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Soil: Interpreting Your UMass Soil Test Results

Contributed by Ellen Weeks, Master Gardener

 
 

Soil testing is the most accurate way to determine whether your soil pH needs to be adjusted, what your nutrient needs may be, and helps save money by not applying lime and/or fertilizer unnecessarily. It is also useful for identifying sites potentially contaminated with elevated levels of lead. The UMass Soil and Plant Nutrient Testing Lab offers a variety of soil tests for home gardeners. Information can be found at soiltest.umass.edu

 

Your soil test results report reflects the properties of the sample you submitted and are compared with soil types common to New England and the Northeastern U.S. The optimum range (or typical range in some cases) is shown in the column to the right of your results. The recommendations are specific to the crop selection that you indicated on your submission form. The information below is based on Interpreting Your Soil Test Results. For the UMass fact sheet Interpreting Your Soil Test Results, go to soiltest.umass.edu/fact-sheets/interpreting-your-soil-test-results

 

NUTRIENTS

Nitrogen (N) – Because nitrogen is quickly depleted from soil by plant growth, rainfall, and the rate of microbial breakdown of organic matter, it is not possible to predict the amount of nitrogen that will be available to plants over the course of a growing season. For vegetables and annuals, an annual application of a nitrogen containing fertilizer is advised, and is dependent on the crop being grown.

Phosphorus (P) – Provides plants with a means of using the energy harnessed by photosynthesis to drive its metabolism. A deficiency can lead to impaired growth, weak root systems, poor fruit and seed quality, and low yield. Excessive levels are a concern due to the potential negative impact on surface water quality. Most phosphorus losses occur with runoff - phosphorus enrichment is a leading source of water quality impairment of many lakes, streams, and rivers in New England.

If low: If fertilizer is needed, the rate recommended will satisfy immediate crop needs and begin to build soil phosphorus levels to the optimum range. Recommendations are expressed as P2O5 to correlate with the analysis you will find on the fertilizer bag. Once levels are in the optimum range, only a small amount of phosphorus is needed to replace annual depletion and maintain soil levels. If the level is very low, several years of corrective fertilizing may be needed.

If above optimum: Phosphorus application is unnecessary and should be limited. Where soil phosphorus levels are excessive, phosphorus application should be eliminated and additional steps taken to minimize the risk of surface water contamination by limiting loss due to runoff.

Potassium (K) – Potassium rivals nitrogen as the nutrient used in greatest amounts by plants each growing season. Plants deficient in potassium are unable to utilize nitrogen and water efficiently and are more susceptible to disease. While there is usually some potassium in the soil, fertilization is often necessary to maintain optimum yields.

If low: The fertilizer recommendation will satisfy crop needs and build soil potassium levels to the optimum range. Sandy soils tend to lose substantial quantities due to leaching and will require more frequent applications of fertilizer. Even when soils test in the optimum range, some addition of potassium generally is recommended to replace that used in crop growth. Potassium recommendations are expressed as K2O to correlate with the information on the fertilizer package. If above optimum: No action is needed and adding additional potassium is unnecessary.

Calcium (Ca) – Essential for proper functioning of plant cell walls and membranes. Sufficient calcium must also be present in actively growing plant parts, especially in fruits and roots. Properly limed soils with constant and adequate moisture will normally supply sufficient calcium to plants. If levels are low and lime is not required, gypsum (calcium sulfate) may be recommended.

Magnesium (Mg) – Magnesium acts together with phosphorus to drive plant metabolism and is part of chlorophyll, a vital substance for photosynthesis. Like calcium, magnesium is ordinarily supplied through liming. If magnesium levels are low and lime is required, dolomitic lime (rich in Mg) will be recommended. If Mg is low and lime is not required, Epsom salts (magnesium sulfate) may be recommended.

