Agricultural chemistry often aims at preserving or increasing the fertility of soil with the goals of maintaining or improving the agricultural yield and improving the quality of the crop.  Soils are analyzed with attention to the inorganic matter (minerals), which comprise most of the mass of dry soil, and organic matter, which consists of living organisms, their degradation products, humic acids and fulvic acids. 
Fertilizers are a major consideration.  While organic fertilizers are time-honored, their use has largely been displaced by chemicals produced from mining (phosphate rock) and the Haber-Bosch process.  The use of these materials dramatically increased the rate at which crops are produced, which is able to support the growing human population.  Common fertilizers include urea, ammonium sulphate, diammonium phosphate, and calcium ammonium phosphate.