Frequently Asked Questions about Soil Testing and Fertilizer
Q: What is a soil test?
A: 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.
Q: Where can I get a soil test kit?
A: There is not really a "soil test kit." To take a soil sample, you need a clean, plastic bucket; a stainless steel soil probe, shovel, spade or garden trowel; a sample information form; and soil boxes. Information forms and soil boxes are available from NCDA&CS regional agronomists, the Agronomic Division office in Raleigh, county Cooperative Extension offices, or local agribusinesses. Information forms are also available online.
IMPORTANT NOTES: Do not use brass or bronze tools because they will contaminate samples with copper. Galvanized buckets or tools will contaminate samples with zinc. When filling out the sample information form, specify the crop(s) you plan to grow in order to get appropriate lime and fertilizer recommendations. Instructions and crop codes are shown on the back of the information form. Home lawns, vegetable gardens and flower gardens are considered crops and have their respective crop codes.
Q: How much soil is needed for a soil test?
A: The Agronomic Division recommends about 0.5 pound. This amount should be enough to represent the area being tested. The boxes used for submitting samples have a "fill line" that shows the amount of soil needed.
Q: What is a soil test extractant?
A: 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.
Q: When I receive my soil test report, how do I know what the numbers mean?
A: A cover sheet called "Understanding the Soil Test Report" comes with your report and defines all the parameters shown on the report.
Q: What does the term CEC shown on the report mean?
A: 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.
Q: What does the term BS% mean, and what does it tell you about a soil?
A: 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.
Q: How often should a soil be tested?
A: 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.
Q: What does the pH measure?
A: 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].
Q: Can I buy a pH meter to test my soil?
A: 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.
Q: When is the best time to take soil samples?
A: 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.
Q: What is the procedure for taking soil samples?
A: Collect a slice or core of soil (4 inches deep for lawns, 6 to 8 inches deep for gardens) from 8 to 10 random locations across the lawn or garden. Mix these cores together in a plastic bucket. Fill a soil box with this mixture and label it so you know which area it came from.
For most garden areas, one sample/plot should be adequate. However, certain areas may need to be sampled and labeled separately if soil types are obviously different or if plants with different requirements are being grown. In such cases, label samples from each unique area differently: e.g., FRONT, BACK, SIDE, etc.
Q: Can samples be taken when soils are wet?
A: Soil moisture does not affect soil test results. Samples are dried before they are analyzed. However, extremely wet soils are difficult to collect and mix. Therefore, allow soil to drain before sampling. Soil too wet to till is too wet to sample.
Q: Do soil samples need to be sealed before shipping?
A: There is no reason for individual sample boxes to be sealed. The laboratory provides boxes that give adequate sample protection. To mail sample boxes full of soil, pack them inside a sturdy box that prevents them from being crushed and mixed with other samples. DO NOT ship samples in paper envelopes.
Q: How deep should samples be taken?
||6 to 8 inches
|Shrubs and Trees
||6 to 8 inches
|No-Till Field Crops
* Note: If you are taking a sample to diagnose a suspected nutrient problem, take the sample within the plant root zone.
Q: What is the most desirable pH for growing plants?
A: It depends on the plant.
||Optimum Soil pH
|Azalea, rhododendron, camellia, mountain laurel
|Most shrubs and shade trees
|Fescue, bermudagrass, zoysia, St. Augustinegrass
|Berries and most fruit trees, except blueberries
|Most vegetable and field crops
Q: What is lime and how does it benefit the soil and plant growth?
A: 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.
Q: What is the difference between calcitic and dolomitic lime?
A: 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.
Q: What is gypsum, and why can't it be used in place of lime?
A: 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.
Q: Why would a crop turn yellow after lime is applied?
A: 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.
Q: Why do the soils of North Carolina need lime?
A: 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.
Q: How often should lime be applied?
A: 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.
Q: What happens when too much lime is applied?
A: 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.
Q: Does pelletized lime work as well as powdered lime?
A: 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.
Q: Why is it important to mix lime into the soil?
A: 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.
Q: Does surface-applied lime provide any benefit to established lawns and shrubs?
A: 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.
Q: When lime is mixed into the soil, how long does it take to change soil pH?
A: 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.
Q: Why do clay soils require more lime than sandy soils at the same pH?
A: 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.
Q: How much lime is enough?
A: 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.
Q: Why do clay and organic soils hold nutrients better than sandy soils?
A: 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.
Q: Is it possible to increase the nutrient-holding capacity of a sandy soil?
A: 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.
Q: Will lime rid my yard of moss problems?
A: 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.
Q: What do the numbers on the fertilizer bag mean?
A: In order, the three numbers represent the percent nitrogen (N), percent phosphate (P2O5) and percent potash (K2O) contained in the bag. This formula is a national standard for fertilizer labeling. For example, a bag of 15-10-20 contains 15% nitrogen, 10% phosphate and 20% potash. Every 100 lb of this fertilizer contains 45 lb of plant nutrients.
Q: If a November soil test recommends 10 lb of 10-10-10 per 1000 ft² for shrubbery, when should this fertilizer be applied?