Sulfur (S) – A component of several enzymes that regulate photosynthesis and nitrogen fixation. The vast majority of sulfur in soil is stored in soil organic matter and is converted to available mineral form by soil microorganisms. In New England, atmospheric deposition from burning fossil fuels has historically contributed significant amounts to soil each year, though, with improved emissions control and the use of cleaner fuels, sulfur deposition has been reduced. Still, sulfur deficiencies are rare in New England. When sulfur levels are low, several sources of sulfur are available to ensure adequate plant nutrition including: gypsum (calcium sulfate), potassium sulfate (0-0-50), and sul-po-mag (0-0-22-11 Mg-22 S). Moderate applications of animal manure or compost will generally result in adequate soil sulfur levels.

Micronutrients – Micronutrients are elements essential to plants that are required in very small amounts. Five of these (iron, manganese, zinc, copper, and boron) are tested routinely. Micronutrient deficiencies are most likely to occur in sandy, low organic matter soils. High soil pH may also bring about micronutrient deficiencies, especially in sandy soils. Micronutrient deficiencies and need for micronutrient fertilizers are rarely seen in the Northeast. When levels are well below the optimum range, the UMass Soil & Plant Nutrient Testing Lab recommends collecting a plant tissue sample to determine if a deficiency exists and a micronutrient fertilizer is required.

 

TOXIC ELEMENTS

Aluminum (Al) – Aluminum is not a plant nutrient and, at elevated levels, can be extremely toxic to plant roots and limit the ability of plants to take up phosphorus by reducing phosphorus solubility. Aluminum sensitivity varies greatly with plant type. Acid-loving plants, such as rhododendrons and blueberries, can tolerate moderately high aluminum levels, whereas lettuce, carrots, and beets are very sensitive. Available aluminum increases greatly at soil pH below 5.5, so maintaining proper pH will lower aluminum solubility to acceptable levels. High levels of aluminum are not considered toxic to humans.

Lead (Pb) – The UMass Soil & Plant Nutrient Testing Lab routinely screens all soil samples for elevated levels of lead, which is naturally present in most New England soils at low concentrations of 15-40 ppm total lead. At these levels, lead generally is thought to present minimal danger to people or plants. Soil pollution from lead-based paint and leaded gasoline have increased soil lead levels to several thousand ppm in some places. Lead is primarily a concern with growing children, who are much more sensitive to it than are adults. To be harmful, lead must be ingested by eating or breathing the dust. Plants do not typically accumulate dangerous quantities of lead internally. Surface contamination of edible parts from dust, rain spatter or direct soil contact (as with root crops) is usually the most significant source of plant contamination.

 

The UMass soil test report indicates the amount of extractable lead. When a soil test indicates extractable levels of lead are greater than 22 ppm (equivalent to approximately 300 ppm total lead), the UMass Soil and Plant Nutrient Testing Lab advises the gardener that their lead level is elevated and to get a Total Sorbed Metals test as well (soiltest.umass.edu/services).

 

Good Gardening Practices to Reduce Lead Exposure

1. Locate gardens away from old painted structures and heavily travelled roads.

2. Give planting preferences to fruiting crops (tomatoes, squash, peas, sunflowers, corn, etc.).

3. Incorporate organic materials such as high quality compost, humus, and peat moss.

4. Lime soil as recommended by soil test (a soil pH of 6.5 to 7.0 will minimize lead availability).

5. Wash hands immediately after gardening and prior to eating.

6. Discard outer leaves before eating leafy vegetables. Peel root crops. Wash all produce thoroughly.

7. Protect garden from airborne particulates using a fence or hedge. Fine dust has the highest lead concentration.

8. Keep dust in the garden to a minimum by maintaining a well-mulched, vegetated, and/or moist soil surface.

 

Recommendations (using results from the Totals Sorbed Metals Test)

Low – less than 400 ppm total lead

• Follow the good gardening practices listed above.

Medium – 400 to 999 ppm total lead

• Follow the good gardening practices listed above.

