How Might the 2019 Growing Season Impact Soil Test Results

It is not uncommon to have years with abnormal weather that influences soil test results. Soil tests are by nature a picture of soil fertility levels at one point in time, that is why it is wise to keep soil samples at the same time of year each sampling cycle, in repeated locations at a standardized depth while following the same crop each time. Most years have a weather pattern that will bias soil test results slightly and that is why repeated sampling on a regular basis is a key aspect of a successful fertility management program.

For example, in the fall of 2012 much of the region had been under the influence of a drought. The extended period of abnormal dryness could have reduced soil test potassium levels because of potassium ion trapping in between clay layers. Soil pH reported may have been 0.2 to 0.3 units lower than reality because the dry soil lead to a concentrating of hydrogen ions in the remaining soil solution leading to a lower pH reading. If the drought was severe enough to reduce yield, the crop uptake may have been less than predicted and thus the resulting soil test values may have been higher than if a normal crop was harvested. This can be easily reduced by using past yields for the crop removal aspect of your fertility recommendations rather than predicting future yields. See the blog post on this topic on the ALGL website. Click Here

In years like 2012 it is relatively easy to look back and evaluate if it was dry enough to impact K and pH soil tests or if the yield was off enough to reduce crop removal. Those impacts were relatively uniform across a farm, county, state and can be accounted for as part of a long-term fertility management program.  Remember the cycle of management is to measure, plan out what action to take, implement the plan, remeasure, evaluate the impact/effectiveness of the plan and create a new plan to restart the cycle.

The weather patterns of 2019 also have the potential to impact soil test levels this fall, but the impacts may vary by field. Like trend line yields, repeated soil sampling allows for the trending of soil test values over time. There are a wide range of weather and management variables during the 2019 to shift soil sample values above or below the trend line of the past few sampling cycles. 

Prevent Plant/Unplanted Acres

  • If fertilizer was applied in the fall of 2018 or spring of 2019, but no crop was planted to utilize the nutrients, the soil fall 2019/spring 2020 soil test levels may be higher than trend.
  • If fertilizer was not applied fall 2018 or spring of 2019 due to wet soils, the soil test levels should be near trend.

Cover Crops

  • If a cover crop was planted in June or July in soil that was fertilized and allowed to grow through sampling time, the cover crop will take up a relative portion of what the intended crop would have. While the results will vary by the planting date and species of cover crop, this should reduce the potential for the 2019 fall soil test level to be higher than trend in fields where fertilizer was applied.
  • Fall seeded cover crops may not have much growth before sampling and will have little impact soil test results.
  • If the cover crops were terminated before soil sampling, nutrients like potassium that release from crop residues quickly may be captured in the soil sample.

Management of Prevent Plant/Unplanted Acres

  • Fields that have been repeatedly tilled through the summer months to control weeds may prove challenging to maintain a constant sampling depth unless the soil is allowed to settle for 4 to 8 weeks before sampling with a few rainfall events. Soils that have been tilled frequently, allowing the topsoil to dry out too much, can mimic drought soil conditions may result in lower than trend soil pH and K
  • Chemical termination of plants can lead to a quick release of nutrients, chemical termination just prior to soil sampling may lead to temporary increases in soil test values.
  • Mowing of fields would simulate plant uptake while maintaining a slow release of nutrients from plant residues back to the soil. Mowing may have the least impact on soil test values in relation to trend line soil test values.

Crop Removal

  • If a different crop was planted than intended in the soil test recommendations, the actual crop removal will be different. Be certain to compare the crop nutrient removal for the yield goal of the anticipated crop vs. the actual yield of the crop that was planted. This difference my impact soil test results.
  • If you elected to harvest a cover crop for forage, the crop removal may be relatively high depending on the species. The removal of a whole plant is significantly higher than removal of grain. For example, if corn was planted in mid to late June with the anticipation of harvesting the corn for silage, the crop removal will be much higher than the expected grain crop removal, thus leading to lower soil test values.
  • If the crop was planted later than normal, lower than expected yields will be common. Less crop removal means higher soil test values.

Variable Yields Across the Field

  • Less than ideal planting conditions have led to a wide range of impacts from field traffic. Many fields exhibit row to row variability due to the negative impacts of driving across wet soils in the push to plant the 2019 crop. These patterns may persist into 2020 as well. The variability in plant growth will lead to variability in plant yield and can potentially increase sampling error.
  • No-till fields suffered more from traffic patterns than tilled fields, but many of the crop planted in tilled fields have more gradual plant growth variations due to restricted roots resulting from shallow tillage compaction and have been more vulnerable to short dry spells this summer.
  • Fall tillage and fertilization after the 2019 harvest will have to be done carefully to keep from making the impact of tillage and traffic compaction worse if soils are wet.

Summer weather

  • Regions with near normal rain fall will see near normal soil test values.
  • There have has been a isolated region in southeastern Indiana and west central Ohio that experienced a lack of rain fall for over three months in the later half of the growing season. While dry enough to negatively impact crop yields, it was dry enough to have a large impact on soil fertility levels. However, like the other impacts of 2019 it may be enough to bias soil pH and potassium soil test levels slightly lower. This may also reduce crop nutrient uptake with actual yield being lower than predicted.

The interpretation of the 2019 data will lead to valuable knowledge as part of an overall soil fertility management program. Every growing season has factors that can potently influence soil fertility results, however 2019 will have a greater potential to influence you soil test results than most. The key to success will be field notes about crop growth, yield, and management of the field as part of a total fertility management plan. Field notes with a bit of soil fertility knowledge will be very valuable in the coming years. the higher seasonal variability has led many wise fertility managers to sample more frequently, commonly once per crop rotation, to be able to identify and address variability soil test data more effectively.

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