New laboratory testing procedures lay the groundwork for improving soil health
When you start thinking in terms of soil health, soil testing takes on a whole new significance. You move beyond an exclusive focus on nitrogen (N), phosphorus (P), potassium (K) and pH levels to an acute awareness of the overall well-being of your soil. Once you reach that point, you can start to correct the weaknesses that limit the soil’s productive potential.
"Improving soil health involves assessing all three aspects—chemical, biological and physical," says Farm Journal Field Agronomist Ken Ferrie. "Some aspects of your soil might be healthy while others have room for improvement. The first step is to establish baseline levels. Then you can monitor your progress as you take steps to correct yield-limiting factors."
You can conduct several soil health tests yourself—water infiltration, soil respiration, aggregate stability, pH, surface hardness, compaction and density. Other aspects require the help of a soil-testing laboratory, which offers some tests you might not have encountered before, such as aggregate stability, texture analysis, active organic matter and mineralizable nitrogen.
"Some of these new soil health tests are not widely available yet," Ferrie notes. "As more farmers integrate soil health into their management plan, they will become more common."
Soil chemistry. The basic soil test you’ve probably been using for years measures the chemical aspect of soil health. A soil health test is similar but more comprehensive.
"Besides testing for macronutrients—N, P and K—we now need to test for micronutrients," Ferrie says. "Overapplying or underapplying any macro- or micronutrient might be detrimental to soil health."
Take soil health samples in the spring when soil moisture is close to capacity.
Misapplication of one nutrient can trigger a chain reaction. For example, if you apply excess nitrogen to a soil where pH is optimum, the extra nitrogen stimulates microbial populations, Ferrie explains. Those microbes release soil carbon, which might be lost as carbon dioxide.
"Down the road, you might need to apply more nitrogen fertilizer because you reduced the supply of nitrogen stored in the soil," he adds. "If you apply an ammoniacal source of nitrogen, which creates acidity, soil microbial activity might be reduced. Then you will need to apply more limestone to correct the acidity."
Farmers who irrigate or apply manure should also test for sodium. "Excess sodium prevents clay particles from flocculating (just like hydrogen in an acid soil), so the soil particles run together and structure is destroyed," Ferrie explains.
In other words, balanced fertility, which includes the right amount of each nutrient and proper pH, is a key component of healthy soil.
Water pH versus buffer pH. Since proper pH is essential to soil health, you need to understand how much lime you need to apply. That requires a more sophisticated soil test that reports not just the traditional water pH reading but also the buffer pH reading.
"The water pH reading (which is the only one reported on many soil tests) measures soil acidity as it affects plants and microbes," Ferrie says. "But you can’t use that reading to determine how much lime to apply because the amount needed to neutralize acidity varies with different soil types. The reading that tells you how much to apply is buffer pH, which takes into account the soil’s buffering ability."
It works like this: A light soil, with low organic matter and low cation exchange capacity, might have a water pH of 4.9. A heavier clay loam soil might have a water pH of 5.5. Because of the difference in the soils’ buffering ability, the light soil might only need 1 ton of lime per acre to correct its acidity, while the heavy soil might need 3 tons. Liming both soils at the same rate would be a mistake, Ferrie adds, because the wrong rate on one field would reduce microbial activity.
Cation exchange capacity. Until now, you probably have viewed cation exchange capacity (CEC) as an indication of how well your soil retains nutrients—whether you could apply a larger amount every few years or whether you needed to apply a smaller amount every year.
"But the CEC reading also helps identify the texture of your soil," Ferrie says. "The higher the CEC reading, the more clay particles and the fewer sand and silt particles. This gives you an indication of the productive capacity of your soil so you know what you can expect it to produce in a perfect state of health.
"A very light soil, even in the best of health, might not match the yield of a darker soil in poor health. It’s like comparing a 17-year old to an 80-year old—even though both are healthy, the teenager is more athletic.
"Even if the 17-year-old is overweight, he might still outrun and out-jump a fit 80-year old," he adds. "By making all our soils as healthy as possible, we maximize each one’s productive capacity."
You put this knowledge to work when you implement a soil health management plan. "For example, consider CEC when selecting a cover crop," Ferrie says. "If your soil has a high CEC, you have a lot of clay, so the soil is probably tighter, with higher bulk density. Radishes would be a good choice because they create biochannels that improve air and water flow.
"If your soil has a lower CEC, it probably has a sandy loam or sandy silt loam texture. A cereal rye or annual ryegrass cover crop would help improve aggregate stability," Ferrie says.
Using GPS to record each sample location allows Brad Beutke, who works in the Farm Journal Test Plots, to return and document soil health changes.
Physical soil aspects. You can measure aggregate stability and soil texture in the field, but both can be analyzed in a laboratory for increased precision and repeatability of results, Ferrie says.
The aggregate stability test determines how well soil structure is likely to withstand rainfall or tillage. "In the lab, soil is placed on top of a sieve and water is applied with a rainfall simulator," explains Thomas Zerebny, Ferrie’s assistant. "The volume of the total sample remaining on the sieve is the percentage of aggregate stability."
