4 BULLETIN 6 2, HAWAII EXPERIMENT STATION 
This process of soil formation is often referred to as lateriza- 
tion and the product designated as laterite. This latter term, which 
has been applied to a certain more or less well-defined tropical soil 
type and is constantly used in pedology, has not so far been given 
an exact definition, although several proposals concerning it have 
been made. It was first used in connection with certain porous 
tropical red clays, notably in India, which upon drying became hard 
and refractory and were used for making bricks for building pur- 
poses. Later on, the definition of the term was concerned chiefly 
with the processes of formation. Campbell (11, pp. 120-128) sepa- 
rates the processes of tropical rock decomposition into two groups — 
alteration and weathering — depending on the state of saturation by 
water of the decomposing rock material. Attempts have been made 
to define laterites on the basis of chemical composition. Fermor 
(IS, pp. 559-566) proposes that the term " laterite " be restricted 
to soils containing 90 to 100 per cent lateritic constituents (oxides 
of iron, aluminum, titanium, and manganese more or less hydrated) ; 
that soils containing 50 to 90 per cent lateritic constituents be called 
" siliceous laterites " ; that those containing 25 to 50 per cent of 
these constituents be called " lateritic soil " ; and that soils having 
less than 25 per cent receive no designation on this basis. Recently, 
Martin and Doyne (25, p. 546) proposed that the classification of 
laterite and lateritic soils be made on the basis of the composition 
of the clay fraction. They suggest that soils of which the silica- 
alumina ratio in the clay fraction falls below 2 be called lateritic 
and that when this ratio falls below 1.33 the soil be described as 
laterite. 
It should not be inferred from the foregoing that all the soils 
of Hawaii are of the same average composition and exhibit similar 
physical characteristics. Even though the parent material from 
different sources exhibits marked similarity in composition, dif- 
ferences in age and variations in weathering agencies are so large 
as to bring about a wide variety of decomposition products. Tem- 
perature changes may range from tropical heat to that approaching 
Temperate Zone conditions, and rainfall may vary from a few inches 
to several hundred inches a year. Examination of the Appendix 
(p. 43), giving certain climatic conditions and corresponding soils 
with some of their physical and chemical properties, show that the 
soil types of the Hawaiian Islands vary widely. Kelley and his 
coworkers (21) and McGeorge (24) show that soil types exist in 
Hawaii which are not laterites nor even lateritic in any of the 
senses mentioned above. The soils of Hawaii in large part may, 
however, be classified as laterites or lateritic. 
Organic matter modifies the soil complex to no small degree and 
varies considerably both as to amount and state of decomposition. 
In very humid regions where the organic material decomposes under 
partly anaerobic conditions, the rate of decomposition is slow and 
there is likely to be an accumulation of partly decomposed organic 
matter. In this case large amounts of organic matter are present 
not only in the clay or colloidal fraction, but in the different tex- 
tural subdivisions of the silt fractions as well. In the less humid 
areas, or where excess moisture is rapidly removed by good drain- 
age, the organic matter is present in smaller amounts and decom- 
position is more thorough. From a biological point of view there 
