How you can improve the accuracy of your fall soil test
Written on 09/13/2021
Soil testing is the foundation of a sound agronomic crop plan. To maximize the genetic potential of the crop and hit that target yield goal, we need to understand what the plant’s nutritional needs are – and that starts with knowing what is in the soil.
In an ideal world, every field would be soil tested right before the seed goes into the ground each spring. But – logistically, this isn’t always possible. Often the tight spring window leaves too little time to develop custom fertility plans for every field and not enough lead time to order fertilizer. In these circumstances, fall soil testing – if done correctly - is a viable option that can still generate the soil analytics necessary to build an agronomically sound fertility plan for the upcoming spring crop.
A few considerations should be made to ensure that fall samples are representative of what the soil nutrient levels are when the seed goes into the ground in the spring:
- Soil temperature should be less than 8oC:
When surface soil temperature is high, microbial activity is high. An active microbiome has a significant impact on nutrient levels in the soil. Waiting until soil temperatures are lower than 8oC ensures microbial activity is low, reducing the chances of potential nutrient level changes over winter. If fall sampling is combined with a mild winter, fields should be re-sampled in the spring and fertility plans should be adjusted to account for any nutrient level changes throughout the winter.
- Consider your farm management practices:
Different farm management practices can have an impact on soil nutrient levels and should be considered when soil sampling. For example, application of herbicides, pesticides, or fungicides can impact soil nutrient status. If a fall burndown is in the plan, ensure there is enough buffer time between sampling and application (refer to herbicide label). Tilling can also impact soil nutrient levels so sampling should be avoided immediately after turning the soil.
- Importance of sampling collection and handling:
Whether you are sampling in the fall or spring, a soil sample is only as good as its collection. Ensure you are collecting a sufficient number of subsample cores (generally 10-20 – the more variable the soil, the more samples that should be collected). Areas that differ in soil type, appearance, past treatment, or productivity should be sampled separately, provided the area can be treated separately. If samples are not being tested that day, store samples in the freezer in an air-tight sample bag until the day the sample is being tested.
Using these considerations, you can take advantage of your fall soil analytics to generate a solid agronomic fertility plan for your spring crop.