There is some confusion over where to allocate fertilizer inputs. Some feel they should put out the same amount of fertilizer on all their land. Some feel that they should fertilize their poorer quality soils more heavily so that they will yield better. Others feel that they should spend the majority of their fertilizer money on their best soils and apply little, if any, fertilizer on the rocky, thin soils on hillsides. I fall into the last camp and feel that you should concentrate fertilizer inputs onto your best land. I'll try to explain why in this article.
First, remember what makes one soil more productive than another a more productive soil receives and holds more water than the poor soil and releases it more completely to plants. These are vital points to remember.
You may ask, "Why does one soil receive more water than another when they're close together?" Think about it for a minute. A flat bottomland soil is usually more productive than a hill soil. Why? One reason is that it actually receives more water than the hillside soil. Much of the water that falls on a hillside does not soak in, but runs off. Where does the water go? It goes to the flat bottomland soil, where most of it soaks in. So, the bottomland soil not only receives all the rain that falls on it, it also receives a lot of the rain that falls on the hillside soil.
Let's use, for example, a bottomland soil and a hillside soil that are side by side in a 30 inches per year rainfall environment. The hillside soil sees 30 inches of rainfall, but at least half of it runs off (the actual amount of runoff will vary depending on soil texture, percent slope and intensity of rainfall), leaving only 15 inches of water that goes into the soil. The bottomland soil sees 30 inches of rainfall, plus the 15 inches runoff from the hill, for a total of 45 inches of water that goes into the soil. Thus, the bottomland soil actually receives three times as much water as the hillside soil, even though they are side by side.
The amount of water a soil can hold is strongly related to depth of topsoil. The deeper the topsoil, the more water a soil will hold and release to plants. Obviously, a hillside soil has suffered more erosion over the years than a flat soil. Thus, the flat soil will usually retain more water for plants.
Does this affect the amount of fertilizer you should apply on these soils? Of course it does. The hillside soil has very little yield potential since it receives and holds very little water. I would advise using either low rates of fertilizer on this soil, or none at all, depending on the stocking rate desired. Putting high rates of fertilizer on this soil would be like feeding a Clydesdale draft horse a racehorse ration and expecting him to win the Kentucky Derby. It ain't gonna happen because the Clydesdale doesn't have racehorse genetics (although he might become faster than the average Clydesdale). In the same context, applying high rates of fertilizer to a rocky, thin hillside soil to try to make it do as well as a productive soil is unlikely to work because this soil doesn't have high yield potential.
The other factor in the equation is how much water the soil releases to plants. A sandy soil does not hold much water, but most of what it does hold is available to plants. A clay soil holds much more water than a sandy soil, but much of it is unavailable to plants. That's why a clay soil is sometimes more drought prone than a sandy soil.
A silt loam soil is the epitome of compromise between holding water and releasing it to plants. Organic matter is very effective at holding water in the soil, but still releasing it to plants. Obviously, the most productive soil will usually be a flat bottomland silt loam soil with high organic matter.
Let's look at the other extreme. What would be the most unproductive soil? To me, it would be a clay soil on a hillside. Why? First, the clay soil seals over when it gets wet. More water runs off it than off a sandy soil. Second, it's on a hillside, so it is getting less effective rainfall. Third, clays do not release water well to plants.
The bottom line is this concentrate your inputs on soils that have the potential to do well. Don't spend money trying to improve a poor soil until you have maximized the production on more productive sites. Instead, add the fertilizer you planned to put on the poor soil onto the better soil. It has the potential to use the nutrients to grow more grass. You'll spend the same total amount of money, but spend it smarter and have more grass.