I believe the answer is decomposers. Does that make sense in the context of what you have learned in class? If you look up "the circle of phosphorous," you can see images that show that decomposers, like fungi and microbes break down dead animals and plants using some of their phosphorous, but what isn't used is returned to the soil. The same can be said for sulfur. Decomposers like fungi and bacteria break down dead plants and animals and return sulfur to the soil.
I want to warn you that there is a slight possibility that I might be incorrect. I am still pretty sure I am right. The only thing that makes me question the correctness of my answer is another brainily question I saw when i was looking up this stuff. Two people had responded to that Middle schoolers question and had said the answer was producers. They didn't give any explanation for their answers. So, I am confused as to how they both got the same answer that was different from mine. No material that I have seen regarding either the phosphorous cycle or sulfur cycle has shown plants as RETURNING the most sulfur and phosphorous into the soil. Usually, it has been depicted that producers, such as plants, UTILIZE the most phosphorous and Sulfur from the soil.
