ENDINS, n.' 13.1987. Ciutat de Mallorca. |
THE PROBLEMS OF SOlLS IN
LIMESTONE AREA OF THE NANSEI SHOTO,
by Kazuko URUSHIBARA-YOSHINO *
En les terrasses forrnades a Nansei Shoto (SW del Japó) a causa de I'aixecarnent de I'escull
coral.li, els sols rnés antics tenen les següents caracteristiques: tonalitat rojenca, quantitats rnés al-
tes d'oxids de ferro Iliures (Fez03 % ) aixi corn una rnajor intensitat del pic corresponent a I'hernatita.
En les terrasses rnés recents els SOIS són groguencs, pero els oxids de ferro Iliures (Fez03 % ) són
encara elevats arnb una intensitat mes alta del pic de la goethita.
Les arees cobertes per sols rojos (anornenats Shimajiri-mSji) en els esculls coral.lins aixecats, es
veuen sovint afectades per la sequedat en les zones de conreu de canya de sucre. Per evitar els
danys causats per I'aridesa s'ha rnesclat Jagar (originat a partir de roques argiloses terciaries) arnb
rojos de les arees de Shimajiri-m&. No obstant aixo, la introducció de maquinaria agrícola,
les rnillores del sol a gran escala i I'augrnent de la superficie conreada, han tengut per resultat una
erosió accentuada del sol en aquests carnps. Endernés, els SOIS
arrossegats pels rius resulten perju-
d i c i a l ~
per a la pesca costanera i ocasionen I'extinció dels corals.
Soils on the terraces forrned frorn uplifted coral reef in the Nansei Shoto, SW Japan, have cha-
racters that hue of soils is more reddish, arnounts of free iron oxides (Fe203 % ) are higher, and peak
intensity of hernatite is higher on the older terraces. On the younger terraces, soils becorne yellow-
ish hue, but free iron oxides (Fez03 %) are still higher with higher peak intensity of goethite.
The areas covered by red soils, called Shirnajiri-mSji, on the uplifted coral reef suffer frequently
frorn drought on the sugar cane cultivation. For avoid the darnages by drought, Jagar, originated
frorn Tertiary rnuddy rock, has been rnixed into the red soils in the Shimajiri-máji areas. However,
introduction of rnachines for cultivation, soil arnelioration in a large scale, and enlarging unit area of
each fields have resulted in a large arnount of soil erosion at the fields. Soils flowed frorn the rivers
hinsder coastal fishery and corals becorne extinct.
1. Characteristics of red soils
soils on the younger terraces formed in the perioa
formed from limestone in the
from 125,000yr B.P. to 38,000yr B.P. Soil layer is
thicker on the older, higher terraces and thinner on
Nansei Shoto and their
the younger, lower terraces.
The older the soils are, (1) the more reddish be-
come, (11) the greater the Fez03 % and (1111 the hig-
There are several terraces composed of uplift-
her the peak intensity of hematite. These relations-
ed coral reeí in the Nansei Shoto. The highest coral
hip between the properties of soils and hue is
terraces seem to be formed older than 230,000yr
shown in Fig. 1. On the younger terraces, yellowish
B.P. and then the formed ages of the coral terraces
hue becomes stronger, but Fez03 % is still greater
are; 125,000yr B.P., 100,000yr B.P., 80,000yr B.P.,
and the peak intensity of goethite is higher. From
60,000yr B.P. and 38,000yr B.P. Reddish soils are
this, it is suggested that the free iron is contained
formed on the older terraces, but reddish-yellow
as hematite in the soils on the older terraces, whe-
reas as goethite in the soils on the younger te-
* Departrnent of Natural Sciences, The Kornazawa University.
Setagayaku. Tokyo, 154 Japan
Climate conditions in the Nansei Shoto at pre-
R E D
mer rainy season, and with the infrequent typhoon
visit during the months of August and September.
If we calculate the normals of water deficiency by
the rnean values 1941-1970 based on the Thornth-
waite's rnethod, it is shown that the whole area of
the Nansei Shoto is covered by O rnrn of water de-
ficiency; which rneans a hurnid clirnate as a long-
year mean state. However, in sorne years, there oc-
cur water deficiency and drought in the field of the
limestone area forrned by the uplifted coral reef.
This will be dealt with in detail in the next part.
\\ goe. -a
2. Red soils formed from
limestone and agriculture
Taking an exarnple frorn the Okinawa island
Figure 1. Relationship between hue and properties controlling
located in the central part of Nansei Shoto, agricul-
the hue in the 82 horizons in the Nansei Shoto.
tural landuse is described with special regard to
the soil groups. In the Okinawa island, there are
sent are given by year clirnate for 1941-1970 by
four soil groups as shown in Fig. 3(a): (I) the red-
calculating water deficiency for each year based on
dish-yellow soil formed frorn the rocks excepting
the method by Thornthwaite (1948). The distribu-
lirnestone. This is called ccKunigami-rndji)). (11) Red
tion shown in Fig. 2 reveals a tendency that the
soils formed from lirnestone. They are called ccShi-
water deficiency is greater in the southern part of
majiri-mdjin. (111) Bluish-brown ccJagar)) forrned
the Nansei Shoto. The high values of water defi-
from the Tertiary rnuddy rock. And (IV) alluvial soil.
ciency in the Nansei Shoto occur in the years with
Since frequency of drought occurrence is very
scarce rainfall during the Baiu season, early surn-
high in the island in surnrner, sugar cane, which is
during 1941-1970 around the Nansel Shoto.
