37 
0.153 per cent soluble iron and aluminum phosphates. The weak 
solvents apparently exert a very limited solvent action, as is shown 
by the small percentage of the total phosphoric acid dissolved. All 
the soils, irrespective of type, contain iron and aluminum phosphates 
far in excess of calcium phosphates. Citric acid is a much more 
effective solvent than nitric acid. 
The humus content of Hawaiian soils varies between wide limits 
according to locality and climatic conditions. Since humus is a 
factor in the availability of phosphoric acid, some determinations 
were made of the humus content of various types and also of the 
phosphoric acid combined with the humus. There is apparently no 
relation between the amount of humus in the soil and the phosphate 
content of the humus. Stoddard 1 found that as the amount of humus 
decreases the percentage of phosphoric acid in the humus increases. 
This is not true of Hawaiian soils, but, as a general rule, it may be 
said that those high in lime or magnesia and humus contain a large 
percentage of phosphoric acid organically combined. Attention is 
called to the high phosphate content of soil No. 1, in which the humus 
contains 22.98 per cent phosphoric acid. In view of the fact that 
this soil has been found not to respond to phosphate fertilization and 
yet to have a very low content of calcium phosphate, as measured by 
fifth-normal nitric acid, the conclusion is evident that the organic 
phosphate in this soil is present in a readily available form. 
The soils having the strongest fixing power are likewise those con- 
taining phosphoric acid in least available form as measured by plant 
growth and also those containing the least phosphoric acid soluble in 
weak solvents. These phosphates are only slightly soluble in citric 
and nitric acids, but are more soluble in weak alkali. The soils of 
lowest fixing power, due primarily to lower clay content, are high in 
lime and magnesia. These soils are also high in phosphate, and while 
a large percentage is in the form of iron and aluminum phosphate, the 
proportion soluble in citric acid is equal to or more than that soluble 
in alkali. An exception to this, and one to which attention should be 
called, is soil No. 11. This soil is high in lime, magnesia, and humus, 
but was taken from a very humid district. The analysis shows the 
major part of the calcium phosphate to have been washed out, leaving 
the iron and aluminum phosphates. This is probably similar to the 
action of weathering agents upon lava in the original Hawaiian soil 
formation, as a result of which lime has decreased from about 10 per 
cent to less than 1 per cent in the average soils. The phosphoric acid 
has decreased quite often also during this process of disintegration. 
Soils Nos. 2, 3, 5, 6, 7, and 8 are all clay soils. Nos. 5, 6, 7, and 8 
were chosen for the fact that in the two former the iron content is in 
i Wisconsin Sta. Research Bui. 2 (1909). 