Soil texture is determined by the percentage of sand, silt and clay particles (from largest to smallest). "Texture affects the size of pore spaces, water infiltration rate, available water capacity, permeability and cation exchange capacity," Zerebny says.
"When soil structure is destroyed, clay and silt particles percolate downward, leaving the sand particles," Ferrie explains. "If you find that the percentage of sand particles in the top 6" is increasing, it means the aggregate stability has failed and the healthy crumb-like structure has been lost."
Biological testing. The biological components of soil health are organic matter (or carbon) content and microbial activity. Organic matter, which is made up of decomposed plants and microbial organisms, holds nutrients, helps build soil structure and improves water-holding capacity.
"The amount of total organic matter, which you’re familiar with on soil tests, is an indicator of soil health, but it is slow to change," Ferrie says.
"Although it might take many years to make significant improvements in total organic matter content, it’s worth the effort in terms of higher yield and lower commercial fertilizer cost," Ferrie says.
You can measure the amount of organic matter in soil by comparing it to a simple color chart. "But a laboratory test is better because the results are repeatable in future years," Ferrie says.
Different laboratories might use different organic matter tests, so it’s important to stick with one laboratory for repeatable results. Sampling depth is also important. Soil within 2" of the surface will show changes in organic matter content faster than soil at 7".
Active organic matter. One portion of total organic matter can help farmers measure their changes in soil health over a relatively short time period. That fraction is called active organic matter (or active carbon).
"Active organic matter is the portion that breaks down fastest and makes nutrients available to plants," Ferrie says. "Unlike total organic matter, the active organic matter value can change fairly rapidly in response to changes in crop rotation, the use of cover crops and aggressive tillage."
Mineralizable nitrogen. Mineralizable nitrogen is nitrogen in the ammonium form, which plants and microbes can use. It might be provided by soil organic matter during the growing season.
"When you learn your soil’s potential to mineralize nitrogen, temperature and moisture conditions during the growing season will give you an indication of how much ammonium nitrogen might become available," Ferrie says. "That will help you decide whether to make a late-season application of nitrogen fertilizer.
"I think of soil with high mineralizable nitrogen values as being nitrogen-friendly," Ferrie adds. "On those soils, it is easier to pull back on nitrogen fertilizer rates."
Root health assessment. In a root health assessment, technicians plant a seed in a soil sample to determine the diversity of soil microorganisms.
"After four weeks, the plant’s roots are washed and given a score based on their health," Zerebny explains. "The healthier the roots, the more diverse the microbial population and the healthier the soil."
"A more diverse microbial population results in fewer disease issues for the crop because the beneficial microbes prevent pests from building to damaging levels," Ferrie says.
At the day-to-day management level, the test might suggest whether or not you need to apply a seed treatment, Zerebny adds.
There are numerous laboratory tests to evaluate soil health, which in turn helps you develop a health management plan:
- Nitrogen, phosphorus,
- potassium and pH
- Water pH
- Buffer pH
- Cation Exchange Capacity
- Aggregate stability
- Total organic matter (carbon)
- Active organic matter
- (active carbon)
- Mineralizable nitrogen
- Root health assessment
New Assessment Tools
- Soil Biological, Respiration and Nitrification (BRAN)
- Soil Health Tool
Collecting your samples. "Take your soil samples and measurements in the spring when soil moisture is close to capacity," Ferrie advises. "Record the locations using GPS, so you can
return every three to six years and measure your progress as you implement your soil health improvement strategy."
The soil health tests mentioned in this story are offered by some commercial laboratories and Cornell University (http://soilhealth.cals.cornell.edu). (See "New Soil Health Tests" below.) Ferrie expects that additional laboratories will begin offering more soil health testing services in the near future.
New Soil Health Tests
Soil-testing agencies are responding to the growing interest in soil health by developing new tests that add knowledge, precision and repeatability. Two of the newest tests include:
- The Soil Biological Respiration and Nitrification (BRAN) test measures carbon dioxide respiration by soil microbes, which is fueled by the active carbon portion of the organic matter in the soil. It predicts the approximate quantity of nitrogen that will be released per year and estimates the degree of biological activity in the soil. It is available from Nebraska-based Midwest Laboratories (www.midwestlabs.com).
- The Soil Health Tool, developed by the USDA–Agricultural Research Service, uses factors such as carbon dioxide respiration rate, active carbon and water-soluble carbon as indicators of the soil’s nutrient-supplying power. It uses measurements of five soil biological properties to produce a Soil Health Score, which represents the overall health of the soil system. The tool comes from at least three laboratories—Woods End Laboratories (http://woodsend.org) in Maine, Brookside Laboratories (www.blinc.com) in Ohio and Ward Laboratories (www.wardlab.com) in Nebraska.
Building on the Systems Approach, the Soil Health series will detail the chemical, physical and biological components of soil and how to give your crop a fighting chance.
You can e-mail Darrell Smith at firstname.lastname@example.org.
- March 2014