68 Principles of Plant Culture. 



The carbonic acid these remains took from the air during 

 growth is also set free to slowly disintegrate the mineral 

 soil constituents, rendering these soluble and thus avail- 

 able as plant food. In winter, the frost separates the 

 compacted particles of clods, making the latter perme- 

 able to air and rootlets, or flakes off new fragments of 

 rock, thus unlocking new supplies of mineral fertility. 



93. The Importance of Organic Matter in the Soil. Crops 

 secure a large part of their nitrogen, as well as of other 

 food substances, from dead organic matter, i. e., animal 

 or vegetable materials. The application of such matter 

 to the soil is, therefore, of great importance, where large 

 crops are expected. Stable and barn -yard manure, the 

 offal from slaughter-houses, tanneries, breweries etc., are 

 all valuable for this purpose, when wisely used, ^ot 

 only does organic matter in the soil furnish plant food, 

 but while in a partially decomposed state (humus), it 

 renders the soil porous and greatly increases its water- 

 holding power. 



<^_ 94. The Soil Needs Ventilation. The roots of growing 

 plants and the decomposition of organic matter in the 

 soil tend constantly to exhaust the latter of its free oxygen, 

 and to replace this with carbonic acid, which is not used 

 by the roots. Hence, without some interchange between 

 the contents *of the soil cavities and the atmosphere 

 above, the roots sooner or later become smothered and 

 perish. In sufficiently porous soil, changes in tempera- 

 ture and in atmospheric pressure, aided by wind and rain, 

 furnish the needed soil ventilation, but in poorly -drained 

 soils, and soils not thoroughly tilled, the roots of plants 

 often suffer from insufficient oxygen. A puddled crust 



