The water used to irrigate crops has dissolved minerals in it. As the water is taken up by plants (or evaporated), it leaves those minerals behind. This then raises the salinity of the soil. Eventually, the soil becomes too salty/mineral encrusted to support crops. This happened in the Middle East. What is now Iraq used to be the main grain growing region of the Middle East several thousand years ago. Over time, the salt in the soil rose to the level that wheat could not grow, but barley could. Eventually, the soil became too salty for barley. Today, some of the Iraqi desert shines in sunlight because of the salt crystals. It has never recovered and will never recover. California is progressing faster.

Selenium is one of the minerals that ends up in the runoff of Californian agriculture. In some coastal areas, the selenium content is so high that bird eggs laid by wildlife never harden. Or the birth defects are so high that the hatchlings don't live.