Soil Fertility and Nutrient Use: Soil Testing
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Table of Contents
Soil testing is the most accurate tool available to determine nutrient requirements for a crop. It is actually made up of three discrete steps:
Figure 9-1. Trends in Retail Fertilizer Sales
The OMAFRA-accredited soil testing program is the main guide, along with help from plant analysis and nutrient deficiency symptoms, in determining the fertilizer requirements for a specific crop on a specific field.
The OMAFRA-accredited soil-testing program provides assurance of appropriate analyses to support recommendations for nitrogen, phosphate, potash and magnesium fertilizer, along with recommendations for the amount and type of lime to apply. The analytical methods used were chosen to provide accurate results on the range of soils found in Ontario. Table 9-1, OMAFRA-Accredited Soil Tests, lists the tests provided on an accredited soil test in Ontario.
Nitrate nitrogen can be measured from a separate, deeper sample. See Chapter 1, Corn, for further information.
1 Soil organic matter can be useful for herbicide recommendations but is not an accredited test.
Units on soil test reports
Soil test results for phosphorus, potassium and magnesium are reported in units of milligrams per litre of soil (mg/L), which is approximately equal to parts per million (ppm). Plant-available phosphorus is measured with a sodium bicarbonate extract (also known as the Olsen extract). Plant-available potassium and magnesium are measured with an ammonium acetate extract. Manganese and zinc are reported as indexes that account for both the amount of nutrient extracted from the soil and the soil pH. Soil pH has a huge impact on the amount of these nutrients that is available to plants.
These analyses have been found to be the most accurate estimates of plant-available nutrients for soils in Ontario.
The OMAFRA-recommended fertilizer rates are provided in this publication. They are designed to produce the highest economic yields when accompanied by good or above-average management. Research shows that higher-than-recommended rates of fertilizer will occasionally produce higher yields, but that these increases in yield are likely to be small and not profitable. Some fertilizer is recommended at soil test levels slightly above those where crop response is profitable. This is done to maintain high soil nutrient levels and to allow for some error in sampling the field. For high-value crops such as tomatoes, the maximum profit aimed for will require essentially the same amount of fertilizer as maximum crop value (maximum yield with high quality). For low-value crops, there may on occasion be a slight yield response to rates of fertilizer above those recommended for maximum profit. However, trials on farm fields with corn have shown no yield response to rates of fertilizer above those recommended by an accredited soil test. For further assistance in interpreting soil test reports, contact a Certified Crop Advisor or call the OMAFRA Agricultural Information Contact Centre (1-877-424-1300).
See Appendix C, Accredited Soil-Testing Laboratories in Ontario, for a list of accredited labs in Ontario.
Soil Tests From Other Laboratories
Each year, a number of growers ask OMAFRA staff to interpret results from laboratories that are not accredited. Provided the laboratory uses the identical test used by the OMAFRA-accredited service and expresses its test results in the same units, the OMAFRA fertilizer requirements for phosphate and potash can be determined, but there is no assurance how accurate the analyses are.
Only OMAFRA-accredited soil tests can be relied on to provide accurate fertilizer recommendations. To be accredited, a laboratory must use OMAFRA-approved testing procedures, must demonstrate acceptable analytical precision and accuracy, and must provide the OMAFRA fertilizer recommendations.
A number of laboratories provide soil tests such as cation exchange capacity, aluminum, boron and copper. These tests are not accredited by OMAFRA because they have not been found to contribute to better fertilizer recommendations. Research has shown that on Ontario soils, use of cation exchange capacity to adjust potash recommendations can lead to less reliable recommendations than are currently provided.
Sample soils with a sampling tube or a shovel. Sample each field or uniform section of a field separately. Take at least 20 soil cores, 15 cm (6 in.) deep, from any field or area sampled up to 5 ha (12.5 acre) in size. For fields larger than 5 ha (12.5 acre), proportionately more cores should be taken. See Figure 10-1, Scouting Patterns, for appropriate sampling patterns for soil sampling. The more cores taken, the more likely the sample will provide a reliable measure of the fertility in the field. One sample should not represent more than 10 ha (25 acres).
