54 Journal of Agricultural Research vol. xx, no. i 



or completely overcame the chlorosis, although small amounts were 

 without appreciable effect. 



In view of the nonavailability of the concentrated organic iron com- 

 pounds, it seems probable that the beneficial effect of the bulky organic 

 materials was not due primarily to the addition of certain iron compounds 

 that were available in the calcareous soil as a whole. It is more probable 

 that the particles of organic material formed isolated centers or points 

 where iron was more available than in the rest of the soil. The plants 

 were not able to secure all the iron they needed from these points for the 

 reason that plants are apparently not able to absorb a maximum amount 

 of iron with only a portion of their roots (18). 



It may seem that the results of the last two tests negative the con- 

 clusions arrived at in the experiments with rice grown in solutions con- 

 taining carbonate of lime where organic iron compounds supplied sufficient 

 available iron. Conditions in the nutrient solutions, however, were some- 

 what different from those in the soil. To begin with, in the nutrient 

 solutions the plants obtained their iron from an ordinary solution that 

 was more or less sterile and that was frequently renewed. In the soil, 

 on the other hand, the plants probably obtained their nutrients from 

 aqueous films surrounding the soil particles, and there is evidence that 

 in films reactions may occur which do not take place in ordinary solu- 

 tions. Furthermore, bacterial action in the soil might have destroyed 

 rapidly certain of the organic compounds supplied. 1 



EFFECT OF WATER CONTENT OF SOIL ON THE AVAILABILITY OF IRON 



At present we know little of the true soil solution or film moisture. It 

 is evident, however, that the nature of the soil particles must influence 

 the composition of the solution or substances dissolved in the enveloping 

 film. In the films surrounding particles of calcium carbonate the amount 

 of iron in solution must be greatly reduced, since the iron would be pre- 

 cipitated as ferric oxid. 



If it is assumed that each particle in the soil is isolated and that the 

 moisture films surrounding the individual particles are discontinuous, it 

 would follow that the larger the proportion of particles which were carbon- 

 ate of lime the less soluble iron there would be in the whole medium. 



This assumption would explain why carbonate of lime is more effective 

 in inducing chlorosis the more finely divided it is and why a certain 

 quantity of carbonate of lime exerts a stronger influence in a sandy soil 

 containing relatively few particles than in a clay soil containing a large 

 number of particles. 



However, the case is not so simple as is assumed above. The moisture 

 films are not discontinuous but more or less continuous, the continuity 



1 The fact that ferric citrate and ferric tartrate were no more effective when applied in frequent small 

 doses than when applied all at once is some evidence against the idea that the organic iron compounds were 

 unavailable because they were destroyed by bacterial action. 



