ABSORPTION OF ASH-CONSTITUENTS 103 



ferrous sulphate (FeS0 4 .7H 2 0). This is one of Shive's nutrient solutions. Three 

 single-salt solutions (with trace of an iron salt) may be used in rotation, if concen- 

 trations and time periods are properly chosen. 



2. Importance of Essential Ash-constituents. — Little is known as to just how the 

 small amounts of essential ash constituents are used in the plant, but all must be 

 supplied. Sulphur occurs in proteins, phosphorus in nucleins (a special group of 

 protein-like substances), magnesium occurs in chlorophyll, and iron is essential for 

 the formation of chlorophyll. 



3. Importance of Non-essential Ash-constituents. — Although plants grow well 

 with only the essential elements supplied, yet they generally contain many non-essen- 

 tial elements, and these are not without influence upon growth and development when 

 they are present in the right amounts. Grasses accumulate silica in the epidermis and 

 are thus more or less protected, from fungi, etc., by a glassy layer on the exterior. 



4. Ash Analysis of Plants. — The chemical analysis of the ash of a plant shows what 

 elements are present and in what proportions they occur. Different species differ in 

 these respects, and also in the amount of total ash per unit of weight, etc. The nature 

 of the soil influences the ash content of the plant. Different parts of the same indi- 

 vidual plant differ in ash content. Leaves are generally richer in ash than stems and 

 roots. The ash content alters with the age of the organ or tissue. 



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 absorbing 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 and kind of climate that are 

 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 supply 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 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. 



7. Oxygen Absorption from the Soil. — Ordinary plants generally need a low rate of 

 oxygen supply from the soil to their roots. 



