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Variable Rates And Your Soil

Why are drainage and water holding capacity of the soil important in developing benchmark maps for variable rate fertility?


The answer comes from Terry Aberhart, Agri-Trend Agri-Coach and farmer from Langenburg, Sask.:

Drainage and water holding capacity of the soil are very important for developing benchmarks when looking to develop a variable rate fertility program. Drainage patterns and issues within a field can in many areas be one of the biggest factors affecting crop yield. In areas of the field that have poor drainage, we will usually see an accumulation of sodium and or salts. In severe cases, these areas may have issues with crop germination. These areas will have very low yield potential and in most cases have very high levels of nitrates and other nutrients because there is not enough crop being grown to utilize these nutrients.

In most cases the first action that should be taken is to see if the drainage issue can be improved. The next step is to make sure you are not adding to the salt or sodium issues by applying too much fertilizer.

In many cases we can see these areas improve in yield over time by reducing nutrients that are compounding the problem and focusing on nutrients that will help the crop deal with stress and develop a strong rooting system. In most cases these areas need higher seeding rates to account for increased mortality rates. Areas that have moderate drainage issues can be a bit tricky because in dry years they can yield very well and on a wetter year they will not do well.

Knowing the drainage patterns and issues in the field is important because when looking at NDVIs or yield maps to create management zones, many times areas that show a lower yield value can be the result of poor drainage, or may be the result of compaction or a lighter soil. If this is the case, you would want to be able to manage those areas differently.

Water holding capacity is important for the same reasons. Soils within the field that can hold more water will, in most cases, be the best areas of your field — unless you are in a area that has high rainfall and you have some drainage problems. Lighter soils that do not hold as much water will suffer quickly under dry conditions and in most cases will have lower yield potential.

If you have areas with very low water-holding capacity in a sandier soil, you may also have issues with nutrient leaching. These areas may really benefit from slow release nutrients or split fertilizer applications.

We have also seen cases under wet conditions and saturated soils — using NDVI images — that show the best areas of the field are the very sandy areas. They were the only areas of the field where crop roots were not water logged and therefore, at the time, the crop was the best in those sandy areas. This would not be typical of a normal year.

In most cases drainage and water-holding capacity are the biggest factors affecting yield potential and variation within a field on any given year. These factors usually play a bigger role in influencing yield patterns than nutrient variations alone in a field. Having a good understanding of these areas along with NDVIs or yield maps to help determine crop yield and potential across these areas in your field will help you develop benchmarks for variable rate fertility with a higher level of confidence.

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