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.
These are obtained from Picroena excelsa. They should be boiled for two or three hours to extract the bitter principle. An excellent all-round insecticide is made from 1 lb. quassia chips, 1 lb. soft soap, and 10 gal. water. The quassia chips, after boiling in a gallon or two of water, should be strained off through muslin or sacking, and mixed with the dissolved soft soap, making the whole solution up to 10 gal. Trees in fruit, Cucumbers, etc, should not be sprayed with quassia solution, as it imparts a bitter flavour.