Vol. XII. No. 301. 



THE AGRICULTURAL NEWS 



415 



THE SOIL 



NON-AVAILABLE WATER IN 



THE SOIL. 



The following .irticle is a suimiiary appended to 

 Research Bulletin Xo. 3 of the University of Xi'braska. 

 It deals with the results of experiments conducted 

 with the object of determining the relation of the final 

 water content of the soil to the hygroscopic coefHcienC, 

 and to determine to what e.xtenc the moisiure is 

 removed from the different portions of the subsoil not 

 penetrated by plant roots: — 



Water tight cylinders, feet long and holding about 

 100 D). of soil, were either filled with dry soil, saturated 

 with water, and drained before sealing at the bottom, or 

 filled with soil already containing the desired amount of 

 moisture. These were removed to a greenhou.se, seeds of 

 wheat, mile, beans, or maize planted in the moist surface 

 soil, and no more water added, the resulting plants being 

 allowed to grow until they matured normally or died. 

 Upon the death of all the plants in a cylinder it was opened, 

 both the total and the free water in each 3-inch section of 

 soil determined, and the distribution of roots observed. In 

 other cylinders three perennial desert legumes were grown 

 until they died or were near their limit of endurance, then 

 the cylinders were opened and the moisture content and root 

 distribution determined. 



In all the cylinders bearing plants, a hard crust devel- 

 oped below the surface mulch of dry soil but it seemed to 

 have no injurious effect. The formation of such a crust is to 

 be regarded as unavoidable where during a prolonged period 

 of rainless weather, plants with a well-developed root system 

 and a very limited amount of moisture in the subsoil are 

 transpiring a large amount of water. 



In their ability to exhaust the moisture of the subsoil 

 before dying, l!ed Fife wheat. Kubanki wheat, milo, Mexi- 

 Ciin beans, and maize showed little diflerence; but in their 

 ability tn continue alive after first showing serious injury 

 from drought they exhibited marked dift'erences. The 

 interval between wilting and death in the case of the 

 beans amounted to only a few days, but in that of wheat and 

 milo it often extended to many weeks. Where there was 

 a well developed root system and no remarkably unfavour- 

 able conditions occurred before the death of the plants, the 

 moisture content could be reduced by any of these plants 

 almost to the hygroscopic coefficient. 



In experiments with perennial desert legumes, the plants 

 remained alive after the water content had fallen slightly, 

 but distinctly, below the hygroscopic coefficient, even to the 

 point at which all the above-mentioned annual crop plants 

 had died. Under favourable conditions these legumes 

 adjusted themselves to the gradually increasing dryne.^s of 

 the soil by dropping their leaves one by one, but where, with 

 the subsoil moisture already reduced to near the hygroscopic 

 coefficient, conditions causing an abnormally high transpira- 

 tion suddenly set in, death occurred without the leaves having 

 dropped. While the experiments furnish no evidence of any 

 ability on the part of these legumes to utilize /or j^roic^/i the 

 last portion of free water, they indicate that this portion has 

 a very high value for the maintenance of life and that even 

 some of the water below the hygroscopic coefficient may be 

 available for the maintenance of life in these plants. 



In the portions of a semi-arid subsoil where roots are well ' 

 developed, the final content of free water is independent of 

 the distance from the surface, except where the stored water 

 is much in excess of the amount required for the complete 

 maturity of the plant. 



When the portion of the subsoil, in immediate contact 

 with the Tootf, contains only a comparatively small amount of 

 free water, crop plants may die quickly if conditions are such 

 as to Ciuse an unusually rapid transpiration. An abundance 

 of free water in deeper portions of the subsoil, into which but 

 few roots have been developed, may not avail to carry the 

 plant over such a critical period. 



The economy in the use of a certain amount of free water 

 stored in the subsoil may be nuich affected by its distributirm. 

 A high content confined to the portion of the .>oil near the 

 surface may induce a rapid growth of the plants and an 

 economical use of the water in so far a3 the production of 

 foliage alone is concerned, but the resulting large transpir- 

 ing surface may cause the death of the plants before they 

 reach maturity. The same amount of free water distributed 

 through a greater depth may induce a slower growth, a!lo,v 

 a longer lease of life, and so permit of the production of seed, 

 while if the same amount be distributed through a still 

 greater depth the content of free water may everywhere be 

 too low to permit of the development of roots, thus render- 

 ing the free water non-available. 



The loss of water from the subsoil of dry lands under crop 

 seems to take place almost entirely through transpiration. In 

 the absence of plants the loss from the subsoil is small. The 

 stored moisture of the different depths of subsoil in the 

 field becomes available to the plants by the roots being 

 developed into these depths, but little moisture being 

 elevated to the roots by capillarity. However, in the case 

 of subsoils saturated in cylinders or pots and comparable 

 to such field subsoils as are only a few feet above the water- 

 table, the content of free water is so high that large amounts 

 of moisture may be elevated to the roots by capillarity. The 

 amount of water retained by a soil saturated in pots or 

 cylinders is far in excess of the amount retained by a similar 

 soil saturated in a field where the water-table is at a consid- 

 erable depth below the surface, as in ordinary dry-land soils. 



To obtain a basis for comparing the available moisture 

 in soils, either the hygroscopic coefficient or the wiltini' 

 coefficient may be used. In general the one may prove as 

 satisfactory as the >-ther, but in considering the germination 

 of seeds and the development of roots and hence the whole 

 of the earlier portion of the life of annual crop plants, the 

 wilting coefficient appears the preferable; while in consider- 

 ing the production of seed in the case of annual crop plants, 

 and the inaintenance of life and even the growth of perennial 

 plants, the hygroscopic coefficient appears much the pre- 

 ferable. 



In the case of ordinary dry-land soils, the water-table 

 being at a considerable depth below the surface, the miximum 

 amount of soil water available to plants, for growth and for 

 the maintenance of life, is apjiroximatdy equal to the frte- 

 ivater^ihQ diti'erence between the total water and the hygro- 

 scopic coefficient — in these portions of the soil and the 

 subsoil occupied by the roots. l>'or some plants the avail- 

 able water appears to be somewhat greater and for others 

 somewhat less than the free water. 



