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The Garden FAQ Index |

What kind of soil do I have and how can I improve it?

Gardeners often have problems with the way their soil "handles." At one extreme is the heavy clay soil, which is sticky when wet and like concrete when dry. At the other extreme is the "beach sand," which needs constant watering and just doesn't seem to grow a good garden. These are known as problems of soil texture . Fortunately, soils with texture-related problems can be improved.

Soil Texture Classifications

Soil texture is determined by the relative proportion of sand, silt and clay found in a given soil. The term "texture" refers to the size of the individual soil particles and has nothing to do with the amount of organic matter present in the soil.

Sand is gritty to the touch and the individual grains or particles can be seen with the naked eye. It is the largest of the three size classes of soil particles. A soil in which sand predominates is classified, logically enough, as a sand-textured soil or simply a sandy soil. Sandy soils are coarse in texture.

Silt is smooth and slippery to the touch when wet, and the individual particles are much smaller than those of sand. These individual particles can only be seen with the aid of a microscope. Silt-textured or silty soils contain relatively large amounts of silt.

Clay is sticky and plastic-like to handle when wet. The individual particles are extremely small and can only be seen with the aid of an electron microscope. As you might guess, clay-textured, or clay soils, are rich in clay and fine in texture.

The three main soil texture classifications, then, are sandy, silty and clay. Sandy soils are coarse-textured, clay soils are fine-textured, and silty soils intermediate in texture. "Loam" is another soil texture classification. A loam soil contains considerable amounts of sand, silt and clay. It is the preferred texture for horticulture because of its ease of workability. Textures between these classifications are also often used in describing soil types, e.g., sandy loam or clay loam soils.

You can roughly estimate the approximate amount of sand, silt and clay in a soil by a simple method called "manual texturing." The texture is determined by the feel of the moist sample when rubbed between the thumb and forefinger. If the soil sample is predominantly sand, it will feel very gritty. If it is predominantly silt, it will feel smooth or slippery to the touch. And if it is predominantly clay, it will feel sticky.

Horticultural Characteristics of Soils

From a horticultural point of view, it is very important to be able to identify the type of soil you will be working with as noted above, you can do this yourself by the manual texturing method or have it done at the Soil Testing Laboratory. The horticultural properties of a sandy soil will be quite different from those of a clay soil.

A sandy, coarse-textured soil drains easily and quickly after a rain, is easily worked, and warms up quickly in the spring.

But sandy soil also has some disadvantages. It has a lower moisture-holding capacity than a clay soil and therefore must be watered more frequently. It has a lower nutrient-holding capacity than a clay soil and must be fertilized more often. When vegetative cover is lacking, it is subject to wind and water erosion.

A clay, fine-textured soil has the advantages of high moisture-holding and nutrient-holding capacity. If the soil in your vegetable garden has a large proportion of clay you'll probably need to spend less money on fertilizer and water.

As you might expect, a clay soil has disadvantages as well. A clay soil often has poor drainage. That means that the soil remains saturated with water after the spring thaw and after heavy rains. If this happens, plant roots will be deprived of oxygen and the general health and vigor of the plants will be reduced.

Clay soils warm up very slowly in the spring. This characteristic can mean a delay of a week or more in planting seeds in a vegetable garden or setting out annual bedding plants.

Because of their high water-holding capacity, clay soils are also subject to alternating expansion and contraction due to alternating freezing and thawing. This can result in "heaving," whereby plants are pushed out of the soil, as well as the breakage of plant roots.

Crusting and cracking (due to drying) are also problems in clay soils. Crusting impedes root penetration and prevents seedling emergence. Cracking causes the tearing of roots and other plant parts.

Finally, clay soils are difficult to work if the moisture content is not "correct": If too wet, they are gummy and impossible; if too dry, somewhat like concrete.

Soils of all textures, but particularly clay soils, will compact under "heavy traffic" conditions, as when a pathway is made across a lawn or garden.

Improving the Soils

Both sandy soils and clay soils can be enormously improved by the generous addition of organic matter such as compost, well-rotted manure, or peatmoss. Spread a layer of organic matter 7 to 10 cm thick on the surface of the area to be improved, and then THOROUGHLY INCORPORATE IT INTO THE EXISTING SOIL. If you do not incorporate the organic matter, water will not percolate well and thus plants will grow poorly.

Keep in mind that excessive amounts of manure, especially if fresh, can raise nutrient and salt levels to a degree that may be toxic and therefore restrict plant growth.

As well, very large amounts of straw or peat moss may induce temporary nutrient deficiencies, especially of nitrogen. For this reason, nitrogen fertilizer is usually added at the same time to compensate for the temporary nitrogen deficiency.

By Sara Williams Department of Horticulture - University of Saskatchewan

Credit for information from the following sources: Denver Botanic Gardens, MSU Extension Office, Colorado State University Cooperative Extension