HEATING SOIL BY ELECTRICITY

Otago Daily Times, Issue 22742, 30 November 1935, Page 23

HEATING SOIL BY ELECTRICITY

OBTAINING EARLIER CROPS Every gardener is desirous of raising plants in advance of their normal season. Until recently, this necessitated the use of greenhouses, hotbeds, cold frames, germinators, etc., with the necessary boilers and manure pits. Manure has now become increasingly scarce, and .also this mode of warming the soil is neither easy nor very reliable. Two types of electric soil-heating cable have been devised which do away with all the old difficulties. These are described as non-regulating and self-regu-lating. Both cables are of special construction, and comprise, a conductor of resistance wire, in which the heat is generated, and are electrically insulated with material capable of withstanding the high working temperatures. . They are also provided.with a lead covering to prevent moisture penetrating to the wire, and to protect from mechanical injury; and, finally, a braiding of jute, impregnated with compound, is applied to prevent corrosion. REGULATING THE HEAT. The non-regulating cable must be provided with either a thermostat or time switch, so that heating can be maintained for a definite period. The knob extending beyond the thermostat ease is turned to lower or raise the temperature setting for any point from 30 to 100 degrees (60 to 65 degrees is the usual hotbed temperature). The self-regulating cable, made by Messrs Siemens Bros, and Co., Ltd., is even simpler. It is suitable for small frames, and maintains an even temperature without any attention whatever being given to it. Buried in the ground, it automatically makes adjustment for variations in the atmospheric conditions. The loading is approximately 200 watts, and it may be connected to the normal supply voltages of 200-240 volts, either alternating or direct current. At the present time, it is only made up in units of 10yard lengths, and is capable of supplying neat for frames up to six feet by four feet. This cable is available in three temperature ranges, namely. 50, 60, and 70 degrees, respectively.

INSTALLING. The method of installing the cable is very simple. It should be laid in zig-zag fashion, the spacing between the turns being dependent upon the size of the frame. The cable is covered with a layer of sand, about an inch thick (this ensures uniform heat distribution), and? then protected by some wire mesh to prevent damage when digging the ground. The remainder of the soil can next be addedusually six to nine inches will be found adequate. To economise current,.it has been found an advantage to protect the bottom and side qf the bed with heat-insulating material. If about six or eight inches of coke or clinker is placed well below the depth of the soil, the efficiency of the loading is materially increased. Covering the frame lid with canvas or sacking to conserve the heat during the nights when the temperature drops will also save much useful energy. Cable buried in the soil to a depth of five or six inches will deal effectively with 30 square feet of soil, allowing one 60ft length on a 100-volt supply. Two 60ft lengths in parallel will provide for 72 square feet: or 120 ft on 230-volt supply will deal with the same area. The thermostat is connected in series, and can handle 432 square feet at once.

For comparison purposes, a manure hotbed of precisely the same size was arranged immediately adjoining an electric hotbed. Both beds were ready for use on the same day, and were planted with exactly the same kind of plants. At the end of 20 days it was found that the tomato plants in the electric hotbed were nine inches in height, while those in the manure bed were only an inch and a-half. Growth in the latter bed was retarded by an exceptionally cold spell, which did not affect the electrical installation.

Another grower experimented to ascertain if it were possible for lettuce grown during the winter to be marketed in this country at a time when we were dependent on overseas supplies for this produce. He accordingly sowed seedlings in elec-trically-heated and also in unheated beds in a large glasshouse at the beginning of May. The seedlings in the heated bed made rapid progress for the first four weeks, then tha growth was slower. By the middle of August the first lettuce was cut, and by the end of the first week in September all the lettuces were sold. These were of excellent size and quality. Those set in unheated beds were not ready til) mid-October. This glasshouse is equipped with an ordinary fuel-fired hot water pipe system, and from July onwards the minimum temperature (air) was 46deg. The heated bed had 150 ft length (700 watts) of soil-heating cable, spaced so as to give 10 watts per square foot. The supply of energy was controlled by a time switch, and could only be taken at “ off-peak ” times —that is, between 10 p.m. and 8 a.m., and noon and 3 p.m., when special cheap rates were available. The temperature of the soil was maintained at 55deg to 60deg.—D. Dyson Rayner. A.M.1.E.E., in Amateur Gardening.