The problem of obtaining maximum crops Is centuries old but the agricultural world Is still awaiting Its solution.
It is known now that the crop any soil will yield under particular climatic conditions depends on the character and conditions of that soil. If the soil is poor and infertile, the crop may be expected to be small; if it is rich and climatic conditions are favorable, the yields should be large. The real problem, then, is how to make the soil fertile and how to keep it so. If it is poor, then improvement is necessary; if it is good, further improvement may pay. There are probably few soils so poor that proper methods cannot put them on a paying basis; on the other hand, there are no soils so rich that they will always continue to be fertile.
Brown, P. E., "Bacteria and Soil Fertility" (1913). Circular (Iowa State College. Agricultural Experiment Station). Paper 7.