Rain Washes Away Fertilizer Into Local Waters: Myth Debunked

A persistent lawn myth is that rain washes away fertilizer. It isn’t too hard to understand the reason for the myth: Everyone has seen a heavy rain runoff the lawn and go down the storm drain – presumably carrying the fertilizer with it. But wait! The grass turns green after a rain! If all the fertilizer is gone, why would the grass turn green – shouldn’t the grass turn yellow?

The reason the lawn turns lush green is because the fertilizer did not wash away. What happened is rather complicated organic chemistry. The simple way to think of it as a magnet and iron filings. The organic matter – grass leaves, debris and even the soil is magnetized. The fertilizer is like iron filings – it clings to the organic matter and soil. Rain water does not wash away the fertilizer.

Why Does Rainwater Contain Nitrogen and Other Fertilizers?

When you fertilize your lawn you saturate the soil with all the fertilizer the organic matter and soil can hold. Rainwater naturally has some nitrogen and other fertilizers in it – the atmosphere is 78% Nitrogen and it is absorbed by the rain along with dust and other “stuff”. With the soil saturated with nitrogen no more can be held by the magnetized organic matter and soil. So what runs off in the rainwater is a natural excess of nitrogen and other fertilizers – not the fertilizer you just put on the lawn.