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soil test part 2

By Harry Brook

Once you’ve successfully soil sampled and received the results, how can you best use the information for fertility planning for the next crop? Interpretation is important.

Test labs make it easy for you to see where you sit for most nutrients with letters behind the numbers signifying Low or High or Very High. They will also make recommendations based on the anticipated crop and yield planned. However, more can be done to fine tune these results into action.

When fertilizer levels are reported in parts per million, double those numbers to get the pounds per acre. This is based on the assumption that 6 inches of topsoil weighs roughly 2 million pounds per acre. Organic matter is a bank account for nutrients that can be drawn on in times of plentiful moisture. A rule of thumb is that there is about .7 pounds of nitrogen for every .1% organic matter in the soil available from organic matter, for the crop, during a growing season. Under-fertilizing, over several years, can draw down this fertility bank account and, sooner or later, the piper must be paid. Low organic matter leads to poor soil structure, puddling and crusting of the soil surface.

Soil pH is a measure of alkalinity and acidity. It also affects the availability of nutrients. A soil pH of seven is neutral, above seven is alkaline and below seven is acid. Most of our soils tend to be acidic and using chemical fertilizers will tend to acidify the soil, slightly.

diagram_copy.pngCation exchange capacity (CEC) is often reported on soil tests. Simply put, it is an indirect measurement of soil clay content. The higher the clay content, the higher the CEC. There is better information from sodium and calcium numbers. High sodium can mean you have swelling clays in the subsoil. There are a lot of soils in the county with high sodium. These soils swell and prevent root penetration when wet, limiting the root zone, making crops more susceptible to drought. When dry, the subsoils can be rock hard, still a barrier to root growth. Signs of these soils are the variable crop height in dry years.

Electrical Conductivity, also known as soluble salts, is valuable. It is reported in millisiemens/centimetre or decisiemens/metre (they are equivalent). As a direct measurement of salinity, it is usually linked to high sodium, calcium and/or magnesium. A rating under 2 is non saline; 2 – 4 slightly saline. Greater than 4 starts to limit the crops you can successfully grow. Above 16 is severely limiting crop choices.

Trace mineral levels are not usually a problem. The only trace mineral that may need fertilizer is copper as it is found to be low in many areas in central Alberta. Although boron is often sold, there are very few circumstances where supplementation actually pays. Meet the needs of the macro-minerals first before looking at trace fertilizers.

With the information from the soil test you can use AFFIRM,(https://www.alberta.ca/alberta-farm-fertilizer-information-and-recommendation-manager.aspx) a free, Alberta government program to determine the nutrients you need based on the fertilizer budget and crop you wish to get. Get the best value from your soil test and maximize every dollar spent on fertilizer. Here’s hoping for rain, the biggest nutrient of all.


Harry Brook is Flagstaff County's Agricultural Fieldman. He can be reached via email at: This email address is being protected from spambots. You need JavaScript enabled to view it. or by phone at: 780-384-4138.