8 POPULAR SCIENCE MONTHLY 



formation of the four feet of soil next to the rock over our average 

 domain, where such depth obtains, it will probably be none too con- 

 servative. To preserve a good working soil-depth, with such an esti- 

 mate, surface wastage should not exceed some such rate as one inch in 

 a thousand years. If one chooses to indulge in a more liberal estimate 

 of the soil-forming rate, it will still appear, under any intelligent 

 estimate, that surface wastage is a serious menace to the retention of 

 our soils under present modes of management. Historical evidence 

 enforces this danger. In the Orient there are large tracts almost 

 absolutely bare of soil now, which formerly bore flourishing popula- 

 tions. Long-tilled lands generally bear testimony of like import. 

 Much more than mere loss of fertility is here menaced; it is the loss 

 of t the soil-body itself, a loss almost beyond repair. When our soils 

 are gone, we too must go, unless we shall find some way to feed on 

 raw rock or its equivalent. The immense tonnage of soil-material 

 carried out to sea annually by our rivers, even when allowance is made 

 for laudable wash, and for material derived from the river channels, 

 is an impressive warning of the danger of excessive soil-waste. Nor 

 is this all; the wash from one acre often buries the fertile portion of 

 another acre, or of several. Sometimes one's loss is another's gain, 

 but all too frequently one's loss is another's disaster. 



If the atmospheric waters may not run off the surface freely without 

 serious menace, where may they go and what may they do consistent 

 with our welfare? The answer lies in a return to the study of the 

 origin and internal work of soils. For necessary brevity, let us neglect 

 all secondary soils, or overplacements, and consider simply the origin 

 and activities of primary soils derived from primary rocks. The 

 action of air and water in producing soil from such rock is partly 

 chemical and partly physical. Certain rock substances are made 

 soluble and become plant food or plant poisons, while others remain 

 relatively insoluble, but are reduced to a finely divided state and form 

 the earthy element of the soil. 



Some of the soluble substances thus formed at the base of soils are 

 necessary plant food, while some are harmful; but what is more to 

 the point, all are harmful if too concentrated. There is need, there- 

 fore, that enough water pass through the forming soil, and on down 

 to the ground-water and out through the under-drainage, to carry away 

 the excess of these products. An essential part of the best adjustment 

 is thus seen to lie in a proper apportionment of the amount of water 

 which goes through the soils. If this be not enough, the plants will 

 suffer from saline excess ; if it be too much, the plants may suffer from 

 saline deficiency. 



When evaporation from the surface is active and prolonged, waters 

 which had previously gone down to the zone of soil-formation and 



