Reviewed June 2008
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Figure 1
If your lawn has areas with distinctly different soil conditions that need to be considered separately, collect soil samples from each area (at points in a zigzag pattern).
Taking a soil sample
Soil testing provides an estimate of the plant-available nutrients in the soil and is an essential tool for a sound fertilization program. Periodic soil testing will help to correct nutrient deficiencies, avoid excess fertilizer applications and maintain a healthy lawn.
A routine soil fertility test (pH, neutralizable acidity, phosphorus, potassium, calcium, magnesium, organic matter, and cation exchange capacity) is recommended under the following circumstances:
After processing your sample, the testing lab will send you a soil test report. Analyses in the following areas are usually included:
Soil pH
Soil pH is a measure of the acidity or alkalinity of the soil. A pH of 7.0 is neutral, less than 7.0 is acid, and greater than 7.0 is alkaline. Most of the essential plant nutrients are available at optimum levels between a pH of 6.0 and 7.0. Acidic soils (pH < 6.0) and alkaline soils (pH > 7.5) may require amendments that modify pH.
Soil pH below 6.0
If a soil pH is less than 6.0 (acidic), then a lime requirement will be calculated. Limestone is used to raise the pH of acid soils, resulting in greater availability of soil nutrients. Do not apply lime unless it is recommended. If lime is recommended, review the following guidelines before making the application.
Soil pH from 6.0 to 7.5
The pH range between 6.0 and 7.5 is the optimum range for turfgrass. Thus, there is not a need for any amendments at this range.
Soil pH above 7.5
At higher pH (>7.5) the soil is alkaline and not favorable for plant growth. Applying elemental sulfur at the recommended range given on the soil test report prior to lawn establishment will bring the pH down. At higher pH levels, phosphorus and iron availability may become limited, and yearly soil tests may be required to correct the problem by applying appropriate amounts of nutrients or amendments. Do not apply more than 10 pounds of elemental sulfur per 1,000 square feet on established lawns, as it may cause leaf burn.
Phosphorus
Soil test results represent the amount of phosphorus (P) available to plant roots from the soil. Established lawns generally require lower quantities of available phosphorous than vegetable and field crops. Turfgrass, with its dense, fibrous root system, is efficient at extracting the immobile phosphorus from the soil. However, new lawns have a limited root system and require higher phosphorus levels for root growth and healthy seedling development. Refer to Table 1 for phosphorus fertilizer recommendations. The phosphorous content of fertilizers is expressed as percent phosphorus oxide (P2O5); therefore, phosphorus fertilizer recommendations are given in pounds of P2O5 per 1,000 square feet.
There are a number of different phosphorus fertilizer sources available. Relatively inexpensive phosphorus-only sources are super phosphate (0-18-0) or triple superphosphate (0-45-0). Use these sources when a soil test calls for large amounts of P2O5 on an established lawn. For example, if the soil test reports a need to apply two pounds P2O5 per 1,000 square feet, then 4.4 pounds of 0-45-0 would be required per 1,000 square feet.
Other phosphorus sources are starter fertilizers that contain relatively high percentages of P2O5 such as 20-27-5 or 16-25-12. At establishment, incorporate these sources into the surface 1 to 2 inches. For example, if your soil test indicates that you should apply three pounds P2O5 per 1,000 square feet and you have purchased some 20-27-5, how much of the fertilizer should you apply? Dividing 3 by 0.27 (the proportion of P2O5 in the fertilizer bag) gives you the answer: 11 pounds per 1,000 square feet of the 20-27-5 is needed to supply the required three pounds of actual P2O5 per 1,000 square feet.
For general lawn maintenance, many complete fertilizers are available with analyses such as 29-3-4, 26-3-5, or 13-13-13. For the first two, each time you apply one pound of nitrogen per 1,000 square feet, you are applying approximately 0.1 pound of P2O5. These two sources would be good to use if your lawn soil falls in the medium category. For the 13-13-13, each time you apply one pound of nitrogen per 1,000 square feet, you are also applying one pound of P2O5. This would be a good fertilizer to use if your lawn falls in the low available phosphorus category.
