ABSORPTION OF ASH-CONSTITUENTS 103 



5. Microchemical Ash Analysis. — Small amounts of plant tissue may be studied by 

 microchemical methods, to determine what chemical elements are present. 



6. The Plant and the Soil. — Ordinary plants obtain all their ash constituents from 

 the soil, but a chemical analysis of the soil is of little value in determining whether a 

 plant can thrive in any given soil. The essential elements must be present as the 

 proper salts, and these must be supplied to the obsorbing roots at proper rates. 

 Soils may generally be much improved for growing plants by the addition of certain 

 inorganic salts, or of material that will produce these when it is decomposed by soil 

 microorganisms. To determine the value of a fertilizer, it must generally be actually 

 tested with the given soil and with the kind of plant that is under consideration. 



Many plant roots are normally accompanied by fungus hyphae as mycorhiza, 

 these hyphae either forming a weft about the root or occurring in the cavities of the 

 superficial cells. Mycorhiza is necessary for many plants, especially when growing in 

 humus soils. It appears that the fungus hyphae facilitate the movement of substances 

 from the soil into the roots. There is little or no nitrification in humus soils, and it is 

 possible that the mycorhiza in such soils may furnish the roots with some nitrogenous 

 substances other than nitrates. 



A soil may be unproductive because it contains too much (or too little) of the 

 soluble mineral salts, or because it contains very injurious substances in toxic amounts. 

 "Soil sickness," often resulting from repeatedly growing the same crop on the same 

 soil, appears to furnish an example of this, the toxic materials being probably organic 

 in such cases. They seem to be produced from the decay of plant roots, etc., or from 

 substances emanating from the roots, and they appear often to be related to the activi- 

 ties of microorganisms in the soil. Such a " toxic " soil may produce good growth of one 

 kind of plant (as wheat) while it is very injurious to another kind (as tomato). Bog 

 soils are toxic to many forms of plants, although characteristic bog plants thrive in 

 them. 



