Chemical Properties of Soil, Soil Fertility and Nutrient Management

Educator Background

Essential Plant Nutrients

An essential nutrient is a nutrient element that a plant (or living organism) needs to function, grow or complete its life cycle. If that element or nutrient is lacking or deficient, there is a negative impact on the plant (or organism). As much as this is true for plants, so it is for humans, animals, microorganisms. Some essential plant nutrients are referred to as macronutrients because they are needed (and are found) in large amounts in the plant tissue. Examples are nitrogen (as NO3- or NH4+), phosphorus (as HPO42- or H2PO4-), calcium (Ca2+), magnesium (Mg2+), Sodium (Na+) and potassium (K+), and sulfur (S). Those that are required in trace quantities are referred to as micronutrients. Examples are iron (Fe2+, Fe3+), manganese (Mn2+), boron (as borate H3BO3), copper (Cu2+), zinc (Zn2+), etc. Although small amounts of these are needed, they are essential for different reasons such as metabolism or enzyme activity.

Nutrient Supply to Plants and Nutrient Cycling in Soil

The Law of Conservation of Mass states that matter is neither created nor destroyed. It's easy to see that in the cycling of nutrients in soil. Nutrients in the soil, which consists of elements, exist as ions in soil solution. They are moved around or cycled through release from exchange sites on the soil solids and organic matter, and through dissolution into soil solution where they can be absorbed or taken up by plants. Plants are consumed by animals and humans, and the nutrients are released back into the soil through the decay of plant and animal matter. Chemical reactions occur in the soil to maintain a balance or buffer nutrients in the soil solution through adsorptionThe process by which atoms, molecules, or ions are taken up from the soil solution or soil atmosphere and retained on the surfaces of solids by chemical or physical binding. and desorptionThe migration of adsorbed entities off of the adsorption sites. The inverse of adsorption. on clay particles and organic matter.

When excessive nutrients are released into the soil solution through decomposition or microbial metabolism of organic matter, and addition of fertilizer, either precipitation as soil minerals or adsorption to soil solids can occur. When nutrients are removed from solution by plants, more nutrients are released to the solution to maintain a balance between the amount on the soil solids and soil solution however, when crops are harvested away from the farm, nutrients in those crops are removed from the soil and the soil needs to be replenished. Both human activity and environmental factors can influence nutrient balance and affect the mineral and biological processes in soils.

Nutrient Mobility in Soils

Plant nutrients exist as ions and there are positive ions called cationsAn atom or atomic group that is positively charged because of a loss in electrons., and negative ions referred to anions. The anions are not strongly attracted to the exchange sites of the soil particles and organic matter, so they are quite soluble and readily move through the soil. This means that they are readily accessible to plant roots and can be picked up or can leachThe removal of soluble materials from one zone in soil to another via water movement in the profile. to the groundwater. The immobile ions are not readily accessible to plant roots but interact more with the mineral and organic surfaces. Some are more immobile than others. Calcium, ammonium, potassium, and magnesium are more soluble and mobile than micronutrient cations, phosphate, and molybdate. Due to the general immobility of these cations, plant roots access them through a small volume of soil surrounding individual roots. Understanding nutrient mobility in soils is essential to managing nutrient applications to maximize plant growth.

Soil Properties Influencing Nutrient Supply to Plants

Several properties and processes have an impact and influence nutrient availability to plants. Two major chemical factors influencing nutrient availability to plants are cation exchange capacity (CEC) and pH.

Cation Exchange Capacity

Soil particles consisting of sand, silt, clay, and organic matter have charged surfaces that attract cations (positively-charged ions) and anions (negatively-charged ions). The ions adsorbed on these charged sites can be exchanged by other ions through a process called ion exchange. Cation exchange capacity is much larger than anion exchange capacity of most agricultural soils. Most cations involved in ion exchange are plant nutrients except for hydrogen, aluminum, and sodium. Ion exchange reactions in soils are very important to plant nutrient availability and retention in soil because the surface charge on soil particles allows the soil to store large quantities of nutrients and release small amounts into soil solution as they are depleted by plant uptake.

