Understanding your Soil Test Report

You’ve taken your agent’s advice and taken a soil sample on time.  Now you have a report, and it is time to make some decisions, but first, what does this report mean?  Let’s take a look at what’s reported on a North Carolina soil report.

First, it is important to understand what a soil test does.  A soil test extracts and measures elements essential for plant growth.  In North Carolina, soil testing can be done through NCDA&CS for free from April through about November.  During peak testing dates, December through March, there is a $4 per sample fee.  Other states offer similar services and testing can also be performed by private labs, but this article will focus on the NCDA&CS test.

After the test has been performed on a pasture sample, you will receive a report that will look similar to this:

This is an actual report from Lee County, NC.  This grower sampled an established fescue pasture to receive recommendations on maintenance fertility applications.  In some cases, you may see comments from the agronomist, but most often the first part of the report will show the recommendations.  Recommendations are made for lime, nitrogen, phosphate, potash, magnesium, sulfur, manganese, zinc, copper, and boron.  You may also have a link to more information.  In this case, note 12 is linked, which is a publication with more information on fertilization of pasture crops.   Let’s go through each of these recommendations and what they mean for you, as well as how they relate to the numbers given in the test results section of the report.

Lime

The lime recommendation is given for pastures in units of tons per acre.  This value is calculated using soil pH, exchangeable acidity (Ac), target pH, and residual lime credit.  The lowest rate recommended will be 0.3 tons/acre.  Soil pH and exchangeable acidity are both reported in your results.  The soil pH is a measure of active acidity in the soil solution.  Exchangeable acidity represents the portion of the CEC which is occupied by hydrogen or aluminum ions which must be neutralized to raise soil pH.  Each of these numbers are required to accurately determine liming needs.  The CEC is reported and represents the nutrient holding capacity of the soil.  This value will be lower in sandy soil and higher in clay or organic soils.  The base saturation (BS%) is reported and represents the portion of CEC occupied by bases, mostly calcium, magnesium, and potassium, which are all essential plant nutrients.  Target pH depends on the plant being grown and the soil classification.  Soil classification is also reported in your results and will be either mineral, mineral-organic, or organic.  This is determined by the humic matter (HM%), which represents soluble soil organic matter, and the weight/volume ratio (W/V).  For maintenance of fescue on a mineral soil, the target pH is 6.0.  Remember, the proper soil pH is critical for plant growth and development as it directly relates to nutrient availability.

Nitrogen (N)

The NCDA&CS soil test does not test for nitrogen in a standard predictive soil test.  This test in only done in certain situations because nitrogen does not persist long in soils, and in general the amount in soil at the time of testing is not representative of the actual amount in the field.  Recommended nitrogen rates are based on field research studies, and the recommended rate represents the total nitrogen that should be applied in a year.  In general, nitrogen will be split into several applications and rates may vary depending on your pasture management practices.  Talk with you agent to determine the proper application rates for your situation. 

Phosphate (P₂O₂) and Potash (K₂O)

These recommendations are based on an index determined by your soil report, the P-I or K-I respectively.  These indexes represent the amount of available phosphorus or potassium in the soil.  An index value of 0-10 is very low, 11-25 is low, 26-50 is medium, 51-100 is high, and 100+ is very high.  Recommendations are given in pounds per acre.  Phosphorus is relatively immobile in soils, and an application can be made at once to correct low phosphorus levels.  Potassium will leach, so a split application can be beneficial in sandy soils.  Phosphorus and potassium can be applied to fescue in early fall or early spring.

Magnesium (Mg)

Magnesium recommendations are based on the percent of CEC occupied by Mg, expressed as Mg %.  If your soil is below critical level, 25 pounds per acre of Mg will be recommended.  If lime is needed, the use of dolomitic limestone will supply the required Mg.

Sulfur (S)

Sulfur is recommended when the sulfur index (S-I) is below 25.  Sulfur leaches rapidly, so applications should be split in sandy soils.  Sulfur is required by the plant in order to properly utilize nitrogen. 

Manganese (Mn)

The manganese availability index (Mn-Al) is calculated based on the soil test manganese index (Mn-I).  Mn availability is closely correlated to soil pH.  For crops such as pastures, where data determining the response to added Mn is limited, you may see a $ in the recommendation line if levels are low.  This will link to a note with more information about Mn rates.  If pH is high enough to affect Mn availability, you may see either pH$ or $pH in the recommendation line.  These again will link to notes with more information.

Zinc (Zn) and Copper (Cu)

Zinc or coper are generally recommended when either the zinc availability index (Zn-AI) or copper index (Cu-I) are below 25.  Zinc is generally recommended at a rate of 6 pounds per acre if the crop is known to respond, regardless of soil type.  Copper rates are recommended based on the crop and soil type.  However, with pastures, there is generally less data on crop response, so instead of a recommendation you may see a $ which will link you to a note with more information.  In some situations in North Carolina, you may have an area with excess zinc or copper.  If you see a Z on the zinc recommendation line, or a C on the copper recommendation line, this indicates that the Zn-AI or Cu-I is above 2000.  If you see either of these situations, refer to the agronomist comments section of the report for more information. 

Boron (B)

Boron is recommended for crops with a known response.  The only forage crop with a known response to boron which will have a recommendation is alfalfa. 

Your sample also reports sodium (Na), exchangeable sodium percent (ESP), soluble salt index (SS-I), and nitrate nitrogen (NO₃-N).  Sodium is evaluated for all samples.  Levels below 0.4 are considered inconsequential to plant nutrition, but higher can cause root injury or interfere with plant growth.  ESP represents the total amount of sodium on the CEC.  Values below 15% are considered acceptable.  SS-I or NO₃-N will only be given on certain reports, and will only be reported for a pasture sample if it is submitted as a problem diagnostic sample.  You agent will be able to work with you if a problem sample is submitted and either of these values are a contributing problem.  

I hope this can be used as a guide to help you understand your soil test report.  If you would like more information about understanding you soil test, please see the NCDA&CS document “Understanding the Soil Test Report” at http://www.ncagr.gov/agronomi/uyrst.htm.  Remember, each situation is unique, so your county extension agent is your best resource for determining your forage fertility needs.