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An evaluation of soil warming for increased crop production Public Deposited

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  • In regions where soil temperatures limit plant growth, artificial soil warming may be an economically feasible practice. This hypothesis was evaluated in a soil warming experiment near Corvallis, Oregon. This experiment was prompted by the observation that multiple use of waste heat discharged in the condenser cooling water of thermal power plants may become an important consideration in the development and siting of these plants. The thermal discharge might be used to achieve increased soil temperature by circulating warm water through a subsurface pipe network. Objectives of this investigation were: (1) to determine the effect, if any, of buried line heat sources on air temperatures within a crop canopy; (2) to determine the extent to which soil temperature can be elevated with buried line heat sources maintained at various temperatures; (3) to establish the effect of subsurface heating on soil water regimes and to evaluate a subsurface irrigation system as a means of maintaining high soil water content and hence high rates of heat transfer in the vicinity of heat sources; (4) to evaluate a theoretical model for prediction of energy dissipation rates; (5) to establish the yield response to soil warming for numerous crops; and (6) to evaluate the influence of subsurface heating on soil and air temperatures and crop production in a wood frame, plastic covered greenhouse. Six individually controlled electrical heating cables were used to simulate a buried pipe network. Thirteen different crops were grown on heated and unheated areas during the four years of this study. Air and soil temperatures were monitored at over 200 locations with thermistors. Readings were taken with a computerized digital data acquisition system. Soil water content was monitored with electrical resistance blocks. Energy inputs were measured for each heating cable with kilowatt-hour meters. Air temperatures at four heights above the soil surface over bare soil and in a field corn canopy were not appreciably affected by soil warming. Statistically significant temperature increases due to soil warming were observed but they were too small to be of consequence for crop growth. Soil temperatures in the upper 25 centimeters were more responsive to solar heating than to subsurface heating. Temperature increases due to soil warming were one to five degrees centigrade at the five centimeter depth, depending on heat source temperature, time of year, time of day and crop canopy conditions. A major portion of the root zone was maintained above 20 degrees centigrade during most of the growing season. The greatest temperature increases were observed on a plot where subsurface irrigation was used to maintain high soil water content near the heat sources. During the summer substantial soil drying occurred in the vicinity of the heat sources, particularly under a field corn crop. Thermal gradients prevented rewetting by sprinkler irrigation. A subsurface irrigation system maintained a wet soil near the heat sources throughout the growing season. The rate of heat loss from buried heat sources was found to respond to changes in depth and spacing of sources, source temperature, soil surface temperature and soil water content, as predicted by theoretical considerations. A high correlation between mean monthly air temperature and mean monthly heat loss rates was found. The results indicate that the area required to reduce the temperature of circulating warm water, from a 1,000 megawatt thermal power plant, by 10 degrees centigrade would range from 10,000 hectares in the winter to 20,000 hectares in the summer under Willamette Valley climatic conditions. This requirement could be reduced by design modifications or subsurface irrigation. A wide range in crop response to soil warming was observed for different crops and for some crops in different years. The results obtained with field corn and bush beans suggest that the response to soil heating depends on the degree of adversity to which the crop is subjected. When climatic conditions and management factors are optimum soil heating has a limited effect on crop yields. When one or more of these factors are limiting soil heating becomes more effective and greater yield responses occur. In nearly all cases soil warming resulted in more rapid germination and early growth, and earlier maturation. Double cropping of bush beans and double cropping with summer and winter annual forage crops appear to be feasible with soil warming. Yield increases due to soil warming were above 50 percent for several forage and vegetable crops. Several cropping sequences were suggested. Additional input from agricultural economists and engineers is needed to determine those crop combinations which will result in the greatest economic returns from a soil warming system. Soil heating did not result in higher air temperatures in a plastic covered greenhouse. Soil temperatures were substantially increased and this resulted in an increase in tomato production of 64 percent compared with a crop grown in the greenhouse with no soil warming. Strawberry yields did not respond to soil warming in greenhouse culture and this was attributed to high air temperatures due to solar heat trapping during daylight hours. The results of this investigation suggest that soil warming with condenser cooling waters from thermal power plants is feasible. Additional information is needed to evaluate the economic and engineering aspects of a soil warming system. It is unlikely that a soil warming system can fulfill all the needs of a thermal power plant cooling system. Additional studies to evaluate other beneficial uses of waste heat to be used in combination with a soil warming system will be required.
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