[ 93 ] 



i%, while Gramineae, Umbelliferse, Compositae, and Chenopo- 

 diaceae showed only about \/ Q . These results, however, are 

 very important in showing how some orders of plants, such 

 as Rosaceae, Cruciferae and Leguminosae thrive on poor soils 

 while others such as Solaneae, Liliaceae, Gramineae, Umbelli- 

 ferae, Compositae and Chenopodiaceae, need liberal manuring. 

 Some plants of the same natural orders differed widely from 

 others in this property of acidity of root-secretions and the 

 figures should be judged according to this reservation. 



116. (6) Fertility is governed by the minimum of a neces- 

 sary ingredient. A soil may be rich in all essential ash cons- 

 tituents of plants but deficient or wanting only in one, and 

 this deficiency or want may result in its barrenness. Soils 

 derived from several rocks (e.g. alluvial soils) are better than 

 soils from one rock, as there is no likelihood for such soils 

 being deficient in any necessary constituent. 



117. Barrenness. (i) Soils containing an excess of Fer- 

 rous salts, e.g., those formed by the oxidation of Iron pyrites 

 (FeS 2 ). Land newly reclaimed from the sea contains Ferrous 

 salts and are therefore temporarily barren. Tank earth freshly 

 put on soils also makes them temporarily barren. FeSO 4 is 

 soluble in water ; but Ferrous salts combined with organic 

 matter are soluble only in HC1. Soils which are too acid may 

 therefore become barren when there are organic Ferrous salts 

 present. Draining, liming and cultivation and exposure to 

 the action of sun and air (which convert ferrous salts into 

 ferric salts) are the means of reclaiming lands containing 

 these poisons. 



(2) Ammonium-Cyanate (NH* CNO) is poisonous to 

 plants. 



(3) Ammonium-Sulpho-Cyanate ( CS Ajo 2 ) is also poi- 



sonous to plants. 



(4) More than 2% of soluble salts in a soil makes it 

 barren , but a very much less proportion of common salt 

 would make a soil barren. Lands reclaimed from the Sun- 



