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Why You Should Soil Test and Variable-Rate Your Fertilizer



Why Should you Soil test and Variable Rate your Fertilizer?

Given the dry 2021 growing season, it is expected that we may be seeing some higher than usual residual nutrient levels within the soil compared to other years. Not only may you potentially see an unusual increase in residual levels, but also a more bold fluctuation of residual levels between different zones. With high fertilizer prices this fall and lower moisture levels, there is potential in soil testing to see where to allocate nutrients to where they are needed.



Field Variability Based on Mineralization and Mobilization

Not only may residual nutrient levels be variable this year due to the drier conditions and lower yields, but they will also differ amongst different zones due to the fluctuating levels of mineralization and mobilization of nutrients. In areas with higher organic matter (typically lower lying areas of the field/depressions) mineralization of nutrients becomes higher and therefore the availability of the nutrient to the plant also becomes higher. Higher areas of the field (hilltops and shoulder slopes), typically have a higher rate of nutrient movement for nutrients that are mobile (Nitrogen and Sulfur). With soil tests we can see how the the different levels of mineralization and mobilization of nutrients within different zones vary. From this we can allocate nutrients to where they are needed and best suited.



Why Soil Test and the Benefits of Zone Testing

Given the dry year this year, how does one know how much nutrients are left over from the low yielding crops without soil testing? Are you still going to flat rate 130lbs of nitrogen again this year when your field still has 40-70lbs of nitrogen left over? If yields were good in your area, could have they been better if you would have known which nutrient was limiting the crop from a higher yield? Soil testing takes all the what if’s out of the question and allows us to properly manage our fertility within our fields based on what is physically left in the soil.


What about just composite sampling a field and then flat rating based on the one soil test result? Composite sample does not take into consideration the variance of nutrients in the soil. For example, a hilltop vs a depression. If you sit back and think on it, you would assume that a hilltop is going to have much less nutrients then a depression. One can say then just composite sample the in-betweens/mid slopes to get a field average. But then how do you manage your fertility on your hilltops and depressions with a field average soil sample from the mid slopes? We would be under applying on the hilltops and over applying in the depressions. With a zone SWAT MAP you can properly allocate these nutrients to where they would be most effective and efficiently used by applying different nutrients rates per zone. Zone soil testing using SWAT MAPS allows us to accurately gather soil nutrients levels from different zones in the field, then allowing us to treat all the different zones accurately and more effectively based on the different soil test results gathered.

2021 Soil Test Results So Far

This year we have been seeing very different results from field to field and farm to farm. Some fields have higher nitrogen residual levels (Figure 1) and other fields have low nitrogen residual levels (figure 2). The difference amongst fields goes to show the variability that is unknown until soil test are performed. From this point we can see what zones are lacking what nutrients and allocate the desired nutrients to where they are most needed and would show the greatest crop benefit.


Figure 1. High residual nitrogen levels

Figure 2. Low residual nitrogen levels

Two Main Reasons Why You Should Soil Test:

  1. Learn more about your soil. Not only do you test for NPKS when soil testing but pH, organic matter, salinity, chloride, boron, zinc and copper are also tested for. These are essential baselines to help properly manage your field based on the fundamental building blocks that make up the soil.

  2. Allocate nutrients appropriately. As mentioned above, nutrients can vary significantly from zone to zone and we will not know the degree of variance until soil tests are done. After understanding the different levels amongst the soil and zones we can properly apply nutrients to where they are needed the most and the least.


Michelle Liebrecht

Precision Agronomist

michelle@swatmaps.com

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