A lot of factors go into keeping farmland healthy, but sunlight is one of them.
No light, the ecosystem breaks down at its most fundamental levels.
No foliage, expect soil erosion.
If we must have these, put the photovoltaics over parking lots and buildings, not over arable land.
Cattle can graze near oil and gas wells and even windmills...land can be farmed near those, too, but you don't see agriculture in and among the elements of solar farms.
