0.05 mm-0.002 mm

A lens or microscope needed to see any but the coarsest silt particles


<0.002 mm

May absorb water, swell, and later shrink, causing the soil to crack as it dries

plastic in nature because the water that adheres tightly to the surface of the particles acts both as a binding agent and a lubricant. Physically, clay is a colloid, that is, a suspension of particles that are larger than molecules but that do not settle out of a fluid medium.

The best agricultural soils are usually loams, which are a mixture of sand, clay, and organic matter. The better loams have about 40% silt, 40% sand, and 20% clay. Light soils have a high sand and low clay content. Heavy soils have high clay content. Coarse soils, which have larger particles, are porous and don't retain much water, while clay soils have high water content and allow little water to pass through.

Over half the composition by weight of mineral matter is oxygen. Other elements commonly present are hydrogen, silicon, aluminum, iron, potassium, calcium, magnesium, and sodium. However, soil inherits hundreds of different mineral combinations from its parent material.

Soil Structure

Soil structure refers to the arrangement of the soil particles into groups called aggregates. Aggregates in sands and gravels show little cohesion, but most agricultural soils have aggregates that stick together. Structure develops when colloidal particles clump together, mostly as a result of the activities of soil organisms, freezing, and thawing. If the individual granules do not become coated with organic matter, they may continue to clump until they become clods.

Productive agricultural soils are granular soils with pore spaces that occupy between 40% and 60% of the total volume of the soil. The pores contain air and water, and their sizes are more important than their total volume. Clay soils, for example, have more pore space than sandy soils, but the pores are so small that water and air are restricted in their movement through the soil. When the pores are full of water, air is kept out, and there is not enough oxygen for root growth. Sandy soils have large pores, which drain by gravity soon after they are filled. The water is replaced by air, but too much air speeds up nitrogen release by microorganisms. Plants can't use the nitrogen that quickly, and much of it is lost.

Water itself, as we have noted, can be harmful. Under anaerobic conditions (marked by the absence of oxygen), too much water leaches mineral nutrients and slows the mineralization process. Too much water also slows the release of nitrogen, interferes with plant growth, and accelerates the breakdown of nitrates to the extent that virtually all the nitrates may be lost in as little as half an hour.

Water in the Soil

Water in the soil occurs in three forms. Hygroscopic water is physically bound to the soil particles and is unavailable to plants. Gravitational water drains out of the pore spaces after a rain. If drainage is poor, it is this water that interferes with normal plant growth. Plants are mainly dependent for their needs on the third type, capillary water, which is water held against the force of gravity, in pores of the soil. The structure and organic matter of the soil—which enable the soil to hold water against the force of gravity—the density and type of vegetational cover, and the location of underground water tables largely determine the amount of capillary water available to the plant.

Stern-Jansky-Bidlack: I 5. Roots and Soils I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

Stern-Jansky-Bidlack: I 5. Roots and Soils I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

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