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Comparing Soil Test Results



Soil testing is an important part of farm management. Results from tests inform decisions for amendments and fertilizers. You want to get the most reliable test results possible since soil sampling and testing can be time-consuming and costly. The following article discusses how to select a laboratory, appropriate soil analysis tests, and compares 9 different soil testing laboratories in the northeast.


The topics emphasized below are:


 

Selecting a Lab and Services

SELECT A LAB THAT USES THE RIGHT SOIL TESTING METHOD FOR YOUR SOIL

Different soil types require different soil testing procedures. Laboratories across the country use either a Mehlich 1, Mehlich 3, Morgan, Modified Morgan, Olsen or Bray soil testing procedure; each procedure uses different extractants and methodology to move the soil nutrients from the soil particles into a solution that can be analyzed. Each methodology works best and is calibrated for a certain soil type. To figure out which soil testing procedure to use, one can either contact their local land-grant university soil analysis laboratory or simply use a more local lab. In the northeast, the Mehlich III procedure is the most commonly used and recommended methodology. The laboratories reviewed below all employ this procedure, with the exception of the Cornell Soil Health Lab.



CONSIDER WHAT TYPE OF ANALYSIS YOU NEED

When you send a sample into a soil testing laboratory, you can generally select additional, optional testing services. Below you will find a summary of the most common add-on services and when farmers should consider using them.


THE BASICS: pH, Potassium (K), Phosphorus, (P) Calcium (Ca), Magnesium (Mg), and Cation Exchange Capacity (CEC) Almost all soil testing services will report the 6 parameters listed above. Deficiencies or imbalances of these parameters will have the greatest impact on plant health and growth and should therefore be monitored at least once every three years.

  • Soil acidity is reported as pH and should be monitored regularly. The application of fertilizer and manure will naturally lower the pH of the soil, making it more acidic. When the pH is out of its ideal range, plants will lose their ability to access and absorb the nutrients in the soil.

  • Phosphorus, potassium, magnesium, and calcium are macronutrients and are required in greater quantities compared to micronutrients. Regularly monitor the concentration of macronutrients in the soil to ensure the plant has the necessary nutrients to sustain growth. Fun Fact: Sulfur is another macronutrient that is often not reported in the basic analysis. Historically, sulfur was always found in adequate concentrations in the northeast because it was deposited on the soil in high quantities through acid rain from coal-burning power plants. However, as we reduce air pollution, agronomists are starting to see sulfur deficiencies in certain plants.

MICRONUTRIENTS: Micronutrients: Boron (B), Iron (Fe) Manganese (Mn), Copper (Cu), and Zinc (Zn), are required in lower quantities but are also essential to plant health and growth. In general, most farms opt to only analyze micronutrients when trying to diagnose a problem, as deficiencies in micronutrients tend to be uncommon.


ORGANIC MATTER: Organic matter is a measure of the amount of decomposing plant matter in the soil. Organic matter is important because it is the soil's primary means of holding onto nutrients and water and it creates the loose crumbly structure that is ideal for plant growth; when organic matter is depleted, the soil becomes compact and nutrients will either run off or be leached out of the soil profile. Farmers looking to improve their soil health and quality typically aim to boost organic matter concentrations by reducing tilling, using cover crops, and adding compost and manures. Farmers can monitor their progress by regularly testing organic matter concentrations.

HEAVY METALS: Heavy metals like lead and mercury are often found in high concentrations in the soil of sites that were previously under industrial use. Crops grown in these soils will become contaminated and potentially dangerous to consume. If farming in an urban area or area that may have had some historical industrial use (e.g. tannery or old orchard) consider testing soil for heavy metals.

SOLUBLE SALTS: At low levels, soluble salts generally do not negatively impact plants and excesses are uncommon in most northeast soils. However soluble salts can accumulate under some circumstances -- most commonly in a greenhouse where rainwater and snowmelt don’t leach them from the soil and nutrients are applied through irrigation water. Consider periodic testing in 4-season high tunnels (covered with plastic year-round).


OTHER SOIL HEALTH PARAMETERS A soil’s health is a function of its physical characteristics (porosity, level of compaction), chemical properties (nutrient concentrations and pH), and biological parameters (biological activity). Cornell University has developed a soil analysis that looks at all of these features of the soil and compiles the data into one comprehensive report. For farmers intent on improving their soil health, consider a Cornell Soil Health Assessment (CASH) as a baseline assessment that can be used to monitor changes over time.

NITROGEN: Nitrate Nitrogen, Total Nitrogen, Ammonium Nitrogen, and Pre-Sidedress Nitrate Test for Corn (PSNT)

Many people are surprised that Nitrogen analysis isn’t included in a standard soil test given that nitrogen is the most important plant nutrient. Instead, laboratories often offer, as an add-on service, a number of different nitrogen testing options (Nitrate Nitrogen, Total Nitrogen, Ammonium Nitrogen, and Pre-Sidedress Nitrate Test for Corn (PSNT)). This is because nitrogen concentrations are very difficult to measure. Nitrogen, more than other soil nutrients, is constantly changing forms. Plants can only use mineralized forms of nitrogen (ammonium (NH4+-N) or nitrate (NO3--N)); but depending on the soil and season, nitrogen often exists in high concentrations in the soil in decomposing organic matter (like composts, manure, or other plant residues) or living bacteria and fungi. These act as “slow-release” nitrogen sources and aren’t captured in a standard ammonium or nitrate test. Conversely, a total nitrogen test can be used to measure the total amount of nitrogen in the soil, but as much of this nitrogen is likely immobilized or tied up in some organic form, the results also cannot be used to determine plant needs.

Rather than test nitrogen, most farmers simply annually apply the amount of nitrogen the crop will uptake based on estimates of crop yields. The one exception is that corn farmers can use the Pre-Sidedress Nitrate Test for Corn (PSNT) to estimate the crop needs at a certain time in plant growth.

 

Comparing Soil Lab Services


It can be daunting to decide which lab to use. To simplify the farmer’s decision-making, we collected soil from a single field in northern New Jersey, thoroughly mixed it, and sent it into nine laboratories for their analysis.


We’ve created a chart with the nine labs we sent soil samples to. All the labs, with the exception of the Cornell Soil Health Lab, used the Mehlich III soil test. Cornell Soil Health Lab utilizes the modified Morgan test. The chart compares prices, what is offered in the basic package, and how long it took for results to come back when mailing samples from NJ.


It should be noted that samples were sent out during Covid in the spring of 2021. This could have resulted in longer than usual times to receive results back from labs.


Rating System for "Ease of Readability": 1 (Low) to 5 (High)

As is often the case, there is no right answer in choosing a lab. Each farm is different so working with a local expert to help you interpret results in the context of your farm and soil can be of great benefit.


Two ways to keep test results reliable are to sample at the same time every year and to use the same lab each year. Most farmers aim to conduct their soil sampling in the fall. This allows for adequate time to apply additional amendments before spring planting.

Different soil labs use different equipment and techniques to analyze your soil. If you stick with the same lab, you are more likely to receive results that have greater precision when comparing results from year to year. This helps you understand if the amendments and management practices being applied are creating the changes you are looking for.

To illustrate the differences in soil lab results, North Jersey RC&D pulled soil samples from a farm field in the spring of 2021. We sent the samples to nine labs. You can see from the bar charts below that results varied from lab to lab.