26 MANUAL OF GENEEAL AGRICULTURE. 



to determine when enough has been used to make it neu- 

 tral. Use distilled water. What might be applied to an 

 acid soil ? As in the case of alkali, draining is an effectual 

 remedy. 



SOIL ANALYSIS. 



There are in the earth's crust about eighty simple 

 substances called elements. Of these only ten are neces- 

 sary for plant growth. 



They are nitrogen, phosphorus, potassium, calcium, 

 iron, surphur, magnesium, carbon, oxygen and hydrogen. 

 In addition to these, sodium, silicon, chlorin and alumi- 

 num are -found in many plants, but are not essential to 

 plant growth. None of the above elements are found in 

 the plant or in the soil in the elemental form, but are 

 always in combination with other elements to form com- 

 pounds. 



Carbon is derived from the carbon dioxid of the air ; 

 hydrogen and oxygen from the water taken up by 

 plants, and the others from the soil. Of the soil elements 

 potassium, phosphorus and nitrogen, and sometimes cal- 

 cium, are used by plants to a much greater extent than 

 the others. In fact, if the soil is well supplied with these 

 four, so far as plant food is concerned, it may be consid- 

 ered a rich soil. For this reason in a short analysis of soils 

 the amounts of other elements are never considered. 



The manner in which these four most important ele- 

 ments exist in the soil is: nitrogen as humus (vegetable 

 mould), phosphorus in phosphoric acid, potassium in pot- 

 ash as in leached wood ashes which by the removal of im- 

 purities furnish potassium carbonate, and calcium as lime. 



The soil-humus is the chief depository of soil nitrogen 

 and the main source from which plants receive their sup- 

 ply. True, the air about us is composed of four-fifths 

 nitrogen, but abundant evidence shows that plants cannot 

 draw from this bountiful supply. For the most part, 

 humus is derived from decayed vegetable matter, and as 

 there is not a rank vegetation in the arid regions, it fol- 

 lows that the humus content is one of prime importance 



