Clays and heavy alluvial soils are not usually considered desirable soils on which to build. Water does not run from such soils; they hold moisture, and hence are always damp, and marshes are very apt to exist in the vicinity.
Effects of cultivation.
It was formerly thought that extensive cultivation was objectionable from the standpoint of health, that manured fields in the vicinity of a house were undesirable, and that the turning up of a well-manured field with a plow in the spring was a very likely source of fever. It is a very common belief to-day that when water pipes are to be laid in city streets, thereby disturbing the soil and bringing fresh earth to the surface, typhoid or other fevers may be expected. There is, however, no ground for this belief, and the fact that laborers and their families live healthily in the midst of the thousands of acres of sewage-irrigated fields near Berlin, where the heavily manured fields are constantly being plowed, is a sure proof of this. The earlier text-books on hygiene all assert, however, the contrary; Parkes, for instance, says that irrigated lands, especially rice fields, which give a great surface for evaporation and also exhale organic matter into the air, are hurtful, and in northern Italy the rice grounds are required to be three quarters of a mile from the small towns to protect the village inhabitants against fevers. There is no ground, however, for such a requirement.
No evidence can be found that men who work in sewers and who breathe sewer air all the time are especially unhealthy. Statistics show that the laborers on the sewage fields of Paris and Berlin are actually healthier than the average person living within those cities.
No reason can be assigned, based on our present knowledge of bacteriology, why upturned earth or manured fields should be unhealthy except as the breeding of insects may be encouraged thereby. The two essentials, however, which should be considered are: first, the topography or the formation of the soil in order that the surface water may run off freely, and second, the character of the soil so that ground water may not remain too near the surface. Whether the soil is rock or gravel makes very little difference.
Made ground.
One kind of soil, however, is distinctly objectionable, although, fortunately, in the country such a soil is unusual: That is, a soil made up of refuse, whether it be garbage, street sweepings from a near-by city, or factory refuse.
The writer has in mind one enterprising landowner and farmer who offered a near-by city the free privilege of dumping the city garbage on his land. This was done for several years, and the low-lying districts of his farm were all filled to a more advantageous level. This garbage was then covered with about a foot of dirt and the land sold in building lots to enterprising laborers determined to own their own homes. According to the old theories of hygiene, the occupants of such houses should have died like rats, but no particular excess of sickness in the one hundred houses so located could be observed. One must, however, believe, as we shall see later, that the repeated breathing of air drawn from such polluted soil must be unhealthy, even though the mortality records fail to show it.
It is interesting in this connection to note that the organic matter in soil gradually disappears, just as a body buried in a grave will finally decompose. Experiments show that such organic matter as wheat straw or cloth in small pieces rots and decays in about three years. But this depends very largely on an excess of air. If the soil is open and the organic matter loose, oxidation takes place rapidly; but if a large pile of organic matter is buried in clay soil, it will take decades for it to disappear. The vegetable matter in soil is usually produced by the decay of plants which have either grown on the soil or have been washed down into its voids. A great deal was formerly written on the relation between this organic matter and the prevalence of malaria, and some earlier writers believed that the amount of malaria in a district was dependent upon the amount of vegetable débris in the soil. Since we have learned that malaria is carried by mosquitoes, we are less interested in the amount of organic matter in the soil. Its mere presence is not likely to be injurious.
Water in the soil.