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A History of Natural Soft Soap

It is thought that soap has been around since the days of the ancient Egyptians, and there are records of it being produced from tallow and beech ashes by the Phoenicians in 600 BC. It even gets a mention in the Bible: Jeremiah 2:v22.

Soft soaps have been used to control insects for more than 200 years, but recently there has been increased grower interest in using them again. There is a genuine desire at least to try substances that are safer to use than the array of chemicals on offer.

Soft soap consists of salts of potassium fatty acids and is biodegradable. It is readily broken down by bacteria and therefore not considered a pollutant. Soaps can be made from various sources of natural fats and oils; sodium making hard soap and potassium soft. Soaps are different from synthetic detergents, which are made from petroleum oil.

The mechanism by which soap kills insects is in fact not well understood. It appears that soap washes away the insect's cuticle, and then blocks its spiracles preventing oxygen transfer. In other words, the soap washes off the insect's protective waxy covering, and stops it breathing, so it drowns! Some researchers have suggested that soap can rupture cell membranes inside an insect, causing it to dry out.

Although soap is more commonly known for its ability to kill insect pests on plants, there are many uses for it against pests in animal houses, and on animals themselves, even humans. Soap has also been shown to have an effect on fungi. The mechanism is thought to be very similar: the soap causes the fungal cells to shrink preventing oxygen diffusion.

A lot of work was done in the late nineteenth century on the use of soft soaps against crop pests, and the old references make for enjoyable reading! Much of the work is difficult to interpret because the exact constituents of the various ‘insecticidal soaps' tested were not made clear. Believe it or not, plants would be physically ‘washed' in tubs of soft soap solution to remove insects! In 1860, the horse-drawn ‘Dutch Squirt' enabled growers to ‘spray' plants from a more convenient distance, but the term ‘washing' has stayed with us. Of course, spraying could never be as effective as a tub wash, so additives were soon tested to improve spray performance. Nicotine and quassia were added to soft soaps, and in many cases were shown to improve efficacy and persistence, but not in all. Some researchers even reported that these were unnecessary: in 1877, Symonds wrote … “soft soap is all that is required”!

One of the main draw-backs from using insecticidal soaps was the risk of phytotoxicity, especially from repeat treatments, and applications made under hot conditions. Some of the plant species most commonly noted for showing damage symptoms were the cucurbits, fern, nasturtium, tulips, hawthorn, and plums among others. The use of soft soaps has always been restricted by plant sensitivity, but the degree of damage caused by the soft soap itself, the insecticidal additives or the application conditions is anything but clear. We know that nicotine can be particularly phytotoxic, especially in its older less-refined forms, and we know that the addition of alcohol, or synthetic surfactants can be damaging to plants. Unfortunately, there is a limit to the assistance we can expect either from the literature or field experiences of only a generation ago. The best source of information is with individual entomologists and crop specialists, and from our own research on our own crops.

Benefits of natural soft soap

  • Soft soap can kill insects, provided enough soap covers enough of the insect.
  • It may also be useful against plant fungal diseases (at reduced rates).
  • It can be used in a way to minimise effects on beneficial insects.
  • Risk of spray scorch is much reduced in the absence of alcohol and detergents.
  • It is regarded as environmentally safe, and is fully biodegradable.
  • It is not persistent – an environmental and consumer benefit.
  • Can be a useful water softener (and hand cleaner).
  • Crop residues do not appear to be an issue.

Limitations of natural soft soap

  • Soft soap is contact action only: there is no persistence of activity on the leaf.
  • Repeat applications are usually necessary.
  • Pests vary in their sensitivity.
  • There is very little reliable information on the use of natural soft soap.
  • Properties can vary depending on the source of oil ingredients, and soap making process.
  • Spray applications may cause some leaf scorch (usually from rapid drying or spraying in hot conditions).
  • Additions of alcohol, synthetic surfactants, nicotine etc will increase risk of spray scorch.
  • Increased rates may be required where hard water is used for spraying (some of the soap will be used in precipitating an insoluble scum).
  • Soft soap cannot be readily mixed with other chemicals and fertilisers, (it is likely to affect the performance of other chemicals mixed with it, and the addition of some chemicals can alter the structure of the soft soap itself).
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