17 
ROTATION' OF CROPS. 
The rotation of crops is essential to a permanent upkeep of the soil. 
It is not necessary to enumerate the reasons for this fact. Some of 
them still remain in obscurity. Crop rotation is of very great 
importance in the proper management of Hawaiian clay soils. As 
stated above, the humus is considerably above the average, but the 
peculiarities of the clay necessitate still greater amounts. The fine 
particles of iron and alumina become hydrated when wet and adhere 
to all the soil particles. Only by the maintenance of large amounts 
of humus can such soils be kept in a suitable condition for plant 
growth. 
The pineapple growers, following the lead of the sugar planters, 
have attempted to grow the same crop continuously, but in many 
cases with poor results. The reason is found in the need for humus. 
Crop rotation will supply this need, if a green manuring crop be plowed 
under. Humus is the key to successful soil management in Hawaii 
as is the case elsewhere. At the present time this fact is being rec- 
ognized by the sugar planters as never before. Instead of burning 
the trash and crop residue, they should be plowed under, thereby 
increasing the humus content. Organic matter undergoes rapid 
decomposition in the Tropics, and the necessary condition of aera- 
tion hastens the decomposition. It is necessary, therefore, to plow 
under green manure frequently. The details of crop rotation will 
not be discussed in this bulletin. With the pineapple lands a green 
manuring crop can be grown between crops. The choice of the 
green manuring crop will depend on the locality. It should be a 
legume if possible. The single-crop system can not be permanent, 
and it is of doubtful economy at any time. Sooner or later it must 
inevitably give way to diversification. 
- The retention of moisture during dry weather can be greatly 
increased by increasing the humus content of the soil, so that suffi- 
cient moisture will be retained for the growth of crops during con- 
tinued dry weather. The success of crops in the semiarid sections 
of Kula and the Parker ranch is due very largely to the high humus 
content of the soils. On one of the fields at the experiment sta- 
tion, where a legume has been plowed under each year for the past 
several years, the structure of the soil has been materially modified 
and the moisture-retaining power greatly increased. Similar effects 
can be had with any of the clay soils throughout the islands. 
Hawaiian clay soils require a higher percentage of moisture than is 
usual, due to their high hygroscopic capacity and the slow move- 
ment of moisture through the soil. Hygroscopic moisture will not 
sustain plant life. 
98554°— Bull. 40—15 3 