• Restrict access of children to these soils by maintaining dense cover.

• Do not grow leafy green vegetables or root crops in this soil; instead, grow them in raised beds built with non-contaminated soil and organic amendments.

High – 1,000 to 2000 ppm total lead

• Follow the good gardening practices listed above.

• Do not grow food crops in this soil and do not allow children access to it.

• Keep soil covered and take steps described above to reduce lead availability.

• Grow food crops in containers filled with growing media or clean topsoil; or create lined, raised beds filled with non-contaminated soil and organic amendments.

Very High – Greater than 2,000 ppm total lead

• Contact your local Health Department or the Department of Environmental Protection office for advice on lead abatement measures.

 

A UMass soil test report indicates a threshold for “low” as a Modified Morgan extractable level of 22 ppm for soils (less than 299 ppm total lead). This level is low and can be considered safe (assuming the sample submitted is representative of the area of concern). Estimated total lead levels above 300 ppm are a concern. For more details, see the UMass fact sheet at http://soiltest.umass.edu/fact-sheets/soil-lead-testing-interpretation-recommendations-0

 

CATION EXCHANGE CAPACITY AND SOIL ACIDITY: Cation Exchange Capacity (CEC) – This is a measure of the soil’s ability to hold and release nutrients, specifically the positively charged nutrient ions called cations. These include calcium, magnesium, potassium, ammonium, and many of the micronutrients. Cations are attracted to the negatively charged surfaces of clay and organic particles in the soil called colloids (opposites attract).

 

CEC is reported as milli-equivalents per 100 grams of soil (meq/100 g) and can range from below 5 meq/100 g in sandy, low organic matter soils to over 15meq/100 g in finer textured soils and those high in organic matter. Low CEC soils are more susceptible to nutrient loss through leaching. Generally speaking, a sandy soil with little organic matter will have a very low CEC while a clay soil with a lot of organic matter will have a high CEC. Base saturation is the percentage of the soil’s cation exchange capacity occupied by calcium, magnesium, and potassium. It is an indicator of the pH and lime status of the soil. As pH increases, the base saturation % also increases. Your UMass report includes the base cation saturation values for your sample and compares it to the 3 ranges typically observed in New England soils. When base saturation is well outside of these ranges, it is typically associated with deficient or excessive potassium or very acidic or alkaline soil conditions. Following the fertilizer and lime recommendations provided with your report will typically result in base saturation values within normal ranges.

 

SOIL PH AND EXCHANGEABLE ACIDITY One of the most valuable pieces of information you can get from soil testing is a measure of soil acidity. Soil pH is an indicator of the soil’s acidity, which is a primary factor controlling nutrient availability, microbial processes, and plant growth. A pH of 7.0 is neutral, less than 7.0 is acidic, and greater than 7.0 is alkaline. The lower the pH value below 7.0, the more acidic the soil is. The higher the pH value above 7.0, the more alkaline the soil is. Maintaining proper soil pH is one of the most important aspects of soil fertility management.

 

Most New England soils are naturally acidic and need to be limed periodically to keep pH in the range of 6.0 to 7.0 desired by most crops and ornamental plants. When soil is acidic, the availability of nitrogen, phosphorus, and potassium is reduced and there are usually low amounts of calcium and magnesium in the soil. Under acidic conditions, most micronutrients are more soluble and are therefore more available to plants. Under very acidic conditions, aluminum, iron, and manganese may be so soluble they can reach toxic levels. Soil acidity also influences soil microbes. For example, when soil pH is below 6.0, bacterial activity is significantly reduced.

 

When soil pH is maintained at the proper level, plant nutrient availability is optimized, solubility of toxic elements is minimized, and beneficial soil organisms are most active. While most plants grow best in soil with a pH between 6 and 7, there are some notable acid-loving exceptions, including blueberry and rhododendron, which perform best in soils with a lower pH.

