Fertilizing Gardens and Lawns

Before you plant your garden or fertilize your soil, it’s important to test the soil and see if it needs fertilizer or lime. If you haven’t tested the soil this year, you still can do a soil test and use the soil test results and recommendations as a guide in fertilizing your lawns and gardens. Soil test kits are available at the University Extension Offices or at the University of Missouri Soil and Plant testing laboratories.

Don’t guess when it comes to fertilizing your gardens and changing the soil pH. Over applying can harm the soil and plants as well as pollute the environment. Use your soil test report as a guide and follow the instructions in applying fertilizer and lime.

Soil tests measures nutrient amounts and pH, level of acidity or alkalinity. Most soils in Missouri have pH levels ranging from 5 to 7.5. Most vegetables, fruits, and flowers grown in home gardens grow best in soil pH between 6.0 to 7.0, while rhododendrons, azaleas, laurels and blueberries prefer acid pH (4.5 - 5.5). Nutrients are most available for plant uptake between the pH range of 6.0 to 7.0.

Depending on the pH preference of the plants you are growing, you can either treat the soil with lime to raise the soil pH, or apply sulfur to reduce the soil pH as recommended by your soil test report. Make sure to till the recommended amount of lime or sulfur to about 6 inches into the soil. Tilling and incorporating the material into the soil will quicken the reaction time of these soil amendments due to increased surface area contact with soil particles.

All fertilizer recommendations given in a soil test report are based on the amount (lbs.) of nutrient (N, P₂O₅, 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.

Fertilizers are sold in many grades. Complete fertilizers such as 10-10-10 or 5-10-10; contain all three primary nutrients (nitrogen-N, phosphorus-P, and potassium- K). Single nutrient fertilizers contain only one, but they are generally a high analysis, economical source of that nutrient. (e.g., 46-0-0, 0-46-0, 0-0-60).

Numbers in the fertilizer bags indicate the exact percentages of nutrients by weight. For example 100 lbs of 5-10- 10 fertilizer contains 5 lbs of nitrogen (N), 10 lbs of phosphate (P2O5), and 10 lbs of potash (K₂O). Most garden fertilizers are complete fertilizers and are convenient to use. However, it may be difficult to find the one that exactly matches to the ratio in the fertilizer recommendation provided in the soil test report. Since it is difficult to meet the exact amount of nutrient required from the garden fertilizer blends available in the market, it is important to match the nitrogen required.

An example for calculating fertilizer rates to a garden

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

Step 1 is to measure the area to be fertilized which is accomplished by multiplying the length by width. The area of the garden to be fertilized is 40 x 10 = 400 sq. ft

Step 2 is to select the fertilizer to be used based on the soil test by matching 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 as 25-0-12 best matched the ratio of 2-0-1 recommended by soil test.

Step 3 is to 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 area of garden: 2 lbs of N/1000 sq. ft x 400 sq. ft/ garden = 0.8 lbs of N per 400 sq. ft garden. One hundred pounds of the 25-0-12 garden fertilizer blend will have 25 lbs of N and 12 lbs of K₂O. To provide 0.8 lbs of N for the 400 sq. ft garden you would require: 100 lbs for fertilizer blend /25 lbs of N x 0.8 lbs of N = 3.2 lbs of the fertilizer blend. Since the fertilizer blend ration is same as the recommended ratio. it will provide the required amount of K (1.6 lbs of K₂O) to the garden. NOTE: 2 cups of dry fertilizer weighs 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.

Never apply fertilizers directly on the plant. Ideally, you should apply it about three to four inches from the plant to let the roots absorb the nutrients.

Garden soil needs plenty of organic matter. In addition to applying commercial fertilizer, it is recommended to apply manure, compost and organic sources of fertilizers that would add considerable amount of organic matter to the soil. Calculating the fertilizer rates using organic sources is often difficult. However, some types of organic fertilizer can be purchased in bags that are labeled with their fertilizer grade. Fertilizer rates for these sources can be calculated in the same manner as for inorganic fertilizers.

Manjula Nathan,
Director MU soil and plant testing laboratories,
573-882-3250