THE IRRIGATION AGE. 



51 



in soil formation. The humus increases the power of the 

 soil to retain water and to supply it to the plant, and 

 as all the chemical changes by which plant food is made 

 available take place more readily in the presence of suf- 

 ficient moisture, it will be seen that this moisture hold- 

 ing power of humus is a very important factor in soil 

 fertility. During the decay oif the organic matter, car- 

 bonic acid and other acid substances are produced and 

 these help to dissolve the mineral ingredients of the soil 

 and change them into forms in which they can be absorbed 

 by plants. 



Gradually the lower forms of plants are replaced by 



Vegetation Begins With Very Simple Forms of Plants Like Lichens and Mosses. 



the higher root-bearing plants and the plant roots in 

 their turn play a part in soil formation. The roots act 

 both mechanically and chemically. Every one has seen 

 examples of the enormous' force exerted by plant roots in 

 breaking apart rocks, when the plant gets started i-n a 

 crevice or fissure. This same mechanical action is doubt- 

 less exerted by roots on the rocks underlying some soils. 

 The roots also seem to secrete an acid substance which 

 acts on the rocks with which they come in contact, and 

 when the plants die and the roots decay they leave numer- 

 ous little channels in the soil allowing the 

 passage of air and of water laden with car- 

 bonic acid. 



Sooner or later in Nature's process of soil 

 building, plants belonging to the clover family, 

 the so-called leguminous plants, are intro- 

 duced. These plants, through the bacteria 

 which grow in the nodules or tubercles on 

 their roots, are able to use the free nitrogen of 

 the airas a source of food supply. When these 

 plants die and become incorporated with the 

 soil the nitrogen which they have "fixed" be- 

 comes a part of the soil and is made available 

 to succeeding plants. It has been estimated 

 that from fifty to one hundred and fifty pounds 

 of nitrogen per year may be fixed in this way. 

 and this power of the leguminous plants to ac- 

 cumulate nitrogen is probably Nature's most 

 important method of increasing the nitrogen 

 content of the soil. 



In addition to the processes described above 

 should be mentioned the action of the earth 

 worms and other forms of animal life found in 

 the soil. These organisms are supposed by 

 some authorities to play a very important part 

 in the working over of the soil and its prepa- 

 ration for plant growth. 



It will readily be understood that the vari- 

 ous agencies concerned in the formation of 

 the soil do not act separately nor necessarily 



in any such order as that in which they have been discussed. 

 As a matter of fact all the process described take place simul- 

 taneously. The lower plants do not wait for the rocks to be 

 pulverized, for we see such organisms as the lichens growing 

 on rocks from which one would think it impossible for them to 

 obtain food. If the lichen is removed grooves or furrows will 

 be found on the surface of the stone ,due to the action of the 

 plant. 



Nor are all soils formed directly from the original rocks, 

 for one of the effects of weather, etc., is to separate such rocks 

 as the granites into simpler substances, with the result, for ex- 

 ample, that huge deposits of limestone are found in one place 

 and in another whole hills of sandstone. 



The soil is almost constantly moving, for 

 some of the same agencies which form soils 

 are continually carrying them away. Running 

 water grinds the rocks, but at the same time 

 transports the fine particles to lower levels. It 

 cuts deep valleys in the surface of the earth 

 and carries away the debris, depositing it at 

 various distances from its source. Notice a 

 stream muddied by a recent rain; the mud will 

 be deposited somewhere to help form a soil. 

 The soil is always moving from a higher to a 

 lower level, consequently, it is thinnest at the 

 top of a hill and deeper in the valley. Lakes 

 and ponds are gradually filling up and in time 

 will become fertile fields. 



From what has already been said it will be 

 seen that the completed soil consists of rock 

 particles, mixed with decaying vegetable mat- 

 ter, the remains of animals, and the various 

 substances formed by chemical action from the 

 rocks and organic matter. The rock particles 

 are classified according to their size into 

 gravel, sand, silt and clay. Gravel is sub- 

 divided into coarse and fine gravel, and at least 

 four grades of sand are recognized, i. e., coarse, 

 medium, fine and very fine sand. Silt is also 

 divided by some authorities into two grades 

 known as silt and fine silt. 



The individual rock particles of a clay are the smallest 

 recognized by the soil physicist. They are so small that 

 they cannot be distinguished by the naked eye, nor, in- 

 deed, can they be felt between the fingers. In other 

 words, the clay is entirely without grit and when rubbed 

 between the fingers one can feel only the smooth mass 

 of clay and cannot feel the individual particles. Clays are 

 very adhesive when moist and adhere to tillage imple- 

 ments, and while they absorb large amounts of water, yet 



Nature's Method of Increasing the Humus and Soil Fertility. Notice the Rotten Log 

 and the Leaves Decaying, Thus Returning Plant Food to the Soil. 



