80 Mineral Nutrition of Plants 



Primary minerals 



Phosphorus, sulfur, boron, and molybdenum are components ot 

 many primary minerals. These elements may constitute a definite part 

 of the crystal lattice, as in the case of apatite, pyrites, or tourmaline, or 

 they may be occlusions or isomorphic substitutions. From the stand- 

 point of an immediate source of anions for plant growth, these minerals 

 are of little interest. Minerals such as apatite and tourmaline (4) when 

 ground to pass a 100-mesh sieve do not supply phosphorus or boron at 

 a rate sufficient for the normal growth of most plants when supplied 

 in cultures in amounts comparable with which these elements usually 

 are found in soils. 



Organic compounds 



Probably one of the most significant differences between the availa- 

 bility of soil anions and soil cations is that nitrates, sulfates, and phos- 

 phates are produced in soils as a result of biological activity. Also, the 

 state of oxidation of nitrogen and sulfur is altered by biological activity. 



It is generally assumed that most of the organic nitrogen in soils is 

 protein in nature. The protein character of soil nitrogen has been 

 variously studied. The carbon-nitrogen ratio is frequently used as an 

 index of the nitrogen status of soils. This ratio (varies between 8 and 20) 

 is a function of specific environmental conditions. The rate of nitrate 

 formation in soils usually correlates more closely with the total nitrogen 

 supply (/) than with measurements of the quality of the nitrogen 

 compounds present in the humus. 



A relatively large proportion of the total soil phosphorus frequently 

 is associated with the soil organic matter. For example, Dean (to) 

 measured the organic phosphorus content of 34 surface soils from 

 widely separated parts of the world. The organic phosphorus content 

 of these soils varied from 8 to 50 per cent of the total phosphorus and 

 the carbon-organic phosphorus ratio from 44 to 160. The identity of 

 the organic phosphorus compounds in soils is somewhat obscure. How- 

 ever, it is commonly believed that nucleic acids phytin and their deriva- 

 tives (7, 49) account for a considerable part of the soil organic phos- 

 phorus. Incubation studies (46) have indicated that important amounts 

 of the organic phosphorus are mineralized. 



