MARYLAND WEATHER SERVICE y I 



nesium plays a role analagous to that of sodium chloride in the salt 

 marsh. 



Kecent work in the Bureau of Soils of the Agricultural Depart- 

 ment has shown that toxic substances may be present in soils as a 

 result of their excretion by the roots of plants. They are relatively 

 complex organic substances and each of them exerts its toxic effect 

 only upon the same species of plant as that by which it was excreted. 

 Much of the value of the practice of crop rotation would appear to be 

 due to the accumulation of these toxic substances rather than to the 

 exhaustion of any of the food elements of the soil.* 



The size of the rock particles of the soil and the proportion of 

 humus material that it contains are the only physical characteristics 

 which are of universal importance. The fact that the physical 

 texture of soils determines so completely the control which they exert 

 iiver the supplies of water furnished them by rain-fall or under- 

 ground seepage, makes very important a knowledge of the behavior 

 of soils of different texture with respect to the receiving, retention 

 and lifting of water. The gravels and sands are in all respects the 

 reverse of the finer loams and clays in their relation to the soil- 

 water. The former are highly receptive to the water which falls 

 upon them as rain, while clays may be so unreceptive as to often 

 "puddle" at the surface while their lower layers are not yet satura- 

 ted. The capacity of the gravels and sands to retain water against 

 the pull of gravity is very poor, while clay soils hold water so re- 

 tentively that it may even be impossible for plants to secure it from 

 moist clay owing to the strong retention which prevents the move- 

 ment of water toward the root hairs of plants to replace that which 

 has been absorbed. With respect to the capacity of the two classes 

 of soils to lift water, the sands are capable of doing so poorly and 

 only to inconsiderable heights, while clays lift slowly but to great 

 distances. The loams such as form the bulk of the surface soil of 

 agricultural lands are intermediate in character between the sands 

 and clays, and present, therefore, the optimum, soil conditions for 

 plants, — a moderate amount of water received with ease and re- 



*For the original papers on root excretions see Bulletins 28, 36, 40 and 47 

 of tie Bureau of Soils. 



