Soil Chemical, Physical, and Biological Health

Factors Affecting Soil Chemical Health

pH

Soil pH indicates a soil’s level of acidity or alkalinity, which directly influences the availability of nutrients to plants and microbes. A balanced pH range, typically between 5.8 and 7.5, ensures optimal availability. Deviations from this range reduce the availability of certain nutrients to plants and microbes, negatively affecting the health of agricultural soils.

Management Considerations:  Additions such as lime to raise the soil pH or sulfur-based compounds to reduce the soil pH are often used to make corrections. Regular soil testing and corrective actions help to ensure a healthy soil pH.

Cation Exchange Capacity (CEC)

CEC is a metric that reflects a soil's ability to hold and supply cations (positively charged ions) to plant roots. A higher CEC indicates that a soil can retain more essential cations such as calcium, potassium, and magnesium, which support plant growth and microbial populations.

Management Considerations: CEC is largely dependent on soil texture, with clayey soils typically having greater CEC values than sandy soils. Changes in soil CEC are very gradual, though maintaining optimum pH and implementing practices that build soil organic matter, such as cover cropping and reduced tillage, can help promote CEC buildup. Soils with greater CEC will generally require less frequent applications of essential cations, such as potassium, which tend to leach in soils with low CEC.

Essential Nutrients

Essential macronutrients, such as nitrogen and phosphorus, and essential micronutrients, such as zinc and boron, are nutrients that are irreplaceable and imperative to plant growth and development. Soils rich in essential nutrients promote healthy biology, from plants to microbes. Conversely, soils deficient in essential nutrients lead to stunted plant growth, reduced yields, and disease-promoting plant-microbe relations.

Management Considerations: Regular soil testing is critical to managing essential nutrient levels. Targeted fertilizer applications are used to correct soil nutrient levels and any imbalances present, benefiting soil and plant biology alike.

Organic Matter

Soil organic matter is composed of plant or animal tissue at various stages of decomposition and is commonly thought of as the cornerstone of soil health. Organic matter promotes healthy soil and plant biology by improving soil structure, facilitating aeration, increasing water-holding capacity, and increasing CEC. Soils depleted of organic matter oftentimes experience extreme fluctuations in available water and temperature, resulting in soil biology and plant stress.

Management Considerations: Cover cropping, reducing tillage, judicious use of fertilizers, and organic amendments all help ensure soil organic matter levels are maintained or elevated. Extensive tillage, overuse of fertilizer or chemicals, and exposing soil to direct sunlight may result in organic matter depletion.

Salinity

Salinity is a metric that indicates the concentration of salt in soil water. Elevated soil salinity inhibits water and nutrient uptake by the plant by altering the directional flow of water in the soil-microbe-plant continuum, which can cause water and nutrient stress even when adequate moisture and nutrients are present. Prolonged exposure to salinity can reduce soil fertility and crop yields, and disrupt beneficial soil microbial populations.

Management Considerations: Regular soil monitoring of soil and irrigation water salt levels and practices that promote good soil drainage, such as those that promote organic matter buildup, are crucial for soil salinity management. Salt accumulation is a particular problem in arid and semi-arid cropping systems, where a lack of rainfall can lead to salt accumulation.

Factors Affecting Soil Physical Health

Soil Texture

The relative proportion of sand, silt, and clay present in the mineral fraction of the soil determines the soil texture. Soil texture influences various factors related to soil health, such as water infiltration, drainage, nutrient availability, and more. Sandier soils typically have greater infiltration rates and gas exchange, are more well drained, and lower CEC, while clayey soils typically have reduced infiltration rates and gas exchange, are more poorly drained, and higher CEC.

Management Considerations: Regional farming practices are often, at least in part, guided by soil texture. For example, sandy soils typically require greater irrigation and more frequent fertilizer additions than clayey soils to maintain soil and plant health. Recognizing the unique challenges and strengths of a given soil’s texture and adjusting soil management practices accordingly is a critical component of soil health management.

Soil Structure

Soil structure refers to the arrangement and aggregation of soil particles. A well-structured soil is conducive to water infiltration, drainage, retention, and gas exchange, which promotes microbial and root health alike. Moreover, soil aggregate stability, a measure of the resistance of soil aggregates to breakdown, is strongly linked to erosion and compaction resistance.

Management Considerations: Soil structure management involves practices that reduce compaction and improve soil aggregation. Heavy machinery traffic commonly leads to soil compaction, so reducing the number of passes through the field mitigates the risk of compaction. Furthermore, practices that improve soil organic matter, such as cover cropping and organic amendments, improve soil aggregation and structure.

Water Infiltration

Water infiltration is the process by which water seeps into the soil and is typically measured by the velocity of infiltration, or infiltration rate. Adequate infiltration rates ensure that rainfall and irrigation properly wet the soil rather than result in runoff, which may cause erosion. Fields with poor water infiltration are often indicated by drought-prone upland areas, due to runoff leaving these areas, and ponding-prone lowland areas, due to runoff accumulating in these areas.

Management Considerations: Optimization of water infiltration includes addressing limiting factors, such as compaction or surface sealing. The inclusion of cover crops or organic mulches, reducing machinery traffic, reducing tillage, or the inclusion of permeable barriers all help reduce the risk of compaction and surface sealing.

Available Water Capacity (AWC)

AWC refers to the soil’s capacity to store plant-available water. Soils with higher AWC can support crops and microbes for a longer period of time during drought periods, enhancing resilience against stress, while soils with lower AWC may need greater supplemental irrigation to protect against stress. Soil texture has an inherent bearing on AWC, wherein sandy soils typically have the lowest AWC, while soils rich in both silt and clay typically have the greatest AWC.

Management Considerations: Soil moisture sensors and lab-based techniques can be used to determine the AWC of a given soil. Understanding a soil’s AWC can support more informed irrigation management decisions, ensuring plants receive adequate moisture but not in excess. Improvements in soil organic matter composition complement AWC, especially in drought-prone, sandier soils.