Soil Testing

Routine soil analysis is the most accurate way to determine lime and fertilizer needs. The shotgun approach to nutrient management, applying all nutrients annually whether needed or not, is neither practical nor economical. Vegetable growers must know the nutrient status of the soil and then match application rates to crop needs. This is important to achieve optimum yield and quality, to maximize return on investment, and to limit nutrient losses to the environment.

Soil Test Methods

Soil test methods used for vegetable production are designed to provide a measurement of soil pH and nutrient availability. Soil testing labs use different methods, and it is best to select a lab using analytical methods appropriate for New England soils that provide soil test interpretations based on field correlation and calibration under local conditions. A list of New England soil test laboratories is provided at the end of this section. In New England, routine soil analysis typically includes a measure of extractable phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S), plus several of the micronutrients (e.g., boron, manganese, zinc, copper, and iron). In addition to routine soil analysis, other tests and analytical services are available including: soil organic matter, soluble salts (conductivity), Pre-sidedress Soil Nitrate Test (PSNT), plant tissue analysis, manure and compost analysis, and irrigation water testing.

Several labs have recently adopted Soil Quality, or Soil Health test packages. These packages test the biological activity in soils (active carbon, soil respiration, extractable protein), and physical characteristics (soil texture, aggregate stability, compaction, available water capacity), in addition to the traditional soil chemistry results (soil pH, organic matter, P, K, micronutrients). If using these tests, keep in mind that soils vary considerably from site to site. These results should be used as a baseline against themselves year over year, as opposed to comparing your results any listed “average”, or neighboring farms. These test results can be used as an indicator to evaluate the long-term effects of various soil health practices that you may be implementing. 

The appropriate soil test methods for a given region are selected based on soil characteristics and climate. Different analytical procedures can result in vastly different results. This is especially true of the different extraction solutions used to estimate a soil's nutrient supply. The two extraction solutions used in New England are the weak acid modified Morgan extract and the strong acid Mehlich 3 extract. These are both universal extraction procedures, meaning they are used to determine all major nutrients and many of the micronutrients simultaneously. Most New England Land Grant University labs (ME, VT, MA, and CT) use the weak acid modified Morgan extract. University of New Hampshire uses the strong acid Mehlich 3 extraction solution. A saturated media test using water as an extractant is used to measure nutrient availability in greenhouse potting media and in some cases, high tunnel soils. See Soil Testing Labs in New England for contact information.

The quantity of extractable nutrients may be reported in different units. Some labs report the concentration of nutrients in ppm (parts per million) which is equivalent to mg/kg (weight basis) or mg/dm3 (volume basis). Other labs report nutrient values in lb/acre (weight per area basis). Nutrient concentration in ppm can be converted to lb/acre by multiplying by 2. This is based on the assumption that there are approximately 2,000,000 lb of soil in the surface 6 inches of one acre. Differences in reporting units are important to be aware of, but they are of little consequence to our interpretation of soil test results. Soil test results only provide an index of nutrient supply that must be interpreted based on correlation and calibration data developed by nutrient response research in local soils. It is important to understand what the nutrient interpretations mean.
 

Soil Test Interpretation

Soil test results are of little value without an appropriate interpretation. To be useful, extractable nutrient values must be shown to relate to: 1) the soil's ability to supply that nutrient to crops (correlation); and 2) crop response to application of that element (calibration). Figure 3 illustrates the conceptual relationship between soil test level and yield. This relationship is determined by conducting nutrient response research, under local conditions with representative soils ranging from deficient to adequate for each nutrient of concern. These data form the foundation of our interpretation of soil test results. The exact relationship between soil test level and crop response to different nutrients may vary considerably, but the general shape of the response curve is relatively consistent. At low soil test levels, yield is limited by a lack of the nutrient. As the soil test level increases, yield increases until a point where the nutrient is no longer limiting and the curve levels out; this point is known as the critical soil test level. The critical soil test level is defined as the extractable nutrient concentration in soil above which an economic yield (or quality) response to added nutrient is unlikely. Nutrient levels are considered sufficient when the concentration is just above the critical soil test level. This is known as the Optimum soil test range. Soil test values are interpreted based on how they compare to the critical soil test level and optimum range.  

SoilFig.1

Figure 3. Conceptual relationship between soil test level and crop yield or relative yield. The critical level is defined as the soil test level for a given nutrient above which there is a low probability of a response with addition of that fertilizer.

