Soil Testing - Soil Diagnostics
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|
|Greenhouse media||Plant-available nitrogen, phosphorus, potassium, calcium, magnesium|
|Nutrient solutions, water||Plant-availabe nitrogen, phosphorus, potassium, calcium, magnesium|
|1Soil 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. The period following wheat harvest is often a convenient time in the crop rotation to take samples. 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. It also gives recommendations for the amount and type of lime to be applied, if required. OMAFRA-recommended fertilizer rates are provided in the commodity sections of publication 363, Vegetable Production Recommendations. These recommendations can produce the highest economic yields when accompanied by good or above-average crop production management.
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). 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. 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