Agronomic Services - Soil Testing - FAQ's

A soil test is a process by which elements (phosphorus, potassium, calcium, magnesium, sodium, sulfur, manganese, copper and zinc) are chemically removed from the soil and measured for their "plant available" content within the sample. The quantity of available nutrients in the sample determines the amount of fertilizer that is recommended. A soil test also measures soil pH, humic matter and exchangeable acidity. These analyses indicate whether lime is needed and, if so, how much to apply.

An extractant is a mixture of various chemical reagents dissolved in water. It removes plant-available nutrients from the soil and dissolves them in a liquid phase. Most extractants can remove phosphorus, potassium, calcium, magnesium, manganese, zinc and copper from a sample.

CEC stands for "cation exchange capacity." It is a relative measure of the soil's capacity to hold positively charged nutrients, such as potassium (K+), calcium (Ca++), magnesium (Mg++) and hydrogen (H+). As this value increases, the capacity of a soil to hold nutrients also increases. The following soils are listed in order of lowest to highest CEC values: sands, sandy loams, sandy clay loams, clay loams, clays, and organics.

BS% stands for "base saturation percent." It represents the percent of the CEC that is occupied by basic nutrient elements, such as calcium (Ca++), magnesium (Mg++) and potassium (K+). BS% indicates pH and lime status of soils. As the soil pH increases, BS% also increases. On soils that are properly limed, BS% may range from 70 to 90. On soils that are extremely acid (low pH), BS% ranges from 50 to 60.

As a rule, test sandy-textured soils every 2 to 3 years and clay soils every 3 to 4 years. However, if problems occur during the growing season, send in a soil sample for analysis.

Soil pH indicates how acid or basic a soil is. It is a logarithmic function of the hydrogen ion concentration [H+]: pH = -log [H+]. A pH of 6.0 is ten times more acid than a pH of 7.0; a pH of 5.0 is 100 times more acid than a pH of 7.0, and so on. A pH of 7.0 is neutral; below 7.0, the soil is acid; above 7.0, the soil is basic. For example, battery acid has a very low pH (<3.0); household ammonia is basic and has a high pH (>10.0). [Acidic < pH 7.0 > Basic].

Most pH meters sold in hardware and garden centers are not very reliable for measuring soil pH. Accurate pH meters are available but are quite expensive for homeowner use. Consistently reliable results can be obtained by submitting samples to a soil testing laboratory.

You can take soil samples any time throughout the year. Late summer or early fall is a good time to sample for most crops. Any lime that is recommended can then be applied and have enough time to react and change the pH before the next season's crop is planted.

Collect soil samples 3 to 6 months before planting. For lawns, late summer (mid-August) is appropriate, particularly for cool-season grasses.

The turnaround time for samples submitted from mid-October through mid-April may be several weeks due to the large numbers of samples usually being handled at this time. Samples submitted from mid-April through October are usually analyzed within a few days.

Lime is any substance with the capability of neutralizing soil acidity. Most soils contain appreciable amounts of acidic components (hydrogen H+ and aluminum Al+++) that can be toxic to plants. Indeed, lime is the "anti-acid" for soils.

No soil amendment provides as many benefits as lime. Lime raises soil pH, providing a more favorable environment for microorganisms. Dolomitic lime provides the major portion of calcium and magnesium required for plant growth. Furthermore, plants utilize applied fertilizers more efficiently on soils that are properly limed.

Both kinds of lime have the same acid-neutralizing value when applied at equivalent rates. However, they differ chemically. Calcitic lime contains calcium carbonate only. Dolomitic lime is a mixture of calcium and magnesium carbonates that contains at least six percent magnesium. Most of the lime used for agricultural purposes is dolomitic.

Gypsum, also known as landplaster, is calcium sulfate. It is an economical source of calcium and sulfate but cannot be used as a liming material because it does not have the capability to neutralize soil acidity. In fact, applying gypsum to an acid soil (pH less than 5.5) can have adverse effects on certain crops by displacing soil aluminum, which is toxic to plant roots. Gypsum is frequently applied to crops to provide supplemental calcium. There is no substitute for lime in neutralizing soil acidity.

If plants appear chlorotic after a lime application, low manganese is usually the problem. Applying lime raises the soil pH and reduces the availability of manganese. In such cases, manganese should be applied to the soil. Soils with adequate manganese levels should not show symptoms following lime application. The yield increase attributable to lime should more than pay for applying manganese as well—not to mention the residual benefits to be received in following years.

