Tilth

Tilth is a physical condition of soil, especially in relation to its suitability for planting or growing a crop. Factors that determine tilth include the formation and stability of aggregated soil particles, moisture content, degree of aeration, soil biota, rate of water infiltration and drainage. Tilth can change rapidly, depending on environmental factors such as changes in moisture, tillage and soil amendments. The objective of tillage (mechanical manipulation of the soil) is to improve tilth, thereby increasing crop production; in the long term, however, conventional tillage, especially plowing, often has the opposite effect, causing the soil carbon sponge to oxidize, break down and become compacted. Soil with good tilth is spongy with large pore spaces for air infiltration and water movement. Roots only grow where the soil tilth allows for adequate levels of soil oxygen. Such soil also holds a reasonable supply of water and nutrients. Tillage, organic matter amendments, fertilization and irrigation can each improve tilth, but when used excessively, can have the opposite effect. Crop rotation and cover crops can rebuild the soil carbon sponge and positively impact tilth. A combined approach can produce the greatest improvement. Good tilth shares a balanced relation between soil-aggregate tensile strength and friability, in which it has a stable mixture of aggregate soil particles that can be readily broken up by shallow non-abrasive tilling. A high tensile strength will result in large cemented clods of compacted soil with low friability. Proper management of agricultural soils can positively impact soil aggregation and improve tilth quality. Aggregation is positively associated with tilth. With finer-textured soils, aggregates may in turn be made up of smaller aggregates. Aggregation implies substantial pores between individual aggregates. Aggregation is important in the subsoil, the layer below tillage. Such aggregates involve larger (2- to 6-inch) blocks of soil that are more angular and not as distinctive.