Soil Testing

Soil testing is an effective tool in determining fertilizer requirements for annual as well as perennial crops. Soil testing allows us to understand soil fertility in fields which is used as the basis for fertilizer requirements for new as well as established plantings. Soil fertility recommendation in field crops, vegetables and new orchards largely based on soil tests; however, leaf and petiole tissue samplings are effective tools for tree and tender fruits to manage deficiency or toxicity of nutrients in established orchards. Specifically, soil tests help to determine: 1) fertilizer requirements, 2) soil pH and lime requirements, 3) suitability for biosolids application, 4) suitability for specific herbicides, and 5) diagnose soil related issues in crop production.

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

Soil ammonium and nitrate tests are also available from most accredited laboratories. A list of OMAFRA accredited soil testing laboratories can be found at Soil, leaf and petiole tissue, and forages and feed testing labs | ontario.ca

Taking a Soil Sample

Sampling a representative soil is the first step to get accurate and useful estimates of the average field fertility. 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.

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.

Soils can be sampled at any time of year, but summer or fall are often ideal. 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. However, for Nitrate-N (NO3-N) and ammonium-N (NH4+N), soil samples are collected as close to planting time as practical (e.g., before planting or prior to side-dressing in corn).

Where to Sample

Sample each field separately. Separate large fields and fields with considerable variation into smaller sections with individual samples for each. Each field section or management zone 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. If sampling in a field that has had fertilizer placed through strip-till, take one core within the tilled zone for every 2 to 3 out of the zone.

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

Soil test values are usually reported in parts per million (ppm), which represents 1 milligram of extracted nutrient for each kilogram of soil. We can also estimate the value in pounds per acre. For example, soil test values for NO3-N and NH4+N can be converted to pounds per acre of available N by multiplying the test results by 4 when the sample depth is 1 foot (12 inches). For 6 inches (15 cm) soil sampling depth, we multiply reported ppm by 2 to estimate lbs per acre. The other point regarding reported nutrient ppm is to highlight the fertilizer equivalents. For example, a K soil test value of 180 ppm = 360 lbs/acre of exchangeable K in the top six inches of soil = 360 x 1.21 = 436 lbs K2O/acre [conversion factor from elemental potash (K) to potassium (K2O) is 1.21]. That’s why a potassium soil test value of 180 ppm means there is no need to add more K fertilizer - there is lots there in the soil.

There is no uniform soil analysis reporting form since results and analysis can be different depending on the lab you use. Labs accredited by OMAFRA provide OMAFRA-accredited test value (if asked for) for nitrogen, phosphate, potash, magnesium, zinc and manganese fertilizers along with a crop response code and a fertilizer recommendation (usually in lb/ac). This also gives recommendations for the amount and type of lime to be applied, if required. The crop response code will give you a sense of how likely you are to have a profitable response to a fertilizer application of that nutrient (e.g., HR means that the likelihood of response to addition of that nutrient is high since the reported soil test value is relatively low).

OMAFRA-recommended fertilizer rates are provided in the commodity sections of publication 811, Agronomy Guide for Field Crops, or publication 363, Vegetable Production Recommendations which is continues to be available until Pub 839 (the Ontario Field Vegetable Guide) is available in 2024. These recommendations can produce the highest economic yields when accompanied by good or above-average crop production management.

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/organic amendments and cover crops are used. This information is essential for a reliable fertilizer recommendation.