The Pulaskian., 1915-12-03
II.-How PLANTS FEED.


 Plants require for their growth, me chanical support, air, heat, light, wa ter and food. All of these, with the exception of light, are supplied wholly or in part by the soil. Composed as it is of rock fragments and decaying vegetable matter, the soil offers a place for root development and af- lords physical support for the plant. The air present in the spaces be- tween the soil particles is used by the roots of plants. Heat is supplied in part by the soil as result of it's physical condition. Water, while not component part of soil, is furnished to the plant by it, and the physical condition of the soil determines the amount of water that may be reserved for use by plants. The water sur rounding the soil particles dissolves plant food and the solution is absorb ed by the plant roots. Thus the soil must not only furnish a physical SUD- port for plants, but it must supply plant food and afford a means for transferring the food to the plant.
 Ten elements of plant food are con. sidered to be absolutely essential for the growth of plants. Three of these, carbon, oxygen and hydrogen, are ob- tained from the air, while phospho- rus, potassium, calcium Qime). mag nesium, iron and sulphur are obtained directly from the soil itself.
 Carbon and oxygen are obtained from the air, where carbon is pres ent in the form of carbon-dioxide gas and where oxygen exists in free con- dition. These gases are taken into the plant through the leaves, and there combine with water to form the complex vegetable compounds.
 The nitrogen used by plants is secured through the soil. Through decay of the vegetable matter in the soil the nitrogen-bearing compounds in it are broken down into more sim pie forms, which, through the action of various soil bacteria, are soon changed into form that can be used by plants. With these processes go- ing on all the time (except when the soil is frozen or waterlogged), a con- stant supply of available nitrogen is provided. The content of vegetable matter in the soil must be kept up at all times to insure a sufficient quantity of nitrogen for good crops.
 Nitrogen from the air can only be secured by growing leguminous crops on the roots of which nodules are formed. The bacteria living in the nodules gather nitrogen from the air in the soil, and store it in organic combination in the nodules. Some of this is used by the host plant, but most of it is left in soil to become available through decay of nodules.
 The mineral clements essential for plant growth are supplied by the soil. They are part, to a greater or less degree, of the rock fragments which constitute the body of the soil.
 Lime in soils becomes soluble so readily, and our rainfall is so heavy, that it has been washed from even the best limestone soils in Arkansas. Most of our soils, therefore, are lacking in lime sufficient for the sat isfactory growth of many lime loving crops This, with other changes in the soil, has led to soil acidity.
 Phosphorus and potash are present in all soils, usually in quite insoluble form. They form a part of certain minerals found in the rock fragments of the soil These minerals are brok- en down and the phosphorus and pot ash made soluble through exposure to weather, the atmosphere, the decay of vegetable matter, and slight solv- ent effect of plant roots. Potash in many of our soils is present in suf- ficient quantities to last for hundreds of years, but because of improper til lage and an insufficient amount of de caying vegetable matter it does not in all cases become soluble fast enough to support good crops.
 Phosphorus is the mineral element most seriously lacking in Arkansas soils. For satisfactory crops of any kind it is necessary to. replentish the phosphorus supply by the application of phosphorus bearing fertilizer. raw rock phosphate, acid phosphate, or bone meal.
 When the supply of any one of the ten clements of plant food is limited, the crop is limited. Since all of these are furnished either by the air or 5y the soil we know where to look or the cause of an unsatisfactory rop. As pointed out in the above paragraphs, nitrogen, phosphorus and potash are the essential elements to