By Kim Stonehouse, PAg, Regional Crops Specialist, Tisdale
Although the 2017 crop is not yet in the bin, some items should be considered immediately after harvest. For example, post-harvest soil testing is always a good practice in order to make the most efficient use of the fertility dollar, and fall is a good time to do this. This year, some unusual conditions could make soil testing even more important.
In most years, under good growing conditions, nutrients in the soil will have been used up by crops or may have been displaced due to leaching and denitrification. This year, due to the lower precipitation, some nutrients may not have been completely used or have otherwise been displaced. This makes it very difficult to estimate exactly what has been left behind and the only way to know for sure is to do a soil test.
The most important thing to remember is that a soil test is only as good as the soil sample taken. It is generally recommended that a minimum of 16 samples per field be taken. The more samples you take, the more your accuracy will improve, especially in larger fields. Avoid taking samples from irregular areas of the field. If you hire someone to collect your soil samples, it’s a good idea go with them to point out unrepresentative areas in the field, or give accurate directions to avoid these areas.
Soil testing on a regular basis has certain advantages. It will not only help to establish upcoming crop nutrient needs but it will also monitor changes in nutrient levels over time and will assist in nutrient management planning. Taking this one step further, you may want to pick representative sites, mark them with GPS and sample these same sites each year. This is known as fertility benchmarking and it will allow you to observe the effects of different crops and fertilizer rates on the residual nutrient load in that field.
Another benefit of soil testing is the report can indicate other factors that may need to be addressed or considered when developing a nutrient management plan. In addition to macro- and micronutrient levels, a soil test will indicate a host of soil characteristics, such as soil texture, pH, salinity rating, organic matter content and the cation exchange capacity.
When soil sampling in the fall, care should be taken to ensure that the soil temperature has dropped below 10° C. This will minimize the risk of nitrogen mineralization before freeze-up. Mineralization is the process where nitrogen, in its organic form, is converted to a plant-available form by soil microbial activity. Lower soil temperatures ensure that this activity has ceased and that the nitrogen levels measured in the fall are less likely to change before spring. Soil phosphorus and potassium are generally less affected by sampling date.
While soil testing can be done in the fall or spring, fall testing ensures that results will be available for fertility planning long before they are needed. Additionally, knowing fertility needs in the fall can give the option to purchase some or all of the crop nutrients early, as it is difficult to predict fertilizer prices.