The proper management of soil nutrients in soils for human food production boils down to a simple requirement: the need to replace nutrients that are "subtracted" from soil during production. These subtractions occur as nutrients are taken up by crops from the soil and then exported as food products in crops and livestock. Nutrients can also be lost to soil erosion and in dissolved forms, by drainage of water from the soil (called leaching). The goal of incorporating manure, plant material, and chemical fertilizers by farmers is to add back these subtracted nutrients. In the case of soil erosion, the idea is to avoid such losses completely by protecting soils. Human-managed fields and farms can be compared to nutrient bank accounts, where withdrawals must be balanced by deposits, and where it is better to have a substantial balance than a minuscule balance. Natural systems like forests or prairies lose some nutrients as does a farm field, but to a comparatively minor degree (fig 5.2.1 below). The need for humans to replenish nutrients is much greater in any managed system like a crop field or pasture than in unmanaged forests or grasslands. This is especially true in intensive production systems of crops or animal forages, for example, the corn, vegetable, and hay fields and pastured rangelands that are typical in agriculture of the United States and around the world. In systems where soils are tilled to grow annual crops on hillsides, the combined exported nutrients in food and those lost to erosion can quickly rob a soil of most of its nutrients. Protecting a soil from these losses, and regenerating the nutrients lost by adding crop residues (straw, cornstalks, other stems, and roots), manure, and fertilizer materials (ash, phosphate rock, bone, chemical fertilizers) are therefore important strategies used by food producers to sustain production. We’ll devote more focus to the important role of crop species, crop rotations, tillage, and soil erosion as part of agroecosystems in modules 6 and 7. For now, we want to understand the basics of these principles of soil regeneration.