Terms

There are two kind of acidity which are important for soil testing. There is the "active" acidity which is the acidity which the roots "see". The active acidity is measured with litmus paper, indicator solutions or a pH electrode. This is the acidit y people refer to when they want to know their soil pH value. The other kind of acidity is referred to as the reserve or stored acidity. Clay and organic matter in the soil tends to store acidity. It takes more lime to raise the pH value of a soil with clay or organic matter present than it would for a sandy soil with very little clay or organic matter. Therefore, soils with more clay or organic matter have a greater ability to resist pH changes.

The buffer pH is measured with a weak base that starts out at pH 8.00. Soils that will require lime and have an "active" acidity pH value of 5.8 or below are set aside in the lab to have this solution added. The more the solution decreases from pH 8. 00, the more stored acidity the soil sample has. The computer program will then recommend lime according to the determined stored acidity of the soil.

As an example, a sandy soil from the central part of South Carolina and a a soil from the Piedmont with plenty of clay each have a pH value of 5.5. This would be the "active" acidity mentioned earlier. The laboratory determination of the buffer pH va lue of the sandy soil, though, could be 7.80 while the buffer pH value of the soil with clay could easily be 7.30.

Since the sandy soil has decreased the buffer solution pH value of 8.00 less than the clay soil (7.80 vs. 7.30), the sandy soil will get a lower lime recommendation since it has a smaller amount of stored acidity. In this particular case, to bring these two soils up to a pH value of 6.5 the sandy soil will get a lime recommendation of 0.50 tons per acre while the lime recommendation for the clay soil will be 1.00 tons of lime per acre...twice as much lime, even though they both have the same pH value.

Starting October 1, 1996, the Clemson University Agricultural Service Lab began sending out soil test reports with a revised format. In addition, more soil test information has been added to the reports in response to requests from farmers, fertilizer dealers and several county agents.

The two micronutrients which have been added to the soil test report are copper (Cu) and boron (B). Neither of these have a sufficiency rating.

Research and experience have shown that copper deficiency is rarely a problem in South Carolina. There is also limited research data to interpret soil test results. Soils where copper deficiency may be a problem are the high organic soils (gre ater than 10% organic matter) or very sandy soils with a high pH value and subsoil clay deeper than 20 inches. Recommendations for copper should be based on plant tissue analysis.

Boron deficiency is a concern for certain crops on Coastal Plains soils. The appropriate comment will be triggered in the soil test recommendation according to the specified crop and soil type. For example, comment 103 for commercial watermel ons says that applying 0.5 to 1.0 pounds of boron per acre as a foliar spray prior to bloom MAY help fruit set. For cotton, comment 12 says to apply 0.4 pounds of boron per acre in the fertilizer or in the insecticide spray in either one or several appl ications so long as the total amount applied does not exceed 1.0 pounds per acre.

Since boron is mobile in the soil like nitrate, a blanket recommendation is given with the assumption that at the beginning of the cropping season, most of the residual bor on from the previous year's crop will have leached through the soil past the root zone. As with copper, a tissue test will give the best assessment of available boron.

We hope that those who have requested this additional information will find it helpful. For those who have been satisfied with the soil test reports issued by the Agricultural Service lab in the past, they will be pleased to find the same information still on the new reports.

The CEC is the abbreviation for the cation exchange capacity of the soil. Any element with a positive charge is called a cation and in this case, it refers to the the basic cations, calcium (Ca+2), magnesium (Mg+2), potassium (K+1) and sodium (Na+1) and the acidic cations, hydrogen (H+1) and aluminum (Al+3). The amount of these positively charged cations a soil can hold is described as the CEC and is expressed in milliequivalents per 100 grams (meq/100g) of soil. The larger this number, the more cations the soil can hold.

A clay soil will have a larger CEC than a sandy soil. In the Southeast, where we have highly weathered soils, the dominant clay type is kaolinite which has very little capacity to hold cations. A typical CEC for a s oil in the Coastal Plains region is about 2.0 meq/100g of soil and the typical CEC for a soil in the Piedmont region is about 5.0 meq/100g of soil. The CEC gives an indication of the soils potential to hold plant nutrients. Increasing the organic matter content of any soil will help to increase the CEC since it also holds cations like the clays. Organic matter has a high CEC but there is typically little organic matter in our soils.

Sample Soil Test Report Data

The percent base saturation tells what percent of the exchange sites are occupied by the basic cations. If calcium has a base saturation value of 50% and magnesium has a base saturation value of 20% as shown above, then calcium occupies half o f the total exchange sites (CEC) and magnesium occupies one-fifth of the total exchange sites (CEC). In our example where the soil has a CEC of 5 meq/100g, 2.5 meq/100g of the CEC is occupied by calcium and 1 meq/100g of the CEC is occupied by magnesium. If all the exchangeable bases (Ca, Mg, K and Na) total 100%, then there is no exchangeable acidity.

