Problem Soils Types, Causes, Consequences, and Management: A document for farmers and land managers

Soil is the foundation of plant production and a vital resource for food security and environmental sustainability. However, not all soils are equally suitable for growing crops. Some soils pose significant challenges for farmers and land managers due to their physical, chemical, or biological properties. These are called problem soils, and they require special management practices to improve their productivity and prevent further degradation.

Problem soils can be classified into two broad categories: soils with inherent limitations and soils with induced limitations. Soils with inherent limitations are those that have unfavorable characteristics due to their natural formation and composition. Examples of these are heavy soils, sandy soils, rocky soils, steep soils, eroded soils, exposed subsoils, peat soils, and marsh soils. Soils with induced limitations are those that have become degraded or impaired due to human activities or natural disasters. Examples of these are compacted soils, soils with chemical constraints, such as low fertility, high salinity, acidity, or alkalinity, and soils with physical constraints, such as poor structure, low water holding capacity, erosion, or compaction.

This document aims to provide an overview of the main types of problem soils, their causes and consequences, and the effective practices for managing them in global plant production. The document is organized as follows: Section 2 describes the soils with inherent limitations, Section 3 describes the soils with induced limitations, and Section 4 summarizes the key recommendations for problem soil management.

Soils with Inherent Limitations

Soils with inherent limitations are those that have unfavorable characteristics due to their natural formation and composition. These soils are often found in regions with extreme climatic conditions, such as arid, semi-arid, humid, or cold environments. They may also be associated with specific geological formations, such as volcanic, glacial, or alluvial deposits. The main types of soils with inherent limitations are:

• Heavy soils: These are soils that have a high proportion of clay particles, which make them dense, sticky, and difficult to work with. Heavy soils tend to have poor drainage, low aeration, high waterlogging, and low permeability. They are prone to cracking and hardening when dry, and to water erosion when wet. Heavy soils are often found in tropical and subtropical regions, where they are also affected by high temperatures and rainfall. Heavy soils can limit plant growth by restricting root penetration, water uptake, and nutrient availability.
• Sandy soils: These are soils that have a high proportion of sand particles, which make them loose, coarse, and easy to work with. Sandy soils tend to have good drainage, high aeration, low water retention, and high permeability. They are prone to wind erosion, leaching, and drought. Sandy soils are often found in arid and semi-arid regions, where they are also affected by low rainfall and high evaporation. Sandy soils can limit plant growth by reducing water availability, nutrient retention, and organic matter content.
• Rocky soils: These are soils that have a high proportion of stones, gravel, or boulders, which make them hard, rough, and uneven. Rocky soils tend to have variable drainage, aeration, water retention, and permeability, depending on the size and shape of the rocks. They are prone to mechanical damage, frost heaving, and water runoff. Rocky soils are often found in mountainous and hilly regions, where they are also affected by steep slopes and low temperatures. Rocky soils can limit plant growth by reducing soil depth, water storage, and nutrient supply.
• Steep soils: These are soils that have a high slope gradient, which make them unstable, risky, and inaccessible. Steep soils tend to have poor drainage, low aeration, high water erosion, and low water retention. They are prone to landslides, gully formation, and soil loss. Steep soils are often found in mountainous and hilly regions, where they are also affected by high altitude and low temperatures. Steep soils can limit plant growth by increasing water stress, nutrient depletion, and soil degradation.
• Eroded soils: These are soils that have lost their topsoil layer, which is the most fertile and productive part of the soil. Eroded soils tend to have low organic matter, low nutrient content, low water holding capacity, and low biological activity. They are prone to further erosion, compaction, and crusting. Eroded soils are often found in regions with high rainfall and wind, where they are also affected by deforestation, overgrazing, and cultivation. Eroded soils can limit plant growth by reducing soil quality, water availability, and nutrient supply.
• Exposed subsoils: These are soils that have their subsoil layer exposed, which is the deeper and less fertile part of the soil. Exposed subsoils tend to have high clay content, high acidity, high salinity, and high toxicity. They are prone to waterlogging, nutrient imbalance, and plant stress. Exposed subsoils are often found in regions with low rainfall and high evaporation, where they are also affected by land clearing, deep plowing, and irrigation. Exposed subsoils can limit plant growth by inhibiting root development, water uptake, and nutrient absorption.
• Peat soils: These are soils that have a high proportion of organic matter, which make them dark, soft, and acidic. Peat soils tend to have poor drainage, low aeration, high water retention, and low nutrient availability. They are prone to subsidence, decomposition, and fire. Peat soils are often found in regions with high rainfall and low temperatures, where they are also affected by waterlogging, flooding, and drainage. Peat soils can limit plant growth by causing oxygen deficiency, nutrient deficiency, and toxicity.
• Marsh soils: These are soils that are permanently or periodically flooded by water, which make them wet, muddy, and anaerobic. Marsh soils tend to have high organic matter, high salinity, high acidity, and high toxicity. They are prone to waterlogging, leaching, and sedimentation. Marsh soils are often found in regions with low elevation and high rainfall, where they are also affected by tides, waves, and currents. Marsh soils can limit plant growth by creating water stress, nutrient imbalance, and plant disease.

