Answer

How does a controlled release fertilizer work?

Controlled release fertilizers, like Osmocote, are made by coating balls of water soluble fertilizer with resin. Water can be drawn into the granule by osmosis and as the pressure builds up the dissolved fertilizer is pushed out. The coating expands and contracts depending on the temperature, opening and closing minute cracks. The higher the temperature the more water can enter the granule and the more fertilizer is released. This relates to plant growth where plants grow faster at warmer temperatures and need more fertilizer.