14 
From the data showing the profit and loss it is noteworthy that the 
application of 300 pounds of ammonium sulphate proved the most 
economical, either when applied to the spring crop only or to both 
spring and fall crops, and that in the latter case the profits were very 
large. So far as these experiments go, they show in addition that the 
yields can be maintained at a high point and good profit be made 
under the system now employed, provided the proper fertilizer be 
used. This is not to be interpreted, however, as being a recommenda- 
tion of the system now in use, since it has been shown (p. 19) that 
with the rotation of crops, involving the plowing under of a legume, 
still greater yields can be obtained. The rotation system is far more 
rational and permanent and ought to be employed on all rice lands. 
It has been found in other countries that the continued application 
of ammonium sulphate tends to produce acidity in the soil due to the 
fact that the sulphate ion tends to accumulate in the soil. The oc- 
casional application of lime, however, will correct this defect. The 
highly basic character of Hawaiian soils, on the other hand, partic- 
ularly the rice soils, justifies the belief that the production of acidity 
from the use of ammonium sulphate will be far removed in point of 
time. It is of interest in this connection that the annual application 
for over 60 years of 300 pounds of ammonium sulphate per acre at 
Eothamsted to a soil containing considerable amounts of calcium 
carbonate (probably 100 tons per acre in the first 7 inches) has not 
produced injurious acidity. The soil on which the above rice ex- 
periments were conducted contains a relatively high percentage of 
lime and magnesia, particularly the latter, but neither of these is 
present as carbonate in more than very limited amounts. The carbon 
dioxid content of the soil is low, not more than 0.10 per cent. The 
iron and aluminum, however, occur largely as hydrates which give to 
the soil its basic character, and which we may reasonably believe will 
prevent the accumulation of injurious acidity. It is of further 
interest to note that the application of lime has been shown to cause 
a decrease in the yields of rice on this soil. 
It would not be safe, however, to recommend ammonium sulphate 
as the only fertilizer to be applied to the rice lands of the islands 
generally, since the effects of fertilizers frequently vary widely on 
different soils. In order to throw further light on this question some 
experiments have been conducted cooperatively on other rice lands, 
which resulted in showing that ammonium sulphate produced practi- 
cally as large increases as a complete fertilizer. At Kailua, for 
instance, approximately 60 per cent increase in yield was produced 
both by 150 pounds of ammonium sulphate and by a complete fer- 
tilizer containing an equal amount of ammonium sulphate. 
As already pointed out, the rice soils, as a rule, are rich in phos- 
phoric acid but contain relatively small amounts of potash. While 
it is true that rice takes up a large amount of potash only a compara- 
