Editor’s note: The following was written by Meaghan Anderson, Iowa State University Extension field agronomist in Central Iowa, and Rebecca Vittetoe, field agronomist in East Central Iowa, for the Extension Integrated Crop Management News blog.
Fall is a great time to take soil samples. Soil testing is the only way to determine soil nutrient levels, and the results of these tests can help make critical management decisions for the next growing season.
This article will discuss when to soil sample, different soil sampling methods, how to sample, what to test for and where to send soil samples for analysis.
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.
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 two to four years or once in a crop rotation. Fields should be in the same crop each time when sampled to help reduce variability of test results.
Method to use
Soil samples can be taken in the following manner:
- Whole field sampling: Collect a small number of samples to represent the whole field. Not recommended for crop fields because you are not able to capture field variability.
- Grid sampling: Field broken into square grids (1.1 acre to 10 acre grids) and a sample represents a grid.
- Management zone sampling: Field separated into management zones for 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, it is ideal that a sample should not represent more than approximately 10 acres, unless available information suggests larger field 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.
Cores should be taken from a combination of locations — in the row, between the row, mid-way between the row — 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.
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, around old feedlots it is not uncommon to see high phosphorus and potassium levels. Why? When manure was hauled it wasn't commonly hauled to the corners of the field, but the area closest to the feedlot.
Technology has advanced to 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 and may help explain differences found in some soil test results. Find the tool at https://ortho.gis.iastate.edu/.
How to collect
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 a plastic bucket, especially if you are testing for zinc (Zn)
- Field map or GPS coordinates of soil sample areas
Having a general plan of attack prior to heading out to the field is helpful; 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 phosphorus and potassium fertilizer recommendations are based on a 6-inch sampling depth, pulling cores consistently at a 6 inch depth is important for calibration.
Liming decisions are also made from these samples, but application rates should be adjusted for expected depth of incorporation.
Helpful tip: Draw a line at 6 inches on your soil probe to help you stay consistent with the depth when pulling cores.
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 one.
What to test for
So what exactly do you need to test your soils for? We want to be able to use the soil test results to make fertilizer decisions — particularly phosphorus (P) and potassium (K), and lime (pH and buffer pH). Additional tests can be completed but may add to the cost.