During the previous war (WWI), a shortage of sulfur necessary for ammonium sulfate fertilizer production resulted in the increased production of ammonium nitrate as a replacement. Concentrations of ammonium nitrate increased over time in the mixture of the two compounds. Consequently, the fertilizer mixture began to clog and compact under its own weight because of ammonium nitrate's hygroscopic nature (tendency to absorb and hold water). Workers resorted to small charges of dynamite to loosen the paste back into powder form, a tried and true method for non-explosive ammonium sulfate.
Analysts say that on the day of the explosion, poor or inconsistent mixture of the fertilizer had resulted in an abnormally large concentration of ammonium nitrate in sections of the stockpile.