14 JAMESTOWN CONGRESS OF HORTICULTURE 



provides time and opportunity for the most valuable plant food mate- 

 rials carried in the water films about the soil grains to diffuse out into 

 the moving water and so become lost in the drainage. Thus we have 

 an explanation of a* seeming paradox, namely : properly drained fields 

 lose less of their soluble plant food by underdrainage than do those 

 poorly drained. 



Next in importance to internal soil surface, among the physical 

 factors which determine the productive capacity of soils, is the segre- 

 gation of their soil particles into granules, crumbs or kernels. With- 

 out it all but the extremely sandy soils must be sterile, even though 

 they carry an abundance of plant food. Without segregation we have 

 the puddled soil or clay, but with segregation highly developed we 

 have the light, deep, tractable, mellow fertile loams so congenial to the 

 widest range of crops. 



The low producing power, or absolute sterility, so invariably asso- 

 ciated with puddled soils and with those too close in texture, we 

 believe to be primarily due to a lack of available moisture, notwith- 

 standing the seeming paradox that they are carrying an excess of it. 

 It is a familiar fact that crops wilt and cease to grow in close textured 

 clayey soils still carrying 8 to 12 per cent of water, while they may 

 grow luxuriantly in coarse sandy soils possessing but 1 to 3 per cent. 

 So, too, we often find desert types of vegetation growing in humid 

 climates on extremely close grained clayey soils and more strangely 

 still in peat swamps where the water content is excessively high. To 

 understand these facts it must be remembered that there is a certain 

 thickness of water film which is held so firmly to the soil grain sur- 

 faces as to be wholly unavailable to the crop. Portions of this layer 

 cannot be driven off completely even at the temperature of boiling 

 water. When all of the facts shall have been worked out we believe 

 it will be found that the thickness of the unavailable water about the 

 surface of soil grains is essentially the same whether these be large, 

 as in the coarse sandy types, or very small as in the finest clays ; and 

 if this is the case the absolute amount of unavailable water must 

 increase as the internal surface of the soil becomes greater and as the 

 diameter of the soil particles decreases. 



The coarse sandy soils, with their relatively small internal sur- 

 face, carry a correspondingly small amount of unavailable water and 

 hence in them small rainfalls in dry times have a relatively high 

 efficiency. So, too, must soluble plant food and fertilizers, when 

 applied to them, for the same reason, have a relatively high efficiency. 

 But in the finest clay soils, especially if they are not strongly granu- 

 lated, the amount of unavailable water is very large and hence it is 

 that heavier rainfall during drought periods and more liberal applica- 

 tions of fertilizers are required to produce the same relative increase. 

 But it is possible to have the finest clay soils so completely puddled, 

 or separated into their ultimate grains, and the effective soil surface 

 thereby so enormously increased, by the minuteness of the particles, 

 that nearly the whole of the water, even when the soil is saturated, 

 becomes unavailable to plants and for the simple reason that the water 



