil degradation has to be referenced to the initial soil properties. We need to know if reduction in agricultural productivity due to soil erosion is reversible or not. A soil is degraded if its present or potential utility cannot be restored by improved management for the intended or other uses. A soil is degraded if the loss of intrinsic qualities, which manifest in reduced crop yields, is permanent.
SUGGESTIONS FOR FURTHER READING
Lal, R. 1989. Conservation tillage and sustainable agriculture: tropics vs. temperate environment. Adv. Agron. 42: 85-197. Lal, R., Hall, G.F. and Miller, F.P. 1989. Soil degradation. I. Basic processes. Land Degradation & Rehabilitation. 1: 51-69. Mbagwu, J.S.C. 1989. Effects of organic amendments on some physical properties of a tropical Ultisol. Biol. Wastes 28: 1-13. Mbagwu, J.S.C. 1992. Improving the productivity of a degraded Ultisol using organicand inorganic amendements. 1. Chemical properties and maize yield. Biores. Technol. 42: 149-154.
252 J.S.C. Mbagwu
SOIL DEGRADATION
PROCESSES FACTORS (Actions & Interactions) (Agents & Catalysts)
DETERIORATION OF LEACHING DECLINE IN BIOMASS C SOIL STRUCTURE - Compaction FERTILITY REDUCTION IN SOM - Crusting DEPRESSION CONTENT - Accelerated Hard-setting SODIFICATION DECREASE IN POPULATION IMBALANCE OF WATER/AIR TOXIFICATION ACTIVITY & SPECIES RATIO -Al+3 DIVERSITY OF SOIL FAUNA - Wetness -Mn+3 - Drought - Heavy Metals ALTERATIONS IN BIOLOGICAL PROCESSES FROM FAVORABLE TEMPERATURE EXTREMES TO UNFAVORABLE TRENDS - Permafrost - Supral Optimal
DEFORESTATION ACID RAIN URBANIZATION - Application of City EXCESSIVE & UNTIMELY WASTE DISPOSAL Wastes PLOWING - Conversion of Land to Non-Agricultural INTENSIVE ROW CROPPING Uses & MONOCULTURE
HIGH STOCKING RATES & EXCESSIVE GRAZING
Figure 4. Factors and processes of soil degradation