Issue: February 20011

For high-quality vegetables and fruits, test the soil

Most gardeners want more than just a patch of vegetables and fruit. They want high-quality produce that tastes good and contributes to a well-balanced, nutritious spread on the dinner table.

To get those high-quality crops, the gardener needs a good soil fertility program, which begins with soil testing.

Soil testing helps assess the fertility level of soil before planting to predict a crop's fertilizer requirements. Sampling technique is the key to an accurate soil analysis. A composite soil sample should be taken from at least five sites in the garden.

The idea is to get a representative sample of the soil. A slice of soil 6 to 8 inches deep and a half-inch thick can be removed from each site with a shovel. A strip of soil an inch wide is then cut from the center of the slice. All samples are combined in a bucket and mixed thoroughly. A pint of this composite soil sample can then be sent to a soils lab for analysis. (The county Extension office will have a list of soils labs and their fees.)

Most analyses include evaluations of soil texture, pH, salt content, percentages of sodium and organic matter, and the amounts of nitrogen, phosphorous and potassium in the soil, usually expressed in parts per million. Don't panic! Most analyses also include an interpretation by a s oils expert on how much fertilizer is required for optimum growth in the garden.

Texture refers to soil particle size. Sandy soils have large particles and tend to be droughty. Clay soils often crack and drain more slowly. Ideal soils are mixtures of sand and clay called loams. Organic matter will improve the structures of almost all soils, improving the water holding capacity of sands and drainage of clays. Unfortunately, most soils in New Mexico have rather low levels of organic matter. Organic matter levels can be raised in most gardens with compost.

Don't let the term "electrical conductivity" (E.C.) shock you. It's only a fancy term used to describe the amount of soluble salts in the soil. The higher the number, the more soluble salt you have. High salt content can burn plants. High percentages of sodium in the soil mean trouble. Water and air will have a hard time moving freely in such soils. The addition of gypsum (calcium sulfate) will help to loosen up such soils.

Most soils in New Mexico are either neutral or alkaline (basic) in terms of soil reaction. The higher the pH (above 7), the more alkaline the soil. Soils that are alkaline tend to make phosphorous, iron and zinc unavailable for plant uptake. An intervenial chlorosis (yellowing between veins) of younger leaves is probably a sign of iron deficiency. You can correct iron or zinc deficiencies with either soil or foliar applications of iron or zinc chelates.

Most forms of phosphate fertilizer--necessary for good flower and fruit development--are somewhat insoluble and must be applied before planting to incorporate it into the soil so plant roots can grow to them. On the other hand, nitrogen fertilizers--needed for good growth of leaves and stems--are highly soluble and should be applied in split applications, part before planting and partly as a side dressing later in the season after plants emerge. Nitrogen fertilizers should be incorporated into the soil 4 to 5 inches from the plant to keep nitrogen from vaporizing into the air as ammonia and to keep it from burning the plants. Side dressing the fertilizer will help it dissolve in the soil water and slowly move down to the plant roots.

Most soils in New Mexico contain sufficient potassium. A good application of compost will not only help improve soil structure, but also supply most of the other minor elements needed for optimum plant growth.

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For more gardening information, visit New Mexico State University's Cooperative Extension Service publications world wide web site at

George W. Dickerson, Ph.D., is is a horticulturist with New Mexico State University's Cooperative Extension Service. New Mexico State University is an equal opportunity/affirmative action employer and educator.

Also Please join us on Southwest Yard & Garden, a weekly garden program made for gardeners in the Southwest on: KNME-TV Albuquerque at 9:30 p.m. Saturdays, KENW-TV Portales at 10 a.m. Saturdays, and KRWG-TV Las Cruces at 11:30 a.m. Saturdays (repeated at 1 p.m. Thursdays.)