Soil pH

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Source: https://upload.wikimedia.org/wikipedia/commons/a/a3/Soil_pH_effect_on_nutrient_availability.svg

Some cations, such as H+, Fe2+, Al3+, are acidic while others, such as Ca2+, Mg2+, K+, Na+, are basic. The proportion of acidic and basic cations determines a soil's pH. In soil, when the acidic cations increase relative to the basic cations, soil pH will decrease or become more acidic. On the other hand, when the exchange sites are occupied only by basic cations, the soil will have a neutral to higher pH. The percentage of exchange sites occupied by base cations is referred to as base saturation. Nutrients become less available to plants when the soil is either too acidic or too alkaline. Farmers typically add lime (calcium-containing compounds) to fields to adjust acidity issues so as to enhance nutrient availability and plant growth.

Biological Processes

Microbial activity and nutrient cycling through soil organic matter have a substantial effect on plant nutrient availability. Microorganisms mediate the decomposition of organic matter and microbially-controlled transformation processes. Humus, the most stable form of organic matter resulting from the decomposition process is involved in storing and releasing nutrients through the cation exchange process. During degradation, organically-complexed ions in the residue can be released (mineralization process) and if there are insufficient nutrients in the residue to meet microbial demand, then inorganic ions in soil solution are immobilized or withdrawn into microbial cells.

Physical Processes

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Roots in compacted soil

Arrangement of soil particles provides soil structure that allows passages (pores) for water movement and space for roots to grow and access nutrients. A well-structured soil provides adequate pores, promotes storage and movement of water, and allows roots to explore the soil volume to pick up necessary nutrients. When the soil is compacted, large pores are lost, roots have difficulty growing through the soil, and are not able to branch out as they would in a well-structured soil.

Assessing Nutrient Deficiency in Plants

Nutrient deficiency occurs when the soil nutrient supply is so low that the plant cannot function properly. Plants show that they are deficient in one or more essential nutrients by showing a reduction in yield or visual deficiency symptoms such as a change in leaf color different from normal healthy plants, death at the growing tip, stunted growth, poor fruit development, etc. Each nutrient deficiency exhibits a different symptom in a given plant, and each visual symptom is related to the function(s) of that nutrient in the plant. FertilizersAny organic or inorganic material of natural or synthetic origin (other than liming materials) that is added to a soil to supply one or more plant nutrients essential to the growth of plants. are often added to soils to supply deficient nutrients to plants. It is important to know what nutrients to supply (i.e. supply only what is deficient), when to provide additional nutrients to plants (when most beneficial to plants or convenient to get in the field), how to supply (e.g. foliar, broadcast, or bands) it, and how much to supply to the soil. Soil and plant analyses are tools that are used to identify the specific nutrient that might become deficient so that it can be corrected and thereby prevent future yield losses.

Nutrient Management

Not all nutrients removed from soil are returned or cycled back to the soil. When crops are harvested, nutrients are removed and often do not cycle back to soil immediately, or to where it might be useful to the soil. Eventually, the soil is not able to supply a sufficient amount of nutrients to growing plants and a deficiency of specific elements is created. Fertilizers are commonly applied to soils to supply the deficient nutrients in soil, but they can be lost through soil erosion, surface water runoff, and leaching to groundwater, all of which create environmental impact or damage. Soil erosion and surface runoff are accelerated when the soil is disturbed or plowed and there is no ground cover on the soil respectively. To make it more efficient, useful to the plant and avoid potential environmental issues, it is important to manage the fertilizer addition. Cycling of nutrients in the soil is completed when plant and organic residues are recycled or returned to the soil. As these plant residues decompose, they release the elements and nutrients contained in them back to the soil. These enter into the nutrient balance and are made available to growing plants and microorganisms.