SOBRALIA 



SOIL 



1675 



!). L6wii, Rolfe. An imperfectlj' known species in- 

 troduced iibout 1892 from Colombia. It grows about 

 1>2 ft. high and has fls. of a bright uniform purple. 



The following trade names are not accounted for : 5. inag- 

 nifica.—!!). PfauU.—S. virylndlis. 



Heinrich Hasselbrinq. 



SOIL. The soil is a superficial covering of the earth's 

 crust, more or less well adapted to the growth of plants. 

 It is usually only a few inches thick. Below this is a 

 subsoil often differing, especially in humid climates, 

 from the soil proper in color, texture, or chemical com- 

 position. A verj^ striking definition has been suggested 

 by Sir John B. Lawes, who considered the soil to be 

 rotten subsoil, and the sul)soil rotting rock. The term 

 soil is occasionally used in a more comprehensive way 

 to include both the soil and the subsoil. 



The soil adapted to the growth of the higher plants 

 consists of fragments of rocks or minerals, organic 

 matter, soil solution, and a soil atmosphere. The 

 mineral fragments vary in size from the finest clay 

 particles to gravel and even boulders. The organic 

 matter is derived from low organisms, from previous 

 vegetation, or from growing plants; as also from stable 

 manure, and occasionally fish or animal matter added 

 to the soil by man. The soil solution consists of water 

 carrying dissolved substances derived from the soil 

 grains and from the organic matter, as well as from 

 fertilizing materials artificially applied, and constitutes 

 a nutrient solution from which the plant derives its 

 mineral constituents. The soil atmosphere differs from 

 the ordinary atmosphere above the soil in being richer 

 in carbon dioxid and nitrogen, and containing more 

 water vapor and less oxygen. 



In origin there are two main classes of soils: seden- 

 tary soils, formed by the disintegration and decomposi- 

 tion of rocks in place; and transported soils, including 

 those of alluvial, glacial and aeolian origin. The word 

 alluvial is here used to include all water-transported 

 material; the tenn is, however, frequentlj^ used in a 

 more specific sense to indicate the recent flood deposit 

 of rivers. 



Soils are classified according to their origin and their 

 mechanical and chemical composition and properties. 

 Genetically, they are classified according to the rock 

 from which they are derived, as granite soil, lime- 

 stone; or according to the manner of their origin, as 

 alluvial, lacustrian or drift. Mechanically, they are 

 classified broadly into stony, gravelly, sandy, sandy 

 loam, loam, clay loam, clay, adobe, black-waxy, or, 

 according to some other physical property; chemically, 

 into calcareous, humus, alkali, and according to other 

 striking chemical features. In the soil survey of the 

 U. S. Department of Agriculture a local name is adopted 

 for each type under which the specific characters are 

 given; examples of this are Hartford sandy loam, Nor- 

 folk sand, San Joaquin adobe, Glendale loess. 



The physical properties of soils concern the size and 

 arrangement of the particles, and the relation of these 

 to each other and to the organic matter; also the soil 

 atmosphere, the soil moisture, and the physical forces 

 of heat and gravitation. In these there is an intimate 

 relation with physiography or the form and exposure of 

 the surface of the land, as well as to climatology. 



There are undoubtedly constant physical changes 

 going on in the soil, as well as chemical changes, which 

 have much to do with the best development of vegeta- 

 tion. The soil moisture may be looked upon as a 

 nutrient solution dissolving its material from the diffi- 

 cultly soluble compounds in the soil and from fertilizers 

 artificially applied. The amount of substances in solu- 

 tion varies with the moisture content and with the way 

 moisture is supplied to the soil. The dissolved sub- 

 stances, naturally present in the soil or derived from 

 fertilizers, influence the solubility of the soil compo- 

 nents, rendering them more or less soluble according to 

 their nature and existing conditions. It is probable 

 that there is a normal weathering of the soil material 

 which produces a certain concentration in the soil solu- 

 tion which will be maintained on the gradual withdrawal 

 of nutrient material by the plant. However, this 

 natural weathering is often not sufficient in amount to 

 produce the yield and quality of crops desired, and tliis 

 msiy be increased bv methods of cultivation and fertili- 



zation so that crops may annually remove larger quan- 

 tities of nutrient substances without any particular 

 exhaustion to the soil. 



It is certain that these nutrient materials do not ac- 

 cunuilate to any considerable extent in soils in humid 

 countries, as they are liable to be leached away and also 

 to recombine, forming difficultly soluble compounds 

 with the material of the soil grains. A soil is in good 

 heart or good condition when the physical conditions, 

 such as the water supply, soil atmosphere and tempera- 

 ture relations, are favorable, and when the weathering 

 of the material is sufficient to furnish an abundant and 

 constant nutrient solution in the soil moisture. 



One of the most potent agents in the weathering of 

 soils is the organic material contained. This is unques- 

 tionably due largely to the amount of carbon dioxid 

 formed, which renders many of the nutrient matters 

 much more soluble. Moreover, the organic matter forms 

 a culture medium for bacteria, ferments and the vari- 

 ous organized and unorganized agents which assist in 

 breaking down the organic material, and facilitate as 

 well the weathering of the other soil components. Soils 

 in general have remarkable power of absorbing on the 

 surface of the soil grains vast quantities of carbon 

 dioxid, ammonia and other gases, and of other nutrient 

 materials, which while soluble and actually dissolved, 

 do not readily diffuse out into the solution between the 

 soil grains. 



The influence of fertilizers is therefore twofold: the 

 direct addition of plant-food for the immediate use of 

 plants, and the action of the fertilizing components 

 upon the solubility of the otherwise difficultly soluble 

 compounds in the soil. There are other offices which 

 are very strikingly shown in the case of lime. This 

 substance when in the form of either caustic or slaked 

 lime corrects the acidity which is very often present in 

 soils. It changes the structure of soils. It renders 

 some of the soil components much more soluble, espe- 

 cially when the lime is in the form of the sulfate or 

 gypsum, and it has undoubtedly a physiological role 

 which enables the plant to assimilate larger quantities 

 of other nutrient matters even in amounts which would 

 be detrimental if the lime salt were not present in ex- 

 cess. 



The principal objects of the cultivation of the soil are 

 to secvire proper aeration, to conserve the moisture sup- 

 ply, and to improve the drainatre. The irrigation and 

 artificial drainage of soils are treated elsewhere. 





•-■-4';^ 



2333. How the gardener makes his soil, by letting it 



decay in piles. 



The larger pile is composed of sods. 



The physical properties of texture and structure, that 

 is, the size and arrangement of the soil grains, have a 

 greater practical importance with field crops and the 

 relation of crops to soil under extensive cultivation than 

 upon horticultural crops either in the field or green- 

 house, where intensive methods are used. Particularly 

 in the eastern states, where the natural rainfall is re- 

 lied upon for the water supply, these physical proper- 

 ties have great influence in determining the relation of 

 crops to soils. This is due in large part to the influence 



