The benefit
of organic matter in soil is well known. Early agricultural systems
relied on recycling plant residues and composting to increase soil
fertility. As the world demand for food increases, practices that
lead to increased yields on existing farm lands will continue to be
important. Humates are reported to increase crop yields and reduce
drought and high temperature stress. However, these compounds have
not been researched extensively.
Humates occur in three chemical forms: Humic acids, fulvic acids,
and humins. Humic acid is soluble in dilute alkaline solutions and
is precipitated by acidification. Fulvic acid is the fraction that
remains soluble after acidification, and cannot be precipitated. Humins
are insoluble in dilute acid or base. Commercial humate products are
either mined or extracted from naturally occurring sources. Extracted
humates are derived from peat moss, kelp, and seaweed, from shale
and oxidized coal (leonardite) deposits in Texas, Wyoming, New Mexico,
North Dakota, Idaho, and Florida. The humic acid contents of these
deposits usually range from 30% to 60%. Hydroponic experiments have
shown that when equal amounts of essential nutrients are supplied
from humates or traditional sources, tomato (Lycopersicon esculentum
Mill. ‘Mountain Pride’) seedlings grown with the nutrients
supplied by humates out perform those receiving the same nutrient
levels supplied by traditional sources. The beneficial effects of
humates in foliage plants could not be soley attributed to their fertilizer
value. Humates can have beneficial effects on plant growth by stimulating
the plant to absorb greater quantities of nutrients and by inducing
a more efficient use of the absorbed nutrients. Hydroponic experiments
have shown that low concentrations of humic acids had a positive effect
on nitrate and ammonium uptake in olive, potatoes (Solanum tuberosum
L.) and teak (Tectona grandis L.f.) seedlings.
This experiment was designed to determine if leonardite applications
had an effect on plant growth and if there was a residual effect after
multiple leonardite treatments. Three planting sequences were established
and leonardite applied at 0, 50, 100, 200 and 400 lb/acre (0, 56.1,
112.1, 224.3 and 445.6 kg-ha-1). Subplots were treated at the first,
the first and second, or all at three planting sequences. ‘Purple
Top White Globe’ turnip (Brassica rapa L.) and ‘Florida
Broadleaf’ mustard greens (Brassica hirta L.) were used as the
indicator crops in the first two and last sequences, respectively.
Results from first harvest were statistically analyzed using analysis
of variance. No statistically significant differences (p <0.05)
in fresh weights of roots or leaves, soluble solids, percent dry weight
or size distribution at any leaonardite rate. Results were similar
for the second harvest. Additionally, no significant interactions
occurred among the number of applications and leonardite treatments
in fresh weight of leaves or roots, soluble solids, percent dry weight,
or size distribution. No statistically significant differences or
interactions due to treatments were observed in the third sequence.
Of nine soil nutrients analyzed, differences occurred only in iron
and potassium. Iron levels in the soil were higher with two or three
applications than one application alone. Soil K levels increased with
all treatments over the control.
Observed differences can be explained with factors other than a beneficial
(or detrimental) effect of leonardite.
In the first planting sequence, there were no statistical differences
of any observed responses. During the second sequence planting, a
total of 13.5 inches (34 cm) of rainfall was received. Of this amount,
6.5 inches (16.5 cm) fell the twentieth day after sowing. Consequently,
a significant amount of soil eroded. This also caused a nitrogen deficiency
in the crop. This event probably confounded data for the rest of the
experiment. Due to the nature of the experiment it was necessary to
keep the plots in the same location. No trends were readily apparent
in the data obtained from this planting sequence.
Detectable amounts of humic acid were not found in the soil after
the experiment was concluded. This is probably due to the light, sandy
soil, small amounts of leanardite applied, and excessive rainfall
during the second sequence planting of the experiment. The soil organic
matter content in the study site was less than one percent. At the
highest rate only 672 ppm of Leonardite was added and of that only
79% was humic acid.
Differences in soil K levels among treatments are difficult to explain.
The K concentration in the leonardite humate was 20 ppm. Increases
in K in treatment plots over the control do not follow the incremental
increases that would be expected with increasing amounts of K in leonardite
applied. Humic acids contribute to the breakdown of soil minerals.
This breakdown may have released some K to measurable forms. The high
CEC of humic acid found in leonardite might have also held more of
the additional K that was supplied to the crop in the form of fertilizer.
Soil Fe increased with the number of applications. The leonardite
had an Fe content of 296.9 ppm. As more iron is added, an increase
in the soil concentration is expected. It may also be associated with
soil microclimate being affected by the lower 4.2 pH of the leonardite.
While there is not enough leonardite present to affect the overall
soil pH, it may increase the amount of iron2+ present in localized
areas of the soil. Humic acid in the soil may also chelate soil Fe.
Soil humic acid can also hold iron in exchangeable or complex forms.
In conclusion, no beneficial plant growth effects from number of applications
or rates of leonardite were found under the conditions outlined in
this study.
