Soil Sampling

Now that most crops are off it’s a good time to review the basics of soil sampling before heading out into the fields this fall and winter. Monitoring soil pH and fertility is the foundation of productive fields and pastures. Soil needs to be sampled regularly and with the proper technique in order to build a successful fertility program on every acre.

Soil Sample Timing

Soil sampling generally occurs every 3-4 years on each field. In the “green-gold” rotation common on many dairy farms (3-4 years haylage, 3-4 years corn/corn silage) the soil is usually sampled in the fall before turning over the haylage field to first year corn. Similarly, farms growing corn, soybeans, and a small grain rotation will often sample after the small grain is harvested before rotating back to corn. In short rotations of corn and soybeans or continuous corn it is not uncommon for soil sampling to occur every 2-3 years. Any new ground acquired by a farm should be sampled as soon as possible. Sometimes crops show possible visual signs of nutrient deficiency, Figure 1. In these cases a soil and a tissue sample should be sent to lab to confirm a deficiency.

Figure 1: 

Corn with N and P deficiency from high rainfall, cool temperatures

Soybeans with K deficiency and downy mildew

 

Alfalfa with K deficiency from early season flooding

Soil Sample Analyses

It is important to use the same commercial lab for testing over time because analysis techniques are slightly different between labs.  In most situations testing for potassium, phosphorous, pH, and organic matter are adequate for planning fertility programs. Nitrogen is not stable in the soil and is lost over the fall and winter months. Testing in the spring prior to nitrogen application, or in a year like 2013 where we had high rainfall after the nitrogen fertilizer was put on, is more appropriate. Sometimes soils are analyzed for calcium and magnesium for specialty crops or those trying to “balance” cation exchange ratios. Regular liming supplies enough calcium and using dolomitic lime will supply magnesium. Additionally many of soils regularly contribute calcium, potassium, and magnesium to crops. While some people have spent their entire careers trying to prove there is a response to a specific Ca:Mg:K ratio in the soil, the consensus across the nation is that there is no response to managing these ratios in agricultural soils. Sulfur is similar to nitrogen and isn’t tested for in soil analyses in most cases. With the cleaner air in recent years most fields now respond to 15-25 lb/A of sulfur per year. Micro-nutrient analysis may be beneficial on sandy or muck soils, along with soils having low organic matter  and extreme pH levels that have not received manure in recent years.

Soil Sampling Methods

Soil sampling can be done on an entire field, in targeted management zones, or on a grid. For fields 20 acres or less the entire field is often the sampling area. Using a soil probe, Figure 2, sample two dozen spots throughout the field in a zig-zag pattern, Figure 3, at 6-8 inches deep and place into a plastic bucket for mixing. Metal buckets will add some micro-nutrients to the soil samples and should not be used. Thoroughly mix all of the samples and then take a subsample (usually about 1-2 cups worth) to send to a laboratory. Be sure to avoid sampling areas of the field that have had manure or lime piles recently, are known to be compacted (the headlands), or are low lying areas wetter than the rest of the field.

Figure 2: Soil Probe

Figure 3: Soil Sampling Pattern

Soil samples also can target management zones within the field. Often these areas will follow differences in soil type or areas mapped by yield monitors. For farmers pursuing variable rate fertilizer application and seeding rates it is best to start with three types of management zones: high, medium, and low yielding. Sample these areas in the same way as described above by taking 2-3 samples per acre in each management zone. A number of farms have done grid sampling on 1-3 acre squares in western New York. This is the method that was widely used initially in the Midwest for precision agriculture management. While a number of farms still take the mountain of soil samples necessary to do this, many have moved toward sampling management zones to save on costs while still having the capability for variable rate management.

Bottom Line:
1. Soil sampling needs to occur on a regular basis (every 3-5 years) in order to apply the proper amount of nutrients required by each crop and maintain a proper soil pH.

2. Standard soil testing measures ph & mineral nutrients (K, P, Ca, Mg, Mn, B, Zn, etc.). Testing soils for nitrogen and sulfur require a different type of test and more careful sampling.