Figure 3. (a) Soil rnap of the Okinawa island (Okinawa Prefec-
ture, 1978, rnodified by Urushibara)
1: Kunigarni-miji, 2: Shirnajiri-rndji. 3: Jagar, 4: Allu-
vial soils and others.
(b) The distribution rnap of the areas where had be
polluted with soils (Okinawa Prefecture, 1978).
stronger against drought, has been cultivated in
the yield when rainfall is abundant. Mean yield of
the areas covered by the first three soil groups
sugar cane in the areas covered by Kunigami-mdji
mentioned above. And rice, the main crops in Ja-
is 6.44 ton/lOa. In this areas, acidophilous crops
pan, cultivated in the area of alluvial soil for long
such as pinapple, tea, and citrus fruits are cultiva-
years. However, the relationship between sugar
ted in addition to sugar cane.
cane yield and precipitation in the areas of each
Shimajiri-mdji in the Karst area shows weak
soil groups as shown in Fig. 4. Namely, the yield is
acidity, but the thickness of the soil layer is rela-
the highest in the areas with Jagar, whenever pre-
tively thin. This makes waterholding-power quite
cipitation is scarce' or abundant in the summer
weak. In this connection, drought occurs frequen-
months from May to September. These months in-
tly. Because of thin soil layer, introduction of ma-
clude Baiu season, a rainy season in early summer,
chines is difficult. Mean yield of sugar cane is 5.07
and typhoon season from late August to September.
ton/lOa. Crops otherwise sugar cane are sweet po-
It is said that the Kunigami-mdji is stronger for
tato, tabaco, etc., which have stronger resistance
drought, but yield decreases sharply in the cases
more precipitation. The fields in the areas covered
Jagar is a weakly alkaline, heavy clay soil. It
by red soils formed from limestone, Karst areas, suf-
forms big soil blocks in the case of plowing and
fer from drought quite easily, but yield increases
becomes very hard under the dry conditions. It is
gradually up to some amount of precipitation. The
therefore very difficult to introduce machines from
Kunigami-mgji has low pH and soil layers are thick.
the viewpoint of physical features of soil texture. In
Accordingly, the lower horizon has a higher density.
spite of such unfavorable characteristics, yield of
This results in difficulty for drainage and decreases
sugar cane in the fields of Jagar is higher; mean
Figure 5. Variation of the estimated volume of transported soils
1; caused by soil amelioration
2; from roads etc.
3; from pinapple fialds
succeeded amelioration of soil texture and the
2400 MM 2
yields are increasing. When Jagar was
the unit areas of the fields were
irnprove Shirnajiri-miji, machines were used and
nally, the unit areas were small and the farmers
P REClPl TATlON (MAY -SEPT )
built stone walls of lirnestone blocks along the
hems of the fields. These way of cultivation protec-
Figure 4. Relationship between yield of sugar canes and the
ted the soil erosion and prevented the wash-out of
precipitation from May to September (Okinawa Agri-
soil. In 19801s, the improvement of Jagar into the
cultural Cooperative Society, 1977).
soils forrned from lirnestone is increasing conspi-
cuously in the other islands in Nansei Shoto.
yield is 9.64 ton/lOa.
These result in the following problems: The
In 19501s, cornpost (artificial manure) was put
arnount of transported materials, such as sands,
in this area, but in 19601s, it changed to chemical
soils and rnuds, through the rivers in the Okinawa
rnanure mainly. Accordingly, it caused acidation
lsland into the surrounding coast changed as shown
and serious shortage of organic rnatters, microele-
in Fig. 5. It is clearly shown in this figure that soil
ments and available bacteria. Since the later half of
erosion caused by enlarged pinapple filelds in the
19701s, farmers have been reviving leguminous
Kunigarni-rniji areas since 19601s, washed-out cau-
green rnanure and dung and urine from animal
sed' by the abrupt development of construction
(mainly for Ocean-Expo) during 1972-1975, and
On the other hand, an attempt has been mak-
washed-out caused by the soil improvement since
ing t o ameliorate soil which is weak to drought
1976. The arnount of washed-out materials caused
since the year about 1976. That is,weathered mate-
by irnprovement of soils in the Shimajiri-miji li-
rials from Tertiary mud was mixed to Shimajiri-miji
mestone areas is outstanding. These threaten
in the limestone area where suffers frequently
coastal fishery and extinction of coral reef, even
from drought, by the governmental support. These
though the yields of sugar cane are increasing as a
sequence is shown in Fig. 5. As a result, they have
result of soil amelioration.
KONISHI, K. et al (1974): Radiometric coral ages and sea level re-
The present study was made possible by the
cords from the Late Quaternaty reef complexes of the Ryukyu
grant-in-aid from the Komazawa University in 1985.
Islands. Proc. 2nd. Intn'l Coral Reef Symp., 2,595-613.
The writer wishes her sincere thanks for it.
MEZAKI, K. (1983): An introduction to middle and northern Okina-
wa (in Japanese). Abstracts of Annual Meeting of Association
of Japanese Geographers, 24,19421 1.
OKINAWA PREFECTURE (1978): The condition of polluted fishety
by soils (in Japanese). 1-152.
OKINAWA AGRICULTURAL COOPERATIVE SOCIETY (1977): Soil
properties in Okinawa Prefecture and the method of soil ame-
lioration (in Japanese). 10-14.