SOIL WASTAGE 7 



water-foods, for power and for navigation. Here is a good work — 

 soil-production — followed by advantageous courses of the water both 

 up and down. On the other hand, the rainfall may rush away on the 

 surface as a foul erosive flood, wasting soil and plant-food, gullying 

 the surface, flooding the valleys, filling the reservoirs, sweeping out 

 the dams, barring the streams and clogging the deltas. If it shall 

 be found that nearly all the rainfall should go into the soil and thence 

 into the under-drainage, coming out slowly and steadily by seepage 

 and by springs into the streams, clear and pure, these streams should 

 present nearly ideal conditions for water-food, for water-power and 

 for stream-navigation. An ideal solution of the soil problem may 

 therefore solve the greater part of the whole complex of problems of 

 which navigation is the last term. It may thus prove to be the key 

 problem. It is clearly the initial problem, for it attacks the rainfall 

 when it first touches the earth. 



To see more definitely if it be the key problem, we must turn to 

 details, and yet, with the brevity that is imperative, we may only look 

 at major details, passing by a multitude of special cases, some of 

 which are even exceptions. 



While soils are formed by the atmosphere and its waters acting 

 upon rock (aided by plants and animals), soil surfaces are carried away 

 by wind and wash. At any instant, then, the depth of the soil meas- 

 ures the lag of removal behind production. We hasten to note that 

 the addition of new soil below and the loss of exhausted soil above are 

 alike tributary to permanent fertility, and clearly the best results 

 spring from the proper ratio of addition at the bottom, to wastage at 

 the surface. 



We have as yet no accurate measure of the rate of soil production. 

 We merely know that it is very slovj. It varies obviously with the 

 kind of rock. Some of our soils are derived from material already 

 reduced to a finely pulverized condition. Such are the lowland accumu- 

 lations from highland wash. Such also is the glacial drift, rock- 

 flour rasped from the face of the rock by the glacial file and ground 

 up with old soils. Soils may be developed from such half-prepared 

 material with relative rapidity, but observation shows that even in 

 these cases, when the slope is considerable, wind, wash and cropping 

 remove the surface much too fast for stable fertility. For average rock, 

 under the usual conditions of our climate, the common estimate of 

 natural loss and gain has been a foot in 4,000 to 6,000 years, which 

 includes channel-cutting and bank-undermining. This seems to me 

 too rapid a rate for ordinary soil production under normal conditions. 

 Without any pretensions to a close estimate, I should be unwilling to 

 name a mean rate of soil-formation greater than one foot in 10,000 

 years, on the basis of observation. If we allow 40,000 years for the 



