The Importance of Proper Soil Sampling

Trying to grow crops without knowing the soil pH or fertility levels of each crop field is like driving a car with a broken speedometer: Sooner or later you’re going to get in trouble.

Trying to grow crops without knowing the soil pH or fertility levels of each crop field is like driving a car with a broken speedometer: Sooner or later you’re going to get in trouble. Most farmers realize this and rely on soil analysis, with the sampling done either by someone on the farm or by its crop consulting firm.

Crop consultants are more involved in soil sampling these days, especially since one of the requirements of the nutrient management plan that’s part of concentrated animal feeding operation (CAFO) law is that all crop fields must be sampled at least once every three years. (Check with your consultant or state authorities on the required frequency of sampling.) Most agronomists recommend sampling cropland every two or three years. Annual sampling probably won’t provide much more useful information so it isn’t needed unless something dramatic is happening in a field – for instance, a nutrient-related problem or unusually heavy manure application.

Crop consultants are aware of the importance of proper sampling. However, too often a farmer assigns the job of soil sampling to an employee. Sometimes the “low man on the totem pole.” This can be a problem, especially if the employee isn’t informed on proper sampling technique.

One in a million?

Most soil test labs request a pint of soil for each field sampled. This is about 1 pound of soil. The soil in a 6- to 7-inch plow layer weighs about 2 million pounds for each acre. Therefore, in a 10-acre field a properly taken soil sample is an attempt for the 1 pound of soil submitted to the soil test lab to accurately represent the soil fertility in 20 million pounds of soil!

That’s why it’s recommended that a soil sample shouldn’t represent more than 20 acres of cropland. If you have fields larger than 20 acres, divide the field in some logical way – perhaps by soil type if the field contains at least two soil types. In doing so, think about what you’d do if the two (or more) samples turn out to be considerably different in pH or fertility. Differences in pH can be managed rather easily by instructing the driver of the lime truck as to what portion of the field he should spread and at what rate. If the differences are in soil fertility you may need to selectively broadcast fertilizer – even if you normally rely on planter-applied fertilizer to do the job.

Soil sampling is serious stuff

If someone on the farm does the soil sampling, consider the following: First, the person must be reliable. When he or she is out “in the back forty” taking samples, you won’t know if the person is taking the recommended 15 to 20 cores (subsamples) per field of 20 acres or less, or simply walking into the field and taking it all from one or two spots. The person should be told how to properly take samples, and that this is a job that could have a significant impact on fertilizer rates, crop yield and quality. During the many years I was agronomist at Miner Institute, soil sampling was done by our field crops supervisor or an experienced, trusted employee. After the farm became a CAFO we employed a crop consulting firm that would do the sampling if asked, but this was one job we continued to do ourselves.

Getting the job done right

Assuming you’ve decided to do the soil sampling yourself or have a trusted farm employee do it, following are some guidelines: Sample when row crops such as corn are not growing in the field. My preference is to sample in the late summer or fall, but if you always sample at the same time of the year, then spring is okay. Changes such as nutrient availability, particularly potassium, are seasonal in nature. So stick with your choice of soil sample fields:

  • Depending on how many fields you have on the farm you may decide on sampling one-third of the fields each year. If CAFO regulations require sampling at least every three years this will meet requirements while not resulting in too much sampling effort each year. Try to sample fields to be seeded to alfalfa (or any crop requiring a high soil pH) at least 6 months ahead of seeding, especially if there’s a chance that a high rate of agricultural limestone will be needed.
  • Avoid sampling fields where lime or manure was recently applied. Brush aside any crop residue and other debris from the soil surface. Using a soil test auger or soil tube (can be purchased from Nasco and other farm supply companies) takes cores to a depth of 6 to 8 inches. If there’s an obvious change in soil type at a slightly lesser depth you might want to sample only to this depth. For long-term no-till or minimum till fields it’s also a good idea to take a separate soil sample from the top 2 to 4 inches, primarily to check on soil pH. Repeated applications of nitrogen fertilizer can acidify the top couple inches of soil – sometimes called an “acid roof.”
  • Put the individual cores in a plastic pail, and when you have the necessary 15 to 20 cores, break them up and thoroughly mix the now-granulated soil. If the soil has a high clay content and is too wet to mix properly wait for it to dry but don’t heat the sample. Remove anything that’s not soil and submit a pint of soil (or whatever amount the soil test lab requires). Be accurate when filling out the soil sample information sheet, and fill it out completely: The more information you provide to the soil test lab, the more accurate the fertilizer recommendations. Also, it’s important to choose a reputable analytical lab and use it every year. This avoids the confusion resulting from the various soil extractants used by different soil test labs.

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