Gardening 101

Understanding a Soil Test Report

The first time you receive your soil test report you may be in for a shock—there are a lot of numbers, letters and elements that you may vaguely remember from the periodic table. In short, the report may as well be written in Chinese. I hope this article will give you the tools to understand what you need to know to amend your soil.

The first thing I look for on the report is pH, which measures how acidic or basic the soil is and ranges from 0 to 14.  This number is found under “Test Results.” In the example report for the “Front” for the “Soil”, the pH is 4.2, which is considered very acidic. Most plants prefer a soil pH between 6.2- 6.5. At a pH of 6.2, most soil nutrients are available to most plants.

The lime recommendation of adding 120 M (M= pounds/1,000 sq. ft) directly correlates to the low pH. Keep in mind, you should not surface apply more than 50 pounds of lime per 1,000 sq. ft at one time. Wait six months between lime applications and then retest before applying more lime. If you plan to incorporate the lime into the soil, the entire amount can be added at one time.

Next take a look at the test results for the P-I (Phosphorus Index) and K-I (Potassium Index). The following index values indicate whether significant plant growth will result from the addition of a nutrient:

0 to 25 (low amount) = high impact on plant growth

25-50 (medium amount) = medium impact on plant growth

50-100 (high amount) = low impact on plant growth

100+ (very high amount in soil) = no impact and sometimes a detrimental impact

In the soil results the P-I = 287.  This means there is a lot of Phosphorus already in the soil and adding more would not be beneficial to the plant. The K-I = 28.  This is in the medium range and adding potassium could have an impact on the plant growth.

Now it’s time to look at the recommendation listed under N: 7 lbs of 15-0-14.  What does that mean? The three numbers stand for the recommended percentages of Nitrogen (15%), Phosphorus (0%), and Potassium (14%).  These rates directly relate to test results. This means that in a 100-pound bag of fertilizer 15 pounds of it would be nitrogen, zero pounds phosphorus, and 14 pounds potassium.

So how much do you need to actually apply? You must calculate the area (length x width) of the place where you plan to spread the fertilizer. Let’s say your lawn is 5,000 square feet. Your equation would look like this:

7 pounds/1,000 sq. ft  x 5,000 sq. ft = 35 pounds of fertilizer (# of pounds/1,000 sq.ft  x total area = # of pounds of fertilizer).

The North Carolina Department of Agriculture does not test for Nitrogen.  Nitrogen leaches quickly from the soil. Plants need about one pound of nitrogen per year. The soil test reports give a standard recommendation for nitrogen.

Since it has been a long time since most of us have had algebra and chemistry, understanding a soil test report is a challenge. Most homeowners find it difficult to translate the results and end up over fertilizing.

The good news is the North Carolina Department of Agriculture will soon be coming out with a new report that is easier to understand. Until then, if you have questions about your soil test report, call your local county extension office for help.

Byline:
Michelle Wallace is the Consumer Horticulture Agent for the N.C. Cooperative Extension Service in Durham Co. For questions, contact the Durham County Extension Master Gardeners at 919-560-0528 or email [email protected]