LAND AND OTHER AGENTS OF PRODUCTION 189 



still very large. It has been estimated that all plant production in 

 this country is annually reduced from 20 per cent to 25 per cent 

 through plant diseases, and there is considerable foundation for this 

 estimate. When we realize that we are dealing with a crop worth 

 annually between six and seven billion dollars on the farm, the mag- 

 nitude of this loss is appalling. Only a part of this can, of course, ever 

 be reached and prevented. Many diseases are physiological, pro- 

 duced by the effect of climatic and soil conditions difficult or impos- 

 sible to change. In the irrigated regions of the West, new types of 

 physiological diseases have caused serious troubles in the orchards of 

 deciduous fruits and in the orange groves. It may take years of care- 

 ful research to even find out the cause of some of these troubles, and 

 they appear to be difficult to remedy even when the cause is thor- 

 oughly known. 



On the other hand, the fungous diseases of plants have yielded to 

 research during the last thirty years in a manner which is really mar- 

 velous. The black rot of the grape, a native disease on American 

 grapevines, attacked our rapidly increasing grape industry in the 

 eastern United States in the early eighties. The Department of 

 Agriculture at Washington started experiments in 1886 and within the 

 next four or five years gave to the grape-growers a complete and suc- 

 cessful routine treatment by spraying through which from 95 per cent 

 to 98 per cent of the crop could be saved. This treatment is the very 

 basis of the grape industry. Without it the vines would bear only 

 ragged and unsightly bunches scarcely fit for shipping to market. 



I should not, however, convey the idea that all plant diseases have 

 been brought under control. Many problems, like the crown gall 

 of fruit and other trees, the root rots, the new citrus canker, and 

 numerous others, still attack vegetation unchecked, or only partly 

 controlled, or, in case of the citrus canker, controlled by heroic 

 methods, such as burning up the entire tree when only a single leaf 

 is affected. 



I have mentioned earlier in this paper chemical investigations of 

 the soil. At first, that was thought to be the important problem in 

 soil studies. Later it was shown that the physical properties of the 

 soil were as important, or possibly more important, than its chemical 

 composition. Still later, within the last twenty years, it has been 

 realized by investigators that the biological properties of the soil are 

 even more important than either its chemical or its physical proper- 

 ties. The soil under our feet is teeming with bacteria, with fungi, with 



