The physical, chemical, and biological properties of soil work together to make it a complex, dynamic, and living substance. The role of organic matter in soil health is vital. Soil organic matter facilitates the interactions between the soil properties.
Based on the chart, it might be tempting to conclude that organic matter is inconsequential to soil health. However, just as a relatively small amount of oil keeps an automobile’s engine running smoothly, organic matter keeps healthy soil functioning optimally.
Soil layers, texture, and structure are physical soil properties. Soil layers are distinct horizontal zones in the soil, arranged like layers of lasagna, from the topsoil down to the bedrock. Soil texture is determined by the proportion of sand, silt, and clay mineral particles. Soil structure refers to the aggregation of soil particles and the arrangement of those aggregates with each other. Well aggregated soil drains water properly and exchanges air with the atmosphere providing an ideal home for plant roots.
Soil pH, cation exchange capacity (CEC), and mineral nutrients are chemical soil properties. Most gardeners are aware of the roles mineral nutrients like nitrogen, phosphorus, and potassium play in plant health. However, CEC and soil pH determine the amount of mineral nutrients the soil can hold and whether soil nutrients present in the soil are available to the plant.
Soil organisms, including bacteria, fungi, nematodes, worms, insects, plant roots, and small mammals, comprise soil’s biological properties. Soil organisms cycle and retain nutrients, aid water infiltration, suppress disease, degrade pollutants, and improve soil structure.
In summing up how organic matter builds healthy soil, high soil organic matter feeds and supports high populations of soil organisms. High soil organism populations produce ‘microbial glue’ through their bodily secretions (think slimy earthworms and the like.) ‘Microbial glue’ binds soil particles and the resulting aggregates together for better soil aggregation. Better soil aggregation improves pore space for air, water, and plant roots. Improved aggregation and pore space result in optimum soil structure for healthy plant growth.
For more information about adding organic matter to the soil, visit HGIC 1655, Soil Conditioning – Establishing a Successful Gardening Foundation.