Soil Sampling

A critical step in soil testing is sample collection. Be sure to read and follow sampling procedures from the specific lab you will be using. For all testing, it is important to obtain a representative sample; a poor sample may result in erroneous soil test results and poor recommendations. The first step is to determine the area that will be represented by the sample. Soil physical appearance, texture, color, slope, drainage, and past management should be similar throughout the area. It may be helpful to draw a map of the farm and identify areas where you will sample separately. Using a clean bucket and a spade, auger, or sampling tube collect at least 12-15 subsamples to a depth of 6-8 inches from random spots within the defined area and place them in the bucket. This may be accomplished by walking a zig-zag pattern across the sampling area and collecting a subsample periodically. Avoid sampling field edges, old fence rows, areas where manure or lime were stockpiled, and other non-representative areas. Next, break up any lumps or clods of soil, remove stones and plant debris, and thoroughly mix subsamples in the bucket. This step is very important, because only a few tablespoons of your sample will actually be used for testing. Once the sample is thoroughly mixed, scoop out approximately one pint of soil and, if specified by your lab, spread it on a clean piece of paper to air-dry (brown paper bags work great). Once the sample is dry, place it in a labeled container provided by the lab (or a zip lock bag), and complete a sample submission form. For each sample, indicate the crop to be grown, recent field history and any concerns.

Soil pH and extractable levels for certain nutrients (e.g., P and K) vary throughout the year. While the seasonal fluctuation is not typically large enough to significantly influence interpretation and recommendations, it can make it difficult to compare soil test values from the same field over time. For this reason, it is a good idea to be consistent about timing of sample collection from one year to the next. Although soil samples can be taken any time, many prefer to take samples in late summer or fall because this allows time to apply any needed lime, plan a fertility program and order materials well in advance of spring planting. Avoid sampling when the soil is very wet or within six to eight weeks after a lime or fertilizer application. Routine soil analysis should be conducted once every two to three years.

Soil Test Recommendations

Nutrient recommendations are determined by soil test results; however, even when two labs use the same methods and generate equivalent results, their nutrient recommendations may differ. These disparities arise due to differences in soil test interpretation and recommendation philosophies. Over the years, three basic philosophies have emerged. These include the sufficiency approach, the build and maintain approach, and the base cation saturation ratio (BCSR) theory. Both the sufficiency approach and the build and maintain approach follow the general concept that there are definable critical levels of nutrients in soil, and that below this level crops are likely to respond to additional nutrients applied. When nutrient concentrations are in the optimum range, just above the critical level, there is a low probability of crop response to the addition of that nutrient. With the build and maintain approach, fertilizer recommendations are made with the goal of building the soil's nutrient levels into the optimum range, then maintaining these levels by applying nutrients at rates that approximate crop removal. The sufficiency approach is a more conservative philosophy where nutrient recommendations are intended to meet crop needs, not build soil fertility. No nutrients are recommended above the critical soil test level. The sufficiency approach is designed to “feed the crop” while the build and maintain approach is designed more to “feed the soil.” In theory, the sufficiency approach is a more profitable system since fertilizer is only applied when there is likely to be an economic return. However, in practice the sufficiency approach is also more risky due to the inherent uncertainty associated with soil testing.

The third philosophy, the BCSR theory, promotes the idea that maximum yields can only be achieved by creating a balanced ratio of calcium (Ca), magnesium (Mg), and potassium (K) in the soil. At one time, many private labs and a few public labs used the BCSR concept to interpret soil test results and make nutrient recommendations. Over time, as the body of research evidence illustrating the flaws of the BCSR concept grew, leading most private labs and essentially all of the public labs to abandon the system. Most of the guidelines developed by Land Grant Universities and used by both public and private soil test labs follow a combination of the sufficiency and build and maintain approaches, the goal being to provide adequate, but not excessive, levels of essential nutrients to promote healthy plant growth. Nutrient recommendations provided in this Guide are a reflection of this compromise. The nutrient guidelines are intended to help growers optimize crop yield and quality, maximize return on fertilizer investment, and minimize nutrient losses to the environment.

The nutrient recommendation tables in this Guide are applicable to the New England soil test results given as very low, low, optimum (medium or high), and above optimum (very high or excessive). Table 5 provides a brief interpretation of each of these categories. Generally, nutrients should be in the optimum range for good yield and quality. When levels are below the optimum range (very low or low), the addition of more of the nutrient will usually improve production and provide a return on fertilizer investment. Nutrient recommendations are intended to meet crop needs and provide enough to slowly (over several years) build soil test levels to the optimum range. When soil test levels are in the optimum range, crop response to application of that nutrient is unlikely, but some amount may be recommended to maintain soil tests levels by replacing a portion of crop removal. In the nutrient recommendation tables for each crop listed in the guide, these build and maintain application amounts are indicated by a range such as 0-50 lb per acre. Crops and even cultivars of the same species vary in their uptake and removal of nutrients and this is accounted for in the nutrient recommendations. If a nutrient is in the above optimum range, crop response is very unlikely and application of that nutrient is generally unjustified. It is important to keep in mind that factors other than nutrients may limit crop growth, and simply adding more nutrients will not improve yield. To optimize yield and maximize response to fertilizer, sound agronomic practices must be used (e.g., crop rotation, timely planting and harvest, pest control, soil health, and water management).  

