I. INTEODUCTION 9 



Glycine or glycocoll is amino-acetic acid and has the constitu- 

 tion 



H 



These compounds will be discussed at greater length hereafter. 



Sulphur. The occurrence and properties of this element are well 

 known and need not be discussed here. In the nutrition of plants 

 and animals it plays a small part, but still it is essential. 



It occurs in small quantity in albuminoids and, in certain plants, 

 in the form of sulphides and sulphocyanides of organic bases. In 

 animals, it is particularly abundant in the hair or wool. 



Plants probably obtain the sulphur they require from the sulphates 

 present in the soil, and in most cases from calcium sulphate. It is to 

 be noted that soluble metallic sulphides are violent plant poisons, as 

 are many other unoxidised sulphur compounds (e.g., sulphocyanides, 

 sometimes present in commercial sulphate of ammonia). Yet it is 

 found that certain plants actually secrete sulphides and sulphocyanides 

 of organic bases and owe their characteristic odour or flavour to the 

 presence of these compounds. Such is the case with mustard, garlic 

 and many other plants. 



Phosphorus. The properties of this element are very remarkable 

 and are well known to all students of chemistry. In agriculture its 

 compounds, the various salts of phosphoric acid, are of the utmost 

 importance. 



Phosphorus is extremely widely distributed, though generally in 

 small quantities. It is present in almost every mineral and rock, 

 though the average amount present in the soils and rocks of the earth's 

 crust probably does not exceed one or two parts in 10,000. From the 

 soil, plants obtain their phosphates ; these in turn pass into the bodies 

 of animals, where they often accumulate in large quantities, so that 

 the bones or shells of animals always contain relatively large amounts 

 of phosphate of lime. 



Some minerals, too, consist mainly of phosphates, e.g., apatite 

 consists of 3Ca 3 P 2 O 8 .CaCl 2 or 3Ca 3 P 2 8 .CaF 2 ; vivianite is essentially 

 Fe 3 P 2 O s .8H 2 O. More abundant are deposits of impure phosphate of 

 lime in the various forms of phosphorite, osteolite, coprolites, etc. 

 These are often used as fertilisers. 



Potassium occurs in many silicates : some contain a relatively 

 small quantity, while in others, e.g., orthoclase or potash felspar, 

 Al.,0 8 .K 2 0.6SiO , the proportion of this element is considerable. It 

 also occurs largely in sea-water, from which seaweeds often accumu- 

 late large quantities of potassium compounds. Another very import- 

 ant source of potassium is the huge saline deposit at Stassfurt ; this is 

 supposed to be the result of the evaporation of a large inland sea, and 



