Fertilizing Your Lawn

Few soils contain enough available nitrogen to maintain healthy turfgrass. Nitrogen shortages lead to very slow growth, yellowing of the plants, thinning of turf, weed infestation, and increased incidence of some diseases. On the other hand, over-feeding with nitrogen leads to excessive shoot and leaf growth, reduced root growth, low plant carbohydrate (food) reserves, increased susceptibility to environmental stresses, and some diseases. In fact, lawns containing grasses adapted to lower water and nutrient inputs, when over-fertilized with nitrogen and/or over-watered, will decline in health rather than improve.

Nitrogen contained in fertilizers comes from either inorganic or organic sources, and may be either quickly or slowly available for plants to use.

Inorganic FertilizersInorganic lawn foods, such as ammonium nitrate, are water-soluble or quick-release nitrogen sources; that is, nitrogen becomes available for plant use soon after water is applied. The response is quite predictable and results are often visible in 5-7 days. However, the effects are relatively short-lived. On sandy soils, high application rates of fertilizers combined with heavy watering or rainfall, usually causes nitrogen to leach through the soil beyond the turfgrass root zone.

Organic FertilizersOrganic lawn foods are either natural or synthetic and contain carbon in their chemical structure. Nitrogen from natural organic products becomes available only after the product begins to break down due to soil microbial or chemical action. These are considered slow-release nitrogen sources because nitrogen is gradually released to the soil and becomes available for plant use over a longer period. Soil temperature and moisture are key factors governing microbial activity and, thereby, nitrogen release. Compared to quick-release sources, these have a lower leaf-burn potential and can be applied at slightly higher rates, less often, and without damaging turf.

The primary synthetic organic fertilizer is urea, which is considered a quick-release nitrogen product. However, urea is often processed and/or combined with other materials to create products with a more or less slow-release characteristic, such as polymer-coated urea. As with natural organic fertilizers, nitrogen release from slow-release sources is dependent on soil temperature, moisture, and chemical and/or microbial action.A soil test will determine if your lawn has adequate levels of phosphorus and potassium to sustain its health and, if not, how much to apply. Unless otherwise indicated by soil test results or by local regulations (phosphorus is not legal in fertilizers in Minnesota unless you are a licensed applicator), fertilizers should contain nitrogen, phosphorus, and potassium (N, P, K) in ratios of 4-1-2 or 3-1-2, respectively. Where soil tests indicate ample phosphorus, purchase a fertilizer with the middle number at zero.

A note on phosphorus in Minnesota: The state has banned the use of phosphorus in fertilizers sold to the public in Minnesota. This is because phosphorus that runs off into the drainage systems (storm water sewers) causes high levels of algae grown in the water sources that collect water runoff such as the lakes, ponds and rivers.