North Carolina soils become quite acid because the warm, humid climate leaches basic cations from the soil. Acid rainfall can also cause soils to become acidic. The basic cations—calcium (Ca++), potassium (K+) and magnesium (Mg++)—are leached from the soil profile and replaced with acidic components (H+ and Al+++) that are toxic to plant roots. The toxic components are neutralized by lime.

Apply lime only when recommended by a soil test. Some soils need to be limed every 2 to 3 years, others every 3 to 4 years. In general, sandy-textured soils need to be limed more frequently than clays because sandy soils are more subject to pH changes caused by leaching and fertilizer treatments.

Ideally, lime should be applied and mixed into the soil before a crop is planted. However, if the need is great, lime can be applied even when plants are already in place. Unlike fertilizers, lime can be spread over growing crops without damaging the crop. When lime has been applied as recommended, do not apply more without taking another soil test.

Soil pH and micronutrient availability are interrelated. Too much lime can raise soil pH to a point where micronutrients become unavailable to plants. Micronutrients affected by pH include iron (Fe), manganese (Mn), zinc (Zn) and, to a lesser extent, copper (Cu). Problems caused by applying too much lime are more difficult to correct than those that result from having applied too little. You can always apply more lime, but you cannot remove it if you put out too much.

When used as specified on the label, pelletized lime is just as effective as powdered lime. Pelletized lime contains finely ground dolomitic lime bound in pellet form. In contact with water, the pellets disintegrate and release the enclosed lime. Pelletized lime is more expensive but easier and less messy to apply than powdered lime. To many homeowners, it is well worth the extra cost.

Lime is only slightly soluble in water and does not move into the soil as effectively as soluble fertilizers, such as nitrogen (N) and potassium (K). Lime reacts with soil acids most effectively when it is mixed with the soil and adequate water is present. However, when lime cannot be incorporated into the soil, a surface application is better than none at all.

Yes. However, surface-applied lime reacts much slower than lime mixed into the soil. Most of the reaction of surface-applied lime occurs in the top 1 to 2 inches of the soil. If you surface-apply lime to established plants, apply no more than 50 lb per 1000 ft² initially, even if your soil test report recommends more. Apply the remainder after six months. On no-till fields, apply only 1.0 to 1.5 tons per acre; apply the remainder within the following year.

With adequate soil moisture, lime begins to react immediately, and pH changes should be evident within a couple of months. However, it takes about 6 to 12 months for the total benefit of lime to be realized. As soil pH increases, the reaction rate of lime decreases due to a lower concentration of soil acids: e.g., the lime reaction rate is higher at pH 5.0 than at pH 6.0.

Clay soils are aluminosilicates and contain more potential acidity than sandy soils. As the pH falls below 5.5, aluminum become soluble at levels toxic to plants. In addition, soluble aluminum reacts with water to produce hydrogen ions, further reducing soil pH. Since clay soils generate more acidity than sandy soils, they require more lime to counteract it.

Soil test measurements of pH and exchangeable acidity indicate whether enough lime has been applied. The purpose of applying lime is to reduce exchangeable aluminum (acidity) to levels that are not toxic to plants. Research shows that nontoxic levels occur at pH values above 5.5. A soil pH of 6.0 is usually recommended to allow for variation in lime application, soil type, climate, fertilizer effects and crop tolerance. Some crops have a higher target pH of 6.0 to 6.5.

Generally not. Moss growth is influenced by other conditions, such as shade, poor drainage, soil compaction, poor fertility (although moss grows quite well on fertile lawns) and very acid soils. Although lime will neutralize soil acidity, the other factors that promote moss growth remain the same. Several chemicals are available for moss control. Follow label recommendations for rate and method of application.

Chemically, clay and organic soils have many negative sites that attract and hold positively charged nutrient ions, such as potassium (K+), calcium (Ca++) and magnesium (Mg++). Pure sand alone, on the other hand, has no negative sites. The nutrient-holding capacity of a sandy soil is directly related to the amount of clay and organic matter mixed with and coating the sand. In addition, because sandy soils are porous, water moves freely through the soil profile carrying nutrients downward with it.

In a home garden setting, the best way to increase a soil's nutrient-holding capacity is to mix organic matter, such as composted leaves, into the soil. As added benefits, organic matter enhances biological activity within the soil, increases water retention and provides essential plant nutrients. In a field crop situation, green manure (cover crops) can be incorporated into sandy soils to increase nutrient and water retention. To maintain beneficial effects, organic matter must be added to the soil regularly.