Turf - Fertilization in the Spring

The following is information on spring fertilization in turf from Virginia Tech.

There is no better money spent than to have a soil test done on the turf areas at least every third year. A soil test is the only way to determine if the soil needs lime, phosphorus (P) or potassium (K). Apply nutrients as recommended by a soil test and you’ll be taking a huge first step towards protecting water quality.

Nitrogen gets the most attention. The one nutrient that won’t be analyzed in a soil test is nitrogen (N). That is surprising since N is the focal point of turf fertility programs. The reason is that N levels change rapidly in a soil and test results usually have little meaning by the time you receive the report. The test results will provide recommendations on appropriate N application levels suitable for the grass and location.

Spring is one of the trickiest times to optimize N fertility. Cool-season grasses such as tall fescue or Kentucky bluegrass have their most significant period of root development in the spring, so some N is beneficial. High rates of N promote a lush, dark green lawn, but there is too much emphasis on shoot growth rather than roots, and this often leads to serious problems with disease, insects, or drought later in the year.

Warm-season grasses like bermudagrass or zoysiagrass don’t initiate much root growth until after shoot greening is complete, so the ideal scenario is to wait at least until 50-75% green-up before applying N. Excessive spring N fertilization that promotes a lot of warm season grass shoot growth can be disastrous to the turf if there is a late freeze.

Choosing a N source.

When selecting a fertilizer for the spring take a close look at the fertilizer label. It tells exactly what nutrients are contained in the product on a percent by weight basis and indicates the N release rate. Most garden fertilizers typically contain large amounts each of N, P, and K (e.g. 10-10-10, 19-19-19, etc.). The N in these sources is almost always entirely water soluble (WSN) and provides quick plant response. Specialty turf fertilizers contain high N percentages and lower levels of P and K (e.g. 29-3-7). This doesn’t mean the P and K aren’t needed by the plant, but history and experience show that repeated applications of garden fertilizers over the years results in soils testing high to very high in P and K. Unnecessary applications of P and K over time can ultimately result in water quality concerns (from excessive P) and nutrient imbalance (likely from excessive K).

Specialty turf fertilizers also often contain different forms of N that provide varying N-release rates and the label indicates the percentages of both WSN and water insoluble N (WIN) making up the total % N in the product. Organic fertilizers are usually very low N analyses (6-10% by weight) and are mostly WIN (often up to 85% of the total N). The higher the % WIN the more slowly available the N is to the plant by way of chemical and microbial reactions in the soil. Any source containing 50% or more WIN can be safely applied at higher application rates with minimal concern for nutrient leaching. However, understand this -- the plant does not care what the source is, only that its needs are met with appropriate application levels and timing.

N rates

The level of N depends on the fertilizer source, the grass, and area of the state. However, here are some basic guidelines. Fertilizer sources that contain more than 50% WIN can be applied up to 1.5 lbs N/1000 sq ft; those that are predominantly WSN should be applied at no more than 1 lb N/1000 sq ft. Recommended N levels from now through the end of May on cool-season grasses should not exceed a total of 1.5 lbs/1000 sq ft for Kentucky bluegrass, tall fescue, or perennial ryegrass, and no more than 1 lb for fine-leaf fescue. The rest of the recommended seasonal N fertility shouldn’t be applied until this fall. For warm-season grasses, allow some greening to occur before applying N, and resist the temptation to initiate standard N fertility programs of up to 1 lb N/1000 sq ft until the average last frost date has passed. Finally, consider that zoysiagrass performs best at 1-2 lbs N/1000 sq ft total per year, whereas bermudagrass can safely receive up to 3-4 lbs total.

Adapted from "Spring Lawn Fertilization—Getting it Right!" by Mike Goatley, Extension Turf Specialist/Associate Professor Crop & Soil Environmental Sciences, Virginia Tech. Go to http://www.weblogs.cals.vt.edu/lawn_garden/ for the podcast.