Satellites are the modern way of agricultural land assessment. Not only is it accurate, but it proves to be much more cost efficient than former techniques such as soil analysis.
By measuring the electro-magnetic radiation reflected from the ground, a whole region can be scanned at once, and farmers can have a review for their whole farmland area. On such maps, they can see the variations of their crop performance as well as soil and fertilization status. From there, they receive advice; they can set up their own action plan and time it with meteorological data.
Here is the link to the article of Economist.com for the whole story.