So far, the budget has taken the amount of nutrients leaving the field in the above ground crop biomass into account. However, crops have elaborate root systems that also take up nutrients. Estimating the amount of nutrients in the root systems is more difficult since the below ground biomass is not readily measured, and because the root system is subject to decomposition. In the short term, nearly all of the nutrients embodied in the below ground portion of the plants remain a part of the plant-soil system. As crops are harvested and fields experience wet-dry cycles and tillage operations, these nutrients will be liberated via decomposition. A portion will be incorporated into the soil organic matter; some will be lost via water movement and gas exchange. The processes that govern decomposition in the soil are complex, and they vary based on the soil type, the temperature, the nature of the wet-dry cycles, and the way the field is managed. This makes blanket statements about the availability and fate of below ground nutrients difficult. Nevertheless, in general, the more a grower does to encourage the incorporation of these nutrients into the soil organic matter, the more fertile the soil will become over time and the fewer losses will result. For more information on practices that promote increases in soil organic matter and decreases in nutrient losses, see the soil management page. For the purpose of the nutrient budget, a typical assumption is that the nutrients taken up by the roots are roughly equivalent to the inherent supply of nutrients made available from the soil, without fertilizer additions.
Return to the nutrient management page.