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Contents:
Soil Testing
Results and Interpretation of a Soil Test
Good Gardening Practices to Reduce the Lead Risk
It is best to do soil testing in the fall: you can use the test results to start preparing the soil in fall/winter for planting in spring; furthermore, the soil testing lab is sometimes backed up with orders in spring, especially in April and May.
Results and Interpretation of a Soil Test
The following article specifically discusses soil test results and amendments to New England soils, but the information contained herein is general enough to be applicable to soil testing anywhere else.
The goal of soil testing is to provide guidelines for the efficient use of soil amendments, such as lime and fertilizer. Those provided with your soil test are the best now available for the crop chosen. Problems directly related to disease, insects, and to some extent weather and cultural practices can not be addressed by a soil test.
The Soil Sample – One of the most important steps in soil testing is obtaining the soil sample. It should represent the soil in which the plants are or will be growing. Randomly take several small samples across the area of concern, through a depth that contains or will contain the bulk of the plants' roots. A poor sample will result in bad recommendations.
Soil Test Results
Soil pH, Buffer pH, and pH Adjustments – Soil pH is a measure of the soil's acidity and is a primary factor in plant growth. When pH is maintained at the proper level for a given crop, plants nutrients are at maximum availability, toxic elements are at reduced availability, and beneficial soil organisms are most active. Most plants prefer a soil pH between 5.5 and 7.5 and the majority do best in the middle part of this range. Some notable acid-loving exceptions are blueberries, potatoes, and rhododendrons.
Due to the climate and rock-types in which the soils of New England have formed, soils here tend to be very acidic (4.5-5.5). For this reason they must often be amended with materials capable of raising the pH. Many products are available to accomplish this, but ground limestone is the most common. Lime recommendations are made in terms of:
Buffer pH is a measure of the soil's capacity to resist pH change after lime has been added. Two soils with the same soil pH may have quite different buffer pHs, and thus, one will require significantly more limestone than the other to obtain and optimal pH. The extent to which the buffer pH is lower than 6.8 is proportional to the amount of limestone needed.
Occasionally the soil pH must be lowered, because either the plant requires acid soil, or the soil was previously over-limed. Incorporating elemental sulfur is the most effective way to lower soil pH. In the soil the sulfur oxidizes to sulfuric acid. One to two pounds of sulfer will lower the pH of most New England soils about 0.5 unit. Unfortunately sulfur is rarely available in garden centers. Contact the Soil Lab for other options.
Cation Exchange Capacity and Percentage Base Saturation – Cation exchange capacity (CEC) is an important measure of the soils ability to retain and supply nutrients. The bulk of this capacity in limed New England soils resides in finely divided soil organic matter. A smaller contribution comes from the soil's clay particles. The basic nutrient cations (positively charged ions) of Calcium (Ca++), Magnesium (Mg++), and Potassium (K+), and the acidic cations of Aluminum and Hydrogen account for nearly all the adsorbed cations in the soil. Very sandy soils, low in organic matter, commonly have CECs less than 5. New England soils with very high CECs (greater than 40) are invariably rich in organic matter. A CEC in between 10 and 15 is typical and usually adequate.
CEC is important because it represents the primary soil reservoir or readily available Potassium, Calcium, and Magnesium and several micronutrients. It also helps to prevent their leaching. The ease with which a plant gains access to these nutrients depends somewhat on the relative percentages of the adsorbed cations. For this reason it is suggested that the percentage saturation levels be held within loosely defined ranges. For example, a soil with base saturations of Calcium 70%, Magnesium 12%, and Potassium 4% would be considered balanced for most crops and has a soil pH of about 6.5.
Individual Nutrients
Nitrogen (N) – Nitrogen is essential to nearly every aspect of plant growth. Nitrogen is absorbed from the soil as nitrate (NO3-) and ammonium (NH4+). This soil test estimates their current levels. Fertilizer recommendations are not generally made on the basis of these measurements because their levels can fluctuate greatly with the soil and weather conditions over short periods of time. Instead, they are used to assess the extremes of nitrogen fertility. For example, very high ammonium levels can be toxic to the roots of many plants, particularly if the soil pH is above 7. Very high levels of both forms may coincide with fertilizer “burn.” Recommendations are made on the presumptions that very little nitrogen remains in the soil after the growing season and that most crops require between 1 and 4 lbs. of nitrogen per 1,000 square feet of soil per year. Adjustments are often made for soils recently or continuously supplied with manure or compost, which contain nitrogen that will be released in the growing season.
Phosphorous (P) or Phosphorous Pentoxide (P2O5) – Among other important functions, phosphorous provides plants with a means of using the energy harnessed by photosynthesis to drive its metabolism. A deficiency of this nutrient can lead to impaired vegetative growth, weak root systems, and fruit and seed or poor quality and low yield. Soil phosphorous exists in a wide range of forms. Some is present as part of soil organic matter and becomes available to plants as the organic matter decomposes. Most inorganic soil Phosphorous is bound tightly to the surface of soil material particles. Warm, moist, well-aerated soils at about pH 6.5 optimize the release of both these forms. Plants require fairly large quantities of phosphorous, but the levels of phosphorous available to plant roots at any one time is quite low. Soil tests attempt to assess the soil's ability to supply phosphorous from bound forms during the growing season.
