This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
This is one of the cheapest, simplest, and at the same time most effective insecticides used for horticultural purposes. Good samples should be free from resin and contain not less than 8 per cent of potash. It dissolves readily in water, and 1 lb. to 4 gal. serves to kill aphides, mealy bug, scale, red-spider, etc. Soft soap is mixed with various other insecticides and fungicides, as may be seen from the formulae given above.