Physical properties of soil are divided into texture and structure. Soil texture is a physical measurement of the percentage of sand, silt, and clay particles in a soil (as determined by grain size, with sandy soils being the largest and clay the smallest). It is a given, and cannot be altered (see more in Unit 2.1, Soil Physical Properties).
Sandy soils usually feature low nutrient- and water-holding capacity and an associated lower organic matter content. On the plus side, sandy soils drain well, warm quickly, and allow early cultivation and planting in the spring. Clay soils are the opposite: they carry high levels of nutrients and water, but are often difficult to work.
You can determine soil texture by a simple field “feel” test called ribboning, or have it measured with a lab soil test. Soil structure refers to the arrangement of individual soil particles (sand, silt, clay) into aggregates or “clumps”; ideally, it takes the form of a granular or crumb structure, much like the cross section of a loaf of good whole grain bread.
Such a structure features an amalgamation of small, intermediate, and large, stable aggregates. Some major contributors to stable aggregates and good soil structure are: • The addition of organic matter—fresh, as green manures, and stabilized, as finished compost.
Organic matter is a feedstock for soil microorganisms that break down the organic materials and in the process exude mucilaginous glues and slimes that help bind soil particles into stable aggregates. Plant roots, both living and decomposed, also contribute “binding” substances to the system. • Timely and skilled cultivation techniques— rough plowing or digging physically forces soil particle contacts, beginning the process of aggregation.
Organic matter contributes to stabilizing the aggregates that form. Note: Too much cultivation (especially secondary cultivation, or pulverizing) damages soil structure, as does working a soil when it’s too wet. See details about cultivation in Unit 1.2, Garden and Field Tillage and Cultivation. Chemical properties of a soil measure its nutrient-carrying capacity and pH (acidity; see Unit 2.2, Chemical Properties of Soil). These are best determined by a soil test.