The use of phosphorus fertilizers is now a recognized necessity in all systems of permanent agriculture. However well supplied naturally, all soils gradually become depleted in phosphorous thru the removal of crops and this element must be returned if crop production is to continue satisfactory. Many soils, especially those under long continued cultivation, have been found to respond very profitably to phosphorus fertilization while even the newer, more fertile soils are approaching a condition where phosphorus will soon become the controlling factor of growth. The interest in phosphorus fertilizers is therefore becoming more widespread and the use of these materials is assuming much economic importance.
Brown, P. E. and Gwinn, A. R.
"Effect of sulfur and manure on availability of rock phosphate in soil,"
Research Bulletin (Iowa Agriculture and Home Economics Experiment Station): Vol. 3
, Article 1.
Available at: https://lib.dr.iastate.edu/researchbulletin/vol3/iss43/1