Fall Soil Sampling


A day off for a farmer is a rare thing. There’s many times back-to-back-to-back shifts are needed just to get the job done. This time of year, in particular, trying to get the crop off and binned can be a 24 hour-a-day, seven day-a-week job. Add to that shorter days, colder temperatures, and a winter fast closing in, getting what has to be done completed is no easy task.

That said, now is the perfect time to start planning next season’s fertilizer management plan. Fall soil testing gives a snapshot in time of the nutrient status of the soil. It also gives the information, along with yield data and field notes from the year, that is used to determine the best approach to take next season.

The truth of the situation is if you don’t know what you have to start with, you simply cannot know how much, or with what to fertilize. A true case of flying blind.

Soil testing is an important tool used to gauge soil fertility, and soil testing can help track the fertility status of fields over the years, but much like taking grain samples, the results are only as good as the sample that is submitted for testing. In a year such as this, soil testing is not the easiest task – soils have decent moisture levels and that can make getting a good core difficult. Nonetheless, the advice is to press on.

Fall is also a good time to soil test from a time perspective. Generally, soil testing can be carried out quite late into fall, and is best done once the soil has cooled to 100C or lower. At that temperature, any bacterial activity and/or mineralization are at a minimum and the status of the soil is best understood.

Manitoba Agriculture, Food and Rural Development has a great resource on their website at http://www.gov.mb.ca/agriculture/weather/soil-temperature.html. They post a 14-day soil temperature history measured at two inches under grass. Those historical records to date show that soils are still relatively warm. John Heard, crop nutrition specialist at Manitoba Agriculture, Food and Rural Development advises that when using these as guides for N application, we suggest farmers assess soil temps at actual depth of N application (for example 4” for ammonia).

If a sampling plan is not already in place, work closely with your local agronomist or fertilizer dealer to draw one up. This plan will ensure soil samples are taken in sufficient quantity in enough areas of the field to produce a representative sample. It’s very important the person most familiar with field topography have input into the sampling plan to ensure the most appropriate locations are sampled. The lynch pin in any management strategy that requires samples is to produce a representative sample.

The samples then need to be handled correctly to preserve them in the same state they were taken until they are tested. The laboratory conducting the soil testing will provide information about packaging the sample and shipping. If samples are not air dried before shipping, they should be delivered to the laboratory the same day they are taken, or if that is not feasible, they can be refrigerated for a few days or even frozen. The laboratory will conduct a soil test based on your instructions. Generally, a typical soil test package will include tests for nitrate-nitrogen, available phosphorus and potassium, and extractable sulphur, as well as pH and salinity. Additionally, a micronutrient scan can be requested to determine levels of elements such as copper, zinc or boron, amongst others.

Once the soil test report is back, understanding and interpreting the results help fine tune the fertilizer management plan for your farm. The recommendations in the report are a guideline, or a starting point upon which to build your plan. You alone know your farm and your operation, the moisture conditions and the historic performance you’ve come to expect. This input, together with that of your agronomist and fertilizer dealer builds out the plan.

A comprehensive program of soil testing will help identify if macro or micronutrients are being depleted and prevents the environmental and economic penalties of putting on too much, too little or the wrong mix of fertilizer.