Soil Tests Demystified
by Joel Reiten 

Nutrients, in the form that plants can use them, are one of the most basic requirements of vegetable production, whether in the home garden or in the farmer’s field. A basic soil test is one of the best ways to find out if you have the nutrients necessary, and in the proper balance, for good production.

A basic soil test will measure the two basic types of the elements in the soil that plants depend on for growth. These elements are the same for both organic and conventional production.

Anions
The first basic group of elements are the anions. These are nitrogen, phosphorus and sulfur. They are called anions because they each have a negative charge. These elements do not bind tightly to soil and are subject to leaching in one or more of their forms.

The first and most important to plant growth is nitrogen (N). A soil test will calculate the amount of available nitrogen by looking at the amount of soil organic matter (SOM) in and the amount of nitrogen, usually expressed in pounds per acre, that the amount of SOM will provide when broken down by soil macro- and microorganisms. For instance, 3.5% SOM in a silt loam soil will release around 100 lbs of nitrogen per acre, adequate for most crops. This breakdown, however, is very dependent on the amount of soil moisture, soil temperature and other biological factors in the soil. The addition of manure, composts and legume cover crops can all add to available soil nitrogen.

Phosphorus (P) in the soil and its availability to plants is a complex matter. While most soils have adequate levels of phosphorus, its availability is very dependent on the soil pH, temperature and level of soil microorganisms. The level of available phosphorus is determined by an acid extraction technique. This technique simulates the weak acids excreted by the plant root and its ability to dissolve the phosphorus and make it available. The level of phosphorus is usually expressed in parts per million (ppm) and a level of 30–40 is usually adequate for most crops. Phosphorus levels can be increased by the addition of rock phosphates but can take up to a year to become available.

The last important anion is sulfur (S). Sulfur is rapidly becoming the fifth major plant food nutrient for crop production. The largest portion of available sulfur is in SOM and is released through bacterial action. Sulfur is a very important part of protein formation. Sulfur levels are usually reported in ppm and should be in the range of 8–12 ppm. Sulfur levels can be increased by elemental sulfur and gypsum.

Cations
The next group of elements reported on a soil test are the cations. These elements have a positive charge and bond to the negative charge of the soil particles.

Soil potassium (K) is usually not a problem as most soils hold adequate levels, although many organic growers apply K Mag, or potassium magnesium sulfate, to make sure that the soil has adequate available amounts of potassium. Potassium is typically reported in ppm and levels of around 150 ppm are usually adequate. Manures also contain higher levels of potassium and can be used to supplement crops.

The levels of calcium (Ca) in soils can vary greatly and are directly related to soil pH levels. Calcium has many functions in plants from the building of cell walls to an important role in seed formation. Calcium is usually reported in ppm and levels of around 2,000 ppm are necessary for healthy plant development.

Magnesium (Mg) is also very important for plant growth and varies greatly in soils. Magnesium is usually reported in ppm and adequate levels are in the range of 50 to 100 ppm. Limestone and K Mag can be used to correct deficiencies.

Potassium, magnesium and calcium are part of the base saturation of the soils and are generally looked at as being in balance when, added together, potassium is around 5%, magnesium is around 15% and calcium is around 75% with other minor bases making up the final 5% of the total.

Soil Testing
Soil tests should be looked at as being a single snapshot in time and will change slightly with time of the year and the lab that the sample is submitted to. Soil samples should represent a random sampling of 4–6 spots in your garden, taken at a depth of 6–10 inches and mixed together thoroughly. Soil samples should be dried prior to shipping.

Most soil test labs will provide recommendations based on the type of crop you want to grow and more and more labs will also give organic recommendations.

Joel Reiten
Seed Production Manager

Soil Testing Resources

ATTRA's list of soil testing labs and other soil testing information. www.attra.org

A&L labs has facilities around the US. They provide good reports and excellent support information. www.al-labs.com

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