Back to Contents4.4 Variable-Rate Management of Inputs
Site-specific management of nutrients has three advantages over traditional approaches; agronomic, economic and environmental. Some of the most visible precision agriculture tools are the variable-rate controllers on application equipment. Schueller (1989) described liquid fertilizer mixing and flow control to minimize material transport lag times. He determined that rate and mixture variation are improved by the following conditions: 1) flow control of each system component, 2) quick response times of the pumps and valves involved, 3) connecting hoses should be as short and as small in diameter as possible, 4) adequate mixing must be provided, and 5) the mixer should be as close as possible to the nozzles. It was also found that flows could be controlled by varying the pump speeds or the recirculation flows.
Smith et al. (1990) interviewed several farmers who had switched to fertilizing based on soil unit needs. One farmer saved over $18,000 on 400 hectares (1,000 acres) of corn when compared to the previous cropping year using conventional fertilizer application practices. Another farmer built his own variable rate fertilizer system for $1000, saving him more than 50% in fertilizer costs. A third farmer fertilized his fields on a two-hectare (five-acre) grid, based on soil maps derived from aerial maps scanned into his computer. He saved 138 to 165 kg/ha (125 to 150 lbs./acre) of nitrogen using this application approach.
Whitney et al. (1995) developed a spreadsheet simulation to predict the dynamic performance of a variable-rate applicator that applied fertilizer based on real-time measurements of nitrogen using the plant-nitrogen-sensor-index (PNSI) method. The authors used this method to predict the error of nitrogen application of the actual machine and the associated costs. Quantilization, control valve speed, valve operating time symmetry, ground speed, and field element size were evaluated for their contribution to measurement error. It was found that weighted binary valves produced greater error than linearly proportional valves due to quantilization error. Valves with asymmetrical on-off times had a high error. The optimal least-cost field element length for 0.76 m nozzle spacing was found to be 0.75 m. This application method was found to save $265.85/ha ($107.40/acre) over conventional nitrogen application methods.
Reichenberger (1990) described a system that simplifies precision fertilizer application. The system utilizes a laptop computer and a fifth wheel to determine placement in the field. An application rate map was stored in laptop memory and machine application rates were controlled with feedback from the fifth wheel. The unit was reported to be ready for modifications that would allow application of liquid fertilizer, chemical injection, planting, and the development of yield maps.
Olieslagers et al. (1996) described the fertilizer distribution of a spinning disc spreader. Many parameters including orifice position and angular speed of disc impact the distribution pattern of disc spreader. VRT application, accomplished by changing the mass deposition rate on spinner discs, leads to a fluctuating spreader pattern which results in large deviation from the intended application rates. Chaplin et al. (1995) investigated the distribution of dry material during field application. They described a methodology based on American Society of Agricultural Engineers (ASAE, 1997), and did testing for a single-disk mounted fertilizer spreader. Petersen et al. (1991) investigated how the distribution pattern of a twin-disc spreader was influenced by fertilizer particle size. They provided a detailed test method for collecting fertilizer samples and used interpolation techniques and computer graphics to get continuous distribution patterns.