Soil is used for a variety of purposes including cropping, pasturing, gardening, forestry, agroforestry, construction, etc. The aim is always to find the most suitable type of soil to get the best outcome.
Especially in agriculture, the desired yield of a crop could be obtained from a type of soil that is physically, chemically, and biologically suitable or productive for that crop. A few soils are naturally very productive; and some soils, which do not have many limitations, can be made productive by appropriate soil and crop management practices.
About 10% of the total soil resources of the world have little or no considerable limitations to cropping. Yet, most of the soils have some kind of limitations in terms of growing crops.
Some soils can be productive for only a limited number of crops, but offer serious limitations for others. Soils that have serious limitations to agricultural and other land uses are traditionally called problem soils. Such soils need special management techniques for their profitable and sustainable use. In this article, we discuss the top 12 soil problems.
Stoniness refers to the presence of rock fragments, such as gravel, stone, and cobbles in high proportion. These materials do not provide or retain nutrients and moisture. They offer physical resistance to the movement of agricultural implements and restrict tillage operations. They reduce the workability of soil and hinder cultivation.
2. Shallowness of Soil
Shallowness of soil refers to the presence of a thin soil layer over the rock bed or a hard-cemented layer or a root restrictive layer near the surface. The depth of shallow soils varies from 30 to 50 cm depending on the root depth of the crops grown. These soils are often stony or gravelly and they are prone to desiccation. Such soils frequently occur on steep lands.
3. Low Strength Soil
Soil strength represents the ability of the soil to support loads. It is related to windthrowing, bearing capacity of farm animals and machineries, and the foundation of buildings, roads, and highways. Generally, the following three attributes of soils are considered the most important ones for low-strength soil:
- Plasticity: Highly plastic soils are unsuitable for the foundation of buildings and road materials
- Low-weight bearing strength soil: Such soil accounts for low capacity to support loads when they are wet
- Organic: Peats or peaty soils have low shear strength and may be subject to contraction.
4. Vertic Properties
Vertic properties are the features of Vertisols, which swell on wetting and shrink on drying. They are also called shrink-swell soils and expansive soils. In India, dark-colored Vertisols are called black cotton soils. The dark color of these soils is due to humus complexes with dispersive clays. The Vertisols produce wide and deep cracks upon drying.
The cause of cracking is a high clay content coupled with more than 50% Montmorillonitic clay minerals. The distribution of Vertisols is highly localized and it is linked with mafic rocks, semiarid climatic conditions, or both. The greatest absolute extents are found in the lavas of the Deccan plateau of India. Twelve countries, in all the major continental regions, have over 10% of their land with vertic properties. Uruguay and India have the highest relative extent.
Hydromorphy or wetness in the soil profile for the entire or a part of the year is generally found in flat and low-lying areas with respect to the surrounding land. These lands include alluvial and coastal plains, deltas, and river valleys, including valley floors, peat bogs, marshes, and other wetlands.
About 20 countries have a substantial proportion (more than 25%) of land affected by hydromorphic constraints, with the highest proportions (more than 50%) in the Falklands, the United Kingdom, and Bangladesh.
6. Low Cation Exchange Capacity (CEC)
Soils that have a low organic matter content, low clay content, clay minerals with low Cation exchange capacity (CEC), or all these properties in surface soil possess low cation exchange capacity. These soils have a low inherent fertility and also a low capacity to retain nutrients added as fertilizers.
Generally, low CEC soils include highly sandy soils, and tropical soils dominated by kaolinite clay and sesquioxide. In humid and sub-humid regions there are extensive areas of sandy or highly-weathered soils with a low CEC.
7. Soil Acidity and Alkalinity
A soil pH value below 7 indicates acidity and a value above it denotes alkalinity. Acidity and alkalinity influence the growth of plants and microorganisms and affect chemical transformation, nutrient availability, and elemental toxicity.
At soil pH below 4.5, aluminum and many heavy metals are released into soil solution. Also, nitrogen, phosphorus, potassium, and other nutrients become virtually unavailable. Different plants have different levels of tolerance to acidity and alkalinity, but most plants thrive at pH levels between 6.0 and 7.5. Levels below 4.5 or above 8.5 present significant limitations for the growth of most plants. Highly acidic or alkaline soils are not suitable for agriculture.
8. Acid Sulfate Conditions
Acid sulfate soils offer serious limitations to most uses. They usually develop in estuarine margins inundated with brackish water and supplied with plenty of organic matter. They are a potential constraint to land uses involving excavation or soil disturbance.
Potential acid sulfate soils contain pyrite, which reacts with atmospheric oxygen to form sulfuric acid on exposure to air through drainage. It creates extreme acidity that causes the release of toxic materials, including aluminum and heavy metals. Acid sulfate soils may also be highly saline.
9. Salinity and Sodicity
Soil salinity and sodicity occur naturally in semi-arid to arid regions. They are also found in coastal lowlands. The state of salinity is caused by the accumulation of free salts in the profile, and sodicity due to dominance of the exchangeable sodium on colloidal surfaces.
A total of 21 countries have more than 15% of their land affected due to salinity and sodicity. Out of that, 13 lie in a broad belt extending from the African Sahara and its bordering Sahel zone through the Middle East and into Central Asia. Sodicity is even more strongly localized in Central Asia for over more than 10% of its extent.
10. Aluminum Toxicity
Soils with aluminum toxicity have the exchange complexes dominated by alumina. This occurs with a soil reaction of pH <5.5. This problem is commonly associated with strongly acid soils. The main cause is intensive weathering and strong leaching under high rainfall.
Aluminum toxic soil is predominantly a problem of the humid tropics. However, it is also found in the subhumid tropics and in a few temperate-zone countries with high rainfall area, e.g., New Zealand. It affects some 800 million-ha in South America and 400 million-ha in both Africa and Asia. The most affected eight countries lie largely or entirely in the rain forest zone. More than 50 countries have more than 25% of their territory affected by aluminum toxicity.
11. Phosphorus Fixation
The problem of phosphorus fixation in soils is generally associated with high content of free iron oxides in the clay fraction, which insolubilizes phosphate ions and renders it unavailable to plants. It is a dominant feature of strongly acid soils, and hence found often in conjunction with aluminum toxicity.
The eight countries with more than 20% of land affected by high phosphorus fixation are also affected by aluminum toxicity. Acidic and high aluminum-containing soils are highly phosphate fixing.
12. Low Soil Fertility
Low soil fertility refers to the inadequate supply of available nutrients due to low nutrient availability, imbalance of nutrients, and presence of some kind of toxicity.
The low fertility results from strong weathering, low cation exchange capacity, low organic matter content, strong soil acidity, strong phosphate sorption capacity, and strong nutrient leaching or nutrient imbalances. Low supply of micronutrients causes considerable constraints to food production in the tropics.
A clayey soil containing free iron oxides insolubilizes the phosphate ions and renders it unavailable to plants. This is called as phosphorus fixation of soil.
A soil that dominates the content of alumina is called aluminum toxic soil. The word toxic refers to the reduction in the ability of soil to support plant growth.