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Choosing a sampling scheme: How to decide

Dustin Sawyer for Progressive Forage Published on 01 October 2018
Soil testing

Are you soil sampling regularly? If so, are you getting the most you can for your money? It sounds easy, right? Just stick a shovel into the field and grab some dirt, send it to the lab, get results and move on.

This approach will get a lab result, but is it really the best way to spend your dollar? There are a few different paths that can be taken to make soil testing work for you and your plan for the land. This short article is aimed at helping you to sort through these different paths.

Potassium management

Soil testing is typically thought of in the context of the environmental management of nitrogen or phosphorus. In a forage production system, however, soil testing is of importance for the proper management of potassium (K) specifically. Because forage harvest removes the whole plant, the potassium that is primarily tied up in stalk tissue is not returned to the soil.

The result is a forage system that removes anywhere from three to five times the soil potassium as a similar grain operation. Proper soil testing can help keep an eye on that potassium reduction and stop it from becoming a limiting factor. Depending upon the type of operation, there are a few different soil-sampling schemes that can be employed to help keep tabs on soil K.

Sampling frequency

As with other seemingly easy processes, protocols have been developed for soil sampling that removes the fun. The good news, though, is that following these protocols will help to ensure that your soil-testing dollars aren’t wasted. Regardless of the soil-sampling scheme you choose, the most important part is to collect a soil sample that represents the field.

The sample should consist of seven to 10 individual cores to a depth of about 8 inches. Any unique or unusual areas should be sampled separately. Stay out of headlands, dead furrows or any other obviously bad spots. Follow these rules, and you’ll have accurate results that represent the field.

The most important thing to remember about soil sampling is that one sample should be collected for every 5 acres of land. This is important because 5 acres is the minimum requirement for nutrient management planning under the NRCS 590 standard.

Even if you are not engaged in nutrient management planning now, having current soil tests that conform to the standard will ensure that you don’t need to resample a field if the 590 standard becomes relevant later.

A second reason for pulling one sample every 5 acres is that it’s simply good practice. Even a flat field with a consistent cropping history can have variability.

Old fencelines, previous uneven manure applications, drainage and compaction can all impact the nutrient content. If one sample is collected for a 40-acre field, for example, that variability will be missed. The result is a poorly spent soil-testing dollar and an even more poorly spent fertilizer dollar.

Conventional versus grid sampling

Sample collection can fall under one of two sampling schemes: Conventional sampling and GPS grid sampling. The names imply the differences. GPS grid sampling uses GPS technology to pinpoint the spot from which a sample was collected while conventional sampling does not.

GPS grid sampling can be further refined by the intensity of sampling. Common intensities are 5-acre and 2.5-acre grids. These mean that one sample is pulled for every 5 or 2.5 acres, respectively. While those are the most common, some people will choose to go as intense as 1-acre grids.

Conventional soil sampling is a great way to track soil nutrients at the field level. It’s generally less expensive than grid sampling, though the cost differences have narrowed over the past several years.

Conventional sampling will usually average the soil test values for all samples within a field and provide one rate of fertilizer application for the entire field. This is a great option for fields less than 10 acres or fields that have little variability.

Grid sampling is the next logical step for the conventional sampler that really wants to take control of their operation. It provides clearer detail and more information than conventional sampling. Because each sample is georeferenced, maps can easily be made that provide a visual depiction of the nutrient levels within the field.

As a component of grid sampling, the sample collector will typically delineate the field boundary on an ATV. This will provide a ground-truthed shapefile of the field, another useful item for when the 590 standard becomes pertinent.

Variable-rate technology (VRT) is the most common use for grid samples. This is technology that will vary the rate of fertilizer as it is applied to the field. It further refines the precision of the fertilizer dollar. While VRT applications will not reduce the cost of fertilizer application, it helps to shift the focus toward maximizing profit by placing fertilizer closer to where it’s needed.

The specific sampling scheme chosen for a field depends on how the data are going to be used. While the results of a soil test are important for the immediate application of the correct amount of fertilizer, there is also great value in the long term. Soil test results provide a record of nutrient status in a field.

Looking to the future is the key to putting soil analysis to work for you. When following recommended protocol, you will only get one data point every four years.

I heard something the other day that put things into perspective for me: A lucky farmer will get only 40 crops in a lifetime. This made me realize that a lucky farmer will only get 10 soil tests in a lifetime. Make them count. Plan for how the field will be operated four, eight, 12 years from now. Your future self will be happy you did.  end mark

PHOTO: Soil probes have become more user-friendly and reliable, but producers need to know how to get the most return for the dollars spent on soil tests. Photo courtesy of Rock River Laboratory Inc.

Dustin Sawyer
  • Dustin Sawyer

  • Laboratory Director
  • Rock River Laboratory Inc.
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