Soil sample pointers

Every year I get questions that focus on fertility. My first response is to ask, “Have you pulled a soil sample?” The typical response I get is, “I think so.”

Just what role should soil testing play in your farming operation?

Soil sampling is not a perfect science. However, it can be a valuable component of your agronomy programs by following a few basic principles.

The standard soil sampling depth is six to seven inches. This depth represents approximately two million pounds of soil—a convenient conversion number. I recommend that you stay close to this depth. Equations that soil testing labs use were developed with that standard depth.

You need a strategy for sampling fields. The general rule of thumb is a “composite” soil sample is not to be greater than 20 acres.

For example: an 80-acre field needs four separate soil samples. Try to divide sampling areas from fields greater than 20 acres into a management zone. These zones can be based upon soil type, history, yield potential, field geography, field use or other logical divisions.

Each composite sample should have 12 to 25 soil cores. This allows you to collect a representative soil sample from that area. Fewer than 12 cores per sample can lead to poor representation if one of the cores is extremely high or low in a given nutrient. If you have banded any fertilizer applications, you must have at least 20 cores per sample.

Another question I often get is, “How often should I resample?”

I suggest every two to four years, depending upon the crop grown, fertilizer utilized, environmental conditions and other agronomic components. Try to resample fields the same time of year, and preferably after the same cropping sequence.

Precision agriculture programs usually sample in a tighter acreage pattern. Some programs use standardized grid samples while others use management zones. Most grid sampling is done on 2.5-acre grids. MFA’s precision program often offers a combination of composite, grid and management zone sampling. This provides producers with the ability to apply the right amount of nutrients to the right acre, and it gives growers a great range of agronomic and economic options. Contact your local MFA to see how MFA’s precision program can help you and your farm increase profits.

Regardless of the collection process you choose, basic soil sampling procedures should be followed. Consistency is the key to an effective soil-sampling program.

Even if you follow these simple guidelines there may be anomalies associated with soil testing. Don’t get frustrated if some of your soil tests come back different than expected or are inconsistent across space or time.

Soil is a complex combination of physical, mineral, biological and organic residue systems working together. Some unexpected measurements will come along.

Soil sampling is just one component of crop nutrient management. On page 10 of this magazine, MFA agronomist Steve Cromley discusses nutrient recommendation programs.

Dr. Jason Weirich is director of agronomy for MFA Incorporated.   See related story on interpreting soil samples HERE.