Potassium
The soil test results represent the amount of potassium (K) that is available to the turfgrass plant. Potassium plays important roles in enhancing turfgrass stress tolerance (i.e., drought, heat, wear, disease) in addition to its essential roles in plant growth. Potassium fertilizer recommendations in the past relied heavily on yield response data from other crops, and, therefore, may have underestimated the quantities needed for healthy, stress-tolerant turf. The recommendations in Table 2 reflect current thinking on potassium's role in turf. Potassium is not held very strongly in sandy soils and can be leached past the turf root zone. Sandy Missouri soils should therefore be tested more frequently -- once a year rather than every three years.
The concentration of potassium in fertilizer is expressed as percent potassium oxide, or potash (K2O). Therefore, potassium fertilizer recommendations are given in pounds of (K2O) per 1,000 square feet. Two common potassium fertilizer sources are available: potassium chloride (0-0-60) and potassium sulfate (0-0-50). To deliver 1.5 pounds of potash to 1,000 square feet, you would need to apply three pounds of potassium sulfate over that area. Both of these potassium sources can burn turf, especially potassium chloride, so apply in cool weather and water the area soon after application.
Calcium and magnesium
Calcium (Ca) and magnesium (Mg) are rarely deficient in Missouri soils, but deficiencies can occur on acid sandy soils. In most instances calcium amounts sufficient for plant growth are supplied by liming the soil to increase soil pH. Use of dolomitic limestone [CaMg(CO3)2] to raise pH will also supply magnesium, along with calcium.
Organic matter
Soils with adequate organic matter (3 to 6 percent OM) will have better structure and therefore better resistance to compaction as well as improved water- and nutrient-holding capacity. The addition of organic matter to soil, at the time of lawn establishment, is the best way to enhance structure, as well as water and nutrient retention capabilities. It is important that the source of organic matter be free of pathogens and weed seeds and that it be low in soluble salt content. Well-aged (1 to 2 years) manure, composted bark/sawdust, leaf and grass clipping compost, and peat are good sources of organic matter. Mix a 1-inch layer (about 2 cubic yards per 1,000 square feet) into the surface 4 to 6 inches of soil as uniformly as possible.
Soil organic matter and nutrient content can also be increased by leaving clippings on your lawn and mowing tree leaves back into your lawn in the fall instead of raking and collecting. Best results occur with the use of a mulching mower.
Cation exchange capacity
Cation exchange capacity (CEC) is a measure of your soil's capacity to hold nutrients; specifically, positively charged ions such as K, Ca, and Mg. Clay and organic matter contribute to cation exchange capacity; thus, soils with high CEC will retain nutrients better than low-CEC soils.
Nitrogen
Nitrogen (N) is the nutrient that is required in the highest quantity for the maintenance of a healthy lawn. Soils generally contain large amounts of nitrogen, but it is mostly present in soil organic matter. Nitrogen is released from organic matter relatively slowly, but the amount is generally insufficient to supply the needs of a medium to high quality lawn. Several soil and weather variables affect the release of soil nitrogen and what happens to the nitrogen in the soil following release. Also, nitrogen can be easily leached from the soil when it is in nitrate form. Thus it is not a stable element in the soil and is not normally test by soil testing labs. Rather, yearly nitrogen applications, based on field experiments, have been developed by turfgrass scientists.
Table 1
Phosphorus fertilizer recommendations for all Missouri lawns based on soil test results.
Soil test available phosphorus**
0 to 20 pounds per acre
20 to 40 pounds per acre
40 to 60 pounds per acre
60 to 120 pounds per acre
120 + pounds per acre
Table 2
Potassium fertilizer recommendations for all Missouri lawns based on soil test results.
Soil test available potassium**
0 to 100 pounds per acre
100 to 200 pounds per acre
200 to 300 pounds per acre
300 to 400 pounds per acre
400 + pounds per acre
G6954, reviewed June 2008