Citrus Station Mimeo Report CES 66-10
March 18, 1966
Bacterial Metabolites Can Cause Problems in Tile Drained Wet Land Citrus Groves
Harry W. Ford
University of Florida
Citrus Experiment Station
It should be mentioned that the bacterial metabolites and associated products
discussed in this report have also been found associated with open ditches.
'Tile lines used for draining citrus groves are installed at depths of 4 or
more feet below the surface. This is usually several feet deeper than the level
of the old water table prior to drainage. Thus, water may flow from the lines
for extended periods during the year. Tile lines laid at such depths in flat-
woodsareas are subjected to problems not encountered with the shallow drainage
systems installed for vegetables and pastures.
In 1959, numerous tile lines were found with considerable amounts of red,
hydrophilic, filamentous, iron oxide sludge at the tile outlets. The sludge was
formed by iron bacteria. They oxidized the ferrous iron found in the ground water
that entered the lines. Lines with the most sludge had been laid with fiberglass
sheets as the filtering material. The fiberglass was clogged with a red deposit
so that certain tile lines ceased to function within 6 months. It was assumed
that the red deposit on the fiberglass was the same iron oxide sludge that re-
duced the internal capacity of certain tile lines by as much as 50%. Tile lines
laid in sawdust contained considerable amounts of iron oxide sludge but not to
the extent found in fiberglass lines.
In 1961, several tile lines were laid with slag gravel, a by-product of the
phosphate industry as the filtering material. Slag had good filtering charac-
teristics. Experiments by Ford and Spencer in 1962 indicated that alkaline fil-
tering materials, particularly slag gravel, did an excellent job of eliminating
the iron oxide problem that was plaguing tile lines by preventing ferrous iron
from entering the lines. Slag buffered the water at pH 8.8 which precipitated
the iron as ferrous hydroxide. It was calculated that the slag should last for
10 to 15 years because of the very low exchange capacity in the subsoils of most
sands that would be subjected to drainage. No other detrimental factors were
known to exist. The experiments also indicated that the iron problem could be
minimized by self-cleaning grades and larger holes in the fiber pipes that were
used for this study.
Slag is a material with extensive surface and consists predominantly of calcium
and silica. The surface is excellent for the growth of bacteria so that slag has
been used extensively in sewage disposal plants as trickle filters. In sewage
plants, the bacterial decomposition reactions are aerobic. Coatings of bacteria
grow on the slag and decompose the liquid raw sewage which contains organic acids.
The slag does not disintegrate. We now know that the environment around a tile
line is practically anaerobic when water is flowing. There are no heavy coatings
of bacterial slimes to prevent destruction of slag by weak acid electrolytes.
From a chemical standpoint, any weak acid electrolyte would gradually increase
the rate of slag disintergration by neutralizing the calcium.
In December 1965, several tile lines were found in which the slag filter
appeared to be disintegrating. The slag residue had various colors of white,
black, and red and resembled a powder when rubbed between the fingers. Laboratory
studies and field observations have now shown that slag has been too efficient as
a tile filter. In addition to precipitating iron, it has also been trapping
hydrogen sulfide, a metabolite of the sulfate-reducing bacteria. The sulfate-re-
ducing bacteria generate sulfides in an anaerobic environment by utilizing sulfates
applied in fertilizers and irrigation waters together with certain organic acids
found in citrus, grass, and palmetto roots.
The problems with tile line filters involve a complex interrelationship be-
tween iron and hydrogen sulfide. U Mer aerobic well-drained conditions, iron in
the soil is found in the ferric (Fe ) form. Ferric iron is insoluble in water.
Also, hydrogen sulfide cannot be formed in a well-drained soil. Under flooded
conditions, when oxygen in the air spaces of the soil have been eliminated, numer-
ous types of bacteria begin to develop with the changing environment. Each
species seems to have its own range of ideal conditions so that during this tran-
sition period, any remaining oxygen in the water will be used up rapidly by
various groups of bacteria. The rate at which oxygen disappears is enhanced if a
source of energy such as fresh organic matter is available for th bacteria. As
the oxygen disappears, ferric iron is changed to ferrous iron (Fe ). Ferrous
iron is caused partly by anaerobic iron bacteria and also by strictly chemical
means due to the reducing conditions. Ferrous iron in acid ground water is
soluble and will flow toward the tile lines. It is converted to ferrous hydroxide
due to the high pH when water passes through a slag filter. This is the method by
which slag controlled the iron oxide sludge problem. Ferrous hydroxide is an
insoluble white powder, occasionally it is green in color.
Anaerobiosis is also responsible for other changes besides iron. The sulfate-
reducing bacteria are able to multiply and under suitable conditions produce
sulfides. In acid subsoils, most characteristic of Florida fine sands, sulfides
exist as undissociated hydrogen sulfide. Hydrogen sulfide is soluble in anaerobic
ground water and will flow toward tile lines along with the soluble ferrous iron.
