844 Handbook oj Nature-Study 



surface and powdering it into stone flour. And in these situations other 

 plants grow later, sending their roots down into every crack and orevice 

 and thus prying off more of the rock. 



The Soil Carriers 



In the study of the brook we have seen how the water lifts, carries and 

 deposits the soils; and since, at one time or another, the entire surface of 

 the earth has been under water, we can see that water has been the most 

 important of the soil carriers and has done the greatest work. The wind 

 carries much soil, especially in the arid regions ; the movements of the sand 

 dunes in the deserts and on the seashores bear witness to what the wind can 

 do as a soil carrier. But in the northern United States, from New England 

 to the Dakotas, much of our soil has been carried by a great ice river that 

 ones upon a time flowed down upon our lands from the North. This great, 

 slow-moving river, perhaps a mile or more high, plowed up the soil and 

 stones, and freezing them fast carried and shoved them along under its 

 great weight. After a time the ice melted and dropped its burden. Many 

 of the stones were of granite taken up from the old mountains of northern 

 Canada and ground off and rounded during their journey. We call these 

 stones which were brought down to us from the North, "boulders ;" and the 

 soils wliich were brought along on the bottoms of glaciers and dropped and 

 pressed down by the tremendous ice weight and thus made compact 

 although unsorted, we call "hardpan." 



The Kinds of Soil 



By the work of these soil makers and soil carriers, the rock flour was 

 made. But if we should take some of it and plant our seeds in it, we shotdd 

 find that they would not grow thriftily, even though we watered them and 

 gave them every care. The reason for this is that most rock flour does not 

 have in it the substances which the plants most need for their growth. But 

 if we should go to the woods and get some of the black woods-earth and mix 

 it with rock flour, we should find that our plants would thrive. This rich, 

 earth mold in the forest is almost wholly made up of matter once alive, but 

 which is now decayed, and which we call "humus." The more humus that 

 we have in the rock flour, the richer it is in plant food, and the more plant 

 growth it will support. 



In general, soils may be divided into clay, sand, gravel, loam and 

 humus. 



Clay in its purest state is kaolinite, the result of weathering of feldspar, 

 or mica. It is finely powdered and is used for pottery, while the less pure 

 clays are used for brickmaking. Clayey soil is sticky and slippery when 

 wet, and bakes hard and cracks when dry. It is hard to cultivate, but it 

 absorbs moisture from the air and holds fast to its fertility, and is especially 

 good for permanent pastures and meadows. 



Sand, in a pure state, is made up mostly of finely broken particles of 

 quartz and feldspar, and is used for the making of glass. A sandy soil is 

 light and open and easy to work. It absorbs little water from the air and 

 has little power for holding plant food, since the water washes it out. It is 

 especially valuable for truck gardening, because it is a warm soil. It is 

 warm because water does not evaporate from its surface rapidly. 



