Atkins — Hydrogen Ion Concentratmi of the Soil. 205 



conditions of the exijcriraent iron and aluminium i'onn blue salts with the red 

 anthoeyanin. Secondh', that the action of the aluminium is in part responsible 

 for the blue colour in nature, though iron also plays a large, or possibly the 

 main, part. Thirdly, that the action of the acidity of the alum and aluminium 

 sulphate was to liberate iron already present either as an insoluble deposit 

 (which in some plants has been shown by Gile and Carrero (1916, 1) to exist) 

 or from organic combination. 



Variation in colour due to the addition of salts to a number of anthoeyanin 

 pigments has been studied by Shibata, Shibata and Kasiwagi (1919). The 

 extracts examined by them do not include that of the hydrangea, yet they state 

 that "the colour chaiige of hydrangea and other flowers caused by iron salts 

 and alum ... is nothing but the complex formation, as we see with the extracts 

 containing airthocyanins. ' ' 



Culture experiments carried out by Kraemer (1906, 1909) with Hydrangea 

 otaksa, which normally has red flowers, showed that plants grown with alum 

 gave blue flowers, as did also those with aluminium sulphate and calcium 

 hydroxide. In the latter case the excess of the aluminium was j^robably the 

 active portion of the mixture. 



Kraemer further discovered that the addition of potassium carbonate to 

 plants grown in sand resulted in the production of blue flowers. It appears, 

 therefore, that the availability of iron may somehow be increased by a markedly 

 alkaline reaction after passing through a minimum value. It is desirable that 

 such experiments should be repeated, and the pH value of the soil examined 

 at intervals durhig the growth of the plants. The possibility of an increase 

 of the absorption of iron in the region of marked alkalinity is in keeping with 

 the results obtained by Arrhenius (1922) for the intake of salts in general. 

 Working with wheat and radish in well-aerated water cultures, he found that, at 

 maximal growth, the intake of the salts is at a minimum. 



Chlorosis and Availability of Iron Salts. 



The relation between chlorosis and a deficiency of iron was established as 

 long ago as 1843 by the work of Eusebe Gris. The disease became of con- 

 siderable economic importance in France through its manifestation in American 

 vines, and numerous x-esearehes showed that treatment with ferrous sulphate 

 sprayed on the leaves was beneficial. The subject has been reviewed at length 

 by Roux (1900). Roux's own experimental cultures are of great interest, and 

 a series of photographs illustrates the growth of nine species in soils containing 

 from to 25 per cent, of calcium carbonate. 



More recently Tansley (1917) has demonstrated how in calcareous soils 

 Galium saxatile becomes chlorotic, and either dies or cannot withstand the 

 competition of G. sylvestre, when the two are grown in absence of other plants. 



Chlorosis has received attention also from Gile and Carrero (1916), who 

 conclude that iron is not easily translocated from old leaves where it has 

 accumulated. Thej=' (1916, 1920), moreover, made an elaborate study of the 

 chlorosis of the rice plant, and showed that the ash of chlorotic rice plants was 

 low in iron; the condition was met with in calcareous soils with a normal 

 amount of water, though not when the same soils were submerged. They 

 suggested that special roots better fitted to absorb iron were developed. An 

 explanation in keeping with present knowledge of the pH values at which 

 ferrous and ferric salts are precipitated is as follows. The water standing 

 over the rice tends to lessen soil aeration, to intensify the reducing action of 

 ihc soil, and possibly slightly to increase its hydrogen ion concentration in 



