Spring weather has arrived and the need for a sprinkle of fertilizer to spark a tinge of green may be realized throughout the region. Lawn fertilizer choices abound: differing brands, synthetic or organic, and quick release nitrogen or slow. A main determining factor that should be considered before selection is the nutrient analysis. This fertilizer feature is commonly overlooked or misunderstood, but can have a profound impact on the health of your soil, and more importantly the health of our streams and waterways.
While some fertilizers also supply nutrients like iron and sulfur, the main three numbers on the bag are N (nitrogen) – P (phosphorus) – K (potassium), representing the percentage of each nutrient in the bag. These three macronutrients are needed in relatively high levels by the plant. Nitrogen is most limiting in a lawn, since nitrogen is mobile in the plant, moves to young leaf tips, and is subsequently mowed off. Nitrogen is more limiting in a young lawn than an older one, since organic nitrogen sources, including mulched leaf clippings, have not built up in a new lawn soil. A soil test can't assess nitrogen needs for lawns, since the nutrient is not stable in the soil and release rates fluctuate wildly.
The level of phosphorus and potassium along with the estimated plant need can be estimated by soil tests. These two nutrients are found in many soils, and as shown in figure 1, phosphorus is found in very high levels in the metro St. Louis area by the MU soil testing laboratory.
In established turfgrass, phosphorus deficiencies are rarely observed for two main reasons. Turfgrass roots secrete an enzyme that degrades organic phosphorus making phosphate available. Also, the turfgrass root system and associated soil organic matter support a diverse microbial community that also produces enzymes making phosphorus available to the plant. Excess phosphorus, on the other hand, can cause many problems. From a plant health perspective, excess phosphorus will act to tie up other micronutrients, including iron, copper, and zinc, and make them unavailable to the plant. A span of 3-5 years may be necessary until this condition is naturally balanced. Excess phosphorus may also end up in our streams and waterways causing damaging eutrophication and harmful algal blooms (see https://extension2.missouri.edu/g9181).
While excess potassium doesn't cause quite as much plant harm as phosphorus, the element can also make micronutrients unavailable to the plant. Micronutrient applications in lawns should not be needed for maintenance, but with an excess of phosphorus and/or potassium they might be. Why spend the extra money?
Staying out of an excessive phosphorus and potassium situation is quite simple following the below guidelines.
Last but not least, clean up after making a fertilizer application or whenever using a spreader. Invariably, fertilizer granules will end up on driveways, roads, or sidewalks. Use a hand-held blower and get those granules back on your lawn where they can be used in the plant system. Otherwise, rain pushes the fertilizer down the concrete conduit to storm sewers, eventually polluting our important water resources.
REVISED: February 21, 2017