Silent Spring: Chapter 5 Summary & Analysis

Soil supports life – its nutrients fuel plants, and by extension, most of the world’s animal life as well – but it also formed by life. It is full of living things, including vast quantities of microbes, bacteria, and fungi that interact in complex cycles of soil development, decomposition, and fertilization. Carson argues that scientists have not fully considered the negative effects of pesticides on this delicately balanced ecosystem – not enough research has been done, and the potential for danger is large.

Microscopic soil inhabitants convert decaying matter into nutrients for plants, which are also reliant on the nitrogen and carbon dioxide generated by bacteria. Of the larger soil-dwellers, the earthworm is perhaps most important. It performs a vital role as soil-mixer, helping to aerate and transport vast amounts of soil each year. But, in fact, each member of the soil “community” has a highly specific task that is related to and enriches the others. So, asks Carson, how can we expect that a nonspecific fungicide or broad-spectrum insecticide—poisons that are not targeted to kill a single pest but in fact will kill all sorts of insects and bacteria—will not destroy the good along with the ‘pests?’

This problem has been largely ignored, by control agencies and scientists alike. Some pesticides have been shown to disrupt nitrification, an important process that produces nitrogen in the soil. Pesticides can also disrupt natural predator-prey checks and balances and lead to the flourishing or destruction of different organisms – sometimes the very pests that insecticides were meant to control. When larger insects, like praying mantises, for example, are destroyed, the insects that were their natural prey can reproduce unchecked. Most alarmingly, these chemicals have been shown to persist in the soil for years, accumulating over multiple single applications of ‘safe’ dosages to eventually reach dangerously high levels.

Arsenic in tobacco fields provides one example. Even though arsenic has been replaced by synthetic insecticides in the production of tobacco, the arsenic content of American-grown tobacco increased 300-600% from 1932-52. This is because residues from previous episodes of arsenic spraying in the soil are breaking down into a form that dissolves into water and is then absorbed into the plants.

This introduces the issue of plants absorbing pesticides into their tissue through the soil. Hop growers in Washington and Idaho were advised to use a pesticide called heptachlor to treat the soil against a pest called the strawberry root weevil, but vines planted after treatment withered and died. Even after the area had been replanted, the poison persisted in the soil and was absorbed by plants. Different plant species also absorb these poisons in the soil at different rates, although research in this area is minimal. Clearly, little is known, and the danger is potentially enormous.