Soil Sampling During a Dry Season
There have been many articles circulating with cautions about dry season soil sampling. Conversations about best practices for collecting soil sampling are important and drought conditions provide a great opportunity to have these discussions. Below are some points for consideration.
- First, evaluate if quality soil cores, to the correct depth, can be collected. The recommended depth of soil cores varies by state or region, and the goal is to stay consistent with Land Grant University guidelines.
- Drought conditions and associated dry soils do not always create issues for nutrient analyses.
- Potassium and soil pH are the indicators of greatest concern under drought conditions.
- Potassium is of greatest concern in 2:1 clays which include smectites, montmorillonite, vermiculite, illite and chlorite. These are clays with a high shrink-swell potential that have large cracks in the soil during drought conditions. The structure 2:1 clays can be described as a deck of cards with cations, like potassium, between the cards. Under normal conditions soil water expands the deck of cards allowing the potassium to be freely extracted in the soil lab. When persistent drought occurs, the 2:1 clays shrink like a new deck of cards, trapping, or fixing, the potassium between the cards and POTENTIALLY making it difficult to extract at the soil laboratory.
- Soil pH can be lower or even higher during prolonged drought conditions. Most of the literature addresses increased salts during drought resulting in lower soil pH (1). It is also not uncommon for carbonates that typically get flushed through the soil profile with precipitation to be pulled towards the soil surface as a result of capillary rise due to transpiration. When this occurs, it is possible to see an increase in reported soil pH.
- Timing of sampling following harvest needs to be considered.
- Fall is typically considered to begin as soon as the crop is harvested which could be August or even December. Because of this, it is difficult to categorize fall soil sampling as a good or bad practice.
- Early in the fall there is a considerable amount of potassium in the crop residue immediately following harvest. It will take some time and/or rainfall to flush the potassium from the residue and into the soil where it can be detected in soil samples. A rule of thumb is to wait either two weeks or after ¾” rain has occurred following harvest before collecting soil samples. These guidelines are likely less stringent as the fall progresses and the plants have fully desiccated prior to harvest.
- Concern about fire hazards with soil sampling equipment driving across dry fields, especially when combined with windy conditions are valid and may result in a delay of soil sampling until field conditions improve.
Summary: How can you evaluate if environmental conditions will impact soil test results during drought conditions?
- Determine if quality soil cores can be collected.
- Identify a few soil sample sites with a history of GPS referenced soil tests and collect a limited number of samples to compare current results with prior results.
- If the newly collected soil test results reasonably match the prior results and management practices, then the current environmental conditions are likely not an issue for doing more soil sampling.
REFERENCES
About the author: John Grandin is an agronomist with over 40 years of experience in the field. He is currently the Director of Agronomy at Trace Genomics.
