Minding Your N, P and K’s- How to Interpret Clemson University Soil Test Results For A Bountiful Vegetable Garden

Zack B. Snipes
Clemson Extension, Commercial Horticulture Agent, Charleston and Beaufort Counties

For years I tried convincing my grandfather, an avid vegetable gardener, to get his soil tested by the Clemson University Agricultural Service Laboratory.  He finally agreed and submitted a sample ($6).

A few years later, I learned he was sending a sample yearly, but not following the fertilizing recommendations returned by Clemson. Instead, he continued using the 10-10-10 he’d been using for years.

For many, like grandpa, interpreting the results of the laboratory analysis can be an impediment to making changes in their gardening practices.  Vegetable gardening requires good fertility management and following the recommendations can pay off in reduced costs and improved yields. 

Here are common questions we receive about interpreting soil test results:

Q: I got my results back, what are all those numbers under the “Recommendations”?

A: Those numbers are codes for a set of standard recommendations, and are explained by code under the “Comments” section.

Q: What is pH and what is the range of optimal pH levels?  And what does “buffer pH” mean?

A: The pH scale measures the number of free hydrogen ions, an indicator of acidity.  Nutrients are taken up by plants within a narrow pH range.  For instance, a soil pH ranging from 5.8-6.5 is best for most vegetables.  However, some plants, such as blueberries, do better with acidic soils. Most soils in the Lowcountry are acidic and will need modification to achieve an ideal pH (more below).

Buffer pH reveals how difficult it will be to change the pH of the soil.  This number is needed to make specific recommendations for adjusting the soil pH. Buffer pH is not a number that should concern the home gardener.

Q: My soil test results do not recommend adding lime.  Why not?

A: Lime raises the pH of soil to make it more basic.  Most vegetables prefer a slightly acidic soil pH.  Adding lime when not needed might hurt plant development and be a waste of money for the gardener. 

Q: What does CEC mean?

A: Cation Exchange Capacity (CEC) relates to the ability of soil to hold nutrients.  Soils with a high CEC are more fertile than those with a low CEC.  Increasing organic matter, for example adding compost, to the soil greatly increases the CEC thus improving fertility. 

Q: Why don’t the results measure nitrogen in my soil?

A: Nitrogen, critical for plant growth, is responsible for the green color in leaves, increased seed and fruit yield and rapid growth.  Most garden soil does not have enough nitrogen available for high fertility plants like vegetables.  Nitrogen is difficult to measure in a soil test because it constantly leaches from soil or turns into a gaseous form and is lost. Because of this, the recommendations for applying nitrogen fertilizers are made based on plant needs and measurable soil fertility indicators such as pH and cation exchange capacity (CEC). Applying more nitrogen than recommended can result in excessive vegetative growth with less flowering or fruit. It can also lead to increased disease and insect damage, add to environmental pollution, and waste the gardener’s money.

Q:  I noticed that the amount of phosphorus was reported as excessive.  What does this mean and can I dilute it?

A:  Lowcountry soils are naturally high in phosphorus, so it is not unusual to see this on a report. While gardeners cannot apply amendments or products to dilute excessive nutrients or minerals, the plants in your garden can utilize excess nutrients, lowering the amount in soil over time. In this case, the lab report will recommend you use a fertilizer that does not contain phosphorus such as 15-0-15.

Fertilizer bags are always prominently labeled with three numbers in the same order. This guaranteed analysis informs consumers that the percentage of the major minerals needed for plant growth, nitrogen (N), phosphorus (P) and potassium (K), have been verified by the Clemson Department of Plant Industry Fertilizer Inspection program.

Q: I have not taken a soil sample in for testing this year.  Is it too late? Where can I take it? Can someone help me interpret the results?

A: It’s not too late, although you may not have time to make significant pH adjustments for the spring. Take the soil sample to your local county extension office, and they will send it to Clemson soils lab for analysis.

More information about soil samples can be found at: http://www.clemson.edu/public/regulatory/ag_svc_lab/

After receiving your soil report, if there is anything you don’t understand, please call the Clemson Home & Garden Information Center for assistance.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.