Soil Sampling to Save Money

by Michael A. Debes, Resource Conservationist,
Natural Resources Conservation Service
Dodge City, Kansas

With current fertilizer prices in excess of $450 per ton, no one needs to remind a farmer that fertilizer is a significant expense. The question to ask is, Are you getting the most benefit from your fertilizer dollars?

Would you make a large investment without doing some sound research? Not many of us would. You would want to know potential returns and risks. Can you earn the same returns with less risk? Many producers are risking more than $20,000 to fertilize a section of dry land wheat without knowing how much they REALLY NEED!!

Using sound soil sampling techniques and knowing how to read the lab analysis is the only way to really know if you are getting the most benefit from your fertilizer dollars. There is nothing magical about soil sampling. It has been described as being as much an art as a science. I feel it is a combination of science and common sense. It can be somewhat complicated, but not when you think the process through.

When should soil samples be pulled? A good time is after the last crop harvest, before any tillage has occurred, and soon enough before planting the next crop to make sound fertility decisions.

What depth should the samples be? The research and science for this has been completed for many years. You actually should pull two separate samples--the first from 0 to 6 inches and the second from 6 to 24 inches at each sample site. The surface sample interpretations are commonly used for pH, phosphorous, and organic matter. The subsurface sample is needed for the nitrogen profile only. Nitrogen is a mobile nutrient, which is the reason for the deeper sample. As you read and analyze more lab reports, you will immediately see the reason for the 24-inch sample depth. There are many “tricks of the trade” that make pulling these samples easier than using a sharpshooter shovel! Any individual can pull soil samples. Visit with a county agent, Natural Resource Conservation Service (NRCS) personnel, or a crop consultant for these options.

How many soil cores need to be pulled? The recommended number is to pull 10 to 20 cores per sample. These cores are thoroughly mixed in a clean plastic container, kept cool and dry, and sent to the lab immediately for analysis. Let the lab know that you need a nitrogen test for the subsurface sample only. Average prices for the lab work vary from $22 to $27 for the complete test.

How many samples should be pulled? There are three common sampling techniques: whole field, management unit, and grid sampling. Whole field is the cheapest and quickest and gives the least information. Grid sampling is the most expensive and gives the most information. Management unit sampling is the middle of the road approach. Management unit sampling is simply looking over your field and dividing it into units based on categories that would give different soil fertility results. These can be soil types, slope changes, and past management history; for example, silt loam versus sandy loam soils; 0 to 3 percent versus 3 to 5 percent slopes; and part of the field had manure applied two years ago, part of the field was broken from pasture 20 years ago versus the rest has been farmed for over 100 years.

The NRCS has recently modified its nutrient management standards to include a modified grid sampling requirement. This method divides the field into subfields or areas no larger than 40 acres based on the above-mentioned variability. Samples should be pulled for at least two years based on these subfields to determine for sure if there is any variability. If the two-year samples show no significant variability, then the sample area can be enlarged based on the producer’s knowledge of the field. Anyone enrolling in a 2008 or subsequent Environmental Quality Incentives Program (EQIP) contract that includes nutrient management or waste utilization will have to comply with this 40-acre subfield sampling requirement.

There are many crop consultants that pull and analyze the results. Many county extension agents and NRCS employees are very knowledgeable in reading these results also. Again, pulling good samples is as much an art as a science.

Contact your local NRCS office, county agent, or local crop consultant for assistance in developing a sound nutrient management program.

For more information about NRCS programs, visit the Kansas NRCS Web site at www.ks.nrcs.usda.gov.

This article is also available in Microsoft Word format.

Soil Sampling to Save Money (DOC; 52 KB)

< Back to Conservation Editions - Fiscal Year 2008 Index

Last Modified: 08/05/2008