Turfgrasses require 14 essential mineral nutrients for healthy growth. Six of those - nitrogen, phosphorus, sulfur, potassium, magnesium and calcium - are considered macronutrients, as they are needed in fairly large amounts. The remainder - iron, chlorine, copper, manganese, zinc, molybdenum, boron and nickel - are termed micronutrients, as they are needed in smaller amounts but are essential nevertheless.
All nutritional elements are equally important, since deficiency in any one of the elements can cause plants to grow abnormally and even die. Macro and micro simply refers to the relative amounts needed by plants.
Nitrogen (N) is the mineral needed in the greatest quantities in turf, so most fertilizer programs are built on nitrogen requirements. N is important because it is a component of amino acids (proteins), nucleic acids (DNA, RNA), chlorophyll, ATP, NADH, NADPH and alkaloids. Nitrogen is also the element that has the greatest effect on color and growth. Nitrogen is abundant in the ecosystem (21 percent of the atmosphere is N2), but it is not plant available until converted to NO3 (nitrate) or NH4+ (ammonium). This conversion process is called the "nitrogen cycle," and it involves input from many forms of N, including atmospheric N, plant residue, manure/organic materials and commercial fertilizer (Figure 1).
Organic nitrogen sources
Supplementing N to turf soils is not new. It has been practiced for centuries through the use of organic N sources. These include animal products and byproducts such as manure, blood/bone/fish meal or guano from bat caves.
Many municipalities recycle sewage wastes in an attempt to partially restore normal function to the cycle, however, caution has to be used with the sewage because it often contains a high proportion of heavy metals from industrial sources.
Many turf managers still use these sources. Some of the organic fertilizers supply N in a form that can be very odiferous when it decomposes. This odor creates problems with neighbors who do not appreciate the finer side of turf management!
Plant materials, like corn gluten meal, yard waste, and leaf or mushroom compost can also be a source of N. Organic N sources typically do not have nitrogen contents higher than 10 percent. Natural organic N fertilizers cost more money per pound of nitrogen than synthetic sources, and they are generally reliant upon soil temperature for release. Still, there is a belief that natural sources not only feed the turf, but that they can improve the soil health in a turfgrass system.
Natural fertilizers that contain bulky organic material may also help to improve a soil's bulk density and nutrient status over time. Organic fertilizer sources are manufactured in a variety of forms. Some, like Milorganite, Easy-Flo and corn gluten meal, are granulated and can be applied through a rotary or drop spreader. Others are more bulky (compost) and are applied by hand or topdresser.
A typical application schedule for compost would be once or twice a year at a .25-inch depth, applied as a topdressing and brushed/dragged in, ideally performed in conjunction with aerification. The benefits of topdressing or amending soils with compost are well documented and include:
<0x2022> An improvement in soil bulk density and water holding capacity
<0x2022> A source of organic slow-release fertilizer
<0x2022> Prevention and suppression of disease
<0x2022> A decrease in pesticide usage
<0x2022> Carbon sequestration
<0x2022> Less landfill waste
<0x2022> It is a renewable resource and considered sustainable
Even under intense sports traffic, applications of compost can improve turf quality and ground cover (Marcela Munoz, MS degree)
Compost quality is heavily regulated by the U.S. and state Environmental Protection Agencies to make sure it is safe for use. It is tested for maturity, pH, and the presence of heavy metals, salts and, in the case of food waste or sewage sludge, harmful bacterium like salmonella and fecal coliform. It is also important that the compost is suitable for turfgrass growth, with an appropriate pH and organic matter content.
Criteria for choosing suitable compost can be seen in Table 1. In addition to making sure the compost is safe, there are several issues with using composts on turf that sports field managers should be aware of:
They typically have a large amount of phosphorus (P), particularly the sewage and manure composts. It is not uncommon to see a N:P ratio of 1:1 or 2:1. For example, a chicken manure in our trial has an analysis of 5-3-2, so for every pound of N being applied, over a half-pound of P is also being applied.
In a trial at Ohio State, we have seen phosphorus levels increase from 63 pounds P per acre (the untreated control plots) to 800 pounds P per acre in just three years using a sewage-based compost topdressing at a .25-inch depth two times a year. Natural organics are not yet regulated for levels of phosphorus as synthetic fertilizers are, but that may change.
If the compost has a high salt content, it can have a detrimental effect on seed establishment, particularly in the summer months.
Manure and sewage-based compost have a foul odor for one to two weeks after application. Because they are dependent on soil microbe activity for nutrient release, it may take several weeks to see benefits after the first application.
Applying compost at a .25-inch depth also adds quite large amounts of nitrogen to the system, resulting in very green turf and excessive top growth (resulting in much more mowing). Lower rates may be the answer to this issue.
Recently there has been interest in the use of compost tea, a liquid extracted from brewing or steeping compost in water for several days. The Internet has exploded with articles about compost teas and the role they might play in turfgrass management, particularly for disease suppression. There are several published articles that suggest that compost teas do suppress turfgrass diseases, however, in a 2012 Crop Science article, Dr. Jason Henderson concluded that he saw no improvement in turf quality, color or ground cover on sports turf under traffic conditions. So, while bulk composts do appear to have a role to play in soil and turf health on sports fields, the compost tea debate goes on.
Synthetic nitrogen sources
Synthetic fertilizers are used in sports turf management for a number of reasons. Mowing and removing clippings removes much of the N-containing plant material that in natural systems is recycled. In addition, many sports surfaces are built on a sand medium, which does not have the ability to retain nutrients like a native soil medium does. Furthermore, sports fields are under intense traffic for much of the growing season and require additional nutrients to enhance recovery.
The manufacturing of synthetic N is through the Haber-Bosch process, whereby N2 is reduced to produce NH3 (ammonia). The process requires natural gas (aka methane, CH4), N2, O2, high temperature (provided by burning a fossil fuel), an iron/osmium catalyst and steam pressure. This process is the basis for N fertilizer manufacturing and results in granulated forms of fertilizer.
What sets most of the synthetic fertilizer sources apart from the natural organic sources is that they are cheaper to buy, contain higher percentages of N (i.e., more fertilizer for your money), and they are packaged in a way that makes them easy to apply with generic spreaders. Synthetic fertilizers are also purpose-made for specific growing conditions and specific plants. Examples of this would be a "starter" fertilizer containing phosphorus, or "late-season" fertilizer that contains a quick-release source of nitrogen.
Synthetic fertilizer can also be offered in slow or quick-release forms and can be combined with control products like crabgrass preventer or broadleaf weed control. Interestingly, just as minerals like phosphorus can be added to synthetic fertilizers they can also be removed. Companies like Scotts have recently removed P from their fertilizer products as a way of being proactive in the war on algae blooms in water.
In summary, a sports turf manager has to take a holistic approach to planning a fertilizer program, based on budget, desired turf quality, and also what's best for the environment and the local community. As defined by the USDA, a sustainable approach to growing plants must include profit over the long term, stewardship of the land, and quality of life for the community. So, while there are pros and cons in both natural and synthetic fertilizer programs (see Table 2), it may be that the best approach is to combine synthetics and naturals to get the maximum benefit from both.
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.