Soil testing

Lawn Care/Basics for Homeowners-Lawn Care/Fertilization/Soil Testing

Soil Testing

Image Source: Texas A&M Turfgrass Program

Evaluating the nutrient status of your soil is an important step in developing a quality turf. The best way to find out what your soil needs is to have it tested every three to five years. You can obtain information about soil testing at most local county Extension Offices. The following should help you interpret the soil test results and provide guidelines to improving your soil's nutrient status.

Taking a Soil Sample

Contact your county Cooperative Extension office or local soil testing lab (check the yellow pages) for information about fees, sampling bags and procedures and mailing containers.
Sample to a 3-inch depth using a small shovel or soil probe.
Take 10 to 15 samples per lawn, first removing thatch and live plant material.
Combine samples from similar areas to form a composite sample, but do not mix soils of different texture and color, from areas with sharply different elevations, or areas from disturbed and undisturbed sites. If you have more than one sample to submit to the laboratory, clearly indicate the area in your lawn it was taken from so you know which areas the lab results apply to.
Air dry the sample before sending.

Click here for a directory of most State Cooperative Extension Service soil testing programs.

Soil Test Report

The following are usually included in the laboratory soil test report:

Soil pH
Phosphorus (P)
Potassium (K)
Calcium (Ca) and Magnesium (Mg)
Cation Exchange Capacity (CEC)

Soil pH

Soil pH indicates whether the soil is acidic, alkaline, or neutral. The availability of soil nutrients to the plant depends heavily on soil pH.

Soil pH Below 6.0

Do not apply lime unless it is recommended by soil tests. Use the following guidelines for liming:

Apply no more than 100 lb. of agricultural lime per 1000 sq. ft. of lawn.
Ground agricultural limestone or ground agricultural Dolomitic limestone are preferred. Hydrated lime should not be used in lawns, because it is very reactive and could burn turfgrass at higher application rates
Applying lime after an aerification increases the effectiveness.
Agricultural lime is very slow acting, so the soil pH will change slowly.
The soil should be retested three years after application to check for pH change.

Soil pH from 6.0 to 7.5

This is the optimum pH range for turfgrass growth. It should not be necessary to adjust a soil pH in this range.

Soil pH above 7.5

Sulfur is the only product recommended for lawns to reduce a high soil pH (alkaline). Unfortunately, adding sulfur to a residential lawn to reduce pH is not economically feasible. To compensate for high pH, increase the annual application rate of nitrogen, phosphorus, and potassium by 25%.

Phosphorus (P)

Below are the normally recommended rates of phosphorus for established turf, based on soil test results. Starter fertilizers are high in phosphorus and should be applied at 1.5 lbs. P₂O₅/1000 sq. ft. when establishing or renovating a turf area.

Annual phosphorus (P₂O₅) recommendations for established turf.
Soil test results			lb. P₂O₅ per 1000 ft² per year
Level	lb. P per acre	parts per million P	lb. P₂O₅ per 1000 ft² per year
low	0-25	0-13	3^a
normal	26-50	13-25	2
high	51+	25+	1
^a To increase the amount of phosphorus applied while maintaining the same amount of nitrogen applied, use a fertilizer high in phosphorus for the late fall application. The phosphorus (P) percentage should be larger then the nitrogen (N) and potassium (K) percentage. An example would be 5-10-15 or 17-23-6.

Potassium (K)

Below are the normally recommended rates of potassium for established turf, based on soil test results. Potassium deficiencies are most common in coarse-textured soils. Muriate of potash (0-0-60) can be used to quickly increase soil potassium levels. But muriate of potash can burn leaf blades, so should be applied in cool, dry weather and the area watered soon afterwards.

Recommended annual potassium (K₂O) applications based on soil test results.
Soil test results			lb. K₂O per 1000 ft² per year
Level	lb. K per acre	parts per million K	lb. K₂O per 1000 ft² per year
very low	0-50	0-25	6^a
low	51-100	25-50	4^b
normal	101-150	50-75	2^c
high	151-200	75-100	1^c
very high	200+	100+	0
^a Apply 0-0-60 (muriate of potash) in the early spring and late fall at 2 lb. of fertilizer product per 1000 ft² for two years in addition to planned nitrogen applications.
^b Refer to notes in (a) above. Only apply the muriate of potash for one year.
^c Use complete fertilizer containing nitrogen, phosphorus, and potassium twice per year.

Calcium (Ca) and Magnesium (Mg)

Calcium and magnesium soil deficiencies can be remedied with the addition of Dolomitic limestone, but are rarely deficient if pH is above 6.0.

Cation Exchange Capacity (CEC)

CEC is a measure of a soil's capacity to hold nutrients. Higher CECs indicate a more fertile, higher organic matter soil. The value for an established lawn will not change significantly over time and cannot be adjusted by adding fertilizer or chemicals. It is used by the soil test laboratory to make nutrient recommendations for your lawn.

Nitrogen

There is currently no reliable soil test for nitrogen. Still, nitrogen fertilization is the key to a successful lawn. Therefore, other factors must be considered in determining how much nitrogen fertilizer to add. Refer to Fertilizing Established Lawns, for more information.

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