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Soil Testing During a Drought: What to Expect



The 2021 farming season was one for the books. With the extreme heat and little to no rain, crops had a hard time growing and producing the yields that were targeted. Some areas had reduced rainfall which resulted in below average yields, while other areas had a total crop failure. One question on many minds this fall is what are the nutrient levels remaining in the soil after this challenging growing season?


Soil sampling should be a practice completed each fall as this helps manage fertility in our soils and is a management 'best practice' when planning for the following growing season. Soil sampling helps quantify the nutrients that remain in the soil after the growing season. From here, the proper amounts of fertilizer can be purchased and applied the following spring. This year with conditions being so dry, we expect soil test results to be higher than usual. With SWAT MAPS and zone sampling, we can tell in which zones of the field the nutrient levels are high or low and the trend through the field. What is the difference between an eroded knoll or depression on a year like this? Not all fields will come back with the same results; they will vary from field to field depending on location, overall field properties, and farm practices.


Figure 1. 2021 nitrate trend through SWAT zones from a single field (note: not all results will show this trend)

In Figure 1, we can see a trend for nitrate (which is the plant available form of nitrogen and the form that is detected in a soil test). The hilltops have higher residual nitrate levels in comparison to the midslopes and non-saline depressions. This trend, which we have been observing this fall, is likely linked to plant uptake. On the hilltops, or a SWAT zone 1 and 2, will be your highest and driest areas of the field - they do not hold moisture well. In combination with the extreme heat and little rain, the plant stand count in these areas was likely lower, which relates lower yields. Less plants and lower yields means less nitrogen being taken up and therefore, more being left behind in the soil.


Switching focus to the saline depression areas - SWAT zones 9 and 10. These areas are sometimes characterized by high levels of salinity which are usually dominated by kochia or foxtail barley during the growing season as crops have a hard time establishing. Residual nitrate levels aren’t as high as what is found in zones 1 and 2 but typically higher than what is found in the midslopes in this year’s testing results. The thought process here is that the crop was likely able to establish early on, however due to the lack of rainfall and poor growing conditions, the saline tolerant weeds (kochia mostly) took over instead of the crop thriving. We typically reduce fertilizer application rates in these zones to begin with; however, the kochia and other weeds also take up fertilizer as well. Due to poor yields in these areas, there should also be higher levels of residual nitrogen remaining.


Now for the midslopes: these zones have less plant available nitrogen on our soil test than zones 1/2 and 9/10 and have been the most depleted zones after the 2021 growing season. The soil in these zones is heavier, retains moisture more efficiently, and has higher mineralization potential. Due to these factors, along with the fertilizer that was applied this spring, these are the areas of the field where the crop was able to establish itself most successfully and likely produced the highest yield, resulting in lower nitrate levels on our fall soil tests results.


There is another factor to be taken into account when analyzing soil test results this fall and that is mineralization. Mineralization is the release of nutrients from a soluble inorganic form into a plant available form. Mineralization can be impacted by many factors, however one of the main factors that influences the rate of mineralization is water or moisture. The lack of rain this season and heat created an environment where little moisture was found and retained in the soil causing very slow mineralization or inhibiting the process completely. If there was little to no mineralization taking place throughout the growing season in certain zones of the fields, this could lead to nutrients not becoming plant available. As a result, they wouldn’t be showing up on our fall soil test results due to lack of rainfall up to soil sample season. Soil tests show residual N and S levels in the forms of nitrate and sulphate. If mineralization was slow this year due to drier conditions, the soil will have released less nitrogen on its own, and thus, could partly explain the lower residual N levels. This is especially true if early in the year, crop conditions looked good and there was lots of biomass. There could have been lots of early season fertilizer uptake, and the soil would not have been replenished via mineralization throughout the year


In summary, soil sampling is a very important practice to implement and carry out every year on every farm, especially after challenging growing seasons. Zone sampling can paint a clearer picture than the standard composite soil sample. In challenging years and with high input prices, it is important to know soil nutrient levels to accurately apply nutrients to your fields the following growing season.


Taylor Ellis

Precision Agronomist

taylor@swatmaps.com

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