Understanding Your Lawn and Garden Soil Test Reports and Calculating Fertilizer Requirements Based on Soil Test Recommendations

Published: May 16, 2018

Soil testing is an important tool for growing healthy lawns and gardens. To get reliable results and appropriate fertilizer and lime recommendations, it is important that you submit a representative soil sample from your lawn or garden. For guidelines in taking a representative sample and submitting to the lab for testing, visit MU Soil and Plant Testing Lab's website at http://soilplantlab.missouri.edu/soil. Once the soil test is conducted and the results are available, the ability to interpret the results and follow the recommendations is an important consideration in correcting the deficiency or imbalance, and growing healthy lawns and gardens.

Example of Soil Test Report from MU Soil Testing Labs for Lawns and Garden Fertility Test:

Comments:

*** Fertilizerrates are given in pounds of actual nutrient per 1000 sq. ft to be applied
*** The soil needs additional organic matter for gardens and crops other than lawns. See MU Publication G6950, "Steps in Fertilizing Garden Soil" and G6956, "Making and Using Compost".
*** Lime takes two to three months to react with the soil. Apply lime three to six months before planting.
*** For blueberries soil needs to be treated with 50 lbs of elemental S per 1000 sq. ft to acidify the soil. It takes 3 months for S to react with the soil and acidify the soil.
--- The soil should be tested every 2 to 3 years to determine the effects of your fertilization practices and to develop a new set of fertilizer and limestone guidelines.

Explanation of Soil Test Report Form

Sample ID: This is information you provided upon submitting your sample. The fertilizer suggestions are based on this.

Ratings indicate how high or low your soil is considered in each category.

pHs is an indication of the acidity or alkalinity of soil. A pH of 7 is neutral, while values below 7.0 are acidic and values above 7 are alkaline or basic. Vegetables and flowers grow best from pHs of 6.0 to 7.0. Most lawns grow well in pHs range of 5.5 -7.0. However, acid loving plants like azaleas, rhododendrons, blueberries and raspberries prefer pH below 5.5.

Phosphorus, potassium, calcium, and magnesium tests results are listed in lbs/a in the next four lines. These are some major essential elements required for plant growth. However, these numbers have little meanings for home owners. The ratings, however, indicate if these nutrients are considered low or high.

Organic Matter is the percent of organic matter found in your sample. Soil organic matter is essential in the formation of soil structure, reducing compaction, and for retaining plant nutrients. It helps in improving the water holding capacity of the soil, aeration, and tilth. While soil organic matter levels between 2% and 3% are fine for lawns, 4% to 6% is better for vegetables and flowers.

Neutralizable Acidity (NA) is a measurement of reserved acidity in soil and reported in mille- equivalents per 100 grams of soil (meq/100 g soil). This number along with pHs is used in calculating the lime requirement in soil.

Cation Exchange Capacity (CEC) is the ability of the soil to withhold positively charges nutrients and is reported in meq/100 g soil. While soils with high CEC values can retain more nutrients, low CEC soils can only retain fewer nutrients.

Fertilizer and Limestone Recommendations indicate how much nitrogen, phosphate, and potash (potassium), Zinc, Sulfur and lime your soil needs for each selected crop option of your choice. This is the most important part of the report for home owners. These rates are in pounds needed per 1000 square feet. Depending on the garden size specific amount of nutrient requirements needs to be calculated and fertilizers to be selected accordingly.

Comments: The soil test reports have comments at the end of the reports with notes on soil test in general, with additional notes for specific recommendations for your soil.

Calculating the fertilizer requirements based on soil test recommendations

All fertilizer recommendations given in a soil test report are based on the amount of nutrient (N, P2O5, and K2O) to apply for a given area. Lawn and garden recommendations are given in pounds per 1000 sq. ft. From the given recommendations it is necessary to select an appropriate fertilizer grade and determine how much of this fertilizer to apply to the garden area. Numbers on fertilizer bags indicate the exact percentages of nutrients by weight: 100 lb. of 5-10-10 fertilizer contains 5 lb. of nitrogen (N), 10 lb. of phosphate (P2O5), and 10 lb. of potash (K2O). Because it is difficult to achieve the exact amount of all recommended nutrients from the garden fertilizer blends available in the market, it is important to match the nitrogen requirement.

Example
A soil test recommendation for your vegetable garden calls for 2 lb. of N/1000 sq. ft., 0 lb. of P2O5 /1000 sq. ft. and 1 lb. of K2O. The garden is 40 ft. by 10 ft.

• Step 1:
  Calculate the area to be fertilized. Multiplying length by width, the area of the garden is 40 x 10 = 400 sq. ft.
• Step 2:
  Select the fertilizer to be used. Match the ratio of nutrients recommended to the fertilizer grades available. The N-P-K nutrient ratio based on the soil test is 2-0-1. Ideally, a fertilizer such as 10-0-5 or 20-0-10 or 30-0-15 should be selected. At the local garden store, fertilizer bags marked 20-10-10, 27-3-3 and 25-0-12 are available. The one marked 25-0-12 best matches the ratio of 2-0-1 recommended by soil test.
• Step 3:
  Determine the fertilizer amount to apply: Divide the recommended amount of nutrient by the percentage of the nutrient (on a decimal basis) in the fertilizer.
• First calculate the fertilizer recommendation for the garden area:
  2 lb. of N/1000 sq. ft. x 400 sq. ft. / garden = 0.8 lb. of N per 400 sq. ft. garden. 100 lb. of the 25-0-12 garden fertilizer blend will have 25 lb. of N and 12 lb. of K2O.
• To provide 0.8 lb. of N for the 400 sq. ft. garden you would require:
  100 lb. for fertilizer blend /25 lb. of N x 0.8 lb. of N = 3.2 lb. of the fertilizer blend required to provide the N requirement of the garden. Since the fertilizer blend ratio is almost the same as the recommended ratio, it will provide the required amount of K (1.6 lb. of K2O) to the garden.

Note
The weight of 2 cups of dry fertilizer is approximately 1 pound. Therefore to meet the garden fertilizer recommendation, you will need about 6 cups of the fertilizer blend (25-0-12) material for the 400 sq. ft. area.
Recommended application rate for various granular fertilizers to apply one pound of nitrogen.