Table 5: Interpretation of Soil Test Level Categories

Category

Interpretation

Very Low

Soil test level is well below optimum. Very high probability of crop response to additional nutrients.

Substantial amounts of additional nutrients required to achieve optimum yield.  Fertilizer rates based on crop response and are designed to gradually increase soil nutrient levels to the optimum range over a period of several years.

Low

Soil test level is below optimum. High probability of crop response to addition of nutrients. 

Moderate amounts of additional nutrients needed to achieve optimum yield. Recommendations are based on crop response and are intended to gradually increase soil nutrient levels to the optimum range.

Optimum

Most desirable soil test range on economic and environmental basis. For most crops, low probability of crop response to addition of nutrients.

To maintain this range for successive years, the nutrients removed by crops must be replaced.

Above Optimum

The nutrient is considered more than adequate and will not limit crop yield. At the top end of this range, there is the possibility of a negative impact on the crop if nutrients are added.

Environmental Critical Level This soil test level is independent of crop response and, due to environmental concerns, is only defined for soil test P. This P concentration is associated with elevated risk of P loss in leachate and runoff at concentrations high enough to impair surface water quality. No P should be applied and steps should be taken to minimize losses from leaching and runoff.

 

 

 
 

Plant Tissue Testing

An additional tool that can be used for long season crops such as eggplant, pepper, potato, tomato, squashes, sweet corn, and pumpkin is plant tissue analysis.  While leaf tissue laboratory analysis can be used to verify symptomatic deficiencies in any nutrients, it can also be used to detect sufficiency levels of critical nutrients such as N, P, K, Ca and Mg.  If performed early enough in the season, corrections can be made by topdressing, side dressing, or fertigation.  Prior to making nutrient corrections, other potential issues should be addressed first, such as incorrect pH, inadequate soil moisture, root disease or insect infestation. Leaf tissue analysis requires collecting an adequately sized sample (from all over a planting) of whole leaves from designated locations in the plant canopy. These locations vary among crop species and specific sampling and handling instructions are available from university and private laboratories.  For the purpose of comparison, laboratory reports will also present results in tabular form alongside reference sufficiency ranges of nutrient concentrations for the crop tested. Nutrient status of some crops can also be determined using laboratory testing of whole leaf petioles, and for nitrate-N and K, it is possible to test petiole sap in-field using a portable meter.

Soil Testing Labs in New England

Connecticut:
Soil Nutrient Analysis Lab
6 Sherman Place, Unit 5102
Storrs, CT 06269-5102
Telephone: 860-486-4274
Website: http://www.soiltest.uconn.edu/
Email: soiltest@uconn.edu

Gregory Bugbee, State Laboratory
The Connecticut Agricultural Experiment Station
123 Huntington St., P.O. Box 1106
New Haven, CT 06504
Telephone 203-974-8521
Website: http://www.ct.gov/caes/cwp/view.asp?a=2836&q=378206
Email: Gregory.Bugbee@po.state.us

Maine:
The Analytical Laboratory and Maine Soil Testing Services
5722 Deering Hall, Room 407
Dept. Plant & Soil & Environmental Sciences
Orono, Maine 04469-5722
Telephone: 207-581-3591
Website: http://anlab.umesci.maine.edu/
Email: hoskins@maine.edu

Massachusetts:
UMass Soil & Plant Tissue Testing Laboratory
203 Paige Lab
161 Holdsworth Way
University of Massachusetts
Amherst, MA 01003
Telephone: 413-545-2311
Website: http://soiltest.umass.edu/
Email: soiltest@umass.edu
 
New Hampshire:
UNH Cooperative Extension Soil Testing Service
34 Sage Way
Durham, NH 03824
Telephone: 603-862-3200
Website: https://extension.unh.edu/programs/soil-testing-services
Email: soil.testing@unh.edu
 
Vermont:
UVM Agricultural & Environmental Testing Laboratory
262 Jeffords Hall, 63 Carrigan Drive, UVM
Burlington, VT 05405-1737
Telephone: 802-656-3030
Website: https://www.uvm.edu/extension/agricultural-and-environmental-testing-lab
Email: AgTesting@uvm.edu
 
Private:
Woods End Research Lab, Inc.
290 Belgrade Road, P.O. Box 297
Mt. Vernon, ME 04352
Website: www.woodsend.org
Email: lab1@woodsend.org