 

Due to its climate and geology, New England soils tend to be naturally acidic (4.5-5.5). The most effective way to reduce soil acidity is to apply limestone. The quantity needed is determined by the target pH (based on crops to be grown) and the soil’s buffering capacity (a soil’s tendency to resist change in pH). There are also acidic cations present which can be released into the soil solution to replace those neutralized by the lime. This is called the exchangeable acidity, which measures the portion of the cation exchange capacity (CEC) occupied by these acidic cations. Soils such as clays or those high in organic matter have a high CEC and therefore a potential for large amounts of exchangeable acidity. These soils are said to be well buffered. Effectively raising the soil pH may require a large quantity of lime. Exchangeable acidity, which is reported in units of meq/100 g, is directly related to the quantity of lime required to increase the pH from its current level to the target level.

Occasionally soil pH must be lowered, either because the plant requires an acid soil or the soil was previously over-limed. Incorporating elemental sulfur (S) is the most effective way to lower soil pH. Applying 5 to 10 lbs. of sulfur per 1000 sq. ft. will lower the pH of most New England soils by approximately half a unit (use the lower rate for very sandy soils). No more than 15 lbs. of sulfur per 1000 sq. ft. should be applied at any one time. Retest after 4 to 6 months to determine if more sulfur is needed.

 

ADDITIONAL OPTIONAL ORGANIC MATTER TEST Soil organic matter (SOM) is composed of materials containing carbon, such as plant and animal remains (including bacteria and fungi) in various stages of decomposition, root and microbial exudates, and humus (the end-product of decay and resistant to further decomposition). SOM content of most cultivated or developed areas of New England is almost always less than 8% and typically in the 2 to 4% range. Several factors control the amount of SOM a soil may have, including soil texture and drainage. Well-drained, coarse textured soils tend to naturally have 4 lower levels of SOM. This is due, in part, to the rapid microbial decomposition rates favored by these soil conditions. In fact, it is difficult to maintain high levels of SOM in these soils without drastic, and sometime unsustainable, measures. Despite the low SOM content of many New England soils, it is an important component of soil for nutrient supply, water holding capacity, cation exchange capacity, and soil structure. SOM supplies nutrients through the process of mineralization, which is the decomposition of organic compounds by microbial action into carbon dioxide and mineral constituents. Three of the macronutrients are made available to plants by mineralization: nitrogen (N), phosphorus (P), and sulfur (S).

The optimum range for SOM for soil health varies across soil types. Generally, lower levels of SOM are sufficient, and practical to achieve, in coarse textured, sandy soils as compared to finer soils with more clay content. For example, 2.5% SOM in a loamy sand soil might be considered ideal while 2.5% could be considered marginal in a silt loam soil where 3 to 5% is more common.

 

SCOOP DENSITY The lab uses a calibrated sampling scoop to dispense an exact volume of soil (5 cubic centimeters) used for nutrient extraction. The weight of this sample is recorded and used to calculate nutrient concentration by mass. Scoop density is the mass per unit volume (grams per cubic centimeter). Values typically range between 1.0 and 1.25 with lower values associated with higher organic matter levels. Knowing scoop density is not particularly helpful to the backyard gardener, though the lab may use that information in the case of a problem to determine whether the soil is very sandy or there is too much organic matter.

 

LIME AND FERTILIZER RECOMMENDATIONS These recommendations and any other useful information appears at the top of page 2 of your report. Note whether the liming and N-P-K recommendation are for 100 sq ft or 1000 sq ft., depending on the crop. Go to the UMass fact sheet Step-by-Step Fertilizer Guide for Home Grounds and Gardening for a brief guide on converting your test results into an appropriate fertilizer application at http://soiltest.umass.edu/fact-sheets/step-step-fertilizer-guide-home-grounds-and-gardening.

 

For the UMass fact sheet Interpreting Your Soil Test Results, go to http://soiltest.umass.edu/fact- sheets/interpreting-your-soil-test-results.

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