Three topsoils
were selected from the different farm areas of Karaj region having cec of 6.2,
13.6 and 18.8 cmol+/kg-1and with different textures (sand,
clay loam and silty clay loam). Available potassium levels in the soils were
164, 256 and 328 mg/kg, respectively. The soils were used for a green house
trial. Twenty pots were filled with 16 kg of each soil for eight potassium
treatments plus one additional treatment (same three soils without K-fertilizer
and without plants). The additional treatment received irrigation water similar
to other pots. The experiment had a complete randomized design as factorial,
plus one additional treatment, containing two factors of soils with various cec
and potassium fertilizer rates, with three replications. Two corn seeds were
planted in each pot. Plants were irrigated with distilled water during ten weeks
growth. Subsequently, the corn plants were cut and analyzed for potassium and
magnesium. Available potassium levels were measured in samples collected from
each pot after harvesting. The statistical analysis of the data showed that
there was a significant difference at 5% level on the potassium concentrations
in the shoots but the dry weight yields of shoots and the total potassium
content of shoots were not significantly affected by the treatments. Similarly,
potassium treatments did not affect magnesium uptake by the corn plants
significantly. A comparison between the soil available potassium levels before
planting and after harvesting showed a decline of soil available potassium for
the three soils, irrespective of the fertilizer treatments. Subtracting the sum
of soil available potassium after harvest plus the amount of potassium measured
in the shoots of corn from the sum of the initial soil available potassium plus
the added fertilizer potassium, a term
ΔK
was defined and used to balance the changes in the available soil potassium
levels. The outputs showed the results of fitting a second order polynomial
model to describe the relationship between the
ΔK
of different soils with various cec and the amount of added potassium to pots.
Results showed a positive statistical significant relationship between the two
factors at the 1% level. The behavior of soils having low and intermediate cec
is similar but different from the soil with high cec. A negative
ΔK
indicated that non-exchangeable potassium had been released during the
experiment, irrespective of the fertilizer treatments. However, increasing the
rates of potassium application resulted in a declining trend in
ΔK
values towards zero. The lighter the soil texture with lower cec, the less
potassium fertilizer was required to drive
ΔK
to zero.