SOIL INGREDIENTS 31 



Lime is a valuable constituent of plants and is beneficial to the 

 soil in many ways. It aids in the formation of nitrates in the soil 

 and promotes the decomposition of vegetable matter. When pres- 

 ent it overcomes the sticky tendency of the particles of clay and 

 renders the passage of water through them very easy. The ab- 

 sorptive and retentive power of sandy soils is improved by it. 

 In many northern climates, like Alaska, lime is necessary to 

 neutralize the acids in the soil, and when lime is not used culti- 

 vated plants will grow scarcely at all in such soils. 



Humus. By humus we ordinarily mean the decaying organic 

 matter in soils made up of carbon, oxygen, hydrogen, nitrogen, 

 etc. The fertility of virgin soils is largely due to the nitrogenous 

 humus present, which is derived largely from the dead roots, 

 branches, and leaves of a former vegetation. 



Humus absorbs a great deal of water by reason of its porosity, 

 and this water tends to keep the soil cool. It also warms some soils 

 by absorbing the sun's rays. It is valuable as a manure because 

 of its power to supply nitrogen. It is also valuable because of 

 its power to absorb and hold ammonia and ammonia salts. It 

 promotes chemical action in the soil and supplies the carbon 

 dioxide needed for the disintegration and solution of some of the 

 mineral matters in the soil used as plant foods. 



Humus greatly improves the texture of certain soils. It binds 

 sandy soils and lightens heavy clays, when applied in proper 

 quantities. For soils naturally too dry for cultivation or likely 

 to bake and crack open during a summer drought, additions of 

 humus are very beneficial. Wet soils are not benefited by it, for it 

 tends to hold the moisture. 



The greatest amount of humus is found in temperate climates, 

 where the soil is too damp and cold to permit the rapid decay of 

 organic matter during a considerable portion of the year. In 

 tropical regions and arid regions the amount of humus found is 

 relatively small compared with the amount in temperate climates. 



The importance of humus as a plant food is still questioned by 

 some authorities. Professor King says: " It used to be held that 

 any soil deficient in humus was, because of this shortage, neces- 

 sarily poor or sterile; but it is now known that in arid regions, 

 where humus in the soil is very scanty or even wanting, large crops 



