Make the most of a valuable investment

Soil core sample.
Photos courtesy of Department of Animal Science at Cornell and Dr. Quirine Ketterings, unless otherwise noted.

In times of uncertain markets and unpredictable prices for fertilizers, farmers are often driven to lower expenses while simultaneously laboring to maintain optimal crop production. A challenging situation, but a reasonable investment in a soil analysis can help meet that objective.

Antonio P. Mallarino, professor of soil fertility and nutrient management, agronomy department, Iowa State University, agrees. “The cost of soil sampling and testing, relative to the crop prices and cost of fertilizers and other inputs, has decreased significantly in recent years. Therefore, this is a great time to use soil testing to improve nutrient management and the profitability of crop production.”

A Cornell pH Test Kit available for growers.

Why test?

Along with pH levels, standard testing generally determines nutrient levels of calcium, magnesium, nitrogen, phosphorus and potassium. Organic analysis often includes microbial and organic matter, along with compost and humus. Testing can also identify some types of heavy metal soil contamination.

Saving the environment and money

Soil testing conserves energy and saves money by determining the correct amount of soil amendments needed.

According to Quirine M. Ketterings, associate professor, nutrient management spear program, Cornell University, “Managing nutrients promotes the high yield of high-quality crops while minimizing nutrient loss to the environment. Nutrients can be lost to surface and groundwater as well as to the air,” she says, citing phosphorus (P) and nitrogen (N) as the greatest environmental concerns. “Both are required for crop production, but if present in excess can end up in the air [for N] and/or on the surface of groundwater [N and P]. Not enough P and/or N [or other nutrients] will impact yield and quality as well, and that negatively impacts our environment as well.”

Between 2000 and 2003, Cornell conducted an on-farm research/extension program that involved farmers, extension educators and consultants throughout New York state. The program included testing the need for phosphorus fertilizer in corn-producing soils. Because soils testing in the optimum (manured) or above optimum in P range (in New York, 45 percent of its acreage), showed no need for extra P, several farmers switched to phosphorus-free fertilizers. This not only met the objective to avoid adding unnecessary nutrients to fields, but created considerable savings, both environmental and economical.

Ketterings says, “This on-farm project contributed to a 25 percent reduction in P fertilizer sales for on-farm use [comparing 2000-2002 with 2006-2007], showing the potential environmental and economic benefits of soil testing. The tendency is to over-apply if you don’t know how much you need, so soil testing is core in fine-tuning soil fertility and crop management.”

Soil sample testing at Cornell.

Emphasizing energy costs associated with fertilizer production and application, Mallarino adds, “Properly used soil testing and research-based interpretations should result in nutrient application rates that are profitable, and that do not result in excesses that may impair water quality.”

When to test

“Soil testing should be done on a regular, planned basis,” says Warren A. Dick of the Ohio Agricultural Research and Development Center, and professor of soil science, Ohio State University. His “planned basis” involves long-term commitment. Dick suggests, “It generally pays to have a plan for soil testing that rotates across all production fields over a period of several years.”

Soil sample testing at Cornell.

Ketterings advises at least every three years, or twice in a rotation, but adds, “Annual testing will result in more reliable records, as it allows us to build these trends over time much quicker than with testing every three years.” Soil testing documents important trends by monitoring increases, decreases and consistent nutrient levels.

Taking soil samples

“The soil sample obtained needs to be done correctly so it is representative of the field from which it is obtained. For example, taking a soil sample for no-till crop production is often different from that where farmers use more intensive tillage,” Dick stresses.

Generally, anywhere from five to 12 random soil subsamples, taken at specific depths, are required from a parcel of land. The final subsample mixture submitted provides a more reliable representation of the overall soil. Reliablility is key, as analysis results from this sample will determine soil amendment recommendations.

Soil contamination is also a consideration. Avoiding it is vital. Using clean sampling tools when taking samples, is one means of doing so.

Testing labs advise avoiding the following when sampling:

  • Waiting too close to planting time to test the soil, when pH adjustments are required.
  • Sampling newly fertilized soils.

For more details on this and other pertinent information, ask the lab for a list of instructions. Dick suggests, “There are many extension resources out there that give good advice on how to take a proper soil sample.”

Soil test limitations

“Soil testing has evolved over many years, but it is not perfect,” Dick says. “Nitrogen, the most important fertilizer nutrient applied to crops, can generally be found in abundance in soil. The problem is that it is unavailable to crops and is only slowly released to the soil and plant over a growing season.” According to Dick, labs are limited in their ability to predict how much nitrogen will be released during a growing season. He cites weather, crop diseases, quality of soil organic matter that releases nitrogen and crop demand as contributing factors to the difficulty of accurately modeling and predicting the nitrogen release. Therefore, he says crop demands can alter the effects of a perfect nitrogen release prediction:

  • If crop demand is greater than expected, we undersupply fertilizer and suffer crop yield losses that could have been obtained.
  • If something causes the demand by the crop to be less than expected, we oversupply fertilizer and have a potential pollution problem.

Even with its limitations, Dick maintains, “I also believe that soil testing has improved our ability to better manage organic manures and nontraditional soil amendments derived from industrial municipal activities.”

Mallarino also suggests soil testing is not a perfect diagnostic tool. “But it is very useful for soil fertility management. This is especially the case when the soil-test method is properly calibrated by field research, and research-based interpretations and recommendations are used,” he stresses.

Soil testing and farming technologies

Mallarino cites another positive, this in regard to soil testing’s connection to new farming technologies. “Soil testing is a site-specific diagnostic tool that adapts very well to new precision agricultural technologies,” he says. “Usually, large spatial variability of nutrient levels is the most common source of error and uncertainty in soil testing. Therefore, its use integrated with tools such as GPS, remote sensing, yield monitors and variable-rate application is key for improving the efficacy of nutrient management and crop production.”

Soil sampling is essential to efficient production.

Alleviating soil problems in specific regions

Referring to the Ohio region, Dick says, “Soil tests that better reflect phosphorus availability for support of crop production, yet conserve the phosphorus on the fields, is one example of alleviating a regional problem. No-tillage agriculture has often been able to make fertilizer use more efficient. At the same time, for phosphorus, the constant application of fertilizers and plant residues to the soil surface have resulted in phosphorus saturation levels where surface water can interact and solubilize the phosphorus and then move it off the field. This is a concern in my part of the country, where excess phosphorus loading into Lake Erie occurs. Studies to link soil testing with phosphorus fertilizer recommendations and responses on no-till fields are important to help solve this problem.”

Finding test labs

Testing labs at land-grant universities, which often provide online soil testing brochures and lists of itemized costs, as well as commercial testing centers, are located throughout the United States. Look for a reliable and preferably local lab, one that meets your needs (e.g., if your focus is organic) and provides a timely soil analysis. Predetermining what you are looking for can narrow down the testing and likewise the cost.

The author is a freelance contributor based in New Hampshire writing on farming and sustainability.