Comment by culi

You can see the black zones everywhere where there's intensive agriculture.

Naturally, plants get over 80% of their phosophorus from mycorrhizal fungi. They typically exhaust the immediately available stock near their roots before they start exuding the hormones that start the mycorrhizal association. Fertilizers (phosphorus being the most common) interrupt this natural chemical handshake.

Tilling, allowing soil to go fallow with no cover crops, and heavy machinery—especially when used in combination—are also well documented to be extremely destructive to soil microbiology.

The central valley is especially heartbreaking because just a few hundred years ago it was home to Tulare Lake which was the largest freshwater lake west of the Mississippi River. It was dried up to strip native Californians of a vital source of food and lifeways (though it occasionally reappears when we get historic rainfalls).

One of the biggest problems with the central valley today is high salinity (partly due to fertilizer use). The lake used to naturally manage its salinity with halophytic flora and periodic high-water flushing