You can, but then you have to transport the product to the farm to use it.

And, of course, practically all the Haber-Bosch equipment is not on renewables, and will not be for a long time. It is better for now to displace it until carbon taxes can be used to force conversion.

Solar power gathering above cropland also increases yield by reducing water and heat stress. So even after they convert, you will still be better off with this.

It appears that fossil fuels are used as a hydrogen source for the reaction. I wonder if you could split water and use that as a hydrogen source? It is probably a much less efficient process to source hydrogen.

You can, and it is done. Just not much yet.