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Soil Fertility Basics: Interpreting Soil Tests

by Steve Peters, Seeds of Change Agricultural Planner

Balanced, fertile soil is essential for producing healthy, nutritious plants. That's why passionate gardeners strive to understand the quality of their soil. Physical characteristics of the soil, such as texture, color and smell are all significant indicators of soil quality that can be directly observed. Plant growth and the health of animals eating these plants are the most important indirect indicators of soil quality. Soil tests are also excellent indicators; however, it's important to understand what a soil test actually means and what its limitations are.

A soil test provides clues about soil. It's not an exact science, but can indicate trends and general nutrient ratios. It tells us whether a specific nutrient is abundant or is lacking and helps us take corrective action. As soil stewards we are trying to balance our essential soil nutrients. The soil test helps us achieve this goal.

Soil tests are generally performed by professional soils laboratories. Perhaps the greatest value of these labs is their interpretation of the test results. As an alternative to the professional soil-testing lab, we offer the Lamotte Soil Test (pH, nitrogen, phosphorus, potassium) which, while perhaps not as precise as a professional test, is more convenient and allows you to test frequently.

Frequent testing combined with good record keeping means you can detect trends early and act accordingly. Complementing the Lamotte kit, we also offer the Solvita Garden Care Test Kit. This kit measures CO2 respiration, which indicates a soil's biological activity. Results will indicate whether additional soil amendments, such as compost, are required. Because biological activity fluctuates over the season, it's a good idea to perform this test regularly.

Taking Soil Samples
For your soil sample to yield meaningful test results, it should represent your entire field or garden site. From each of 10-20 locations throughout the garden, avoiding unusual spots such as wet areas or shaly knobs, take trowel full of soil from the top 6 - 8 inches. Place these in a clean plastic bucket. Mix the sample thoroughly with a plastic or wooden tool. Avoid touching the soil with your hands or an iron tool. Take your sample for testing from this. For a laboratory test you'll need about one pound. Soil tests are most useful if we can observe test results from the same field over several years. Sampling during the same time of year and using the same lab or testing methods will assure the most consistent results.

Below are actual test results from two different soil samples, followed by a discussion of what this means for the gardener. Soil # 1 is from our research farm in northern New Mexico, within the Rio Grande floodplain. Soil #2 is from our northeast trial garden, which is in a forest clearing in southern Maine that was pasture over 80 years ago but had since grown back to pine forest (before being logged).

Organic matter (OM)
Organic matter (OM) is a measure of all plant and animal residues, ranging from raw, undecomposed materials to stable humus. It is a source of nitrogen, the major cations (Ca,Mg,K), trace minerals, and growth hormones. OM also stimulates biological activity and enhances soil structure and water retention. It is, however, perhaps the most difficult factor to interpret on a basic soil test, as optimum OM content can vary widely depending on soil type and conditions. Furthermore, the amount of nitrogen being released by OM depends heavily on the quality of the organic matter (stable vs. unstable compounds) and the soil conditions (texture, temperature, water content). Nevertheless, testing for OM can be useful to observe general trends if it is measured every year or two.

OM levels can also indicate a soil's ability to retain nutrients. (See below in the discussion on cation exchange capacity. (CEC)) Soil #2 has a great amount of raw residues, which are useful for stimulating soil microbial activity, but do not contribute to nutrient retention. In contrast, humus and other more stable decomposed organic matter are very important nutrient sources. Note the higher CEC in soil #1, even though overall OM is considerably lower.

Soil pH
Soil pH is a measure of the hydrogen ion (an electrically charged atom) concentration in the soil solution surrounding soil particles. Pure water (H2O) has a pH of 7.0 which is neutral because it contains equal amounts of H+ (Hydrogen) ions and OH- (Hydroxyl) ions. A pH below 7.0 is acid and has a greater number of H+ ions, while a pH above 7.0 is alkaline and has a greater proportion of OH- ions. Soil pH per se has little direct effect on plant growth, provided that a plant can extract a sufficient quantity of nutrients. The most important concept regarding pH is that extreme acid or alkaline conditions affect the availability of virtually all the essential nutrients. For example, at pH 5.0 (strongly acidic) the amount of calcium, magnesium, potassium, nitrogen, and sulfur available to plants is only half as much as at pH 6.0 (moderately acidic). The availability of phosphorus begins to decline at pH 6.5. Below 6.0, most phosphorous is unavailable to plants. As pH levels drop below 5.0, iron and aluminum are released from the soil in quantities that are toxic to plants. Excessive alkalinity also leads to nutrient imbalances. Many of the trace elements, including iron, manganese, boron, copper, and zinc become gradually less available at pH levels exceeding 7.5. Soils with pH above 7.5 also show dramatic decreases in phosphorus availability.

pH also affects the activity of beneficial soil microorganisms. Although bacteria and actinomycetes prefer alkaline conditions and fungi favor acidic conditions, the optimum overall activity of beneficial microbes occurs at mid-range pH levels. Therefore, a pH that is not extremely acidic or alkaline is best for maximizing soil life and plant-available nutrients. Soil #1 has a rather alkaline pH, and the soil test report indicated low levels of several of the trace elements (not shown here). One of the recommendations was to add gypsum (calcium sulfate) because the sulfur is able to acidify the soil slightly. The pH of Soil #2 probably doesn't need to be adjusted, however, if the pH was much lower, the most effective way of raising it would be through further applications of high calcium limestone (calcium carbonate) or dolomite (calcium-magnesium carbonate).

