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Fluid Journal : Fluid Journal 1993-1995
3 Fluid Journal Spring 1993 system, including mineralized N, illustrates why N use efficiency would tend to be greater for the sprinkler system. Mineralized N is expected to be similar for each field, since cropping histories were similar prior to this study, but leaching and de-nitrification would lead to greater N losses under the conventional system. Estimated N mineralization at an annual rate of 3% of the organic N (soil with 2% organic matter) in the surface foot amounts to approximately 138 lb amounts to approximately 138 lb N/acre/yr. Assuming that only two-thirds of the N that mineralized annually would be available during the growing season, then one could add about 90 lb N/acre to the estimate of N availability. Interpretation of such comparisons involves several assumptions that are not easily documented, but estimates (data from Tables 1 and 2, plus mineralization estimate) show that approximately 632, 482 and 301 lb N/ acre would be avail-able at various times during the growing season for the conventional, surge-flow and sprinkler irrigation systems in 1991, respectively. Similar values for 1992 are 568, 341 and 261 lb N/acre, respectively. Total N uptake by irrigated corn is approximately 41% more than grain N uptake, so on this basis the 1991 corn crop would be expected to use about 175 lb N/acre. Excluding N losses and based on the above annual estimates of N availability, inorganic N use for these cropping systems ranged from 28 to 58% (Table 2). The numeric value of these estimates is probably less important than the relative ranking, because it helps to identify cropping systems that intuitively should reduce the potential for nitrate leaching and ground water contamination. These data illustrate that N management practices that ultimately reduce yields can also reduce N use efficiency (Table 2). Nitrogen use efficiency of fertilizer alone cannot be calculated from this data, because check plot yields were not available. Cost of N fertilizer is only one factor that producers must consider when making N management decisions. The 1991 production data (Table 2) illustrate that reducing fertilizer N costs can add to profitability provided yields do not de-crease significantly. But once yields de-cline very much so does profitability (as in 1992). Profitability considerations also need to include irrigation costs and how the equipment costs are amortized, which are beyond the scope of this article. These data illustrate that a "fertilization-as- needed" strategy can maintain productivity and perhaps even increase profitability. They also show that marginal or N deficient conditions can significantly reduce yields and profits. The above scenarios are an obvious oversimplification of the N dynamics involved in irrigated corn production. At the very least, N availability and crop needs (i.e. synchronization) should be considered for key times during the growing season. When this is done, it will become apparent why producers develop preferences for certain fertilizer practices (i.e. preplant, starter, nitrification inhibitors, etc.) that they find to work well for their soils, climatic conditions and tillage systems. Unknowingly, producers put priorities on the various N sources, based on how they manage fertilizer N inputs. Awareness of the various N sources and how they contribute to the available N pool in soil compared to crop N needs can lead to N and water management practices that improve fertilizer N use efficiency and protect the environment. Why BMPs Improving N use efficiency of crop-ping systems seems to be an implied goal of producers and society alike. Although the reasons we strive to improve N use efficiency of crops may differ, most people intuitively agree that it should help protect drinking water from contamination by nitrate. Implementing management practices that improve crop N use efficiency would seem to be the natural thing to do, but many factors tend to impede adoption of what are commonly termed best management practices (BMPs). Producer priorities affect the implementation of BMPs because adoption requires individuals to make trade-offs. These trade-offs take many forms and are influenced by factors that are hope-fully based on sound scientific facts and prudent economic consideration. Built into all of these considerations is a risk factor that may place producers in a real or perceived state of vulnerability. These risks are the integration of many factors that can, in some way, be quantified, but include other considerations, such as peer pressure, that can only be described or characterized. Perhaps the most immediate concern of producers, when considering modifying a management practice, is that of economics. The combination of short-and long-term economic implications weighs heavily with producers because cropping systems must be sustainable. Beyond that, producers must be able to deal with modified time commitments of any new cropping system and they must have the technical expertise to successfully integrate the ever-increasing number of management considerations. These needs open the door for crop consultants and others who can provide information to help producers make higher-level management decisions. For N management decisions, the options may seem to be quite extensive. But, in reality, changing one management practice can impact a number of subsequent management decisions and can even limit the options available to producers. Nearly all N management decisions affect crop N use efficiency in some way. Traditionally, scientists use tagged-N fertilizers or compare crop N uptake from plots receiving N fertilizer with unfertilized check plots to calculate how much of the fertilizer was used by the crop. Both of these methods require special considerations when interpreting data, because there are other sources of N (i.e. manure, residual N, mineralization, irrigation water) in addition to fertilizer. There are also other demands for N fertilizer within the soil system (i.e. microbial immobilization). These considerations all tend to decrease the measured N use efficiency because they either dilute the fertilizer with other N sources or temporarily reduce the fertilizer availability to the crop. Interpretation of crop N use efficiency data also requires some knowledge of synchronization between crop N needs and soil N availability for the different stages of crop growth.
Fluid Journal 1996-1998