How to Use a Soil Test

 

What a Soil Test Tells You

        A soil test is a "snapshot" of the chemical status of your soil.  The test report shows the pH (degree of acidity), and the available amounts of various plant nutrients that were present in your soil at the time of sampling.  Some soil tests also report total organic matter present in the soil.  The soil test report is an index of the chemical aspect of soil fertility, and therefore tells only part of the story.  It is a valuable diagnostic tool, but it cannot alone tell you the correct way to manage your soil.

 

Three Aspects of Soil Fertility

        Physical – good drainage, aeration, moisture-holding capacity, and ease of penetration by plant roots. A crumbly, porous, deep soil provides all of these, is easy to work, and is said to have good tilth, or good physical structure.

        Chemical - adequate levels of all essential plant nutrients, favorable pH, favorable nutrient balance, freedom from toxic excesses of any one element.

        Biological - diversity of beneficial soil organisms (bacteria, fungi, protozoa, nematodes, tiny insect-like creatures, earthworms, etc.) that digest organic residues (fallen leaves, manure, crop residues, etc.) into humus (the decomposed organic matter that makes topsoil dark brown and enhances its capacity to hold water and nutrients).  This activity is called the organic matter cycle, and it releases nutrients in plant-available form.  In addition, healthy and diverse soil life promotes good tilth and protects plants against soil-borne pathogens.

 

The Soil Life is the Engine of Soil Fertility

        In nature, the organic matter cycle provides all of the nutrients that support the natural vegetation.  In agriculture, we remove some organic matter and nutrients in harvest, and therefore we have to "feed" the soil life to maintain fertility.  This is done with compost, cover crops, crop residues, organic mulches, and sometimes purchased organic fertilizers. 

        In a well-balanced, fertile soil, that is receiving adequate organic inputs each year, the organic matter cycle will provide your crops with most or all of the nutrients they need:  nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) sulfur (S) and micronutrients such as boron (B), copper (Cu), zinc (Zn), iron (Fe), manganese (Mn), molybdenum (Mo) sodium (Na) and silicon (Si).  However, in the real world of gardening and farming, some mineral supplementation is often needed.  This is where the soil test becomes useful.

 

Mineral Amendments – which ones, and how much?

        In addition to compost and other organic matter, the soil often needs certain mineral supplements in order to reach optimum fertility and crop yields.  This may occur because:

            (1) past farming practices have depleted some nutrients,

            (2) the soil is naturally low in certain nutrients, or

            (3) the soil is too acid or too alkaline for the crops you

                  are growing. 

        A soil test can help identify the amendments and amounts your soil needs.  If the testing laboratory gives recommendations for organic gardens and farms, this is most helpful.  Otherwise, you may have to "translate" conventional fertilizer recommendations to organic amendments.

Taking a Soil Sample

        Getting a good representative sample is very important for obtaining a meaningful soil test report.  Collect a soil sample by taking cores 0-6 inches deep (0 to 8 inches for a garden with deep topsoil) from 12 to 25 points evenly scattered through the field or area you are testing.  If you do not have a coring tool, dig a hole about 8 inches deep with a shovel, leaving a smooth nearly-vertical surface.  Use a trowel to take a small, even slice of soil from 0-6 inches depth along this surface.  Combine and thoroughly mix the 12 to 25 samples in a stoneware crock or clean plastic bucket (not metal, which can cause false readings for iron, aluminum or zinc).  Allow the combined sample to dry for a day or two at room temperature (not in direct sunlight).  Then put a pint of air-dry soil into a sample bag provided by the lab, or a ziplock plastic bag, and mail it to the lab with your testing instructions, payment and a brief description of the field.

        If you are farming several fields, do a soil test for each.  Also, if you know that two or more sections of a given field have different soil types, or widely different cropping histories, take a sample for each section.

 

Reading the Report

        Usually, the lab will report each nutrient in parts per million (ppm) or pounds per acre (lb/acre).  NOTE: 1 ppm = about 2 lb/acre in the plow layer (6 inches depth).  These numbers represent an approximation of the plant-available nutrients present, not the total amount of each element in the soil.  NOTE:  some labs report elemental P and K, and others report phosphate (P₂O₅) and potash (K₂O).  1 ppm P = 2.3 ppm P₂O₅;  1 ppm K = 1.2 ppm K₂O. 

