Fall is a great time to take soil samples for testing. Soil testing is the only way to determine soil nutrient levels and test results can help make critical management decisions for the next growing season. Soil testing starts with the soil sample. It is critical to take a collection of representative cores for each soil sample. The small sample of soil collected serves as the basis for the lab analysis and ultimately will be used to interpret the soil test results and make recommendations.
Even if you hire someone to soil sample your fields, it is still important to understand the basics behind the soil sampling procedure. Knowing this process can help you know what questions to ask or what to look for when hiring someone to take your soil samples. A handy resource to help with soil sampling is CROP3108: Take a Good Soil Sample to Help Make Good Decisions.
When to soil sample:
Soil sampling is most often either done in the fall after harvest or in the spring prior to field work but should always be done prior to applying fertilizer. It is important to be consistent in the timing of soil sampling (i.e. always in the fall or always in the spring). Samples should be taken every 2 to 4 years or once in a crop rotation. It may be advantageous to sample more often when fertilizer prices are high or when trying to raise soil test levels substantially. Fields should be in the same crop each time when sampled to help reduce variability of test results.
Be cautious of sampling and making test interpretations after prolonged dry conditions. Soil sampling in extremely dry soil can result in test results that appear lower than they actually are. Read Antonio Mallarino’s article “Beware of Dry Conditions When Soil Sampling and Interpreting Test Results” for more information. Ideally, delay soil sampling for about a week after rainfall that soaks through the sampling depth (6 inch depth).
What soil sampling method should I use?
Land managers have several options for soil sampling methods:
- Whole field sampling- Collect a small number of samples to represent the whole field. Not recommended for crop fields due to field variability.
- Grid sampling - Field broken into square grids (1.1 acre to 10-acre grids) and each sample represents a grid.
- Management zone sampling - Soil sampling based on a combination of prior experience, field history, soil survey maps, yield maps, topography, or other management or historic information.
Whatever method you decide to use, each sample should be a composite of a minimum of 10 to 15 cores. Additionally, a sample should not represent more than approximately 10 acres, unless available information suggests larger areas have little variation, such as in soil type or yield potential. Multiple sampling areas per field help to determine if a uniform application rate or a variable or site-specific application rate for fertilizer, manure, or lime is more suitable.
Pull cores from a combination of locations--in the row, between the row, mid-way between the row and not just in one small radius within the sampling area. This is especially important if fertilizer or manure was banded. Samples should represent the sample area/zone as completely as possible.
Importance of Field History
Knowledge of the field history is important in choosing soil sample areas and understanding soil test results. Soil has a long memory, and past management can greatly influence what we see today. For instance, it is not uncommon to see high phosphorus and potassium levels around old feedlots. Why? Manure was typically hauled to the area closest to the feedlot.
Technological advances allow farmers to better understand field history via the internet. Google Earth and other internet mapping services allow users to look at satellite imagery back several decades, but the Iowa State University Iowa Geographic Map Server allows users to search for historical aerial imagery as far back as the 1930s. This can allow incredible insight into prior uses of current farm fields.
How to collect soil samples:
Make sure that you have the needed materials:
- Plastic-lined soil sample bags (enough for how many samples will be taken)
- Soil probe
- Bucket, preferably made of plastic, especially if you are testing for zinc (Zn)
- Field map or GPS coordinates of soil sample areas
Have a general plan of attack prior to heading out to the field; this plan would include a map of areas to sample, an idea of how many samples to take, and a plan for how to collect the samples. Within each sample area, pull 10 to 15 cores to make a composite sample. Since ISU Extension and Outreach phosphorus and potassium fertilizer recommendations are based on a six-inch sampling depth, pulling cores consistently at a 6-inch depth is important for calibration. Liming decisions are also made from these samples but adjust application rates for expected depth of incorporation.
As you sample, if a core pulls differently or does not look like the other cores in the area, discard that core and pull a different done. The photo below shows two cores that were taken from the same sampling area.
What should I test my soils for?
We want to be able to use the soil test results to make fertilizer decisions - particularly phosphorus (P), potassium (K), and lime (pH and buffer pH). The organic matter (OM) can be an indicator for some nutrients such as sulfur –if a soil has more than 3% organic matter, sulfur deficiency issues are generally not an issue. Additional tests can be completed but may add to the cost.
What lab should samples be submitted to?
Typically if you hire someone to soil sample your field(s), they will send it to a lab they work closely with. It is very important to make sure that the soil samples are being sent to a certified lab. You can check and see if the lab is certified by the state of Iowa by checking to the Iowa Department of Agriculture and Land Stewardship Commercial Feed & Fertilizer Bureau webpage (https://iowaagriculture.gov/commercial-feed-and-fertilizer-bureau/commercial-fertilizer) and select Certified Soil Testing Laboratories on the right-hand side. A certified lab will always report results in ppm (parts per million) and not pounds per acre! Recently, some labs have started to use moist testing procedures for potassium, which improves assessment of crop-available potassium and prediction of yield response to fertilization; farmers, especially those farming poorly drained soils, will want to choose a lab that offers this moist analysis. Read more here.
How do I interpret the soil test results?
The Iowa State University Extension and Outreach document PM1688: A General Guide for Crop Nutrient and Limestone Recommendations in Iowa is a resource for interpreting soil test results for Iowa soils. This document provides nutrient application recommendations for phosphorus (P), potassium (K), zinc (Zn), and lime, based on the 6-inch soil sampling depth. The interpretations and recommendations are based on long-term and short-term field experiments across the state.
If you have any questions on soil sampling don't hesitate to contact your local extension field agronomist!
[AMJNot sure methods is the right word here