Because no chemical fertilizers are used in growing organic food, the soils stay fertile longer. Farmers use a system called crop rotation, which means every time they grow a different type of crop on the same soil. This is beneficial because different plants take different nutrients from the soil and growing the same crop year on year depletes soil from the minerals needed for this crop. This also means that with each next crop grown without rotation, the food has less and less nutrients, as they are depleted.
Pesticides, herbicides, and fungicides all add toxins to the ecosystems, nearby waters, and the food that we later consume. Excessive use of these chemicals kills most of insects and weed, which would reduce the produce; but most does not mean all. There are always types of parasites that survive the poison and develop resistance to it. This then leads to the use of more and stronger chemicals to kill now resistant pests. With organic farming, this is not the case, so this is more sustainable practice in the long term.