Potassium (K) or potash (K2O) – Potassium rivals nitrogen as the nutrient element absorbed in greatest amount by plants. Like nitrogen, a relatively large proportion of plant-available potassium is taken up by crops each growing season. Plants deficient in potassium are unable to utilize nitrogen and water efficiently, and are more susceptible to disease. Most available potassium exists as an exchangeable cation (see above). The slow release of potassium from native soil minerals can replenish some of the potassium lost by crop removal and leaching. This ability, however, is limited and variable. Fertilization is often necessary to maintain optimum yields.
Calcium (Ca) – Calcium is essential in the proper functioning of plant cell walls and membranes. Sufficient calcium must also be present in actively growing plant parts, especially storage organs such as fruits and roots. Properly limed soils with a constant and adequate moisture will normally supply sufficient calcium to plants. High humidity and poor soil drainage hinder calcium movement into these plant parts and should be avoided.
Magnesium (Mg) – Magnesium acts together with phosphorous to drive plant metabolism and is part of chlorophyll, a vital substance for photosynthesis. Like Calcium, Magnesium is ordinarily supplied through liming. Low magnesium levels in many soils will normally not cause problems provided the exchangeable cations (see above) are in good balance. If Mg levels are low and lime is required, dolomitic lime (rich in Mg) will be recommended. If Mg levels are low and lime is not required, Epsom salt (magnesium sulfate) may be incorporated at a rate of 5-10 lbs./1,000 square feet. If Mg is high relative to calcium (Ca:Mg ratio significantly less than 6.5), use calcitic lime (which is slightly more expensive) to adjust pH upward, not dolomitic lime, since dolomitic lime contains Mg. If pH needs no adjustment, use gypsum (Calcium sulfate) to add the calcium.
Micronutrients – The micronutrients are elements essential to plants, but required in very small amounts. In most properly limed soils they are available in sufficient quantities. Five of these (iron, manganese, zinc, copper, and boron) are tested routinely. Micronutrient fertilizer recommendations are not available. Extremely high values, however, are noted.
Aluminum – Aluminum is not an essential nutrient for plants. At elevated levels it can be extremely toxic to plant roots, and limit the plant's ability to take up phosphorus. Extractable aluminum increases greatly at soil pH's below 5.5. Proper liming, however, will reduce aluminum to acceptable levels. Aluminum sensitivity varies greatly with plant type. Acid-loving plants, such as rhododendrons, can tolerate very high aluminum levels. Lettuce, carrots, and beets are very sensitive. Hydrangea, a non-sensitive plant, produces blue flowers at low pH and pink flowers at high pH due to the effect of aluminum on pigment formation.
Toxic Heavy Metals – This laboratory routinely tests lead (Pb) and cadmium (Cd). Lead is naturally present in soils in the range of 15 to 40 parts lead per million parts of soil (ppm). At these levels it presents no danger to people or plants. Soil pollution with lead-based paints and the tetraethyl lead of past automotive fuels have increased soil lead levels to several thousand ppm in some places. Unless the total lead level in your soil exceeds 150 ppm, it is simply reported as low and can be considered safe (assuming the sample submitted was representative of the area of concern). Values above 300 ppm are potentially dangerous to people. In such cases consult the following section on soil lead levels. For more information on high soil lead levels, consult this article from the UMass Amherst Soil and Tissue Testing Laboratory.
Cadmium – Cadmium is extremely toxic to both plants and animals. It is naturally present in soils at safely low levels (less than 1 ppm). Industrial discharges of cadmium, however, often cause municipal sewage sludge to contain elevated levels of cadmium. Composted sludges are often used as soil amendments. Although safe upper limits of cadmium for both plants and animals have not been established, monitoring soil Cd levels helps avoid excesses when such materials are used. Unless the cadmium in your soil exceeds 1 ppm it is not reported.
Soluble salts – Soluble salts (SS), such as those used on road to promote melting, and those present in many commercial (and some natural) fertilizers, can cause severe water stress and nutritional imbalances in plants. Generally, seedlings are more sensitive than established plants to elevated SS levels and great variation exists between plant species. Most soils have values between 8 and 50 by the method used in this lab. The middle of this range is typical of most fertile mineral soils. Values higher than 60 may cause damange to sensitive plants (such as onions, etc.). A SS level can change rapidly in the soil due to leaching (washing out), so evaluating its significance must consider the effects of time and growing conditions. Excessive SS levels can often be corrected by leaching liberal amounts (2 to 4 inches) of fresh water. Normal off-season precipitation will usually correct salt problems resulting from over-fertilization.
Questions regarding soil testing may be directed to the Soil and Plant Tissue Testing Laboratory at (413) 545-2311. The Soil Testing Lab at UMass Amherst welcomes samples from anywhere in the country.
Reprinted with permission from Soil and Plant Tissue Testing Laboratory, West Experiment Station, University of Massachusetts, Amherst, MA 01003.
Good Gardening Practices to Reduce the Lead Risk
If one has grown sensitive produce in a soil heavily contaminated with lead and wishes to know if lead has accumulated in the edible portion of the plants, a plant tissue test for lead can be performed at $10 per sample. Please contact the lab before sending any plant tissue.
UMass Soil and Plant Tissue Testing Laboratory
West Experiment Station
University of Massachusetts
Amherst, MA 01003-8020
Phone: (413) 545-2311
Fax: (413) 545-1931
soiltesting@hotmail.com
http://www.umass.edu/plsoils/soiltest/
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