SOBRALIA 



9. L6wii, Rolfe. An imperfectly known species in- 

 troduced abont 1892 from Colombia. It grows about 

 1% ft. higli and has fls. of a bright uniform purple. 



The following trade names are not accounted for : S. mag- 

 nifica.—S. Pfaidi.—S. virgindlis. 



Heinkich Hasselbring. 



SOIL. The soil is a superflcial 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, espcciallr in humid climates, 

 from the soil proper in color, tcxturr, i.r chemical com- 

 position. A very striking drtiniiiini Ikis Im-.-u suggested 

 by Sir John B. Lawes, who .-uiisidirLMl lliu soil to be 

 rotten subsoil, and the subsoil rottiug 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 tnignients of rocks or minerals, organic 

 iii:iiirr, -ill -'luiiMii. anil a soil atmosphere. The 

 in: I I I :- \:cry in size from the finest clay 



]iii . . .!,..! ami even boulders. The organic 



niaiiii- 1- 1. !i -.1 Irnni low organisms, from previous 

 vegftatn.ii. .11- rr..iii -rowing plants; as also from stable 

 manure, aici .Mca-.iMtially tish or animal matter added 

 to the soil liv man. 'I'hi- soil solution consists of water 

 carrying .ii^sniv..! suli-^taiices derived from the soil 

 grains and fi-uin the i>r-aiiic jiiatter, as well as from 

 fertilizing material ii:i i.ii!l\ ap|ilied, and constitutes 

 a nutrient solution !■ :i i ihc plant derives its 



mineral constitui-iii 1:. sphere differs from 



the ordinary atmos,, in r. ,ii„. , ih,- soil in being richer 

 in carbon dioxid and nitnitceu, 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 pla. < ; ami tran-iorted soils, including 

 those of alluvial, ;;l i il i ! a 'i m origin. The word 

 alluvial is here n- , ' all water-transported 



material; the ttrin .1 :. lia-quently used in a 



more specific sense la luda aie ilie recent flood deposit 

 of rivers. 



Soils are classiiied according to their origin and their 

 mechanical and chemical composition and properties. 

 Genetically, they ai-e 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 

 I I. I.iaiii. rlay loam, clay, adobe, blaek-waxy, or, 



11— hiuilils. alkali, alal aca.ivling to other 



s _ ; ,il Iraiures. Ill the s"il -lu'vey of the 



b. .'^, iKvarliueiit of At,'ricn ll II re a lecal naiile is adopted 



for each type under wbicli il;e s| iiic .Ijuracters are 



given; examples of this are llaitleivl -amly loam, Nor- 

 folk sand, San Joaquin adohe. i il< mlalr Iness. 



The physical properties ot -^il., n m the size and 



arrangement of the particle- . ,M,i| ;1- t. i-.it.a iIm-. 



to each other and to the > r 



atmosphere, the soil moist 111 , i , : 



of heat and gravitation. Jii iles, inere i- an inn n- 



relation with physiography or the torni and exposure .it 

 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 this 

 may be increased by methods of cultivation and fertili- 



SOIL 



1675 



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- 

 cumulate to any considerable extent in soils in humid 

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

 to recombine, forming difllcultly 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 suflicient 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 mntter forms 

 a culture medium for Kactevia, f.-rnients ■^^^.\ the vari- 

 ous organized and uti'" ■ i m - >v im. n assist in 



breaking down the oil' .n: a; : , :, . liiiate as 



well the weathering ol 1 'I' ..i n i ■ - i ,; eis. Soils 



in general have remarkaMi [i,,n, i ..i ,ii. ..i inni,' on the 

 surface of the soil grains vast ipiantities 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 ilieref.ne twofold: the 

 direct addition of plant-food for the iniini iliate use of 

 plants, and the action of the ieriili/iiii; components 

 upon the solubility of the otherwise UJlUculily 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 stnictiii-e of soil.s. It renders 

 some of the soil comjioia nt- niiich uioi-e soluble, espe- 

 cially when the lime is m tin i,,iiii ,,f tbe sulfate or 

 gypsum, and it has ninhaiiiteiliy a jihysiological 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 secure propter aio-at 

 ply, and to iininovi- 

 artificial drainage el" 



inserve tne moisture sup- 

 age. The irrigation and 

 treated elsewhere. 



The physical properties of texture and structure, that 

 is, the size and arrangement of tbe 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 -u]-|'l» . lie -e physical proper- 

 ties have great influeiici ill .h 1, iniiiiin;,' the relation of 

 crops to soils. This is due in lai-e part to the influence 



