All agricultural plants require carbon, hydrogen, oxygen, nitrogen, calcium, potassium, magnesium, sulfur, phosphorus, iron, and probably traces of other elements for their normal growth and fruition. The growth of farm crops is dependent upon an available supply of these elements, and the extent of growth is governed by the one present and available in least quantity in proportion to the plant's needs. Of the essential elements, all plants secure two (carbon and oxygen) from the air, one (hydrogen) from the water, and the others from the soil. Most soils contain sufficient plant nutrients for normal plant production, with the exception of nitrogen, phosphorus, and potassium. These elements are used by the plants from the soil in the largest quantities and are generally the plant nutrients governing the productivity of a soil. In addition, the organic matter of the soil is of prime importance, for it is the matrix which holds the nitrogen and governs the structure and water-holding capacity of the soil. Probably most Utah soils contain sufficient potassium; consequently, the problem of Utah soil fertility resolves itself into maintaining an optimum concentration of nitrogen, phosphorus, and organic material in the soil. Therefore, this work represents a study of the influence of manure and crop rotation on the nitrogen, phosphorus, and carbon in the soil.
Greaves, J. E. and Hirst, C. T., "Bulletin No. 274 - Influence of Rotation and Manure on the Nitrogen, Phosphorus, and Carbon of the Soil" (1936). UAES Bulletins. Paper 236.