Simplify the following text for a Grade 17 native English-speaking person.
Original: Humus can be differentiated from decomposing organic matter in that the latter is rough-looking material , with coarse plant remains still visible , whereas fully humified organic matter is uniform in appearance ( a dark , spongy , jelly-like substance ) and amorphous in structure , and may remain such for millennia or more .
Simplified: Physically , the difference between humus and organic matter is that organic matter is rough looking material , with coarse plant remains still visible , while once fully humified it becomes more uniform in appearance ( a dark , spongy , jelly-like substance ) and amorphous in structure .