- The objectives of this study were to establish a suitable method
for the measurement of the root cation-exchange capacity of conifer
seedlings, and to examine possible influences of this root characteristic
on nutrient absorption from a Bellpine soil under greenhouse
The exchange sites of fresh, excised roots were saturated
with H⁺ by means of ten subsequent 15-second rinses in 0.1 N HC1.
Each acid rinse was followed by a water rinse of equal length. This
technique, together with the use of large volumes of acid and "lowsalt"
roots, permitted complete saturation of the root exchange sites
by preventing readsorption of replaced and leaked cations. The acid
which was subsequently released into a neutral 0.1 N KC1 solution
by these roots was titrated. Neutral pH was maintained by addition
of standard KOH for up to two hours.
The titration value did not represent an accurate measurement of the root CEC, since short titration periods prevented complete
neutralization of the adsorbed acidity. Extended titration times led
to the inclusion of large amounts of H⁺, which are derived from various
other sources within the root. This latter fraction of H⁺, termed
non-exchangeable, was quantitatively determined by titration of the
acidity, which was liberated from Ca-saturated roots. The root CEC
was measured as the difference between total and non-exchangeable
The Bellpine soil was shown to considerably reduce the root
cation-exchange capacity of nursery-grown conifer seedlings. Most
likely, changes in the fungus-root association, variations in the
N-level, or high levels of Mn are the causes of the observed CEC
In a greenhouse experiment, incense cedar, noble fir, and two
Douglas fir varieties were grown singly, in association with each
other, and together with bentgrass and tall fescue. Manganese concentrations
in the seedlings reached toxicity levels in many instances
and induced a Fe-deficiency. The tolerance of the conifer species
to high levels of Mn was correlated to their root CEC. Species with
a relatively high root CEC took up more Ca; whereas, in most cases,
seedlings with relatively low root CEC accumulated more K.
Increases in P uptake were paralleled by rising root CEC.
These correlations, however, do not necessarily imply a causal relationship between root CEC and nutrient absorption. Competing
tree and grass species did not, in all cases,affect the nutrient uptake
by the host species according to expectations from their root CEC.
Other factors overshadowed possible influences of root CEC in competitive