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considerable part of the plant nutritive substances which are washed away 

 into the subsoil. Such soils therefore, conversely, must be kept under a 

 covering of plants. 



Local conditions must show which one of these means can best be used 

 to prevent a lack of moisture. In any case it is evident that we do not stand 

 powerless in the face of drought. 



b. Loamy Soils. 

 General Characteristics. 



In considering physical influences injurious to vegetation, we need not 

 distinguish between loam and clay soils. We are concerned always with 

 mixtures of clay and sand and only the proportions of these two elements 

 differ. The sand content decreases more and more from sandy or "mild" 

 loam to strictly loamy soil and to clay soils, which are plastic in a damp con- 

 dition ; in them predominate the fine particles so easily washed away. In 

 our agricultural land, mixtures of lime and humus will also be of importance 

 as modifiers. Lime will make heavy soils more open by increasing their 

 friability. 



Fertility is directly dependent upon friability, hence plastic clays are 

 sterile. Non-friable clay soils are impervious to water, and, in level places, 

 easily give rise to the formation of swamps. The smaller the size of the 

 soil particles, the greater will be their water absorptive power so that very 

 significant changes in volume occur with extensive, rapidly successive differ- 

 ences in the supply of water. Upon this depends the characteristic cracking 

 of clayey soils when drying out. Soluble salts can be washed out of clay 

 soils only with difificulty. 



This drying out is much more dangerous as the soil approaches pure 

 cla}^ When once dr}^, clay takes water up again very slowly since it 

 can penetrate only with difficulty between the closely packed soil particles. 

 These peculiarities decrease proportionately as the admixture of sand in- 

 creases. Drying out in summer becomes at times more dangerous in heavy 

 soils than in sandy, especially if a vigorous growth of trees has developed 

 in regions which at best are poor in precipitation. The summer rains do 

 not then suffice to make good the loss of water. These soils are dependent 

 on the winter moisture. Hence the plant growth suffers here much more in 

 dry springs, in years when the winter moisture has been less and the snow 

 covering has failed, than on sand. This explains the fact that, after hot, 

 dry summers and winters, poor in precipitation, a blighting of the fops of 

 old trees (i. e., a drying of the branches) sets in because of the lack of 

 moisture, even if the spring has abundant rain. .Sandy soils with moderate 

 spring rains are saturated more cjuickly and the water is at the disposal of 

 the roots. 



Heavy soils remain "cold." This is explained by their high water con- 

 tent which increases with the fine granular structure. In many regions im- 

 ported conifers (Abies Finsapo, Biota orientalis anrea, Taxus hibernica, 