Cation Exchange Capacity (CEC)
Cation exchange capacity is a measure of the soil's ability to retain the cation nutrients, including calcium (Ca+), magnesium (Mg+), potassium (K+), and nitrogen in the ammonium form (NH4+). These positively charged elements are attracted to the negatively charged surfaces of clay and humus particles. Once these cations are bound to these sites, they are protected from leaching away in water, yet they are still available for uptake by plant roots. As plants absorb the cations, their roots release positively charged hydrogen ions (H+), which then attach to the negatively charged sites previously occupied by the other cations. As a plant continues to take up the cation nutrients, there are more H+ ions on soil clay and humus particles and in the soil water solution surrounding the particles (i.e. more acidity). Therefore, more cation nutrients need to be added to assure an adequate future supply.

At CEC levels above 20 or 25 (milliequivalents per 100 grams of soil), the soil can hold many more nutrients than a plant would normally need in a year. If the majority of exchange sites in this soil were occupied by the nutrient cations (and not by H+), then very little or no additional amendments may be required for the next 1 to 3 years. However, once this soil is depleted of nutrient cations, it would require a large input of nutrients to restore its original fertility. On the other hand, if a soil has a low CEC, say below 10, then the nutrient reserves would be quickly depleted, and annual additions of the cation nutrients may be required (although at much lower quantities than in a high CEC soil).

Soil #2 has a low CEC and may need to be monitored more closely than soil #1, which has a moderate CEC level. CEC levels are largely dependent on the amount of clay (fine texture) and humus in the soil. Silty (medium texture) and sandy (coarse texture) soils contribute almost no exchange sites, and hence must be fertilized more often. Soil #2 is interesting in that it has a relatively low CEC but a large amount of organic matter (humus). The humus contributes a large number of exchange sites, yet the texture is silty (few exchange sites), so that the combined exchange site contribution of the humus and mineral portions of the soil is still relatively modest. Since the humus is contributing the lion's share of exchange sites, an increase in the pH will increase the CEC. In most soils, however, the CEC remains fairly constant, yet knowing what it is will help us determine the amount and frequency of fertilizer applications.

The cation nutrients (Ca, Mg, K) are expressed in parts per million (ppm). If you multiply ppm by 2, you will get the approximate number of pounds/acre of the nutrient (in the top 6 inches of soil). Soil labs usually report the level of each of these cations from very high to very low, relative to the CEC of each particular soil. This explains why, although the calcium level of soil #1( higher CEC) is greater than soil #2 (lower CEC), the Nutrient Level Rating of soil #1 is lower. Considering this, the total amounts of the cation nutrients are not as important as the proportion of exchange sites that each of these cations occupies. This is expressed as the "% Base Saturation." Calcium dominates the exchange sites, and for best crop performance, should occupy 65 to 85 percent of the sites . Magnesium is best between 10 and 20 percent, while potassium should be about 3 to 5 percent. The remaining sites are occupied by hydrogen, sodium (high alkaline soils), ammonium, and trace elements. Even these ranges are not necessarily the best in all cases, but they can be used as guides for balancing your nutrients.

Soil #1 is rated very high in potassium, which could lead to nitrogen deficiency. If your plants are not exhibiting N-deficient symptoms (yellowing of leaves, slow growth), then you probably have nothing to worry about. The calcium level of soil #1 is rated medium and the lab recommendation was to add gypsum (Calcium sulfate), which can be helpful in several ways. First, it raises the calcium level. Second, the calcium in the gypsum replaces some of the potassium on the exchange sites, allowing excess to leach away. Third, the sulfur in the gypsum decreases the pH, which increases the availability of several trace minerals. Soil #2 has high or very high levels of the major cations and their ratio appears to be ideal, therefore no major adjustments are needed.

Phosphorus, like the cations, is also expressed in parts per million. Two tests are performed on this element. The P1 (weak Bray extraction) measures the amount of phosphorus immediately available to the plant. P2 (strong Bray extraction) measures the readily available P plus the active reserves, which usually are available later in the season. The P2 levels should be 2 or 3 times the P1 levels. Soil #1 had no P1 reported because this test is unreliable in soils above pH 7.5. P2 levels were very high in soil #1, so presumably no additional P is required. If phosphorus deficiency symptoms show up (purpling of leaves, poor root growth, poor flower/fruit set), the best strategy would be to lower the pH and increase biological activity through the addition of compost and green manure crops. Soil #2 had high P1 and P2 levels, and their ratio indicates no problems for supplying the crop throughout the season. If phosphorus levels are low, a colloidal, soft rock phosphate is best because it can supply adequate P over a relatively long period.

One Important Tool Among Many
Soil tests are not an absolute measurement of soil fertility, but they can serve as a valuable guide for indicating problem areas and what directions we may go to solve these problems. The subject of soil quality, health, and fertility is enormously complex, and the soil test is but one tool to help us unlock a small piece of the vast mysteries that lie below the surface. We, as stewards of the land, will do well to be intimately involved with our soil on a regular basis by feeling, observing, and reflecting. The rewards will be great for us as well as our gardens.