Soils with Induced Limitations

Soils with induced limitations are those that have become degraded or impaired due to human activities or natural disasters. These soils are often found in regions with intensive agricultural, industrial, or urban development, where they are also affected by population pressure, land use change, and environmental pollution. The main types of soils with induced limitations are:

• Compacted soils: These are soils that have a reduced pore space, which make them hard, dense, and resistant to penetration. Compacted soils tend to have poor drainage, low aeration, low water infiltration, and low permeability. They are prone to water runoff, crusting, and root restriction. Compacted soils are often caused by heavy machinery, livestock, traffic, or tillage, which exert pressure on the soil surface and compress the soil particles. Compacted soils can limit plant growth by reducing root development, water uptake, and nutrient availability.
• Soils with chemical constraints: These are soils that have unfavorable chemical properties, such as low fertility, high salinity, acidity, or alkalinity. Soils with low fertility tend to have low organic matter, low nutrient content, and low biological activity. They are prone to nutrient deficiency, nutrient imbalance, and nutrient loss. Soils with low fertility are often caused by nutrient depletion, nutrient leaching, or nutrient immobilization, which reduce the soil's ability to supply or retain nutrients. Soils with low fertility can limit plant growth by affecting plant metabolism, growth, and yield.
• Soils with high salinity tend to have high salt content, which make them white, crusty, and saline. They are prone to salt accumulation, salt toxicity, and salt stress. Soils with high salinity are often caused by salt input, salt mobilization, or salt concentration, which increase the soil's salt level or reduce its dilution. Soils with high salinity can limit plant growth by affecting plant osmosis, water uptake, and nutrient absorption.
• Soils with acidity tend to have low pH, which make them sour, corrosive, and acidic. They are prone to aluminum toxicity, manganese toxicity, and phosphorus deficiency. Soils with acidity are often caused by acid input, acid formation, or acid leaching, which decrease the soil's pH or increase its hydrogen ions. Soils with acidity can limit plant growth by affecting plant enzyme activity, membrane stability, and nutrient availability.
• Soils with alkalinity tend to have high pH, which make them sweet, calcareous, and alkaline. They are prone to calcium carbonate accumulation, iron deficiency, and zinc deficiency. Soils with alkalinity are often caused by alkaline input, alkaline formation, or alkaline leaching, which increase the soil's pH or reduce its hydrogen ions. Soils with alkalinity can limit plant growth by affecting plant solubility, chelation, and nutrient availability.
• Soils with physical constraints: These are soils that have unfavorable physical properties, such as poor structure, low water holding capacity, erosion, or compaction. Soils with poor structure tend to have low aggregation, low stability, and low porosity. They are prone to crusting, sealing, and clogging. Soils with poor structure are often caused by physical disturbance, organic matter loss, or clay dispersion, which disrupt the soil's arrangement or cohesion of particles. Soils with poor structure can limit plant growth by reducing soil aeration, water infiltration, and root penetration.
• Soils with low water holding capacity tend to have low moisture content, low water retention, and low water availability. They are prone to drought, wilting, and water stress. Soils with low water holding capacity are often caused by low organic matter, low clay content, or high sand content, which reduce the soil's ability to store or supply water. Soils with low water holding capacity can limit plant growth by affecting plant transpiration, photosynthesis, and biomass production.
• Soils with erosion tend to have soil loss, soil displacement, and soil degradation. They are prone to sedimentation, nutrient depletion, and organic matter reduction. Soils with erosion are often caused by water runoff, wind action, or gravity, which detach, transport, or deposit soil particles. Soils with erosion can limit plant growth by reducing soil depth, soil quality, and soil productivity.
• Soils with compaction tend to have reduced pore space, which make them hard, dense, and resistant to penetration. Compacted soils tend to have poor drainage, low aeration, low water infiltration, and low permeability. They are prone to water runoff, crusting, and root restriction. Compacted soils are often caused by heavy machinery, livestock, traffic, or tillage, which exert pressure on the soil surface and compress the soil particles. Compacted soils can limit plant growth by reducing root development, water uptake, and nutrient availability.

