RATIONAL NITROGEN-FERTILIZATION IN INTENSIVE CROPPING SYSTEMS

The objective of a rational N fertilization program is to account for the sources and fate of N while estimating crop N needs. Efficiency of N use will vary with cropping systems and N sources. Management techn ologies that affect N use efficiency include the amount of N applied, timing and placement of N fertilizer, and use of inhibitors. One of th e main problems in making a fertilizer N recommendation is to account for the contribution of N mineralization to plant available N. Most la boratory procedures do not account for the environmental factors that affect N mineralization and only estimate the size of the mineralizabl e N pool. However, changes in soil moisture and temperature can dramat ically affect the amount and rate of release of mineralized N. Field a nd modeling techniques are two possible techniques to estimate N miner alization. Field techniques can be divided into soil and plant approac hes. Soil incubations in the field provide a quantitative approach whi le soil nitrate tests during the growing season provide a qualitative approach to estimating N mineralization. The plant is the ultimate int egrator of N mineralization. Plant N uptake by an unfertilized crop ca n provide a quantitative approach with certain precautions. This appro ach may be costly, labor intensive, and site specific. Crop N uptake d uring the growing season can be estimated by measuring the tissue N co ntent or using a chlorophyll meter. The chlorophyll meter measures the greenness of the plant and has been shown to be positively correlated to plant N status. Modeling may provide another option by including t he factors that affect the rate of N mineralization from a known pool. The two most important variables include soil moisture and temperatur e. Realistic yield expectations and accounting for existing and projec ted amounts of available N can improve the accuracy of N recommendatio ns.