Well, most land is either farmland, forest or desert. Forest is out of the question for any solar installations unless you want to cut down the trees. Desert is not easily accessible for most parts of the world and provides terrible conditions for solar cells that have sharply declining efficiency with heat and don't like dust.
Leaves farmland, if you want to do this kind of thing at any sort of required scale. (Sure you can put solar cells on barn roofs, but the premise was scale magnitudes beyond that.)
Hint: solar on farmland does not interfere with use as farmland. Look up agrivoltaics. Solar on pasture is even easier, and protects livestock from weather extremes.
Farmland is literally converting solar energy into sugars. It's natures solar plants. Sure, you can get a bit of shade for your cattle. But the hundreds of millions of acres used for crops are directly competing for sunlight with your solar cells.
You imagine that is so, but the science says otherwise.
In fact, most plants can only use sunlight for a few hours a day, and must then endure the heat for the rest of the day. A few crops -- wheat, corn -- offer slightly reduced yields when shaded, but many others -- particularly peppers -- yield better with partial shade. Even where yield is reduced, the extra year-round revenue and radically reduced water loss may even the score.
