1. Weathering of soil minerals
2. Decomposition of plant residues, animal remains, and soil microorganisms
3. Application of fertilizers and liming materials
4. Application of manures, composts, boisolids (sewage sludge) and other organic amendments
5. N-fixation by legumes
6. Ground rock powders or dusts including greensand, basalt, and rock phosphate
7. Inorganic industrial byproducts
8. Atmospheric deposition; such as N and S from acid rain or N-fixation by lightning discharges
9. Deposition of nutrient-rich sediment from erosion and flooding.
Sources of soluble nutrients in the soil.