Minerals, humus and other organic compounds are removed from the soil in harvested crops or forests or by leaching aggravated by erosion. Soils become impoverished when these depleting processes exceed replacement rates. Soil microorganisms are then unable to obtain the nutrients they need to regenerate soil, rendering the soil increasingly unable to sustain plants and animals.
When fertilizers are added to a crop, the plants absorb not only the extra nitrogen, phosphorus and potassium from the fertilizer, but also proportionately increased levels of micronutrients from the soil, including zinc, iron and copper. Over years, the soil becomes deficient in these micronutrients. Lack of them inhibits a plant's capacity to absorb nitrogen, phosphorus and potassium.