From an agricultural standpoint, problem soils are typically categorized into three primary types: saline, sodic, and acidic soils. Each category has its own specific causes, impacts, and management approaches. Grasping these differences is vital for successful soil management.

Saline soils?

Saline soils are soils that have high concentrations of soluble salts, mainly sodium chloride (NaCl), in the soil solution. Saline soils are common in arid and semi-arid regions, where rainfall is low and evaporation is high. Saline soils can also result from irrigation with poor-quality water, seawater intrusion, or salt accumulation from fertilizers and manures.?

Saline soils can reduce plant growth and yield by creating osmotic stress, which lowers the water potential of the soil and makes it harder for plants to absorb water. Saline soils can also cause specific ion toxicity, which occurs when certain ions, such as sodium (Na+), chloride (Cl-), or boron (B), reach toxic levels in plant tissues and interfere with metabolic processes. Saline soils can also affect soil structure, nutrient availability, and microbial activity.?

The management of saline soils involves the following practices:?

• Identifying and mapping saline soils using soil testing, electrical conductivity (EC) measurements, or remote sensing.?
• Selecting salt-tolerant crops and varieties that can withstand osmotic stress and specific ion toxicity.?
• Applying adequate and timely irrigation to leach salts below the root zone and maintain soil moisture.?
• Improving drainage to prevent waterlogging and salt accumulation.?
• Applying organic matter, gypsum, or other amendments to improve soil structure and nutrient availability.?
• Adopting conservation tillage, mulching, or cover cropping to reduce soil erosion and evaporation.?

Sodic soils?

Sodic soils are soils that have high levels of exchangeable sodium (Na+) in the soil cation exchange complex. Sodic soils are often associated with saline soils, but they can also occur in non-saline soils due to the removal of calcium (Ca2+) and magnesium (Mg2+) by leaching, crop removal, or acidification. Sodic soils are more common in arid and semi-arid regions, but they can also occur in humid and sub-humid regions due to human activities, such as irrigation, mining, or industrial waste disposal.?

Sodic soils can reduce plant growth and yield by affecting soil physical properties, such as aggregation, infiltration, permeability, aeration, and water holding capacity. Sodic soils tend to have poor soil structure, with dispersed clay particles that clog soil pores and create hard and compacted layers. Sodic soils can also cause nutrient imbalances, especially of calcium (Ca2+) and magnesium (Mg2+), and increase the toxicity of some elements, such as aluminum (Al3+) and manganese (Mn2+).?

The management of sodic soils involves the following practices:?

• Identifying and mapping sodic soils using soil testing, exchangeable sodium percentage (ESP) measurements, or soil pH measurements.?
• Selecting crops and varieties that can tolerate high sodium levels and low calcium and magnesium levels.?
• Applying adequate and timely irrigation to leach sodium below the root zone and maintain soil moisture.?
• Improving drainage to prevent waterlogging and sodium accumulation.?
• Applying gypsum, sulfur, or other amendments to replace sodium with calcium and lower soil pH.?
• Applying organic matter, lime, or other amendments to improve soil structure and nutrient availability.?
• Adopting conservation tillage, mulching, or cover cropping to reduce soil erosion and evaporation.?

Acidic soils?

Acidic soils are soils that have low pH values, usually below 5.5, in the soil solution. Acidic soils are common in humid and sub-humid regions, where rainfall is high and leaching is intense. Acidic soils can also result from the decomposition of organic matter, the oxidation of sulfur, or the application of acidifying fertilizers and manures.?

Acidic soils can reduce plant growth and yield by affecting soil chemical properties, such as nutrient availability, cation exchange capacity, and buffering capacity. Acidic soils tend to have low levels of macronutrients, such as phosphorus (P), potassium (K), calcium (Ca2+), and magnesium (Mg2+), and high levels of micronutrients, such as iron (Fe2+), manganese (Mn2+), zinc (Zn2+), and copper (Cu2+). Acidic soils can also cause aluminum (Al3+) and manganese (Mn2+) toxicity, which inhibit root growth and nutrient uptake.?

The management of acidic soils involves the following practices:?

• Identifying and mapping acidic soils using soil testing, soil pH measurements, or indicator plants.?
• Selecting crops and varieties that can tolerate low pH and high aluminum and manganese levels.?
• Applying lime, dolomite, or other amendments to raise soil pH and reduce aluminum and manganese solubility.?
• Applying organic matter, phosphorus, or other amendments to improve soil fertility and nutrient availability.?
• Adopting conservation tillage, mulching, or cover cropping to reduce soil erosion and leaching.?

