Root Development in Aluminous Soils— PLUCKNETT, Moomaw, and Lamoureux 
399 
soils were not made at that time. Two particu- 
larly aggressive plant species, Rhodomyrtus 
tomentosa (Ait.) Hassk. and Melastoma mala- 
bathricum L., are known to grow well on Ha- 
waiian bauxitic soils, where Rhodomyrtus tends 
to form dense thickets excluding all other 
vegetation. These two species were introduced 
on Kauai about 50 years ago and Rhodomyrtus 
has now been declared a noxious weed. It is the 
object of this study to characterize root growth 
and distribution of these and other tap-rooted 
species on the bauxitic soils of Kauai. 
Aluminum has been shown to be toxic to 
plants. One of the frequently-reported toxic ef- 
fects of aluminum is root injury (Lignon and 
Pierre, 1932; McLean et al., 1926; Bortner, 
1935) in which roots may be brown in color 
with few rootlets and discolored root tips (Gil- 
bert and Pember, 1931) or with root tips 
blackened and thickened to twice normal size 
(Bortner, 1935). Restricted lateral root de- 
velopment in rye has been reported using water 
cultures (Magistad, 1925). Trenel and Alten 
(1934) concluded that aluminum may be a 
root poison. Using a divided-root technique corn 
plants were exposed to nutrient solutions with 
and without aluminum. Injury was restricted to 
roots in the high-aluminum solutions. Nagata 
(1954) found that over 5 ppm aluminum in 
culture solutions hindered barley growth and 
that aluminum seemed to accumulate in the 
roots. Growth hindrances were decreased by 
adding phosphorus or calcium. He concluded 
that translocation of phosphorus from the root 
to the top in the barley plant was hindered by 
aluminum in culture solutions. 
The precipitation of phosphate and alumi- 
num in the plant as an aluminum phosphate 
has been suggested. Burgess and Pember (1923) 
proposed that aluminum was fixed as relatively 
insoluble aluminum phosphate in plants, espe- 
cially in roots. McGeorge ( 1925 ) suggested that 
internal precipitation of aluminum by phos- 
phorus may be important in plants but listed 
no specific location. 
Wright ( 1937 ) divided root systems of bar- 
ley plants, placing each half in different culture 
solutions with and without aluminum. Plant 
analysis indicated plant damage resulting from 
poorly developed root systems in solutions con- 
taining aluminum, and internal precipitation of 
phosphorus and aluminum where large amounts 
of aluminum and phosphorus were present in 
roots. Wright (1943) found a higher percent- 
age of phosphorus in aluminum-treated barley 
plants than in nontreated; this was particularly 
marked in the roots. The water-soluble phos- 
phorus in the aluminum-treated plants was low, 
while a H 2 SO 4 solution (pH 3.0) extracted 
practically all P from untreated plants but much 
smaller amounts from plants grown in contact 
with Al. The precipitation was listed as occur- 
ring primarily in roots, and sharp reductions in 
yield were attributed to P deficiency in meri- 
stematic regions due to root precipitates. Wright 
( 1945 ) , using microchemical tests to determine 
inorganically and organically bound P, found 
abundant inorganic P in roots grown in contact 
with Al and little or none in roots from solu- 
tions without Al. 
Problems of plant growth on acid soils have 
long been ascribed to the "active” Al in the soil 
and to problems of phosphate nutrition due to 
fixation of phosphates by Al. Longnecker and 
Merkle (1952) studied root development of 
crimson clover in relation to lime placement 
and found most root growth in layers which 
had been limed. The beneficial effect of liming 
was attributed to decrease in solubility of Al 
and Mn and an increase in solubility of P. Rag- 
land and Coleman (1959) applied lime at sev- 
eral rates to subsoils of the Norfolk catena in 
pots and found grain sorghum root growth 
into unlimed subsoils was related inversely to 
amounts of exchangeable Al. Root growth into 
subsoils increased substantially with lime treat- 
ment. Root development of sorghum grown in 
suspensions of acid clay was restricted severely 
unless 80% of the acidity was neutralized. 
DESCRIPTION OF EAST KAUAI 
The area studied is referred to by McDonald 
et al. (I960) as the Lihue Depression. It is a 
nearly circular basin with the rim being formed 
by the Haupu ridge on the south, the main 
mountain mass of central Kauai on the west, 
the Makaleha mountains on the north, and 
Nonou and Kalepa ridges on the east. The basin 
is floored with lavas of the posterosional Koloa 
