What needs to be considered when soil sampling in wet/dry soils?

By Kevin Otte, CCA, Otte AG, LLC and Luke Baker, Brookside Labs

Soil sampling and the subsequent soil analysis have been performed for nearly a century. Sampling the soil is a time-honored service offered by retailers and independent agronomists, with many public and/or private laboratories providing analytical services. Collecting soil samples is as much a science as it is an art. In today’s technological environment, site-specific sample points/grids/zones can be created and utilized by the person in the field collecting the sample. That’s the science part. The art is how the sample is collected. It is more than just pushing a probe into the ground and filling a bag with soil.

How deep is the soil core? Was the core looked over before putting it into the bag? Was the sample collected manually or automatically? There are different ways to collect a good soil sample that will represent the area that is being sampled and could go down a rabbit hole with the pros/cons of each. The goal of soil sampling should be to provide valuable information and profitability potential to the end user, the farmer.

The “trick of the trade” is interpreting the data provided by laboratory analytical results. The adage “garbage in, garbage out” holds for soil sampling and the subsequent analytical data. It’s similar to yield data. If you don’t have a calibrated yield monitor and the setup is incorrect, the data returned may not indicate the field. Thus, the science and art of soil sampling are vital to getting valuable and trusted data that will guide decisions for soil and crop fertility management. That leads to the interpretation and understanding of the lab results. If a set of samples are collected during a wet spell while another set of samples from the same field is sampled during a dry spell, the results could be different in comparison to each other. Understanding that difference is critical in making those soil and crop fertility recommendations.

Let’s focus on a couple lab analytes that fluctuate between wet and dry soils. The first is potassium. Potassium is a nutrient in the soil solution to be available to the crop. When samples are collected in saturated soils, the potassium values may likely be low on the soil report. Vice versa, when the soils are dry, the potassium values may likely be elevated. As water evaporates, it deposits salts in the root zone leading to higher results, as potassium is a salt. Water moves potassium to the roots via mass flow and without adequate soil moisture, roots can’t access potassium.

During the evaporation process in very dry spells, potassium isn’t the only nutrient being deposited. Calcium and magnesium are also deposited and the plant will take up whatever nutrient is present in whatever ratio it is present in. Thus, you can have reduced potassium uptake if your calcium and magnesium become too high.

Another analyte is pH. When a soil sample is collected during a lengthy dry spell, the pH reported likely will be low due to the accumulation of salts on the soil surface. Salt accumulates when water evaporates at the soil surface, and surface moisture evaporates rapidly during dry spells. Remember that the pH reported on a soil report reflects the day the sample was collected. When the sample is analyzed at the lab and water is added the additional salts remove hydrogen or aluminum on the soil colloid that would typically not be present in the pH measurement. When a sample is collected from saturated soils, these salts are leached lower and the water pH is higher because the salts are not present to react with the soil colloid. Thus, soil conditions and weather patterns are essential when considering pH adjustments.

Collecting soil samples during the same season each time a field is sampled is crucial. Be consistent with the sampling timing, whether in the spring prior to field prep or in late spring/early summer as in season or in the fall following harvest. This may help minimize discrepancies in analytical results compared to years past. Also, shortening the sampling frequency can help mitigate utilizing a sample result for recommendations that may have been affected by wet and/or dry soils at the time of sample collection. The takeaway: be prudent in interpreting the data and understand why and how specific analytes may be affected by the soil conditions at the time of sampling.