104. 



2/i, straw gives 2-4> . &&& Much loss of humus t'ikes place under tilling. 

 The continual raising of one cereal depletes the soil of N. vVhen, in 

 one cage, wheat was raised continually for 8 yc , there was a total 

 loss of 1700# of N per acre for soluble nitrates (Schneider). Only 

 of it was utilized for the product. The huinus loss was 1 ton per acre 

 per ye^r. ''ith rotation an equlibriura obtains with N. 



Humus decreases downwards. Drier soils are apt to contain more 



N. 



All humus with moisture and oxygen is unstable s.nd is worked over 

 by bacteria. Some Pfclypora work the material over to humus and then 

 quit, leaving it to other fungi to finish. First come filamentous fungi; 

 then bacteria; then humus destroyer^ "bacteria which completely destroy 

 the hirrius an such ->nd transform it back to its chemical constituents, 

 and finally th-.re is no humus loft. Or if the humus is not destroyed it 

 becomes sour; this sour huir-us and peat ara injurious to plants: 



1) They prevent plants from getting water. They -produce a 

 physically dry and arid soil. 



2) They are deficient in nutritive salts. 



3) They hold nutritive salts with a treiaenuous tenacity, so 

 that plants are unable to get them. 



Effects of Humus on Soil: 

 1) Beneficial: 



"oisture. As a colloid it takes and holds moisture readily 



Turn! she a material for soil activity, for bacteria. 

 Colloid helps to loosedn soil, and thus increases aeration 

 Colloid helps to hold plant foods. 

 2) , Injurious: 



a) Muck heats up, dries to a powder; sour acids injure plants 

 Humic acid develops to " cement and forms a hardpan. 



I) Physics of the Soil. 



a,. Composition and structure. Discussion was &*&&&&&. omit ted. 



]b. Nature of sand and silt. Sand and silt ar made up of small 

 individual particles, vdiich do not form a lump structure, and therefore 

 will "pack" readily. There are two characteristics of packing: 



1) Individual grain gives a small surface and therefore less 

 wnter holding power, and surface tension; sand has leas surface than clay 



BO clay holds water better than sand. 



2) Bad aeration. 



With sand and silts there are certain characteristics: 



More leaching 

 Less plant food 

 Less humus 



4) Less water 



5) Less organic life* 



. Clay and its part in physical composition; colloidal charac- 

 ter of clay. 



Clays act as a cement and past, enwrapping particles of sand 

 and nodding them together. These particles cling and form lumps, giving 

 good tilth snd a mellow soil . 



Clay is characterized by small particles, good capacity for 

 holding water, good mineral salts; it contains more plant foods, more 

 humus, more organic substances and plant and animal life; it has better 



