Fertilizer spreaders, whether using a drop spreader or a rotary spreader, are calibrated to work correctly only when you walk at a steady pace on a level surface. When you fertilize by walking up and down the slope, or walking across it, it is inevitable that some spots will get too much fertilizer (which can burn your lawn) and some will get too little. If you have no alternative to using a spreader, going up the hill while fertilizing is going to be the most effective (and safest) way to do it.
The best way to fertilize a sloped lawn is with a liquid fertilizer, using a hose-end sprayer. Many such sprayers will allow you to spray out a good 10-15 feet. Simply stand at the top, or bottom, or side of the hill and safely and evenly shoot the fertilizer onto the lawn. Remember: Do not walk on the hill while still wet.
The best liquid fertilizer to use is one with a soil-penetrant or surfactant in it, like our LAWN FORCE 5. This product helps break surface tension on the soil surface and improves aeration, which allows water to soak into the soil instead of run off of it. It also contains root enhancers that help the grass dig in deeper. With one application you can fertilize, deepen roots, and improve the soil structure.
If you or anyone you know has a sloped lawn, pass this article along! Their lawn will thank you for it.