50 Journal of Agricultural Research voi.xx, No. i 



availability of iron in the soil is essential to a complete understanding 

 of this chlorosis. If all the conditions affecting the amount of available 

 iron in the soil were known, it would doubtless be possible to explain why 

 some calcareous soils induce chlorosis when others do not; why in a sandy 

 soil a smaller percentage of carbonate of lime is required to induce chloro- 

 sis than in a clay soil; why a calcareous soil that produces chlorotic plants 

 at one time may not at another; and many other perplexing facts. 



Since a method for determining the amount of available potash or 

 phosphoric acid in the soil is still unknown, in spite of years of work, the 

 prospect is not bright for even roughly determining the available iron by 

 direct means ; and to determine directly significant differences in amounts 

 of available iron seems hopeless when plants obtain their iron from such 

 exceedingly dilute solutions. 



Soils which yield sufficient iron for the growth of plants may not show 

 a detectable amount of iron in the water extract. In some cases the 

 water extract of soils may show considerable iron, but the iron may be in a 

 colloidal state and not in true solution. Colloidal iron was found unavail- 

 able for rice in water culture (14). 



While there are great difficulties in the way of determining the small, 

 significant quantities of soluble or available iron in the soil, it seems 

 from the work of Morse and Curry (34), Ruprecht (40), and Abbott (r) 

 that acid soils may contain much more soluble iron and aluminum than 

 neutral or calcareous soils and may even contain an injurious amount 

 of these compounds. 



The following work on the availability of iron compounds is based on 

 the assumption that the chlorosis and the poor growth of rice in the 

 calcareous soils were caused by a lack of available iron. This assump- 

 tion seems justified by the results presented in the first part of this 

 report. 



AVAILABILITY OF ORGANIC IRON COMPOUNDS 



In work with pineapples it developed that in the presence of a great 

 amount of organic matter a large amount of carbonate of lime was 

 required to induce chlorosis (12). This suggested that in calcareous 

 soils organic iron compounds might be more available than the inorganic, 

 just as iron in solution as a complex ion is less completely precipitated by 

 the usual reagents. The idea seemed substantiated by tests with rice 

 in nutrient solutions containing carbonate of lime, where ferric tartrate 

 furnished much more available iron than equivalent quantities of ferrous 

 sulphate or ferric chlorid. 



Experiment VIII. — Tests were accordingly conducted to determine 

 the effect of various iron compounds and organic materials on the growth 

 of rice in both calcareous and noncalcareous soils. In this experiment 

 the effects of certain pure organic compounds of iron were compared 

 with those of ferric chlorid and ferrous sulphate. A substance which 



