Types and Forms of Nutrient Sources - Nitrogen (N)
Few soils have enough natural nitrogen available to maintain acceptable turfgrass quality and allow rapid recovery from injury and stress throughout the growing season. Nitrogen shortages can lead to very slow growth, yellowing of the plants, thinning of the turf, weed infestation and increased incidence of some diseases.
However, over fertilizing with N can lead to excessive shoot and leaf growth, reduced root growth, low plant carbohydrate (food) reserves and increased susceptibility to environmental stresses and some diseases. In fact, in lawns containing grasses adapted to lower water and nutrient inputs as described in Chapter 4: Turfgrass Selection for Sustainable Lawns, over fertilizing with nitrogen and/or over watering results in a decline in health rather than improvement. Let's look at the characteristics of the two sources of nitrogen fertilizer.
The water soluble nitrogen (WSN) is available rapidly and gives quick results. The 22.5 percent WSN on the fertilizer label example is more likely to "burn" the lawn if spilled or over applied. When more nitrogen becomes available from these sources than the plant is able to utilize, there is an increased risk of water carrying the excess nitrogen down through the soil (leaching) and potentially contaminating ground water sources. This is particularly true where a nitrogen application is soon followed by heavy rains or excessive irrigation.
The water insoluble nitrogen (WIN) is not available to your turfgrass right away, so it can release the nitrogen over a period of weeks, months or longer. The 7.5 percent WIN on the fertilizer label example has a low burn potential if accidentally spilled on the lawn. (However, all spills should be cleaned up as quickly as possible to prevent possible injury to the lawn.) In addition to the above advantages, the WIN will stretch out the time between applications and give greater environmental protection since WIN will not leach as rapidly into groundwater. This is especially important on sandy soils.
Both WSN and WIN sources can be good choices for managing your lawn. However, the higher the percentage of WIN in a fertilizer, the higher the cost, as WIN sources are more costly to include than WSN sources.
Nitrogen Release Rates
Inorganic fertilizers, such as ammonium nitrate, ammonium sulfate and the organic fertilizer urea, are all water soluble or quick-release N sources. That is, N becomes available soon after water is applied to the turfgrass. Their response is quite predictable and results are often visible in 5 to 7 days. However, the effects are relatively short lived. On sandy soils, high application rates of these products, combined with high irrigation or rainfall amounts, will likely result in higher N losses due to leaching. Leaching is the movement of water along with any nutrients being carried by the water beyond the turfgrass root zone.
Organic fertilizer products, natural or synthetic, are those containing carbon (C) in their chemical structure. Nitrogen from synthetic natural organic sources becomes available only after the product begins to break down due to soil microbial action. These are considered slow-release N sources because it is gradually released to the soil solution and available for plant use over a longer period. Soil temperature and moisture are key factors governing the microbial activity and thereby the N release. Compared to quick-release sources, these have a lower leaf-burn potential and can be applied at slightly higher rates, less often, without damaging the turf.
The primary synthetic organic fertilizer product is urea. It is considered a quick-release N product. Urea has been further processed and/or combined with other materials resulting in products with more or less of a slow-release characteristic. Nitrogen release is dependent on soil chemical, soil temperature and/or microbial action. These slow-release fertilizers have a fairly low leaf-burn potential and can be applied at slightly higher rates and less often than quick-release N sources.
Figure 7.1 shows the growth effects of quick release and slow release nitrogen sources on turfgrass. Optimum growth is often best achieved with the combination of both quick and slow release sources.
Characteristics of common turfgrass nitrogen sources are summarized in Table 7.5.
Table 7.5. Characteristics of Common Turfgrass N Sources
Classification, burn potential, leaching potential, low temperature response, and residual effect on common turfgrass N sources.
|Fertilizer Source||Leaching Potential||Burn Potential||Low Temp. Response||Residual Effect|
|Activated sewage sludge||Very Low||Very Low||Very Low||Long|
|Manures||Very Low||Very Low||Very Low||Long|
|Corn Gluten Meal||Very Low||Very Low||Very Low||Long|
|Polymer-coated urea||Low||Low||Low||Moderate to Long|
|Polymer-coated, sulfur-coated urea||Low||Low||Low||Moderate to Long|
|Isobutylidene diurea (IBDU)||Mod. Low||Low||Moderate||Moderate|
|Urea-formaldehyde (often abbreviated "Ureaform")*||Low||Low||Low||Moderate Long to Long|
|Methylene urea||Low||Low||Low||Short to Long|
|Stabilized nitrogen**||Low||Low||Low||Moderate to Long|
*some products may contain urea in addition to the urea-formaldehyde component.
**sources usually contain nitrification and/or urease (a soil enzyme responsible for the initial breakdown of urea fertilizer into ammonia and CO2) inhibitors both of which reduce potential gaseous and leaching losses of nitrogen from the turfgrass rootzone thereby keeping nitrogen available to the grass plant for a longer period of time.
Other Organic Growth Enhancers, Etc.
Recently, a number of organic products have appeared on the market as "biostimulants," "growth enhancers," "soil enhancers" and have claimed other effects to improve both the soil and lawn. This has also been an area of active research to evaluate these materials and their effects. In some instances, there appear to be some benefits while in other situations results appear to be inconsistent at best. As these materials are further tested, evaluated and used more widely, it is likely that some of them will provide consistently beneficial results to our soils and turfgrass communities. It may be best to proceed cautiously when trying these products and evaluate their use and effects on a limited area before expending the dollars and time to apply to large areas.
Grass clippings should be returned to the lawn whenever possible. As grass clippings easily decompose once in contact with the soil, they do not contribute to thatch build-up. Grass clippings are a valuable organic source of nutrients, especially nitrogen (N). As they decompose, these nutrients become available for use by the grass plant. In fact, yearly nitrogen applications may be reduced by one-third to one-half when grass clippings are returned to the lawn.
Proceed to Types and Forms of Nutrient Source - Phosphorus.