10 NATURE AND PROPERTIES OF SOILS 



tissue is fabricated from the soil. Quite the reverse is true. 

 Fresh plant tissue generally carries only from .5 to 2.5 per 

 cent, of mineral material. In spite of this, it is the mineral 

 elements of nutrition that generally limit crop growth since 

 a plant can alwaj^s obtain, except in cases of drought or 

 disease, unlimited amounts of carbon, hydrogen and oxygen. 



8. Primary nutrient elements. — While all of the seven 

 soil nutrients must be available that plants may grow normally, 

 only four or five are likely to become limiting factors. The 

 others are almost always in great sufficiency. These few, 

 nitrogen, phosphorus, potassium, calcium and occasionally 

 sulfur, receive as a consequence especial attention. They 

 may limit growth because they are actually lacking or be- 

 cause their availability is low. These conditions often occur 

 in the same soil. 



Combined nitrogen exists in the soil to a large degree as 

 a part of the partially decayed organic matter present 

 therein.^ As decay proceeds, small quantities of this nitrogen 

 appear as ammonia in combination with some acid radical 

 such as the chloride or sulfate or with the hydroxal group. 

 Later, it is changed through further bacterial action to the 

 nitrate form, united with some bases such as calcium or po- 

 tassium. It is from this latter combination that most plants 

 obtain the greater part of their nitrogen. These inorganic 

 nitrogen compounds, present at any one time in a soil, are 

 but a small proportion of the total soil nitrogen. The air 

 both above the soil and that circulating within its pores has 

 been the original source of all the combined nitrogen. Nat- 

 ural processes have facilitated the combination which has been 

 necessary for such a transfer. The encouragement of such 



^Certain roekSj particularly tliose of a sedimentary nature, carry 

 considerable nitrogen. V^hen sueh rocks weather, this nitrogen tends 

 to become available. The organic matter, therefore, does not absolutely 

 control the amount of nitrogen in a soil. Hall, A. D., and Miller, 

 N. H. J., The Nitrogen Compounds of the Fundamental Socles; Jour. 

 Agri. Sci., Vol. II, Part 4, pp. 343-345. July, 1908. 



