|Abstract or Summary
- Field studies designed to study residual effects from fertilizer
molybdenum were initiated in September 1971 on established grass-clover
pastures at ten sites in Western Oregon. Treatments were 0.0
and 1.12 kg molybdenum /ha on plots receiving 0-35-0-20S fertilizer (elemental sulfur-S) and 0.0, 0.56, 1.12 and 2.24 kg molybdenum /ha on
plots receiving single superphosphate. Treatments were replicated
four times in a randomized block design.
Forage yields were measured during the spring in 1972 and 1973.
Subterranean clover samples were analyzed for nitrogen, sulfur,
copper and molybdenum. Yield response to applied molybdenum was
noted on three harvests of seventeen; these responses were all on
plots receiving single superphosphate. Clover nitrogen and copper
levels were not significantly affected by molybdenum application. Form of sulfur fertilizer had a profound effect on molybdenum
uptake by clover. On plots which received 1.12 kg Mo /ha, application
of elemental sulfur (0-35-0-20S) produced plants much lower in
molybdenum than did single superphosphate application. It was
hypothesized that depression of Mo uptake resulted from one or a
combination of the following phenomena: (1) a decrease in soil pH,
(2) increased competition between sulfate and molybdate for absorption
by the plant, and (3) increased leaching of molybdate due to the sulfate
release pattern of the elemental sulfur material.
Samples of the surface 10 cm of soil from each treatment were
analyzed for anion exchange resin extractable molybdenum. Correlation
between plant molybdenum and soil molybdenum was significant
where all harvests were considered together, but the correlation was
improved by separating soils into two groups. The soils in one of
these groups were found to sorb large amounts of molybdenum, to
release high amounts of hydroxyl anions on treatment with sodium
fluoride, and to contain large amounts of amorphous and exchangeable
iron compared with soils in the other group.
Laboratory studies were carried on to investigate the molybdenum
adsorption patterns peculiar to these soils. Surface samples
were taken from eight of the field sites which showed a large range of
molybdenum responses. Molybdenum solutions (0-20 ppm Mo in
0.01 M CaCl₂) were allowed to equilibrate with soil at 25° C and the supernatant molybdenum concentration measured. Molybdate
adsorption by soils was found to conform to the Freundlich adsorption
isotherm in all instances. Four of the eight soils showed sorption
patterns which followed the Langmuir adsorption isotherm at low
The increase in solution hydroxyl concentration measured two
minutes after mixing soil with 1 N NaF correlated significantly with
molybdate adsorption at an equilibrium concentration of 0.03 ppm Mo.
The fluoride ions may be reacting directly or indirectly with soil
constituents responsible for molybdate retention.
Iron extracted in ammonium acetate, pH 4.8, and ammonium
oxalate, pH 3.3, correlated significantly with molybdate sorbed at an
equilibrium concentration of 0.03 ppm, and with hydroxyls measured
by the above sodium fluoride technique. Neither citrate-dithionite
extractable iron nor aluminum in ammonium acetate and ammonium
oxalate extracts were significantly correlated with molybdate sorption.
Oxalate-extractable aluminum was significantly related to hydroxyls
released with sodium fluoride.
No correlation was found between laboratory measurements of
molybdate retention by soils and the increases in clover molybdenum
following molybdenum fertilization. It is suggested that several
parameters not investigated in the present study such as soil moisture
content, phosphate and sulfate levels, and the soil organic regime may affect rates of molybdate uptake into and translocation through the
plant. Results indicate that it is difficult to predict the effect of
molybdenum application on plant uptake of molybdenum in the field.
A larger number of samples representing a vast range of soils
appears desirable to reliably predict residual effects from applied