Lawn Fertilizer and Rain

Healthy green lawn from proper fertilization.

Fertilizer is an organic or synthetic substance that uses a combination of nitrates to make soil fertile. Lawn fertilizer primarily comes in granular, powder, liquid and pelletized form, which dissolves in moist soil. However, too much rain will wash fertilizer off the grass, which causes no nutrient absorption and fertilizer washout.

Fertilizer Nutrients

Lawn fertilizer contains three primary macronutrients: nitrogen, phosphorus and potassium. Nitrogen promotes cell division among plants and lush growth, phosphorus helps with photosynthesis and fruit production, and potassium helps plants absorb the other two primary macronutrients. Lawn fertilizer also contains a variety of micronutrients and secondary macronutrients to help with healthy growth; however, the quantities and nutrients vary depending on the brand of fertilizer used. Soil testing determines how much micro and macronutrients to add to soil to make it fertile.

Testing

Soil can have different nutrient levels in different areas, but soil tests can determine how much nutrients are needed for each area. An overabundance of fertilizer will cause fertilizer burn, whereas not enough fertilizer will cause less than desired growing results. According to the University of Hawaii's website, soil samples should be tested every one to five acres in each test area. University-based or private labs test soil nutrient levels for a nominal fee, and hardware lawn and garden stores sell soil-testing kits.

Application

Fertilizer application depends on climate, type of fertilizer and grass type. The University of Illinois suggests applying fertilizer in early May, early September and late fall. However, depending on the type of fertilizer applied, the application may be more or less frequent. For instance, the powder variety of fertilizer needs more application during the season than the pelletized fertilizer does because it absorbs into soil quicker. Fertilizer needs moisture to help soil absorb the nutrients; therefore, it is best to apply any type of fertilizer directly after a period of heavy rain or before a period of light rain.

Rain Warnings

Fertilizer applied directly before heavy rain spells will not absorb into the soil. The heavy rain causes a washout of the fertilizer, which removes it from the lawn and into runoff locations. According to the University of Minnesota, phosphorus, a macronutrient in fertilizer, causes the growth of algae and weeds in lakes. The growth of weeds and algae in lakes because of fertilizer runoff is a common ecological problem associated with lawn fertilizer. However, light to moderate rain will not cause a washout and will actually help the fertilizer absorb into the soil.

Liquid Fertilizer

Application for liquid fertilizer is different from the application for granular, powder and pelletized varieties. Liquid fertilizer is best applied directly after periods of steady rain because it is easier for the soil to absorb the nutrients. Liquid fertilizer absorbs immediately into soil within one to two days after applications, whereas the other varieties must dissolve before nutrients are absorbed.

References

• University of Illinois: Lawn FAQs
• University of Minnesota: Preventing Pollution Problems from Lawn and Garden Fertilizers
• University of Hawaii at Manoa: Soil Fertility Evaluation