Where does crop biomass come from?
The matter in plants is mostly carbon, hydrogen, oxygen, and nitrogen, with various metal ions and some sulfur and phosphorus. The carbon, hydrogen, and oxygen are taken from carbon dioxide in the air and water from the soil through photosynthesis. Nitrogen is taken up from the soil as ammonia or nitrates, or fixed from nitrogen gas, but this only happens in legumes. Phosphorus is also part of a nutrient cycle. Metal ions such as sodium, potassium, magnesium, and calcium are taken up through the roots with water.

If you're asking about how this material is replenished, fertilizer is an important answer. For centuries, nitrogen was a limiting nutrient in farms because it would get removed faster than it was replaced, especially if crop rotation was not used or legumes weren't grown. Thanks to the Haber Process we can produce humongous amounts of ammonia from nitrogen gas, this has greatly increased the amount of nitrogen available for plants, but has also resulted in nitrate pollution in waterways and algal blooms and eutrophication. Phosphate and potassium are also used in fertilizer and are usually mined, however phosphorus reserves are limited.