10 
alumina become partially hydrated, and the soil mass swells con- 
siderably, thereby effectively closing the pores. Upon drying out 
contraction takes place, which results in the formation of large cracks. 
Thus the roots are injured and a condition provided for rapid evapor- 
ation of the soil moisture. This can be prevented only by increasing 
the humus and maintaining a mulch on the surface. It is evident 
from the above that a condition of insufficient aeration prevails in 
the clay soils during times of wet weather, and in fact anaerobic 
conditions often prevail. 
Investigations on the physical properties of Hawaiian soils show 
that fertilizers exert considerable physical effect. Phosphates 
materially retard the movement of moisture in the heavy clays, while 
nitrate of soda produces similar effects in some of the highly organic 
soils. A number of fertilizing substances cause a deflocculation of 
the clay. It has been found, on the other hand, that heat causes 
the clay to become granulated and, therefore, is a means of increasing 
aeration and drainage. 
The predominant color of Hawaiian soils is red, due to iron which 
is present in large amounts. Various shades of red often occur close 
together. There are also considerable areas of yellow soil. These 
colors seem to be referable to the state of hydration of the iron. The 
dark red color which predominates in the more arid sections is prob- 
ably due to amorphous hematite and the yellow color to limonite. 
In the humid sections, where the humus content is not high, the color 
of the soil is generally lighter, due to the iron being in the ferrous form. 
The iron is generally so abundant and completely disseminated in the 
soil as to obscure the humus unless present in large amounts. 
CHEMICAL CHARACTERISTICS. 
Soils naturally partake to a considerable extent of the nature of the 
minerals from which they have been derived, and, since Hawaiian 
soils have been formed from basaltic lava, they are potentially basic. 
This does not mean, however, that in all cases the soils are free from 
acidity, as will be pointed out later. In general Hawaiian soils con- 
tain unusually high percentages of iron and alumina, the former 
sometimes exceeding 50 per cent by weight. The silica content is 
low as compared with mainland soils. Lime and magnesia are 
present in quite variable amounts, but usually more abundantly than 
in nonlimestone countries. 
POTASH. 
The potash content on the whole is rather below the average, but 
frequently it is relatively more soluble than usual and consequently 
more available. It is also more constant in different sections of the 
islands than any other of the so-called plant-food constituents. 
