One of the most important functions of soil is to provide nutrients to the root systems of plants to support their growth and development. Soil nutrients needed by turfgrass and other plants are retained on the surfaces of soil particles and organic matter called exchange sites. The total number of nutrient exchange sites is referred to as the cation exchange capacity (CEC) of the soil. The higher the CEC value, the richer the soil is in the contents of nutrients. Turfgrass plants require 16 different nutrient elements for healthy growth. Carbon (C), hydrogen (H) and oxygen (O) are generally provided from water and gas. Nitrogen (N), phosphorus (P) potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) are essential elements (macronutrients) that are needed in large quantities by turfgrass plants. Micronutrients are required in smaller amounts, but are also essential, since a deficiency in one of these nutrients can have a detrimental effect of growth and development. Micronutrients include iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), boron (B), molybdenum (Mo), and often chlorine (Cl) is included. A typical concentration of nutrients in Kentucky bluegrass leaf clippings would be 4.6 percent nitrogen, .6 percent phosphorus and 3 percent potassium, with Ca, Mg and S constituting 1.8 percent and the rest in micronutrients (see Figure 1). This then translates into the types of fertilizer used on sports fields and the natural ratio of N:P:K in the turf plant is used as the benchmark for many of the standard or maintenance fertilizers, which would have an analysis like 5-1-3.
Fields deficient in nitrogen are more likely to get diseases like red thread (pictured), dollar spot, rust, patch diseases and leaf spots.
PHOTOS COURTESY OF PAMELA SHERRATT.
Nutrients are lost from turf in many ways: leaching, runoff, volatilization, denitrification and fixation. Another major way that nutrients are lost from turf is by removing clippings, since they contain about one-third of the turf’s annual nitrogen needs. The key is to minimize as many of these factors as possible. It is also important to note that correcting nutrient deficiencies with applications of fertilizer may not make the deficiency problem go away. Turfgrasses take in nutrients via their roots (foliar feeding is also a sand-based field management practice, but for the sake of this article we can assume that the vast majority of nutrient uptake is from soil solution or colloid surfaces) and as such, if the root system is poor, then uptake is adversely affected. If the turfgrass plant cannot take the nutrients up, applying fertilizer is futile. The key is to maximize root growth and prevent poor soil conditions (drought, saturation, compaction), so that turfgrass roots can take nutrients up. Each spring in Ohio when there are prolonged heavy rains, we see yellowing and chlorosis attributed to nutrient deficiency, because root systems are saturated and not able to function.
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Adapted from Turfgrass Soil Fertility and Chemical Problems: Assessment and Management. Carrow, R.N., Waddington, D.V., and Reike, P.E. 2001. Pub: John Wiley & Sons.
Diagnosing nutrient deficiencies on sports fields can be tricky. Discoloration, poor density and slow growth could be contributed to a whole range of problems like drought, soil compaction, poor drainage, or pest and disease issues. Classic symptoms of nutrient deficiency can be seen in Table 1, but keep in mind that differences in color can be caused by other plant stresses, like photoinhibition.
Figure 1: Turfgrasses need 16 essential elements.
The only way to accurately determine how much fertilizer a sports field needs, or to determine if there are nutrient deficiencies, is to have the soil and/or tissue tested. Native soil fields are typically tested every three years, and they generally do not have nutrient deficiency problems if they are on a fertilizer program. Sand-based fields (\>85 percent sand) require two to three tests per year, as they are much more prone to nutrient deficiency. Conducting a soil test provides a historical record of the soil pH and soil nutrient status. This supplies information on how well last year’s fertility program provided for the needs of the turf. It also provides valuable information that can be used to develop a fertility management plan for the current season. Soil pH and soil nutrient analyses are critical. In particular, soil pH plays a significant role on the availability of nutrients in the soil (see Figure 2). Certain physical analyses are optional. Although most physical parameters don’t change much over time, you should still know them for the fields you manage. The chemical analyses provide information on:
- Soil pH
- Level of available phosphorus, potassium, calcium and magnesium
- Level of available iron, manganese, zinc and copper
- Cation exchange capacity and base saturation levels
These results can then be used to determine the field’s seasonal fertilizer needs and cultivation requirements, to diagnose/predict any turf problems and, ultimately, to maximize turf quality. Why doesn’t the soil test report include nitrogen since it is the most important element for turfgrass growth and recovery? Because soil nitrogen is present in the soil in many forms and those forms fluctuate too rapidly to give an accurate and reliable predication of available nitrogen. Having a tissue nitrogen test or a soil nitrate test done will give the sports turf manager a snapshot of the current nitrogen status, but it cannot be used as a season-long predictor. The Illinois Soil Nitrogen Test (ISNT) is useful for predicting crop fertilizer response to applied nitrogen. However, recent research conducted at Ohio State University suggests that there may be too much variability in thatch mineralization rates to make the ISNT a useful tool in turfgrass.
