Handful of soil
Updated: February 17, 2021
By Patricia Hoopes

A soil test analysis is one of the most important parts of the Nutrient Management Plan. Typical farm management leads to certain expectations from soil testing. Occasionally folks come into the office with questionable results. Perhaps you have received a soil analysis that shows a field to be high in phosphorus and are wondering if it is accurate. If the soil tests differently than expected, we are justified in questioning these results. What should be done?

First, evaluate sampling procedures:

  • Was the management unit a unit with similar soil types, past cropping histories, and the same history of manure or fertilization applications?
  • Were 15 to 20 plugs taken and thoroughly mixed to generate one sample?
  • Was a clean plastic bucket used to collect the samples, avoiding contamination and chemical reactions?
  • Were all samples taken to a depth of 8 inches?
  • Were samples taken randomly throughout the management area, avoiding unusual areas like windbreaks, fence lines, wet areas, or areas near lime rock roads?
  • When were the samples taken? Were there any applications to the field prior to sampling that could affect analyses results? Sampling after manure or fertilizer application, especially in no-till or pasture situations, will make soil analyses highly variable and usually result in highly inflated soil test numbers. The rule of thumb is to wait 6 months before soil sampling.

Once soil sampling procedures are found to be accurate, then it is time to question the lab. If you believe inaccurate results are due to lab error, immediately contact the lab. In general, soil labs keep soil samples for a limited time due to space restrictions. Most labs will retest the sample upon request. Some will charge additional fees, especially if results are similar to the previous one. Ask your lab about their policy.

If you are still not satisfied, resample.

An accurate soil sample analysis is essential to ensure your Nutrient Management Plan provides the best fertilizer recommendations and the best chance for the highest yields.

References: “Soil Sampling Procedures for Nutrient Management” and “Nutrient Manager: Focus on Soil Testing and Nutrient Recommendations.”