56 THE PLANT 1.1 M. OF MARYLAND 



hand, the size of the rock particles. The constant contact of water 

 with the rock particles and humus causes a continued solution of 

 their component substances, and all the importance of the chemical 

 character of the rock particles lies in the substances which they yield 

 into solution and thus make available to plants. The soil-water is, 

 then, a very dilute solution in proportions varying with their solu- 

 bility of all the substances with which it has come into contact in 

 the soil. The existence in the soil-water of salts containing nitrogen, 

 phosphorus, sulphur, potassium, magnesium, calcium and iron is in- 

 dispensable to the nourishment of all green plants. Other elements 

 may be taken up by plants but are used by them only as accessory 

 and dispensable food substances. It is significant that the chemical 

 character of most rocks is so complex that there are only rare cases in 

 which soils lack any of the elements necessary to plant nutrition. 

 It is true that some of the elements are present in soils only in very 

 small quantities, as phosphorus, and may be drawn upon by plants 

 more rapidly than they become available, or may be leached away by 

 the action of the rain. The nitrogen demanded by plants can be 

 used only in the form of nitrates which are very readily soluble 

 salts. The nitrates of the soil are constantly being replenished by 

 the action of bacteria upon the humus, and may be wanting either 

 by the lack of humus or of sufficient aeration of the soil for the 

 activity of the nitrifying bacteria, or by the too rapid leaching away 

 of the products of bacterial action. 



It is evident, therefore, that unfavorable conditions for plant 

 growth may arise through the insufficiency in the soil-water of some 

 of the less abundant or more readily leached salts. Equally un- 

 favorable conditions may arise through the accumulation of per- 

 centages of any of the salts which are so high as to become toxic to 

 roots. This is the condition in salt marshes and in the alkali soils 

 of the western United States, and accounts for the absence of ordi- 

 nary plants from these habitats and their peopling with specialized 

 salt-resisting plants. The fact that a given percentage of salt in the 

 soil-water is not equally toxic to all plants causes a sorting or elimi- 

 nation of species, which may be observed both in the case of salt 

 marshes and in the serpentine barrens, where the excess of mag- 



