18 
EROSION AND DRAINAGE. 
The amount and extent of soil erosion in Hawaii can hardly be 
overestimated. The heavy rains wash away enormous quantities of 
soil. This can be prevented by a combination of deep plowing, 
proper drainage, and green manuring. When the soil is compacted 
rain waters flow over the surface rather than being absorbed, thus 
washing away much soil material and cutting gullies. Where ditches 
are provided they should be arranged close together in the direction 
of the greatest fall, so as to carry away the surplus water and prevent 
overflow. In some places much damage has been done to crops by 
the overflow from inadequate ditches. The use of dynamite, for 
the purpose of shattering the subsoil and allowing better drainage, 
has been practiced with good results in some localities. The plow- 
ing under of coral sand has also been effectively used in some localities 
as a means of improving aeration and drainage conditions, and 
around Honolulu gardeners frequently mix black sand with the soil 
for the same purpose. 
USE OF FERTILIZERS. 
The table of chemical analyses at the end of the bulletin shows 
that, on the whole, Hawaiian soils are well supplied with phosphoric 
acid and nitrogen with a medium content of potash. In many locali- 
ties the soil contains unusually high percentages of phosphoric acid 
and nitrogen. Nevertheless, heavy fertilization is generally prac- 
ticed, due to the low availability of plant food. It is more rational, 
however, to make available the plant food already contained in the 
soil, if this can be done economically. The methods by which this 
may be accomplished have already been suggested. These consist 
briefly in increasing the aeration by deep plowing, thorough drainage, 
and the frequent plowing under of green manures, which, if carried 
out systematically, will materially lessen the need for commercial 
fertilizers. 
The exact fertilizer for each crop varies in different districts. In 
general both nitrogen and phosphoric acid give good results. Nitrate 
of soda, ammonium sulphate, and organic forms of nitrogen each 
produce good effects, but, generally speaking, organic forms and 
ammonium sulphate are recommended in preference to nitrate of 
soda, especially during seasons of heavy rainfall. The soils have the 
power of absorbing large amounts of ammonium sulphate, thus pre- 
venting its being leached out, but nitrate of soda is not fixed by soils. 
When the organic matter is deficient the phosphoric acid is generally 
of low availability and phosphate fertilizers are needed. Soluble 
phosphates will give best immediate returns, but if applied in con- 
junction with decaying organic matter, insoluble forms may be used. 
Pot experiments at the station, carried through several crops, indi- 
