Use of Innovative Tools to Increase Nitrogen-Use Efficiency and Protect Environmental Quality for Temperate and Tropical Regions
Figure 1. Variability of cation exchange capacity (meq/100g) across a center-pivot irrigated sprinkler (54.7-ha) system. Soil samples were collected with a DGPS using 0.405-ha grid sampling technique with one randomized sample collected within each grid.The island of Puerto Rico, with about 2.2 million ac., has a great variability as far as the number of soil orders (classified by the United States soil system) that have been identified (Lugo-Lopez and Rivera, 1977). Topography and position can affect the soils’ physical and chemical characteristics (Schimel et al., 1985), fate of the added N, and its dynamics and compartmentalization (Delgado et al., 1996). We also expect to have the same significant variability across tropical soils as we see in temperate soils (Figure 1). This variability in chemical and physical soil characteristics can affect the yields and crop responses as well as the NUE. New technologies are being developed, calibrated, and tested to improve the management of agricultural resources.Positioning SystemsNew advances in technology are constantly being transferred to the management of agricultural fields. We now have the opportunity to use positioning systems to develop precise records of all agricultural operations, contributing to better management and use of the resources. Generally, positioning systems electronically record the location of equipment, people, objects, or benchmarks. A GPS uses satellite technology to record geographical locations. These technologies are becoming widely used in agricultural management.For security reasons, the US Department of Defense (DOD) introduced an error to the GPS signal that degraded its accuracy. In May 2000, the US government decided to stop this degradation of the signal. Before May 2000, an approach called differential global position systems (DGPS) was used to compensate for or reduce the error. Each GPS receiver determines its unique global position by measuring its distance from satellites in space. Since the satellites are constantly transmitting positions and timing signals, GPS uses a process called ranging, utilizing the time delay to calculate the distance from each satellite. The real-time kinematic GPS can make calculations in seconds with accuracies at the subcentimeter level (Zuydam, 1999). Application of GPS and GIS to Monitor Soil and Crop ResponsesGIS technologies store, organize, and manipulate geographical data. To navigate to specific locations, DGPS can be linked with GIS and a laptop computer or other handheld device, allowing navigation to specific locations where geographic position can be recorded for the sample. DGPS and GIS can be used to create field boundaries for mapping, crop diseases, soil sampling, variable-rate fertilizer applications, and many other agricultural applications. The DGPS antenna can be anchored to a vehicle (truck, tractor, harvester) and connected to a laptop with GIS software, recording sampling locations or crop yields throughout the field. These data can be analyzed and model the spatial variability of the field. These models can be used to develop variable-rate recommendations and create maps for GIS. Variable-rate fertilizer equipment can then be used with these GIS maps to apply specific amounts of fertilizer throughout the field.
Figure 4. NLEAP simulation of residual soil NO3–-N for the 0- to 3-ft. depth of 12 irrigated fields of the San Luis Valley.Computer simulations models can also be used as tools to evaluate the NUE under different combinations of cropping systems and management scenarios. One such model is the Nitrate Leaching and Economic Analysis Package (NLEAP) (Shaffer et al., 1991). NLEAP has been used to predict NO3–-N dynamics across different cropping systems (Delgado et al., 2000 and 2001; Delgado, 2001) (Figure 4) and has been found to perform similarly to other models (Khakural and Robert, 1993; Beckie et al., 1994). Different authors have reported that there is the potential to use NLEAP for evaluating the effect of precision farming on NO3–-N dynamics and NUE (Wylie et al., 1994; Shaffer et al., 1995; Delgado, 1999).Grain- and Potato-Yield Monitoring New technology uses yield monitors and DGPS to measure real-time yield responses at harvest across small-grain and tuber fields. Different sensors can be used to measure the force or displacement of grain as it moves across the grain elevator or conveyor. An impact plate measures the force of the grain or load cells and can be used to measure the weight of the grain as it passes through the combines. Weight sensors are also used for measuring the tuber weight as it passes through the conveyor path of a potato harvester. The grain system also uses sensors to measure the grain moisture content.These systems measure ground speed of the harvesters to calibrate the position where the harvester is moving. Some yield monitors allow for a time delay to account for the position where the crop was harvested. Yield monitors are also calibrated to improve the accuracy of the systems. Farmers can have an instantaneous measurement of the yields as they are displayed in the console. Computers installed in the console record all this information, which can be saved to PC cards and transferred to personal computers for later analysis. Yield maps can be generated daily from this information and used with geostatistical analysis to develop new management approaches for site-specific small-grain–potato rotations.
Figure 7. Georegistration of remote sensing will facilitate the development of precise geographic information maps for application of nutrients during the growing season.Farmers and consultants are using these practices in south central Colorado to develop new management zones. The research effort needs to continue with these new technologies, which are allowing farmers to reallocate the application of products according to their low-, medium-, and high-yield areas in the field. Using the variable-rate technology creates the potential to increase yields in the high-yield areas and minimize the cost by reducing inputs into the low-yield areas. There is also potential to use variable-rate applications of lime to improve the pH across the field and the availability of nutrients. Variable-rate herbicide applications offer other new important uses of these technologies. There is potential to use these practices in the tropics to develop management zones for better use of resources and protection of environmental quality. We will be applying some of these new technologies to develop and implement a management plan to reduce nonpoint pollution in the coastal waters of the Jobos Bay national estuary reserve in southern Puerto Rico.
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