By Kyle Haselman

Harvest season is winding down, and now is a great time to get soil samples pulled for crops for the next couple of seasons. Given the current ag economy, knowing what you have in your soil bank is critical when making large input decisions about nitrogen, phosphorus, potassium, lime and micros. Most growers today are well ahead of the past in maintaining and following routine soil testing practices, but I want to review some of the biggest mistakes that can be made in the process and help those who are not yet sampling on a routine basis to get a plan in place. Consistency and accuracy of the sample are two of the most critical first steps in soil testing that are very often overlooked and not thought about.

When should I soil-sample?
Consistency of samplings or how frequently soils samples are taken is the first step in developing a good soil testing program. Samples should be representative of the field area and should be taken on a regular, routine basis — the same time each year it’s collected. I try to set growers up on a plan to be pulling soil tests at least six months prior to the time fertilizer will be applied. If you spring-apply the bulk of your nutrients, then fall sampling works well. The same applies if you are applying fertilizer in the fall— target spring or early in the growing season to pull samples. There’s not really a right or wrong time to pull samples; the key is to always take them in the same time frame of the year, as levels will fluctuate throughout the year and with varying soil moisture levels. I want to be able to look back at soil test levels over time and develop trends to help make better recommendations; and if samples are not taken at the same time of the year, it makes the data less accurate. Junk data in is junk data out.

How often should I sample?
When setting up a soil testing plan, I try to get growers to think five or more years out on their rotation. I want to make sure we’re sampling in a similar crop rotation. Many growers are in a corn-soy rotation, and in those cases we try to sample every other year to build that trend line quickly. When wheat or cereal grains are still used, we may sample every third year, but try to never go beyond three years. Many intensive grid sampling programs try to go every four years. However, to build a trend line it takes at least 12 years before management recommendations can be based on that information. The point here is to develop a plan and stick with it, and don’t just sample whenever someone offers to do it for free or there is government payment to do it. Several university data studies have some very valid information showing that the adoption of a consistent, repeatable soil testing practice saves more money on inputs, minus the cost of the sampling, than applying strictly on crop removal rates or past fertilizer history.

Who should do the soil sampling?
Soil testing can be done in multiple ways and often is completed by third-party individuals or ag retail facilities. Many growers often struggle trying to complete the sampling task on their own because of lack of time and a plan to do it in a timely manner. There are numerous sampling software programs available today to help complete the task and make recommendations. However, like many software programs, if you don’t use them often enough, it becomes a struggle. Whoever ultimately does the sampling will be using some form of GIS software tool to map out sampling areas. Once again, the key is consistency and planning out the future, not just the here and now. Having the ability to easily look back at past sample results, and then also overlay other data like harvest maps or imagery, adds more validity to the final recommendations.

How important is the depth in a sample?
Accuracy-wise, with soil sampling it all starts in the field. By default, most soil testing labs will assume standard sample depth of 6²/₃ inches unless otherwise noted. At that depth, the average soil on a per-acre basis will be approximately 1 million pounds per acre. What this allows people to do is easily convert a concentration of parts per million into pounds per acre. Depending on the grower’s soils, crops or rotation, that sampling depth may need to be adjusted or changed — but it’s critical to know that when sampling. Nutrient stratification occurs in all soils, both no-till and full tillage, and that needs to be taken into consideration when pulling soil samples. Often, I see nutrient levels start to drop off below a 6-inch depth; so, if pulling deeper cores, the result will be a much lower ppm. A lower ppm number will translate into a recommendation of needing more fertilizer, so its key to know the depth of soil testing and depth of fertilizer placement to make accurate recommendations.

How many samples should be taken, and how close should they be?
Several studies have shown there need to be nine or more soil cores taken to get consistent results. Like everything that gets a calibration, the more soil cores, the more results are consistent. But, with more than 15 cores, the results don’t change much. Fewer than nine cores and results can be sporadic, as there are outliers in every field from past years of tillage, or just natural changes in soils. Soil cores should all be taken in the same representative soil area. They don’t all need to be taken right next to each other, but they need to be representative of the given soil type. Soil sampling by management zone or soil type can help to guide where to pull soil cores from. Soil sampling by grids may produce inaccurate results if zones fall in areas of different soil types. Sampling in smaller grids can help overcome this but will also cost more to perform. Planning out your sampling areas ahead of time is important.

No matter how you choose to do your sampling, just make sure it is done on a routine basis and done in the most accurate, consistent way for your operation.

Haselman, a CCA and independent crop agronomist with Haselman Ag Management, writes from Leipsic, Ohio. Contact him at [email protected] or 419-306-0220.