328 



FORESTRY AND IRRIGATION 



Tune 



off the surface freely without serious men- 

 ace, where may they go and what may they 

 do consistent with our welfare? The an- 

 swer lies in a return to the study of the 

 origin and internal work of soils. For 

 necessary brevity, let us neglect all sec- 

 ondary soils, or overplacements, and con- 

 sider 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 rela 

 tively insoluble but are reduced to a finely 

 divided state and form the earthly 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 passes through the 

 forming soil, and on down to the ground- 

 water and out through the underdrainage, 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 enoip^h, ih^i plants 

 will suffer from saline excess; if too inuch, 

 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 taken up soluble matter, rise 

 again to the surface bringing the soluble 

 matter up and leaving it at the surface on 

 evaporation. Up to a certain point this is 

 favorable to the plant ; beyond the critical 

 point, it begins to be harmful, as abundantly 

 shown in the " alkaline " eflflorescences of 

 arid regions. 



Beside the water that goes through the 

 soil into the subdrainage, and that which 

 runs off on the surface, enough must be 

 held at all times in the soil during the 

 growing season to supply the plants, and 

 yet not enough to water-log the soil. 



The key to the problem lies in due control 

 of the water which falls on each acre. This 

 water is an asset of great possible value. 

 It should be looked upon as such. It 

 should be computed by every acre-owner 

 as a possible value, saved if turned where it 

 will do good, lost if permitted to run away, 

 doubly lost if it carries also soil values and 

 does destructive work below. Let us repeat 

 the story of its laudable paths. A due por- 

 tion of this should go into the underdrain- 

 age carrying azvay harmful matter; a due 

 portion should go again up to the surface 

 carrying solutions needed by the plants; a 

 due portion should obviously go into the 

 plants to nourish them ; while still another 

 portion should run off the surface carrying 

 away a little of the leached soil matter. 

 There are a multitude of important details 

 in this complex of actions but they must 



be passed by; the great features are clear 

 and imperative. 



Experimental studies have shown that, 

 on the average within our domain, crops 

 can use to advantage all the rainfall during 

 the growing season, and that, in most cases, 

 crops are better for all the stored supplies 

 that can be carried over from the non- 

 growing seasons. This greatly simplifies 

 the general problem, for it justifies the con- 

 clusion — to which there are many local ex- 

 ceptions, of course — that the highest crop- 

 values will usually be secured when the soil 

 is made to absorb as much of the rainfall 

 and snowfall as practicable. In securing this 

 maximum absorption and internal soil-work, 

 the run-off, and hence the surface wash, will 

 be reduced to a minimum. It has already been 

 seen that the wash of even this inevitable 

 minimum is likely to be still too great to 

 keep the proper slow pace with soil-genera- 

 tion, when the surface has much slope. Ex- 

 cept on very level ground and on lodgment 

 surfaces, there need be no solicitude about 

 a sufficient removal of the soil surface. The 

 practical problem then lies almost wholly in 

 retaining and passing into the soil the maxi- 

 mum of the precipitation. Obviously this 

 gives the minimum of wash to foul the 

 streams, to spread over the bottom lands, to 

 choke the reservoirs, to waste the water- 

 power, and to bar up the navigable rivers. 

 The solution of the problem for the tiller 

 of the soil essentially solves the zuhole train 

 of problems. 



How is this control to be effected? As a 

 geologist, I naturally turn first to nature's 

 time-tested processes. Nature has been 

 working on this complex problem of bal- 

 ance between soil formation, soil waste, sur- 

 face slope, plant growth, and stream de- 

 velopment, for millions of years, and we 

 have inherited the result, a magnificent in- 

 heritance. The larger part of our domain, 

 when invaded by us, had reached a fair ad- 

 justment of slopes to precipitation, was cov- 

 ever with a soil-mantle of fair depth and 

 high average fertility, and was clothed with 

 rich vegetation. There were exceptions to 

 this, and some of these were large, but limi- 

 tations of time shut out such exceptions 

 here. 



Looking at nature's mr^thods for sugges- 

 tions, we note that a much larger variety of 

 plants are used by nature to cover and pro- 

 tect the soil than we use, and that these 

 have a wider range of adaptation to the 

 special situations where protection is 

 needed. This invites the inquiry whether 

 it is not possible to follow this precedent 

 further than we have done by developing a 

 larger number of profitable plants, among 

 which shall be more that are adapted to pro- 

 tecting the surface, and to growing on 

 slopes specially subject to wash. Forest 

 trees are an important resource of this kind 

 and should be employed as fully as practi- 

 cable, as will, no doubt, be urged with great 

 cogency by those who discuss the problem 



