Complex sentence: The most influential factor in stabilizing soil fertility are the soil colloids, clay and humus, which behave as repositories of nutrients and moisture and so act to buffer the variations of soil solution ions and moisture.
Split into simpler sentences: The most influential factor in stabilizing soil fertility are the soil colloids, clay and humus. Soil colloids behave as repositories of nutrients and moisture and so act to buffer the variations of soil solution ions and moisture.