Aggregation refers to the binding together of sand, silt and clay, and small clods of soil (aggregates), by soil organic matter and organisms associated with it, especially earthworms and fungi. Soils with good aggregation are healthy soils. In addition to storing and protecting organic matter, such soils allow rainfall to rapidly enter the soil. Most water drains through the large pores, but some remains behind in the smaller pores and this is made available to crops when there is no rain.
Residues refer to remains of dead plants (bits of roots, stems, stovers, straw, seed hulls, etc.) and animals (bone fragments, manure, insect skeletons, etc.) that enrich the soil organic carbon pool. Residues have a profound influence on soil biological activity. Communities of bacteria, fungi, earthworms, mites and other decomposers that make up the soil food web, feed on residues, breaking them down into the nutrients that the residues contained.
Soil Organic Carbon is a term roughly equivalent to soil organic matter. It refers to the sum of all the different forms of carbon in the soil. These include: residues, all carbon contained in the soil microbial biomass, charcoal (black carbon) and stabilized (slow release) forms that are bound to clay particles and protected within soil aggregates. The “active” carbon fraction is made up of residues and microbial biomass that are cycled quickly through the soil food web. Particulate organic matter (POM) is made up of the visible small bits of plant and animal remains that reflect recent additions of organic matter. The POXC stands for Permanganate Oxidizable Carbon. It is a chemical test that measures the amount of easily decomposable carbon in the soil. This includes the visible POM and the invisible microbial biomass.
Soil pH: Acidity or Alkalinity of a soil. The soil pH affects nutrient availability and toxicity. Many reddish, clayey soils in humid areas of the tropics are strongly acid, while drier-climate soils are usually alkaline. Crops tend to do well in mildly acidic to neutral (pH 7) conditions, but there are crop and varietal differences in adaptation to pH. Therefore, knowing pH can be fundamental for problem-solving in cropping systems. Soils with pH of 5 and below become problematic for food crop production. At this pH, Aluminum, which is toxic to many plants’ roots, negatively impacts crop growth, and nutrients like phosphorus become unavailable to plants.
Structure refers to the aggregation of soil into crumbs and clods of different sizes. Structure is a physical characteristic of soil, but it is produced by soil biology (e.g., fungal hyphae, microbially produced cements, and root exudates and root remains glue soil together.) A soil with good structure helps soil to store and release water to plant roots, and also makes it more resistant to erosion.
Texture refers to the proportions of sand, silt, and clay-size particles in the soil. Soil texture influences various other soil properties. Whether a soil is sandy, clayey, or loamy (a mixture of the three sizes of particles) influences other soil properties and management strategies. For example, sandy soils may drain better after rainfall, but are more difficult for building up organic mattercl while clay soils can become waterlogged with too much rain but can retain more organic matter.
Available soil phosphorus: Available phosphorus for crops can be a sensitive indicator of soil quality because phosphorus is often limiting in tropic soils, particularly in fields farther from the homestead. Legumes and many vegetable crops that are important in nutrition and income generation approaches produce best in medium to high phosphorus conditions, so P availability is important as a context factor that influences success or failure of cropping innovations .
Adapted from definitions by S. Vanek