September 1995 - Volume II, Issue 9

Important Considerations for Turfgrass Soil Fertility

David M. Kopec, Extension Turfgrass Specialist

The soil component of the turf system is critical in determining the capacity for turfgrass growth. The soil system determines in part the water holding properties, and the mineral nutrition of the turfgrass community. The analysis of soil for turfgrass fertility management is the topic addressed below.

Fertility measurements of soils for turfgrass nutrition is relatively easy to understand. Soil analysis tests are valuable in showing the superintendent extremes in soil conditions. These extremes include above normal amounts of mineral elements, or very low amounts of specific elements, some of which show up as characteristic or classical nutrient deficiencies in turf.

A Look at Soils and the Nature of Soil Fertility
The size of soil particles and the relative distribution of all the soil particles affects soil fertility. Soil particles include sands, clays, silts, and organic matter. Soil particles can be imagined as having 'arms' which stick out of the soil particles. The more 'arms' that stick out, the more nutrients the soil particles can hold. Small soil particles such as clay and organic matter have a very high surface area, and many 'arms' to hold mineral elements. These 'arms' are negatively charged, and hold mineral elements which are positively charged. These positive charged minerals are called cations, and the capacity of a soil to hold cations is called the Cation Exchange Capacity (CEC). The higher the CEC, the higher the soils capacity to hold nutrients. Examples of cations include Sodium (Na+), Calcium (Ca++), Magnesium (Mg++), Potassium (K+) and Ammonium (NH+). Soils with low CEC potentials (sands) generally need fertilization at regular intervals with light application rates. This is so because the mineral elements from fertilizers added are not capable of being stored for long periods of time, and you do not want to wash away the fertilizers you apply. The mineral elements we are most concerned about appear on a standard soil fertility test. These include the elements of Nitrogen (N), Phosphorous (P), Potassium (K), Magnesium (Mg), Calcium (Ca) and Sodium (Na). Other items on a soils test include soil pH, soil salt levels (EC of electro-conductivity) and the amount of sodium relative to the amount of all the cations attached to the soil 'arms'. This is called the Exchangeable Sodium Percentage (ESP).

For general purposes, the relative range of cation exchange capacities for soils are as follows;

Sand = 1-6 Meq/100 gm.

Sand green = 1-14 "

Clay = 80-120 "

Clay loam soil = 25-30 "

Organic matter = 150-500 "

You can see why a clay loam soil or a garden rich in organic compost grows much better
plants than a sand ! Taking soil samples on turf requires that you follow a few rules. Sampling depths for golf courses and lawns should have samples taken to 3-4 inches BELOW THE THATCH LAYER. Almost 90% of the roots occur here (especially on cool season grasses) and that's where the action is for general testing of soils.

How to Take Soil Samples for a Soils Test

1). Use a soil probe or auger and penetrate to 3-4 inches. Remove any thatch and grass tissue.
2) Remove the soil from the probe.
3) Place the soil in a clean plastic bucket/pail.
4) Take at least 6-8 samples, repeating steps 1-3.
5) Mix all 6-8 samples together in the pail.
6) Put about 1 lb. (or 1 pint) of the mixed soil in a labeled soil bag.
7) Do not dry the soil in an oven - let it air dry if necessary.
8) Send to the soils lab for testing.
9) State the type of test you desire.

A standard test normally includes test for:
*salt levels (electro-conductivity or EC)
*pH correction amendments
(limestone - to raise pH)
(sulphur - to lower pH)

Interpretation and Results of Soil Tests
Mineral concentrations of elements in soils provide guidelines for soil fertility programs in general. They are not conclusive or completely interpretive for a prescription fertilization program. Minerals present in soils are almost always "naturally conserved", that is they are stored in soil and released slowly over time. Most soil test interpretations take this into consideration. Typical interpretations for western soils include the following level/nutrient classes.

Nitrate (NO3 -N-) Rating
0-5 ppm low
5-20 ppm medium
20-40 ppm high
above 40 ppm very high

Other tests may include total nitrogen, or may provide a value for total nitrogen only. Total nitrogen includes other forms of soil nitrogen, much of which is not readily available to turfgrass plants.

Co2 Extract Method Bicarb Extract Method Rating
0-3 ppm 0-10 ppm low
3-6 ppm 10-20 ppm medium
6-10 ppm 20-40 ppm high
above 10 ppm above 40 ppm very high
K-Level Rating
0-40 ppm very low
40-175 ppm low
175-250 ppm adequate
over 250 ppm high

Iron is generally abundant in mineral soils, but if free lime (calcium carbonate) is 'high' on the soils report, and iron deficiency may develop on turf. Iron deficiencies on turf appear as a yellow discoloration of the youngest leaf (furthest away from the ground), progressing from the tip downwards. this repeats on each successive younger leaf. A soil test value of 5 ppm or lower (DTPA extraction) may indicate iron availability problems for turf.
pH Adjustments
In our western soils, pH concentration's are almost always above neutral 7.0. Soil pH values of 7.5 to 8.5 are not uncommon. Soil tests for turf often include an interpretation which includes a sulphur requirement to help lower the pH. The sulphur counteracts the chemical reaction of the 'free lime content' (limestone) in the soils, which is a major contribution to high soil pH.

Free Lime Content Sulphur Requirement lbs./1000 ft2
very high 10% 1500
high 5-10% 750-1500
medium 1-5% 150-750
low 0-1% 0-150

You can see from the amount of sulphur required to change the pH of our western calcareous mineral soils is phenomenal! You will burn and most likely kill your turf if you apply more than 15 lbs. of sulphur/1000 ft2. Three applications per year at 1.0 lb. of sulphur/1000 ft2 is about all you can do. If a new construction site is being planned, then higher rates can be incorporated by disking, plowing, and ripping.
Gypsum is calcium sulphate. It is essentially a neutral salt and DOES NOT reduce soil pH. the proper use of gypsum is to correct a sodic (high sodium) soil problem over time. The calcium in the gypsum replaces the sodium from the "soil arm", and the sodium combines with the sulphate (sodium-sulphate) which must be then washed (leached) away. Soil tests which show a soil ESP value of 15% or more requires a gypsum treatment program. 

Return to Turfgrass Research
[Cooperative Extension] [AgInfo] [UAInfo]

University of Arizona Cooperative Extension
4341 E. Broadway Road
Phoenix AZ 85040-8807
602-470-8086 FAX: 602-470-8092

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

All contents copyright © 2004. Arizona Board of Regents.