| Precision agriculture (PA), also called Precision Farming, Site-specific Farming,digital agriculture and information agriculture.which was a new mode for agricultural production and management.It is generated from modern information technology. Totally say Precision agriculture means that inputs of fertilizer, herbicide, insecticide, seed, and others are managed by a site-specific .in this way .it can reduce waste, increasing profits, and maintaining the environmental quality. Automatic variable fertilizting was mainly realized in two ways: One was based on the prescription maps and the other was based on the real-time measuring with sensors. Limited by the technical level and economic conditions in Chinese countryside, automatic variable fertilizating method based on prescription maps was studied in this work. The method conducted by two steps: first was to construct a data control system of fertilizing decision and second was to practice in farmland with an automatic variable fertilizer applicator. A map of the farmland can be obtained through dividing the farmland into plots by GPS. The plots were coded and named by programming in computer. Soil samples for laboratory analysis were selected each plot (20.48m×40m) by GPS. A database of soil nutrient contents has been set up. The fertilization decision was made and a prescription map was constructed. And then the decision-making data were written into an data control system. For the control system on the automatic rate fertilizer applicator, the IC card containing codes and application rate required in each plot was inserted into the controller on the fertilizer applicator. microcomputer read the position signals from GPS, judged the plot where the applicator is, took data of required application rate corresponding to the plot, triggered and controlled the rotational speed of the distribution shaft, so as to realize automatic variable rate fertilization. There are 4 parts in the automatic variable rate fertilizer applicator: DGPS, a computer with GIS, a controller and a set of tractor-seeder and fertilizer applicator. The details are as follows. (1) DGPS. Two AgGPS132 receivers were used for differential positioning in real time during operation of fertilizer in farmland. Here, AgGPS132 receiver can receive signals from the American satellite, 12 satellites can be looked at the same time. Observed value was the coarse acquisition code(C/A) of pseudo-range. While operating in DGPS mode, the positioning error was less than 1 meter. (2) GIS. GIS was a key part of the automatic variable rate fertilization system. With GIS, the databases of soil parameters, such as content of N, P, K and organism, moisture content of soil, was set up. MapInfo Professional 6.0, a tabletop geographical information system, was used for conducting the farmland electronic maps, turning information about soil type, soil quality, soil fertility, application rate and the crop outputs and others over past several years into GIS applying layers. Then the growing condition of crops was analyzed, the fertilization decision was made according to the goal of soil nutrientcontent balance, and the data of fertilization decision were written into the u disk. (3) Controller. With microcomputer as the control center on the board, the position signals were received from GPS; the plot where the fertilizer applicator was identified; the decision data stored in u disk.were taken according to the GPS positioning data. At the mean time, ground speed signals were read and output in pulses to control the rotational speed of the distribution shaft. (4) A set of tractor-seeder and fertilizer applicator. This applicator was improved on the manual variable rate fertilizer applicator developed by JiLin university. Its characters are: hitch, 2 rows, fluted feed. The application rate can be adjusted by change rotational speed of distribution shaft. The field tests were conducted in a farmland of area of 5.87ha, in high -new agricultural technical garden of Dehui city(east longitude 125°33′,north latitude 44°12′). The test results showed that the variable rate fertilization for precision agriculture is competent. Soil analysis and fertilization management were carried out in 2001 and 2002,respectively. A fertilization decision was made according to the balanced method of the soil nutrient content in the spring of 2002. The condition of seedling and crop output were measured and the economic profits of variable rate fertilization was compared in following two aspects: (1) In one farmland, three typical areas were chosen (No.1, 2, 3) for comparison. Basal dressing of 115kg/ha diammonium phosphate and topdressing of 300kg/ha urea were used in the area No.1. Basal dressing of 125kg/ha ammonium and topdressing of 250 kg/has urea were used in the area No.2. The maize outputs of the two areas were 7340 kg/ha and 832 kg/ha respectively. In the area No.3, routine fertilization method was used, i.e. basal dressing of 400kg/hacompound fertilizer of 15:13:12 of equivalent N, P, K and topdressing of 250kg/ha urea were applied. Comparing test data from areas No.1, No.2, No.3, the results showed that the cost of variable rate fertilization was lower than the conventional method,Because of the application of fertilizer using the scientific decision method on considering the existing fertility in the soil. (2) Comparison of the automatic variable rate fertilization with conventional fertilization in Jilin Province. Conventionally, a compound fertilizer is applied with a basal dressing of 400kg/ha and a topdressing of 600kg/ha in Jilin province. The maize output was 10000kg/ha on an average for abundant year. It was shown that the cost of maize production using the variable rate fertilizer method was less by 39.6% than that in abundant years using the conventional fertilization method although the maize output using the variable rate fertilization method was less to some extent. As a result, the economic profit was increased by 33.82%. Considering the present rural technical and economic conditions in China, a system of the automatic rate variable fertilization with virtual GPS was developed. The coordinates of this system were kept the same as the geographical information system in the farmland. The system was not only compatible with precision agriculture, but also can realize variable rate fertilization with no GPS positioning. The characteristics of the system were that positioning accuracy was dependent on the length of the operating plot gird, the sliding rate of the ground, the working speed of the applicator and time interval of speed sample frequency and accumulate error of passing distance, without influence by positioning accuracy of GPS receiver. So, it will also be suitable for the variable rate spraying of pesticide. Because a set of communication and interrupt was not needed in this system, the arithmetic of plot recognition was simplified, and the corresponding program can be carried out both rapidly and correctly. GPS receivers were not required on every operating machine (tractor) because the space data were gathered only with one DGPS in advance. The main work and conclusion of this research are as follows: (1) An automatic variable rate fertilizer applicator was developed for the first time in China;(focal point in this thesis)...
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