The acidity on the report is the amount of the total CEC occupied by the acidic cations (H+1and Al+3). The acidity, like the CEC, is expressed as meq/100g of soil. If the CEC is 5 meq/100g of soil and the acidity is 1 meq/100g of soil (see sa mple above), then one-fifth of the exchange sites in the soil are occupied by acidic hydrogen and aluminum ions. The remaining 4 meq/100g of soil (or 80% of the CEC) will be occupied by the basic cations. The more acidic a soil is and the lower the soil pH value, the closer the acidity number will be to the CEC number. 

You can see a detailed explanation of how the CEC, exchangeable acidity, and percent base saturation are calculated from the routine soil test data.

Sodium is included among the bases to indicate if sodium levels are getting too high. This happens in situations where industrial by-products are applied to the soil or where soils along the coastal region are irrigated with water high in sodiu m. The acceptable base saturation limit for sodium is 15%. This is also called the Exchangeable Sodium Percent or ESP. Sodium levels higher than 15% on the exchange site could result in soil dispersion, poor water infiltration, and possible sodium toxi city to plants. 

So, why do we bother with the CEC, acidity and base saturation? Some consultants and farmers prefer to use the base saturation of the plant nutrients instead of the extractable amounts as a guide for maintaining optimum fertility. For Southeastern s oils with kaolinitic clays, a base saturation of 45 to 65 percent will be satisfactory for good plant growth. The following table gives the approximate base saturation for the soils of a given soil pH:

In South Carolina, if fertilizer and lime is applied to raise the base saturation of a kaolinitic soil to 85 percent as commonly done in the Midwest, the resulting pH would be between 7.1 and 7.5. Soil pH values in that range would result in a major pr oblem with zinc and manganese deficiency. That is why the Clemson University fertilizer recommendations are determined by the amount of each nutrient extracted from the soil (expressed in pounds per acre) instead of using the percent base saturation as a guide.

A favorable base saturation will be obtained if the soil pH is maintained between 5.8 and 6.5. The approach used by Clemson University is also used throughout the Southeast and Mid-Atlantic regions in determining soil fertilizer requirements. The CEC and base saturation is something that many farmers and consultants have asked for to better understand the soil and so it is now available in response to public demand.

When determining the lime requitement for Carolina Bays or soils with organic matter content greater than 10%, the target pH value should be between 5.0 and 5.5. Most field crops in the southern U.S. are grown on soils with less than 2% organic matter but if you suspect that a soil is unusually high in organic matter, you may want to have it tested by the Ag Service lab to see if the organic matter content is above 10%.

Many times organic gardeners will send soil samples to the Ag Service Lab and request the test for percent organic matter. Probably, many of these gardeners have been adding organic matter to their soil for several years and want the soil tested for organic matter content to serve as a report card.

The soil test for organic matter is mostly used by farmers who are about to add herbicides to the soil. The soil organic matter ties up herbicides when they are applied to the soil. Subsequently, the more organic matter that there is in the soil, the more herbicide the farmer will have to apply to compensate for the tie-up. The label on the herbicide container will have a table showing how much extra herbicide is required to compensate for a specified range of soil organic matter. As mentioned earl ier, most soils in the southern U.S. are less than 2% organic matter so the herbicide label should be checked to see if an organic matter test is even necessary.

Nitrogen can be added to soil in many forms. It can be applied as nitrates, ammonium, urea, etc. Soon after the nitrogen is added and if there is some moisture present, the indiginous bacteria called nitrosomonas and nitrobacter convert all forms of nitrogen to the nitrate form. The nitrate form of nitrogen is an ion with a negative charge (NO3-).

The clays in South Carolina soils also have a negative charge and since similar charges repel, the nitrates are not attracted to the clays like the posit ively charged calcium, magnesium and potassium ions. Nitrates are also very soluble in water. As a result, the nitrate ions are very mobile in the soil and move through quite readily as water percolates down into the soil. If a soil is sandy, nitrates move through the soil even faster due to the large pore space and faster percolation of water. These factors are the reason for environmental and health concerns regarding nitrate movement into groundwater.

Because of nitrogen's rapid conversion to the nitrate form and its subsequent movement through the soil, it makes the soil test for nitrogen quite difficult to interpret. By the time you receive soil test results for nitrate nitrogen, it may have alre ady moved through the soil if some rain had fallen between sample collection and reporting of lab results. Some soil tests for nitrogen are done for certain crops but it is still in an experimental stage and not widely used or accepted in our region. Re searchers will collect soil samples at several depth intervals to monitor the nitrate movement during a crop's growing season.

Because of the mobility of nitrogen in the soil, most states in the Eastern region of the U.S. just make a blanket recommendation for each crop with the assumption that most of the residual nitrogen from the previous crop has moved past the root zone f or the new crop. You will notice that for many crops, nitrogen is recommended as a split application. If all of the nitrogen was added at the beginning of a growing season, much of it would move past the root zone before the crop matures. Splitting app lications allows the nitrogen to be applied more in accordance to the crop's needs at different stages of growth and thus reduce leaching.