Plants use nitrogen for many different functions. Without N, plants could not manufacture peptides, amino acids, proteins, enzymes, chlorophyll, or nucleic acids (the building blocks of DNA and RNA). These components are essential for plants to grow and function properly. The reactions that occur during photosynthesis require a lot of nitrogen. If N is deficient during photosynthesis, less chlorophyll is produced and plants begin to lose their green color (chlorosis). Turf responds to many stress conditions by producing special proteins and other metabolites that are rich in nitrogen. Inadequate access to N renders plants more susceptible to stress-related problems. On the other hand, excessive applications of nitrogen will also contribute to problems. Plants respond to abundant N by increasing top growth and arresting root growth. Free amino acids can linger in the leaves that can invite both foraging insects and disease pathogens. In addition, a guttation fluid that is rich in nitrogen and other nutrients can exude from the leaf tips-a phenomenon that is analogous to raising a banner that reads, “EAT AT JOE’S.” When the guttation fluid dries, the leftover salts can burn the leaf tips. Another potential for problems is that when plant growth is pushed by excess nitrogen, the production of their defense compounds is often suppressed.
Applying just the right amount of nitrogen is not only extremely important but it is almost impossible because conditions that determine the amount of N a plant needs at a given time are constantly varying. Air and soil temperature, moisture, HOC, the angle and intensity of the sun, and the variety of grass all affect the rate at which the turf plant will use nitrogen. The optimal amount of N today could be excessive or inadequate tomorrow. Plants have the ability to respond to both deficient and excess nitrogen but it is not always enough. When nitrogen is inadequate, plants produce sugars in the leaves that are quickly transported into the roots. These sugars feed roots and enable them to grow farther into the soil in search of more nitrogen. If too much N is available, the leaves produce amino acids which increases shoot growth. Sugar production is all but arrested and root growth is suppressed. This slows down the absorption of N through the roots.
Plants don’t understand the difference between nitrogen that is biologically released from organic sources and the soluble kind that is applied by modern superintendents. Nitrate is nitrate to the plant. But they also have a hard time understanding and adapting to the rags-to-riches scenario associated with many chemical-feeding programs. During periods when N is inadequate the plant responds by elongating roots. When, all of a sudden, a tidal wave of nitrogen is available from an application of soluble N and an abundance of roots absorbs more than the plant needs, some serious side-effects that include disease susceptibility, insect attraction, burning, and other problems can occur. In a healthy soil ecosystem, however, there are mechanisms that buffer and regulate the amount of N available to plants.
Many soil organisms need nitrogen more than plants but they need it balanced with a certain amount of carbon. This balance occurs in proteins that reside in organic residues and friable humus, and in the prey upon which many soil organisms feed. In fact, one of the main ways in which organisms make N available to plants is through predation of other organisms. Predators such as bacteria-feeding nematodes, for example, don’t need or want the amount of nitrogen inherent in their prey and release the excess as ammonium, which is biologically converted into nitrate that turf roots can absorb. Applications of compost or natural organic fertilizers compliment the functions of soil organisms and they, in turn, regulate the amount of N available to plants. This is not to say that soluble N should never be used but if it is the only source of nitrogen applied, biological activity will likely be suppressed to a point where nutrient regulation will be inadequate. There has to be available digestible carbon if soil organisms are going to regulate applied nitrogen. If the turf ecosystem is healthy and functioning properly, judicious applications of soluble N can be regulated by soil organisms. Many soil organisms can use soluble nitrogen, which, in turn, moderates the amount available to plants. As predation and other biological activities occur, more nitrogen is released for turf roots to absorb. Coincidentally, the activity level of the organisms that release N and other available nutrients respond to many of the same conditions that stimulate plant activity so the availability of nutrients is synchronized with need. Spoon-feeding soluble N is rarely as ideal.
