SOILS AND FERTILIZERS 353 



differences as do occur are not sufficient to explain the observed 

 differences of crop adaption and yield. 



Ten elements are essential to the growth and development of 

 all plants. They are carbon, hydrogen, oxygen, magnesium, iron, 

 sulphur, calcium, nitrogen, phosphorus and potassium. Combina- 

 tions of the three elements, carbon, hydrogen and oxygen, consti- 

 tute approximately 95 per cent of all plants. They form the por- 

 tions spoken of in feeds as fats and carbohydrates, including the 

 oils and starch. Plants obtain their supply of these elements from 

 air and water. The carbon is derived from the carbon dioxide of 

 the air, and the' hydrogen and oxygen from water, which is a com- 

 bination of these two elements. Thus, it is seen that only about 5 

 per cent., or one-twentieth "of the material of plants, actually comes 

 from the soil. Only minute amounts of magnesium, iron and sul- 

 phur are required, and they are present in practically all soils in 

 abundant quantities. The same is usually true as regards calcium 

 although certain crops, particularly clover, require this element 

 in considerable amounts. It is evident that seven of the ten ele- 

 ments essential to plant growth need cause the farmer but little 

 concern. 



Nitrogen, phosphorus and potassium are three elements which 

 in their various combinations constitute the vast majority of the 

 material obtained from the soil by plants. These elements are re- 

 quired by all plants in considerable amounts and their presence 

 in many soils is limited. It is a common but erroneous idea that 

 the soil is a great mass of material, the greater portion of which 

 may sooner or later be used by plants as food. Numerous chemical 

 analyses of soils, representing many different sections, show that 

 a ton of fertile soil to a depth of one foot does not contain on an 

 average more than 10 or 12 pounds of nitrogen, phosphorus and 

 potassium. 



Nitrogen, phosphorus and potassium are always found in 

 small amounts even in the most exhausted soils, but if any one of 

 these three elements is present in too small an amount to meet 

 the full demands of a crop, the growth and yield of the crop will 

 be correspondingly decreased. These elements do not exist in the 

 soil as single elements, but are found combined with other elements, 

 and it has been demonstrated by experiment that plants are capa- 

 ble of appropriating their food only when it exists in certain com- 

 binations. In other words, sufficient amounts of plant-food may 

 be present in the soil, but it may be in such form that plants can- 

 not use it. Herein lies the solution of the soil fertility problem, 

 so far as plant food is concerned, viz, to have present at all stages 

 of the plant's growth an ample supply of each element in a readily 

 available form. 



It is probable that most of the material in the soil, in order to 

 be available to crops, must be soluble in water, at least in the soil 

 moisture which contains carbon dioxide and other compounds coming 

 from the decomposition of vegetable matter. This is indicated by 

 the fact that crops can be grown to maturity in water solutions of 