        Most labs will follow the number for each nutrient with a letter code as follows:  VL = very low (critically deficient); L = low (likely to restrict crop yield); M = medium (sufficient for most crops, may be limiting to heavy feeders); H = high (favorable); and VH = very high (ample, possibly excessive).  Any nutrients that are rated VL or L on your soil test usually needs to be supplemented, whereas nutrients rated VH may indicate a need to cut back on inputs of those elements.

        Direct measurements of soluble soil nitrogen (N) are often misleading, as this element can fluctuate wildly between nearly undetectable and excessive levels (that may leach to groundwater) within a single season.  Therefore, many labs do not report N per se, but give an estimated nitrogen release (ENR) in lb/acre-year, based on the soil's organic matter content, and texture.  If the ENR is 120 or more, the organic matter will provide most or all the N needed by most crops. 

        The soil test will also give soil pH.  A pH of 7.0 is neutral; 6.0 is mildly acid, 5.0 is strongly acid, and 4 is extremely acid, while numbers above 7 indicate alkaline soil.  Most crops prefer a pH of 6.0 to 6.8, although they will thrive at anywhere from 5.5 to 7.5 if soil life is healthy and balanced.  Strawberries and brambles do well at pH 5.5 to 6.0; and blueberries at about 5.0. 

        A good soil test should include a measurement of total soil organic matter (SOM), usually given in percent.  This is a rough estimate of the vigor of the soil life and organic matter cycle.  Generally, good SOM levels for tilled fields are 1.5 to 3% for sandy soils, 2.5 to 5% for loams, and 4 to 6% for heavy, clayey soils.  (These figures are for “wet chemistry” methods for total OM; “loss on ignition” figures are often 1.5 times as much.)  The warmer the climate, the lower the SOM, even if soil life is healthy and active. So don’t let those northern gardeners give you an “organic matter inferiority complex!” If your soil is dark brown, crumbly and well drained, and plants are thriving, the soil probably has a healthy organic matter cycle.  It is actually possible to have too much SOM.  At 7 to 10%, it may tie up micronutrients, leach nitrate to the groundwater, or contain so much available P and K that plant nutrition is unbalanced.

        Some labs also report cation exchange capacity (CEC).  This is an estimate of the soil's ability to hold positively charged (cation) nutrients in plant-available form, especially K, Ca, Mg and Na.  The soil's CEC resides in its clay and humus content; thus rich, clay-loam soils will have a high CEC, and dry sandy soils will have a low CEC.  Building soil organic matter will somewhat enhance CEC.  Some farm consultants and soil test labs pay attention to the relative amounts of Ca, K and Mg held on a soil’s CEC (base saturation ratio), and seek to adjust the cation balance.  For more on cation balancing, see the VABF information sheets, Soil Cation Nutrient Balancing – missing link or red herring?, and Does My Soil Need Cation Balancing?

 

How to Avoid Pitfalls in Reading Soil Tests

        Some soil labs do more accurate work than others; and some provide vital data (e.g. SOM) that others do not.  Different labs use different procedures, so that lab A may report 100 ppm P for a given soil, and lab B only 40 ppm.  Each lab also develops its own ranges for VL, L, M, H, and VH based on experience with its particular methodology.  Find a lab that you trust, and stick with it. 

        Because soil is a dynamic, living system, its pH and the levels of some nutrients can vary significantly during the course of one season.  Retest your soil every two to three years - at the same time of year and using the same lab - and notice the trends.  Are SOM, pH and nutrient levels approaching the optimum range?  Are “low” nutrient levels coming up, and “excessive” nutrient levels coming down?

        Some soil labs are prepared mainly to give recommendations for chemical fertilizers, and will often recommend more N, P, K and lime than is actually needed, even on highly fertile soils.  When this happens, you can learn how to convert their recommendations to organic inputs, or to make your own recommendations.  Or, you could switch to an organic- or sustainable-oriented soil testing lab.