If large doses of soluble N are applied, both plants and organisms can be overwhelmed. High salt fertilizers act like a sponge and draw moisture away from the surrounding area. This can cause osmotic shock, killing organisms in close proximity. Eventually, as the salt is dissipated within a greater solution, the dissolved nutrients change from harmful to helpful and can be absorbed by some soil organisms. Once assimilated into a living organism, the nutrient is somewhat stable until the organism dies or is preyed upon. If it dies, nitrogen may or may not be released depending on the saprophyte that consumes the remains. If the saprophyte requires less N than what is available from its food source, then some will be released; however, if it requires more, then nitrogen may be immobilized from other sources. In a turf soil, the former scenario is more likely than the latter. If a predator eats the organism, there is almost always a release of N in a form available to plants. Not only can this system increase nitrogen efficiency but it can also sustain the release of it for a longer period of time. Unfortunately, very little of this biological regulation will occur on a green constructed of sand, topdressed with sand, and fertilized purely with soluble salts. There needs to be healthy, active, well-fed and diverse population of soil organisms and this is next to impossible to maintain in an environment without adequate levels of organic matter. The sand based environment on most greens serves as an almost inert medium that provides mechanical support for the turf plants. It is rarely cultivated as a habitat for soil life. Old greens, however, may have accumulated enough organic matter over the years to provide adequate habitat for soil biology.
Developing a program that addresses the biological needs of the soil is a logical step toward improving fertility. The use of well-made, well-aged compost in the topdress mixture, compost tea in the irrigation system, and natural organic fertilizers in the spreader can not only reduce the need for soluble N but increase its efficiency as well. In other words, less soluble nitrogen can accomplish more. Applications of compost during spring and fall core cultivation incorporates valuable resources for a diversity of soil organisms. The addition of compost tea and natural organic fertilizers provide more nutrients and even some inoculation. If soluble N is still needed, small doses can react synergistically with biological functions that increase overall nitrogen efficiency. It may not be needed, however. Well-made, mature compost can contain anywhere from 1/2 to 3 percent nitrogen depending on the feedstock from which it was made. Don’t expect a great flush of growth, however. The release of N from compost is slow and sustained and depends on factors such as temperature and moisture. If turf is growing in a very cold soil, some soluble N may be necessary to fulfil the plants’ needs until the soil warms up.
Natural organic nitrogen is synonymous with protein. One of the reasons that it is more expensive than its chemical cousin is that it has a greater value in the animal feed and pet food markets than it does as fertilizer. Protein is not an available nutrient for plants. The chemical structure of protein is too large and complex to be assimilated through plant roots. Protein can, however, be assimilated by soil organisms. When proteinaceous materials come in contact with the soil, organisms begin dismantling it into amino acids and peptides. Most of the nitrogen and carbon is consumed and temporarily immobilized but some is released and mineralized by other organisms into simple nitrogen ions that plant roots can absorb. The population of saprophytes that consumed the protein grows exponentially which results in a relative increase in the number of predator organisms that feed on them. The result of this increased predation is an increase in available nitrogen for plant roots but the release is steady and sustained unlike the tsunami of pabulum that is typical of soluble fertilizers. An additional advantage to using proteinaceous nitrogen is that it is extremely efficient. Very little if any is ever lost to leaching, volatilization, or denitrification and the corresponding increase in biological activity can often reduce plant susceptibility to disease infection and attractiveness to some foraging insects. Taking into consideration its efficiency and its ability to nurture an active soil food web (which can lead to significant savings elsewhere), the value of natural organic nitrogen can be considered more relative to its price.
Even though golf course conditions aren’t found anywhere in nature, contemporary fertilization practices do not in any way resemble natural feeding phenomena. The use of soluble, available nutrients bypasses the digestive system of plants, i.e., the living biomass in the soil. A fertility program that ignores the needs of these organisms is, in a sense, repudiating their value and importance. The assumption that these soil organisms have little to no worth may be the main reason why managers in both agriculture and horticulture are so dependent on chemicals.
The preceding text was excerpted from Ecological Golf Course Management, published by John Wiley & Son, 2002