Collect the soil in a clean plastic pail, break up
the lumps and mix the soil well. Place a subsample into a soil sample
box and forward it for testing. Traverse the sampled area in a zigzag
pattern to provide a uniform distribution of sampling sites. Where
parts of a field differ in appearance (of soil or crop) or in previous
fertilization (manuring or liming), sample separately. This applies
even if they are too small to fertilize separately. Avoid sampling
recent fertilizer bands, dead furrows, areas adjacent to gravel
roads or where lime, manure, compost or crop residues have been
When to Sample
Sample each field once every 2 or 3 years. Potash levels can change quickly where large amounts of nutrients are removed from sandy soils (i.e., alfalfa hay, corn silage, potatoes or tomatoes). Under these conditions, take samples each year.
To allow time for mailing and analysis, take soil samples from fields that will be fertilized for spring-seeding crops in the fall. With the rush of harvest and the frequency of poor weather in late fall, summer may be a more convenient time for some growers to soil sample.
Sample Boxes and Information Sheets
Soil sample boxes, information sheets and information on the cost of various tests are available from any of the accredited labs, or from many fertilizer and farm supply outlets.
Management practices such as manure application or legume sod plowed down can affect a soil test recommendation and the crop to be fertilized. This information is essential for a reliable fertilizer recommendation and should be recorded on the field information sheet that must accompany the soil sample sent in for analysis.
For more information visit the OMAFRA website at www.ontario.ca/crops.
OMAFRA-accredited tests are available for manganese and zinc. In the case of zinc on corn, the soil test is best used in conjunction with visual deficiency symptoms. With manganese, plant analysis, visual symptoms and the soil test are all useful. OMAFRA-accredited tests are not available for boron, copper, iron or molybdenum. Current tests for these nutrients have not provided good estimates of the availability of these nutrients to plants. Plant analysis is generally a better indicator of deficiencies of these nutrients.
Great care is required to prevent contamination of soil samples with micronutrients, particularly zinc. Do not use galvanized (zinc-plated) soil sampling tubes to take soil samples for micronutrient tests. Do not use metal containers to collect and mix samples. Clean plastic containers in good condition should be satisfactory. Soil samples that have contacted galvanized surfaces are unsatisfactory for zinc soil tests. Be careful to keep dust around buildings, etc., out of the samples.
Micronutrient deficiencies frequently occur in small patches in fields. In these cases, soil or plant analysis taken from the entire field is unlikely to identify the problem. Collect separate samples of problem and adjacent good areas to allow for diagnostic comparison.
Plant analysis measures the nutrient content of plant tissue. Comparing the results against the "normal" and "critical" values for the crop can indicate whether nutrient supply is adequate for optimum growth.
Plant analysis is the basis of fertilizer recommendations for tree fruits and grapes and is a useful supplement to soil testing for evaluation of the fertility status of other crops. It is independent of soil testing and can provide a valuable "second opinion," especially for phosphorus, potassium, magnesium and manganese. It is less reliable for nitrogen and zinc. For boron and copper there is not a reliable soil test, so plant analysis and visual symptoms are the methods used for diagnosing deficiencies.
Plant analysis has limitations. Interpreting the results is often difficult since plant-tissue analysis does not usually indicate the cause of a deficiency or the amount of fertilizer required to correct it. Plant analysis is most useful if combined with visual inspection of the crop and soil conditions, knowledge of past management in the field and a current soil test to provide information about soil nutrient levels and soil pH.
There is no formal accreditation process for plant tissue analysis. However, accredited labs that perform plant analysis are monitored and provide quality analysis and interpretation of plant tissue samples.
The time of sampling has a major effect on the results, since nutrient levels vary considerably with the age of the plant. Results are difficult to interpret if samples are taken at times other than those recommended. Nevertheless, sample plants suspected of being nutrient deficient as soon as a problem appears. Samples are best taken from a problem area rather than from the entire field, and it is often helpful to sample an adjacent area of healthy plants at the same time, for comparison.
Take samples for plant analysis from at least 20 plants distributed throughout the area chosen for sampling. Each sample should consist of at least 100 gm (3.5 oz) of fresh material. Sample problem areas separately. Avoid contaminating plant tissue samples with soil. Even a small amount of soil will cause the results to be invalid, especially for micronutrients.
Deliver samples of fresh plant material directly to the laboratory. If they are not delivered immediately, dry them to prevent spoilage. Samples may be dried in an oven at 65°C or less, or dried in the sun provided precautions are taken to prevent contamination from dust or soil. Avoid contact of samples with galvanized (zinc coated) metal, brass or copper.
For more information:
Toll Free: 1-877-424-1300