When the concentrations of iron and hydrogen sulfide reach about 1 ppm, a con-
dition that would occur near tile lines, some of the iron is precipitated as black
insoluble iron sulfide. The black precipitate is first visible as a layer sur-
rounding the bottom half of the filter profile. Iron sulfide in large enough
quantities can reduce the rate at which water can enter the tile line. All of the
hydrogen sulfide is not precipitated as iron sulfide. Some of the hydrogen sulfide
being a weak acid electrolyte, reacts with the calcium in the slag forming calcium
sulfide. Calcium sulfide is partially 'soluble and will flow fnto the
tile line. Aluminum in the slag will not react with hydrogen sulfide because
aluminum sulfide is unstable in water. Thus, most of the aluminum remains in the
disintegrating slag as a white aluminum silicate. As calcium is lost, the buffer-
ing capacity of slag diminishes so that the pH of the discharged water will drop
to values below 7.3. When this happens, any ferrous hydroxide precipitated on
the slag will become soluble and flow into the discharged water of the tile line.
Oxygen is introduced into the system when the water table drops to the
level of the tile. Under these conditions, iron sulfide that has been gradually
building up on the slag gravel, is oxidized to red insoluble ferric hydroxide.
To the naked eye,:.ferric hydroxide looks exactly like iron oxide sludge. How-
ever, the difference is readily apparent under the microscope. The other product
formed when iron sulfide is oxidized in ground water, is hydrogen sulfide which
can continue to react with the calcium in the slag. Ferric hydroxide can be
converted back to iron sulfide by the addition of more sulfide ions which occurs
when the soil water again becomes anaerobic after flooding. Thus, hydrogen
sulfide is the principal weak electrolyte that hastens the destruction of slag
used as tile filters under citrus groves and is also responsible for sulfur slime
found in tile lines. The slime is caused by sulfur bacteria oxidizing hydrogen
sulfide to elemental sulfur.
Disintegrated slag is white in color and consists of a mixture of ferrous
hydroxide and aluminum silicate. Red colors are ferric hydroxide. Black residues
are iron sulfide.
Problems with iron sulfide are not confined to slag gravel. Sawdust lines
have been found to be surrounded with iron sulfide which is probably reducing the
rate of infiltration. Sawdust has also been disintegrating rather rapidly in some
areas except when treated with copper. Sawdust is also a source of food for the
sulfate-reducing bacteria, and many of the tile lines are generating large amounts
of hydrogen sulfide. There has been considerable quantities of sulfur slime and
a significant amount of iron oxide sludge.
Iron sulfide, ferric hydroxide, ferrous hydroxide, iron oxide sludge, and
sulfur slime have been observed in the surface of seepage zone in ditch banks.
It is conceivable that these compounds may have an adverse effect upon hydraulic
Most tile lines have been "blinded" with organic matter from the top soil.
This practice has supplied excellent sources of food and raw materials for the
sulfate-reducing bacteria. Certain soil types such as Scranton, Blanton, and
Ona contain a considerable amount of organic matter in the profile. The iron
sulfide problem has been particularly severe with these soil types. We have found
that iron sulfide accumulates readily in the organic matter placed around the
slag area. Iron sulfide seems to be fixed on the organic matter and does not
oxidize readily when exposed to air.
Studies are in progress to change the type of filtering materials. The new
filters permit iron and sulfides to enter the tile lines where their undesirable
products will be controlled with prophylactic treatments.
Most of the drainage installations showing symptoms of slag disintegration
have been installed for at least 3 years. Changes in the slag may not necessarily
render the tile line less efficient as a means of draining the citrus grove.
The lines may continue to flow for many years before the accumulated iron sulfide
restricts the flow of water. Soil type, soil pH, the amount of organic matter in
the backfill, the presence of an organic pan, and the amount of water normally
flowing from the tile, are factors that have a bearing upon the rate at which the
slag filter will be neutralized. It is worthwhile for the grower to evaluate his
own system,whether it is open ditches.or tile with slag or sawdust filters. Water
table observation wells should be installed midway between tile lines and open
ditches. The well can be made of old 4-inch irrigation pipe, clay tile set on end
vertically, or fiber pipe. There should be some holes spaced at 6-inch intervals
in the zone of the water table. Gravel or sawdust should be placed around the
pipe to prevent sand from entering. The filtering material will also minimize
erroneous readings from subsoil pressures. The well should be installed to a
depth of 3 to 4 feet. The height of the water table should be noted during
periods of heavy rains. The rate at which the water table recedes should be
recorded 24and 48 hours after a rainy period. The values obtained will be an
indication of the rate cfdraw..down for the area in which the water table well has
been placed. For the present, we believe that a draw down of .5 foot or more per
day is an indication of adequate drainage. Rates of .2 foot per day are probably
not sufficient to prevent root damage under severe flooding.
Low readings for water table draw down may be a reflection of inadequate
drainage for reasons other than reduced hydraulic conductivity or the accumulation
of iron sulfide. The grower must recognize that his drainage system may not have
been adequate from the standpoint of depth and spacing when it was installed.
Florida Citrus Experiment Station
Lake Alfred, Florida