SOILS 17 



of the glass with the water. The sand is not soluble and 

 will remain in the glass. 



Only a very small amount of soluble plant food is needed 

 to grow a crop; but while the amount is small, it is absolutely 

 necessary to have enough of it to supply the plants. 



How Plant Food Is Made Soluble. — There are many 

 different ways of making the insoluble plant food in the 

 soil soluble. These are Nature's ways, and the change 

 takes place naturally in soils under favorable conditions. 

 But farmers can do a great many things to assist Nature 

 in this work. 



One very important condition of soil, which favors mak- 

 ing plant food soluble, is to keep the soil well supplied with 

 vegetable matter as it was when the farmer first broke 

 up the virgin sod. Get a small piece of sod from a new 

 piece of breaking, and a handful of soil from an old field 

 that has grown nothing but corn or grain for a great many 

 years. Notice that the first is tough and is held together 

 by many fine roots interwoven among the soil grains. The 

 handful of earth from the old field contains little except 

 the particles of soil. 



The plant roots, as well as other parts of plants found 

 in soil, are called vegetable matter. When this vegetable 

 matter is partly decomposed, it is called humus. 



Decay of Vegetable Matter. — When the weather is 

 warm and the soil moist, the vegetable matter in the soil 

 begins to decay. The vegetable matter is composed of 

 plants, and is made up of the things that growing plants 

 need for food. When the vegetable matter decays, the 

 substances of which it is composed are set free or liberated, 

 thus making plant food soluble. The vegetable matter 

 decaying in the soil not only liberates the plant food of 

 which it is composed, but aids very much in making some 

 of the insoluble plant foods in the mineral particles of soil 

 soluble. It also aids by making the soil warmer, as heat 

 is given off by a decomposing manure pile, and by giving 

 off an acid, called an organic acid, because it is formed from 

 organic matter. This acid acts on the soil grains and dis- 

 solves a small amount of mineral matter off their surfaces. 