A guide to conducting a soil test
1. Take a representative soil sample. Walk in a zigzag pattern from end zone to end zone. Using a soil probe, collect 15 to 20 random soil cores at a 3-inch depth. Remove the upper turf and thatch. Collect the soil in a clean plastic container and mix it to make a composite sample. Send 1 pint of soil to a reputable soil testing laboratory.
3. Use the same laboratory to ensure consistency in test methods and results. The best sampling times are spring and/or fall prior to fertilization, and it is best to sample at the same time every year. Be consistent in the overall sampling procedure from year to year.
4. Allow one to three weeks for test results.
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Table Footnotes: Cation exchange capacity (CEC): CEC measures the capacity of the soil to hold exchangeable cations (nutrients). The cations include hydrogen, calcium, magnesium and potassium. The CEC depends largely on the amount and type of clay present and the organic matter content. The higher the CEC value, the more cations the soil is able to hold against leaching. It is not practical to attempt to increase the CEC of a soil by adding clay or organic matter on a large-scale basis. Liming an acid soil will slightly increase the effective CEC.
Base saturation: % Calcium, % Magnesium and % Potassium: Base saturation is the extent to which the adsorption complex of a soil is saturated with exchangeable cations other than hydrogen or aluminum. It is expressed as a percentage of the total CEC.
Calcium to magnesium ratio: This ratio is calculated on the basis of percentage saturation of the soil CEC by each element. This ratio should be considered when lime is added to the soil. If the ratio is 1:1 or less (less Ca than Mg), a low magnesium limestone should be used. Turfgrasses grow over a wide range of ratios with the ideal ratio being about 6 to 10:1.
Magnesium to potassium ratio: This ratio should be greater than 2:1. In other words, the percent base saturation of Mg should be at least two times the percent base saturation of K. High K frequently results in reduced uptake of Mg by plants. Therefore, to help prevent plant nutrient imbalance, additional Mg may be required to maintain a Mg to K ratio of at least 2:1.
Tissue testing is common on sand-based fields as it allows for close monitoring of leaf tissue nutrient levels, particularly nitrogen. More research is needed to correlate nutrient levels in tissue with turfgrass response, however it can be used as a diagnostic procedure. For example, turf with high dollar spot disease pressure may show a low percentage of tissue nitrogen.
Starter fertilizer applied (right), versus none (left). Fertilizer has a significant effect on turfgrass growth and development, particularly on sand-based fields.
It is important that the soil laboratory chosen is reputable and familiar with the sports turf industry. The lab will produce a concise report. It is worth noting that labs may report different results. This does not necessarily mean that one lab is right and the other wrong; the variation is usually due to the labs using different soil-testing methods that will give different results. For example, an analysis for available nutrients in soils is not absolutely precise, but this is unimportant because no one should be maintaining nutrient status on the absolute verge of deficiency. A turf manager who is concerned with lab results, or who is interested in better understanding soil test procedures, should contact the lab.
Results from the soil test will typically categorize each nutrient as low (deficient), medium, high and surplus. Recommendations for correcting deficiencies are stated in pounds of fertilizer per 1,000 square feet (metric conversion: 1 pound nitrogen per 1,000 square feet = 50 kilograms per hectare). A general guide to the optimum soil-test levels (sufficiency ranges) for sports turf surfaces can be found in Table 2. More highly managed sports fields should target for the upper end of the sufficiency range.
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dr. John Street has been a professor in turfgrass science at Ohio State University for the last 30 years.