56 SOIL CONDITIONS AND PLANT GRO WTH 



convenient to make a distinction between the elements neces- 

 sary in large quantities, and those of which mere traces suffice : 

 the effect of the former can readily be demonstrated in water and 

 sand cultures ; the latter are more difficult to study, as traces 

 are always present in the seed, and often also in the nutritive 

 medium, or the vessel in which the plant is grown. 



The substances needed in quantity are carbon dioxide, 

 water, oxygen, and suitable compounds of nitrogen, phos- 

 phorus, sulphur, potassium, calcium, magnesium, and iron. Of 

 these nitrogen, phosphorus, and potassium compounds are re- 

 quired in such large amounts that they usually have to be 

 added to soils as artificial fertilisers in order to obtain maxi- 

 mum yields in agricultural practice. The other nutrient ele- 

 ments are generally present in air, in soils or in rain, in sufficient 

 amounts to exert their full effect. 



In addition to the above substances, small amounts of 

 manganese and silicon are known to be beneficial : recent 

 evidence suggests that others may be also. The list of ele- 

 ments of which traces only are needed has been much extended 

 in an important investigation by Maze (197), who includes 

 boron, fluorine, iodine, chlorine, aluminium, and zinc. Evidence 

 is steadily accumulating that these substances, in suitable traces, 

 are beneficial to plant growth, as will be shown below. 



Between nutritive effects and toxicity the margin appears 

 to be narrow, and almost all the elements essential to plant 

 nutrition are capable of producing toxic effects under other 

 conditions. In the case of the major nutritive elements toxi- 

 city occurs only when a sufficiently large excess is present 

 to alter considerably the osmotic relationships or the proper 

 physiological balance of the nutrient medium : the transition 

 from beneficial to harmful effects is very gradual and proceeds 

 through the long inert stage shown in Fig. 7. In the case of 

 elements of which only small amounts are needed the transition 

 is much sharper : the limits are easily overstepped and toxic 

 effects set in when the minute beneficial amount is exceeded. 



Carbon. It is generally assumed that plants derive all 

 their carbon from the air, but the French investigators have 