A: Wait until spring to apply fertilizer that was recommended in November. In general, make application about the time foliage begins to grow.
Q: How do you calculate the total amount of lime or fertilizer to apply based on a rate given per 1000 ft²?
A: First, determine how many square feet you plan to treat. Multiply length (ft) by width (ft) and divide by 1000 to obtain the number of 1000-ft² units you have. Multiplying this number by number of pounds recommended per 1000 ft² gives you the total amount of lime or fertilizer needed to cover this area. Example: 40 ft × 30 ft = 1200 ft², divided by 1000 = 1.2 thousand ft² units. If 40 lb/1000 ft² was the recommendation, then 40 lb × 1.2 gives the total amount to apply: 48 lb to cover the 1200-ft² area.
Q: If a soil test report recommends 10 lb of 10-10-10 per 1000 ft², how do you convert this rate so you can use a different grade of fertilizer?
A: The recommendation of 10 lb of 10-10-10 advises an application rate of 1 lb of nitrogen [10% of 10 lb] per 1000 ft². You can substitute any fertilizer grade but calculate the appropriate rate, as follows:
Rate in lb per 1000 ft² = (lb N desired per 1000 ft²) ÷ (%N in fertilizer ÷ 100)
Example: Using a 15-15-15 fertilizer source, the rate to apply is
[(1.0 lb N per 1000 ft²) ÷ (15 ÷ 100)] = (100 ÷ 15) = 6.66 lb of 15-15-15 per 1000 ft².
Q: What if a soil test report recommends an unusual fertilizer grade like 15-0-14 or 8-0-24?
A: Soil high in phosphorus get this type of fertilizer recommendation. When the soil test phosphorus index is greater than 50, no phosphorus is recommended for lawn, garden or landscape samples. Grades 15-0-14 or 8-0-24 can be purchased from fertilizer dealers that cater to farmers. If you cannot find these materials at local garden centers, contact a regular fertilizer dealer who can assist you in blending the proper ratio or provide an appropriate substitute.
Q: How are slow-release fertilizers different from other fertilizers?
A: Slow-release fertilizers have a coating that reduces solubility of the encapsulated nutrients. These fertilizers are effective, though somewhat expensive, nutrient sources that are less subject to leaching. The rate of nutrient release depends on temperature, moisture, and the type and thickness of the coating.
Q: What about "organic" or "natural" nutrient sources for growing plants?
A: Organic and commercial fertilizers are equally successful. Blood meal is an organic fertilizer that contains 12% N. Animal and poultry wastes, composted leaves and municipal waste products are other organic sources. Some municipal wastes are amended with lime (used in the composting process).
Natural (i.e., not man-made) fertilizer deposits include sodium nitrate (NaNO3), 16% N; potassium sulfate (K2SO4), 50% K2O; potassium chloride (KCl), 60% K2O; guano, a deposit of dried bird and/or bat excrement high in nitrogen and phosphorus; greensand, which contains potassium; gypsum (23% Ca), 18% S; and calcitic (CaCO3) and dolomitic lime (CaMgCO3). Lime is a natural source for calcium and magnesium, which are essential for plant growth.
Q: Which are better: organic or mineral fertilizers?
A: To plants, nutrients are the same regardless of their origin. Nutrients in "organic" fertilizers must be converted to an "inorganic" state before they can be utilized by plants. Equivalent amounts of nutrients will provide the same nutritional value to plants.
One major advantage of organic fertilizers is the addition of organic matter to the soil. The added organic matter provides many benefits to the physical, chemical and microbiological environment of the soil. In addition, organic sources contain sulfur and/or micronutrients that may not be present in an inorganic fertilizer. Organic sources generally supply nutrients in a "slow release" manner that reduces loss through leaching.
Q: What are the benefits of water-soluble micronutrients?
A: Water-soluble micronutrients are most effective in correcting deficiencies. Some micronutrients sold for agricultural purposes are in chemical forms that are neither available to plants nor detectable by a soil test. Such products are useless for crop production. They may become soluble under very acidic conditions, but such conditions are not conducive to crop production.
Q: How can small amounts of nutrients (e.g., 2 lb per acre) be applied uniformly?
A: Use a liquid spray applicator with a spray volume of at least 20 gallons per acre. Another option is to apply the nutrient with other liquid fertilizers or chemicals. If you do this, mix a small volume first to make sure all components are compatible. To mix the nutrient with dry-blended fertilizers, dissolve it in water and spray in on the dry fertilizer as it is being blended.
Q: Can heavy fertilization harm plants?
A: Fertilizers are salts and will injure plants in the same manner as table salt sprayed on the foliage. High salt levels in the soil draw water from plants causing them to wilt (dehydrate) and, in severe cases, die. Even normal fertilizer rates can damage plants when soil moisture is low. Irrigation dilutes the salt content and prevents plant injury. As essential as fertilizers are to good plant production, too much can be detrimental. Plant nutrition, like human nutrition, requires a proper balance and level of nutrients to sustain good health.
July 2, 2012