 

Amendments for Specific Nutrient Problems

        Acidity (pH below 6.0):  Use agricultural limestone (not hydrated lime – it is too harsh) to raise pH.  If the soil is rated L or M in calcium, but H or VH in magnesium, use high calcium lime.  If magnesium is also L or M, use dolomitic limestone.  Use 1,000 – 2,000lb/acre (23-45 lb/ 1,000 sq. ft) for moderate acidity on loamy to clayey soils.  Sandy soils should be limed lightly, 500 – 1,000 lb/acre - and only if pH drops to 5.5. Retest soil one to two years later to see if more lime is needed.

        Alkalinity (pH 7.5 or above; rare in eastern US):  Use acidic organic mulch (tree leaves, pine straw, chipped brush), avoid liming.  For strong alkalinity or for acid-loving fruits, use elemental sulfur at 200 - 500 lb/acre (= 5-11 lb/1,000 sq ft).

        Low organic matter:  Use generous amounts of compost (5 to 20 tons/acre, or 250 to 900 lb/ 1,000 sq. ft) the first couple of years.  Grow cover crops and use organic mulch.  You may need an organic N-P-K fertilizer to obtain satisfactory crop yields at first.  Grow a legume green manure or apply composted manure to provide N.  Foliar-feed with fish emulsion or seamix.

        Phosphorus:  If P is low, supplement with colloidal phosphate or rock phosphate at 500 to 1,000 lb/acre (12 to 23 lb/ 1,000 sq. ft), then apply some high quality compost and grow legume or buckwheat cover crops to unlock the P.   NOTE: avoid liming at same time as rock phosphate is applied, since mild acidity (pH about 6.0) promotes P availability. 

        Optimal available P (P1 or "weak Bray") is about 20 to 40 ppm; If available P soars above 100 ppm, it can tie up micronutrients - cut back on manure use, as manure is high in P.

        Potassium:  If K is low, it can often be supplemented with manure compost and/or hay mulch, both of which are rich in K.  Potassium sulfate, greensand and sul-po-mag are three mineral amendments that can add K if the soil is very low in this nutrient.  If K gets very high (over 350 ppm or over 8% base saturation) it can make the soil sticky, upset plant nutrition or reduce vegetable quality.  Cut back on the use of hay mulch and manure compost until K is reduced.  If both P and K are sky-high but SOM is low, the land probably has a history of intensive chemical fertilizer use.  Grow legume cover crops, and use mulch or compost based on leaves or brush (low in P and K) to replenish N and SOM without aggravating P and K excesses.

        Calcium:  If Ca is low but pH is optimal or high, use gypsum at 300 to 500 lb/acre (7-11 lb/1,000 sq ft) once or twice a year until soil tests indicate that Ca levels are medium to high.

        Magnesium:  if Mg is low but pH is optimal or high, use sul-po-mag or Epsom salts at 200 to 300 lb/acre (5-7 lb/1,000 sqft), and retest soil in a year to see if more is needed.

        Sulfur:  If S is well below optimum (10 ppm or lower), use gypsum at 500 lb/acre, or sul-po-mag at 200-400 lb/acre.

        Micronutrients:  A low level of boron (B) should be corrected with about 5 to 10 lb borax per acre (2-4 ounces/1,000 sq ft) - no more, as some crops are sensitive to too much B.  For other micronutrients, moderately low levels can be amended with good compost, seaweed meal or rock powders such as granite dust (check label for micronutrient content).  A critically low micronutrient (VL on soil test, or diagnosed crop deficiency) can be supplemented in sulfate or chelate form.

Mark Schonbeck

 

Recommended Reading

        Building Soils for Better Crops, 2nd ed.  Fred Magdoff & Harold van Es, 1999.  This is an excellent manual of sustainable soil management, written in user-friendly language, and covering all aspects of sustainable management of nutrients and SOM, soil conservation and proper tillage.  Available from Sustainable Agriculture Publications, Hills Bldg, Rm. 10, U. Vermont, Burlington, VT 05405-0082, for  $19.95 + 3.95 shipping.

        How can organic vegetable growers increase soil organic matter without overloading the soil with nutrients?, by Brian Caldwell, Cornell Coop. Extn.  In The Natural Farmer, vol. 2, issue 44 (Spring 2000), page 8; excerpted in Virginia Biological Farmer, Aug-Sept 2000, page 7.