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Ministry of Agriculture, Food and Rural Affairs

Soil Testing

Soil Testing is frequently used to develop fertilizer recommendations and to help manage long-term soil fertility. It can also be used in season to diagnose crop production problems.

Soil testing in Ontario is done by commercial soil-testing laboratories, using tests accredited by OMAFRA. OMAFRA-accredited soil tests are not available for boron, copper, iron or molybdenum. Plant analysis is generally a better indicator of deficiencies of these nutrients.

OMAFRA-Accredited Soil Tests

Materials

What is Analyzed1

Soils for field-grown crops, commercial, turf, etc.

Plant-available phosphorus, potassium, magnesium, zinc and manganese

pH

Lime requirement

Greenhouse media

Plant-available nitrogen, phosphorus, potassium, calcium, magnesium

pH

Total salts

Nutrient solutions, water

Plant-availabe nitrogen, phosphorus, potassium, calcium, magnesium

pH

Total salts

Sulphates

Chlorides

1 Soil organic matter tests can be useful for herbicide recommendations or for evaluating soil quality but are not accredited tests.

Soil nitrate tests are also available from most accredited laboratories. A list of OMAFRA accredited soil testing laboratories can be found at http://www.omafra.gov.on.ca/english/crops/resource/soillabs.htm.

When to Sample

Sample each field once every 2 or 3 years to develop an appropriate soil fertility program. Sample problem areas in season to determine if there is a nutritional disorder. When using soil testing for diagnostic purposes, take a sample from the problem area and a good area within the same field, for comparison purposes.

Sample soils in the fall or in the summer. Make it a habit to sample at the same time each year for more consistent sample results. Late-summer or fall sampling is ideal for fields to be seeded in the spring.

Taking a Soil Sample

A soil-test report's accuracy and the resulting recommendations depend on properly taking, preparing and submitting a soil sample. To start soil sampling you'll need:

  • a soil probe or a shovel;
  • a clean plastic pail (avoid using galvanized metal pails; these will contaminate the sample for micronutrient analysis, particularly zinc);
  • sample bags and boxes, usually available from the soil laboratory;
  • a pen or marker.

Sample each field separately. Separate large fields and fields with considerable variation into smaller sections. Each field section should have relatively the same soil texture, topography, organic matter and cropping history.

Micronutrient deficiencies frequently occur in small patches in fields. In these cases, analysis of soil or plants taken from the entire field is unlikely to find the problem. Sample problem areas separately. When sampling a problem area, be sure to take a comparison sample from an adjacent good area.

For a basic test, take sample soil cores to a depth of 15 cm (6 in.). Nitrate-nitrogen samples are taken to a depth of 30 cm (12 in.). Take at least 20 soil cores for fields up to 5 ha (12 ac) in size. Take proportionately more cores for fields larger than 5 ha (12 ac). The more cores you sample, the more reliable the measure of the fertility in the field. One sample should not represent more than 10 ha (25 ac).

Travel the area sampled in a zigzag pattern to provide a good variety of sampling sites. Avoid sampling recent fertilizer bands, dead furrows, areas adjacent to gravel roads or areas where lime, manure, compost or crop residues have been piled.

Break any lumps and mix the soil well before sending a sample for testing. Approximately 2 mL of each sample are used for the analysis. Fill a clean plastic bag with approximately 500 g of soil and place it into the box. Be sure to clearly mark the sample with all the necessary information (i.e. sample number, farm name, date, etc.).

Interpreting the Results

The OMAFRA-accredited soil-testing program provides recommendations for nitrogen, phosphate, potash, magnesium, zinc and manganese fertilizer, but these are mainly available for traditional crops and certain larger-acreage specialty crops. See the section "Fertility Recommendations for Specialty Crops" for more information on interpreting soil tests for specialty crops. It also gives recommendations for the amount and type of lime to be applied, if required.

In a basic soil test, each element is given a numerical value (usually recorded in ppm), a letter rating and a fertilizer recommendation (usually in lb/ac). Recommendations are only available for some specialty crops. The letter rating of the nutrient is an indicator of the likelihood of crop response.

Soil Test Nutrient Ratings

Response Category

Probability of profitable response to applied nutrients

High Response (HR)

High (most of the cases)

Medium Response (MR)

Medium (about half the cases)

Low Response (LR)

Low (few of the cases)

Rare Response (RR)

Rare (very few of the cases)

No or Negative Response (NR)*

Not profitable to apply nutrients*

*adding nutrients to soils with these levels of nutrients may reduce crop yields or quality by interfering with the uptake of other nutrients.

A soil test recommendation is affected by manure application, plowing down of legume sod and the type of crop to be fertilized. When available, recommended fertilizer rates, especially for nitrogen and phosphorus should be adjusted if manure and cover crops are used. This information is essential for a reliable fertilizer recommendation