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Effects of Problem Soils on Plant Growth and Development?

Problem soils can adversely affect plant growth and development in various ways, depending on the type and severity of the constraint. Some of the common effects are:?

• Reduced nutrient availability and uptake?
• Increased toxicity of certain elements or compounds?
• Altered plant water relations and transpiration?
• Impaired root growth and function?
• Decreased photosynthesis and biomass production?
• Increased susceptibility to pests and diseases?
• Reduced seed germination and emergence?
• Lowered crop quality and yield?

Management Options for Problem Soils?

There is no single solution for managing problem soils, as each soil type and situation requires a specific approach. However, some general principles and practices can be applied to improve the condition and productivity of problem soils. These include:?

• Assessing the soil properties and constraints?
• Selecting suitable crops and varieties?
• Applying appropriate soil amendments and fertilizers?
• Implementing proper irrigation and drainage systems?
• Adopting conservation tillage and mulching?
• Enhancing soil organic matter and biological activity?
• Integrating crop rotation and intercropping?
• Controlling weeds, pests, and diseases?
• Restoring degraded lands and ecosystems?

Key Recommendations for Problem Soil Management

Problem soils pose significant challenges for farmers and land managers, but they can be improved and restored with appropriate management practices. The following are some of the key recommendations for problem soil management, based on the best available scientific knowledge and practical experience:

• Improve soil drainage: Soil drainage is the removal of excess water from the soil, which can improve soil aeration, reduce waterlogging, and prevent salinization. Soil drainage can be improved by installing drainage systems, such as ditches, pipes, or tiles, or by creating surface channels, such as furrows, ridges, or beds. Soil drainage can also be improved by increasing soil permeability, such as by adding organic matter, breaking up compaction, or reducing tillage.
• Enhance soil fertility: Soil fertility is the ability of the soil to supply nutrients to plants, which can improve soil quality, increase crop yield, and prevent nutrient deficiency. Soil fertility can be enhanced by applying fertilizers, such as organic, inorganic, or biofertilizers, or by using legumes, such as clover, alfalfa, or beans, which can fix nitrogen from the air. Soil fertility can also be enhanced by maintaining soil pH, such as by adding lime, gypsum, or sulfur, or by using acid-tolerant or alkaline-tolerant crops.
• Conserve soil moisture: Soil moisture is the amount of water in the soil, which can affect soil temperature, soil structure, and plant growth. Soil moisture can be conserved by reducing evaporation, such as by using mulches, such as straw, leaves, or plastic, or by planting cover crops, such as grasses, legumes, or cereals, which can shade the soil and reduce weed growth. Soil moisture can also be conserved by increasing water retention, such as by adding organic matter, increasing clay content, or using water-harvesting techniques, such as contour farming, terracing, or micro-catchments.
• Prevent soil erosion: Soil erosion is the loss of soil due to water, wind, or gravity, which can reduce soil depth, soil quality, and soil productivity. Soil erosion can be prevented by reducing soil detachment, such as by using vegetative barriers, such as hedges, trees, or grass strips, or by using physical barriers, such as stone walls, fences, or check dams, which can intercept runoff and sediment. Soil erosion can also be prevented by reducing soil transport, such as by using conservation tillage, such as no-till, minimum-till, or strip-till, or by using contour plowing, strip cropping, or intercropping, which can reduce the speed and direction of runoff and wind.
• Rehabilitate degraded soils: Degraded soils are those that have lost their natural functions and services, such as supporting plant growth, regulating water flow, and storing carbon. Degraded soils can be rehabilitated by restoring soil organic matter, such as by adding compost, manure, or biochar, or by planting perennial crops, such as trees, shrubs, or grasses, which can increase soil carbon and biodiversity. Degraded soils can also be rehabilitated by restoring soil biodiversity, such as by introducing earthworms, microorganisms, or beneficial insects, which can improve soil structure, nutrient cycling, and pest control.

?Problem soils are a major challenge for global plant production, as they limit the potential and sustainability of agricultural systems. However, problem soils can be managed and improved by applying appropriate practices and technologies, based on the understanding of the soil characteristics and constraints. By doing so, farmers can increase their crop yields and incomes, while also enhancing the soil quality and environmental services. Therefore, problem soils should not be seen as a hopeless situation, but rather as an opportunity for innovation and improvement.?