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Soil as a natural body
Factors of plant growth
The soil can be viewed as a mixture of mineral and organic particles of varying size and composition in regard to plant growth. The particles occupy about 50 percent of the soil's volume. The remaining soil volume, about 50 percent, is pore space, composed of pores of varying shapes and sizes. The pore spaces contain air and water and serve as channels for the movement of air and water. Pore spaces are used as runways for small animals and are avenues for the extension and growth of roots. Roots anchored in soil suppport plants and roots absorb water and nutrients. For good plant growth, the root-soil environment should be free of inhibitory factors. The three essential things that plants absorb from the soil and use are: (1) water that is mainly evaporated from plant leaves, (2) nutrients for nutrition, and (3) oxygen for root respiration.
Support for Plants
One of the most obvious functions of soil is to provide support for plants. Roots anchored in soil enable growing plants to remain upright. Plants grown by hydroponics (in liquid nutrient culture) are commonly supported on a wire framework.
Plants growing in water are supported by the buoyancy of the water. Some very sandy soils that are droughty and infertile provide plants with little else than support. Such soils, however, produce high-yielding crops when fertilized and frequently irrigated. There are soils in which the impenetrable nature of the subsoil, or the presence of watersaturated soil close to the soil surface, cause shallow rooting. Shallow-rooted trees are easily blown over by wind, resulting in windthrow.
Soil as a natural body
One day a colleague asked me why the alfalfa plants on some research plots were growing so poorly. A pit was dug in the field and a vertical section of the soil was sampled by using a melal frame. The sample of soil that was collected was 5 centimeters thick. 15 centimeters wide, and 75 centimeters long. The soil was glued to a board and a vacuum cleaner was used to remove loose soil debris and expose the natural soil layers and roots. The upper layer, 9 inches (22 cm) thick, is the plow layer. It has a dark color and an organic mailer content larger than any of the other layers. Layer two, at the depth of 9 to 14 inches (22 to 35 cm) diffets from layer one by having a light-gray color and a lower organic mailer content. Both layers are porous and permeable for the movement of air and water and the elongation of roots. In layer three, at a depth of 14 to 23 inches (35 to 58 cm) many of the soil panicles wort arranged into blocklike aggregretes. When moist soil from layer three was pressed between the fingers, more stickiness was observed than in layers one and two, which meant that layer three had a greater clay content than the two upper layers. The roots penetrated this layer with no difficulty, however below layer three, the alfalfa tap root encountered a layer (layer four) that was unpenetrable (too compact), with the root growing above it in a lateral direction. From these observations it was concluded that the alfalfa grew poorly because the soil material below a depth of 58 centimeters:
(1) created a barrier to deep root penetration, which resulted in a less than normal supply of water for plant growth during the summer, and (2) created a water-saturated zоnе above the third layer that was deficient in oxygen during wet periods in the sping. The fact that the soil occurred nalurally in a field raises such questions as: What kinds of layers do soils have naturally. '' How do the flyers form? What are their properties? How do these layers affect how soils are used.'' The answers to these questions require an understating that landscapes consist of three-dimensional bodies composed of unique horizontal layers. These naturally occurring bodies are soils. A recognition of the kinds of soil layers and their properties is required in order to use soils effectively for many different purposes.