284
RUSSELL
plants. Closely linear relationships between the percentages of the strontium
90 removed from such soils and the ratio of strontium 90 to calcium in
plants demonstrate the practical utility of this procedure; in contrast,
markedly curvilinear relationships can occur between the ratios of strontium
90 to calcium in plants and those in soil extracts made with 1 N ammonium
acetate (54, 55). Fredriksson et al. (46) found that extraction with ammonium lactate provides an adequate basis for comparison in an extensive
survey of Swedish soils; the view that this extractant removed appreciable
quantities of both strontium 90 and calcium from surfaces as well from
the solution phase is, however, possible as the ratio of strontium 90 to
calcium in plant shoots was usually about 80 per cent of that in the soil
extract (21).
The effect of the addition of lime on the absorption of strontium 90 from
soil has been considered in manyinvestigations (2). Appreciable reductions,
though seldom by a factor exceeding 3, occur in soils low in calcium. However, if soils contain moderate or high levels of calcium the addition of lime
does not increase the concentrations of calcium in the soil solution and the
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absorption of strontium 90 is therefore not depressed. Fredriksson ef ai.
(56, 57) have shown that liming was of little importance when 50 per cent or
more of the exchange capacity of the soil is saturated with calcium.
Characteristics of soils other than their calcium content usually have
comparable effects on the absorption of both strontium and calcium; thus
the ratio of the two ions in plants is little changed. There are, however,
indications that some preferential absorption of strontium can occur from
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soils low in colloidal minerals or when much organic matter has been
added (2). This may reflect the contrasting distribution of the two ions between the soil and solution phases. Under field conditions where strontium
90 is usually largely present in the upper soil layers, manurial treatments
which alter distribution may markedly affect uptake.
The extent to which the availability of strontium 90 to plants may be
reduced by fixation processes in the soil has also received much attention.
Since fixation is irreversible, or nearly so, its extent will increase with time
until the sites which retain ions are saturated. Experiments in which the
state of strontium 90 has been studied for several years thus provide the
most convincing evidence. Squire (58) compared the extent to which 0.01 Mf
calcium chloride displaced strontium 89 and strontium 90 from a series of
soils in which the former had been present for one week and the latter for
about 3.5 years. The extent to which strontium could be displaced by calcium chloride decreased in different soils by 3 to 10 per cent during this
interval. These figures would overestimate the extent of fixation if the
recently added tracer had not attained equilibrium throughout the labile
phases. Evidence of a small degree of fixation was, however, provided by
the fact that about 1.5 per cent of the strontium 90 which had been present
for 3.5 years could not be extracted by prolonged treatment with concen-
trated nitric acid. Schulz & Riedel (59) used similar extraction procedures
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