Root Development in Aluminous Soils — Plucknett, Moomaw, and Lamoureux 
401 
depth of penetration of tap roots, location and 
length of lateral roots, relation of roots to 
soil profile, and possible evidence of causes of 
thicket formation. A few plants of Norfolk 
Island pine ( Araucaria excelsa) (Lamb.) R. Bn. 
were also excavated in the Wailua Game Refuge. 
Root description and measurement were also 
made of R. tomentosa seedlings used in a pot 
experiment designed to measure plant and soil 
Al. Notes taken included color, thickening, black 
tips, number of lateral roots, length of tap roots, 
and number of active buds. 
Because tap roots were observed turning 
laterally in Halii and Kapaa soils in the field, a 
pot experiment was established using the soil 
layer in which these roots turned as "subsoils.’’ 
These soils were collected from horizons where 
tap roots were observed to turn laterally and 
were sacked carefully to prevent dehydration. 
They were screened through wire mesh con- 
taining approximately 4 meshes to the inch. 
Weighed samples of the screened soil were used 
to form the "bottom” 5 -inch soil layer in 11- 
inch plastic pots, and were treated with six lime 
and phosphate treatments. After the treatments 
were mixed thoroughly in the subsoils, a 5 -inch 
layer of untreated Kapaa surface soil was added. 
Leucaena glauca 5 seeds were planted in the sur- 
face soil and after germination plants were 
thinned to two per pot. 
At harvest the soils were carefully removed 
from the pots and washed from the roots. Meas- 
urements of tap-root penetration and lateral 
root development were made. Roots were ex- 
amined for blunted and blackened tips, and root 
tips from each treatment were preserved for 
staining studies. Yields of tops and roots were 
recorded and plant Al concentrations were de- 
termined. The pH, extractable Al, exchangeable 
calcium, and cation exchange capacity were de- 
termined for each soil. 
Root tips of L, glauca plants from treated pots 
were sectioned on a freezing microtome and 
stained, using hematoxylin without a mordant 
(Johansen, 1940). Slides of the root sections 
were made and photomicrographs were taken. 
6 In a personal communication Dr. F. R. Fosberg 
has indicated the correct name for this plant should 
be Leucaena leucocepbala. 
Fig. 1. Rhodomyrtus tomentosa excavated in the 
Kapaa soil series, Wailua Game Refuge. The tap root 
turned laterally at 4-inch depth and lateral roots pene- 
trated diagonally before ascending toward the surface. 
RESULTS 
Six R. tomentosa plants were excavated in 
the Kapaa soil series. All plants were extremely 
shallow-rooted, with tap roots turning laterally 
at depths of from 3 to 10 inches below the sur- 
face (Fig. 1 ) . Lateral root development irnrhese 
plants was especially pronounced and one large 
10-ft shrub had a lateral root 24 ft long. Lateral 
roots displayed a tendency to grow downward 
and outward for 2 to 4 inches and then to as- 
cend toward the surface. Lateral roots were 
frequently found just at, or slightly under, the 
soil surface. 
Two thickets of R. tomentosa were excavated 
in the Kapaa soil series. These thickets con- 
tained shrubs up to 10 ft in height with trunks 
1.5 to 2.25 inches in diameter. Roots were ob- 
served with diameters up to 2 inches. Tap roots 
turned at a depth of 10 inches and no roots of 
the thickets were found below this depth. There 
was no evidence of root fusion. 
M. malabathricum plants excavated in the 
Kapaa soil were found to have root development 
similar to that in R. tomentosa, with tap roots 
turning laterally at shallow depth and long lat- 
eral roots. One plant (Fig. 2) had a small, 
twisted and deformed tap root with 2 main 
lateral roots 5 ft long. Fibrous roots were almost 
lacking in both R. tomentosa and M. mala- 
bathricum. 
Norfolk Island pine trees (A. excelsa) planted 
about 10 years ago in the Kapaa soils were 
