The best use of kiambu land is real estate to provide housing for Nairobi Metro. It's where the money is - you follow the money always. The money from real estate can be used to launch irrigation schemes say in Narok...if we tap Kiambu/Ngong water - we can flood mai mau/naivasha/suswa and turn them green.
Those land are most ideal because it's warm, sunlight, and fertile.
Most of Kiambu too cold to grow many things.