New Soil Test Categories

The categories for analytical values on soil test reports have changed to reflect the probabilities of response to added nutrients. In the past, the categories grouped together according to the amount of nutrient extracted. Note that this change does not affect the fertilizer recommendations for each crop.

The rating based on probability of response will be specific to each crop (or group of crops), because there are differences in the way these crops respond to added fertilizer. For soil test reports where the crop to be grown has been indicated, the response category shown in Table 1 will be displayed on the report beside the numeric value for the soil test.

Table 1. Probability of response to applied nutrients at different soil test levels

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

Explanation of Categories:

The Ontario soil test ratings are based on the likelihood of having a large enough response to applied fertilizer, in the year of application, to pay for the fertilizer and allow additional profit for the grower. It does not account for residual effects of fertilizer that is not used in the year of application. Response to fertilizer is measured as the amount of increase in crop value generated when the fertilizer is applied, from increased yield, quality or both. For the response to be profitable, the increase in value must be greater than the cost of the applied fertilizer.

Response to added fertilizer is not exactly the same in each field or each year, but average trends can be measured over time. As a general rule, with low soil fertility there will be a high probability that the crop will respond to added fertilizer, and the difference between the fertilized and unfertilized yields will be large. The rate of fertilizer required to get the maximum return will be high, but the yield increase will more than pay for this fertilizer. As the soil test values increase, the probability of a profitable response to fertilizer declines, as does the size of the expected yield or quality response.

In the Low Response category, most crop responses would be improvement to the early growth of the crop from a starter effect. Fertilizer application to soils in the Rare Response category may cause an improvement in early growth, but is more commonly done to replace nutrients removed by the crop, or to provide nutrients to pockets within variable fields that are lower in fertility. Any yield response is unlikely to be large enough to pay for the added fertilizer.

The No or Negative Response (NR) rating signifies that application of this nutrient in fertilizer or manure may lower crop yield or quality. Phosphate additions to these soils can induce zinc deficiency on soils low in zinc and can increase the risk of water pollution. Potash additions may induce magnesium deficiency on soils low in magnesium.

Environmental Implications:

Soil tests were developed as indicators of the availability of nutrients from the soil to growing plants, and the probability of crop response to added fertilizer. Soil testing is an integral part of nutrient management planning.

There is a link between elevated phosphorus in surface water and impaired water quality, and agricultural runoff has been identified as one of the sources of elevated phosphorus levels. A number of factors influence the likelihood of phosphorus in runoff reaching surface water, of which soil test P is only one. The Phosphorus Index is a tool that can identify areas that are at high risk of causing adverse impact to surface water, and suggests management options to mitigate these impacts.

Potassium has not been identified as an environmental risk, although it may contribute to elevated salt concentrations in the soil. If this is suspected, a conductivity test is a simple and accurate way to account for all the sources of soluble salts in the soil.

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