Soil Maintenance
To grow more food the land has to be maintained at its present nutrient level or better so it can grow larger and better crops. Each crop removes some essential nutrients (N:P:K+) from the soil. One of the keys to the Agricultural Revolution was the use of manure and bacteria in symbiosis with legume roots, which could be used to fix nitrogen (N) in the soil. Through crop rotation and use of organic fertilizers the soil can be maintained; but higher yielding crops require either a large amount of organic or commercial fertilizer. Continually growing the same crop on the same fields will deplete the soil in its needed plant nutrients and the crop yields will continually go down or cease—the crop removes the nutrients till they are too low to support good growth. One way around this problem is to move to new land and start over. Even before 1100 Britain was too well developed to find much more arable land with about one third of its about 39,000,000 acres already put to crop use (about the same as today) with about another third in pasture and the final third composed of waste, forest, villages etc. England's slash and burn agricultural stage was largely behind it.
A crop takes from its field 1.3-2.0% of its weight in nitrogen (N), 0.13–0.3% in phosphorus (P) and 1.0–2.0% in potassium (K) and other needed trace minerals. The carbon, hydrogen and oxygen that constitute about 96% of the weight of a typical plant are turned into organic growth by CO2 in the air and H2O from the soil plus dissolved plant nutrients in the soil. Plants grow by undergoing photosynthesis using the sun's energy, CO2, H2O and the major plant nutrients nitrogen (N), phosphorus (P) and potassium (K) plus other nutrients. Twenty bushels of wheat (60 lb/bu) taken from a one acre field takes about 20 pounds of N with it, 100 bu/acre will remove about 100 lbs. N/acre, etc. In nature nearly all of these nutrients are normally in some sort of equilibrium with the plant nutrients returned to the soil when the plants or animals that eat the plants die. When you remove a crop this equilibrium state is disturbed. To restore the soil's fertility after it has been cropped for a while the missing nutrients have to be replaced either by judicious plant rotations or using organic or chemical fertilizers.
Read more about this topic: British Agricultural Revolution
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