Four years' additional experimental work on the application of a high tension discharge to the growth of field crops is described. Ten experiments with spring-sown oats and barley, two with winter-sown wheat, one with spring-sown wheat, and four with clover-hay have now been described.
The discharge was usually given at the rate of 0·5 to 1·0 milliamp. per acre from thin insulated wires stretched above the crop at a height of about 7 ft. and charged to a voltage of 40,000 to 80,000 (crest value). The discharge was usually given for 6 hours a day in two periods, 3 hours in the morning and 3 hours in the afternoon.
If the two experiments with spring-sown oats described in previous papers are included and the experiment of 1919 with spring-sown wheat (which yielded only 8 bushels to the acre) is excluded a series of 18 results is available spread over a period of 6 years. Of these 18 field experiments with various crops, 14 gave positive results in favour of the electrified plots, while 4 showed negative results, i.e. decreased yields compared with the controls. Of the 14 positive results only 3 show increases of less than 10 per cent, while 9 show increases of 30 per cent, and over, some reaching 50 per cent, and over. Of the 4 negative none shows a decrease of as much as 10 per cent.
Of the 12 experiments with spring-sown cereals 10 were positive and 2 negative. Of the positive results only 2 show increases of less than 10 per cent, while 6 show increases of 30 per cent, to 57 per cent.; on the other hand both the negative results are quite small, being 6 per cent. and 9 per cent. respectively.
The effect of electrification in increasing the yield of spring-grown oats and barley has thus been demonstrated. The mean increase in yield for such crops was 22 per cent.
A beneficial effect of the discharge on clover-hay is probable while the effect on winter-sown wheat is still uncertain.
Our knowledge of the proper conditions under which the discharge should be given is still so meagre that there is no reason to believe that the increased yields here described are the maximum obtainable as a result of electrification.
The mode of action of the current in producing increased growth and yield is still obscure. In several cases the electrified field crops showed a deeper green tint than that of the controls, and work already published has shown that in the case of the coleoptile (plumule sheath) of barley minute electric currents are able to bring about an increase in the rate of growth.
The effect of the discharge is of the nature of a stimulus. The additional energy available from the current is too small to have any direct effect since it is only about 1/1000th, or less, of the energy which the plant obtains from sunlight.
There is no evidence that gaseous products of the discharge play any part in the stimulation of growth. The additional supply of nitrogen brought to the soil from oxides of nitrogen produced as a result of electrification must be exceedingly small.
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