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Bradenton GCREC Research Report B'R4-193-21 .
Institute of Food and Agricultural Sciences September 1993 L i y
ROOT MEDIUM COMPONENTS AND FERTILIZER EFFECTS ON pH
Brent K. Harbaugh' f Fl';-r!
Both root medium components and fertilizers can significantly affect pH and,
consequently, your plants' performance. Root medium components include organic
and inorganic substrates, liming or acidifying amendments, and fertilizers
incorporated at mixing. Choice of these ingredients should include consideration
of pH requirements of the crop to be grown, additional soluble fertilizers to be
utilized during the crop cycle, and the quality of irrigation water. All these
factors interact to give an immediate or short term response to the root medium
pH level, and will also affect the stabilizing and long term response to the root
medium pH level.
Organic and inorganic substrates used in preparing root media will probably have
the greatest influence on root medium pH and the need for additional amendments.
The most commonly used organic substrates, sphagnum peat moss and pine bark, are
acidic. As municipalities and industries strive to compost their wastes, we find
an array of composted materials being thrust at the horticulture industry for use
as components in root media. Although the pHs of these composts are as varied
as the source of the compost, most have acidic properties. Vermiculite and some
hardwood barks will provide an alkaline source for root medium substrates, while
perlite and sand are neutral.
Substrate components affect not only root medium solution pH, but also the
buffering capacity of the root medium. The cation exchange capacity (CEC) is a
good measure of the buffering capacity of root media. In general, the higher the
cation exchange capacity, the more stable the root medium pH will be. Cation
exchange capacity levels between 6 and 15, or even higher, are necessary for good
buffering capacity. Peat moss, vermiculite, clay and many composts are all
important for their stabilizing effect on root medium pH.
Liming and acidifying amendments are used to modify root media after substrate
components are selected. The most common materials used to raise root medium pH
are limestone (CaC03) and dolomitic limestone (CaCO3-MgCO3). Dolomitic limestone
is often preferred as it supplies a source of magnesium (Mg) as well as calcium
(Ca) in the process of adjusting pH. Marl and different shells (oyster, etc.)
are also used as calcium carbonate (CaCO3) liming materials but these are more
commonly used more for adjusting field soils.
The effectiveness of the limestone is determined by its purity, how well it is
mixed with the root medium, the cation exchange capacity and moisture of the root
medium, and how finely the limestone is ground. The finer the limestone, the
faster the reaction will be and the resulting change in pH. All the limestone
should pass through a 60-mesh screen, and at least 50% of the limestone should
pass through a 100-mesh screen for use in potting root media.
Other liming materials include hydrated lime (Ca(OH)2) and burned lime (CaO).
These materials are very reactive providing an immediate change in root medium
solution pH, but may not provide season-long pH control. Sometimes a combination
of hydrated and calcitic or dolomitic limestone is used to raise the pH quickly
and yet have residual pH control for the entire growing season.
Iron sulfate, aluminum sulfate and sulfur are used to lower root medium pH. Iron
and aluminum sulfate cause-a quick change in pH but the effects may not persist
for long term crops. Elemental sulfur reacts relatively slowly (3 to 6 weeks)
but provides for long term control. Again, a combination of iron or aluminum
sulfate and elemental sulfur may be desirable for a root medium if a quick as
well as a long term response is needed.
It is important to mix root media far enough in advance of their use to insure
liming or acidifying materials have time to react. Since so many factors are
involved in determining root medium pH, it is impossible to give absolute rates
of liming or acidifying amendments to achieve a desired starting pH for all root
media. In addition, each grower's fertilizer program and water quality will
drive pH up or down during the season and has to be considered. As a starting
place, 2.5 to 3 pounds of calcitic or dolomitic limestone per cubic yard of a
peat-based root medium generally raises the pH about 0.5 unit. Sulfur at 0.25
to 0.5 pounds or iron sulfate at 2.5 to 3 pounds per cubic yard of root medium
has been shown to lower root medium pH about 0.5 unit.
The key for successful pH adjustment for the myriad of possible root media is to
prepare small lots of medium with various rates of amendments. After an
incubation period, the root medium pH can be determined and appropriate rates of
amendments chosen. This can be done by placing samples of root media in pots,
placing pots in a saucers, and adding water from the bottom until no more water
is absorbed after an hour. This insures all components are wetted, such as the
peat moss, and the root medium is at container capacity. A sample of root medium
is then placed in a sealed plastic bag at room temperature (70-80 oF) for two
weeks. A pH measurement is then taken. Two weeks allows for the liming
materials to react and stabilize for an accurate reading. A longer period may
be necessary if sulfur is used.
Fertilizer salts affect root medium pH in several ways. When fertilizer salts
are dissolved, they become chemically acidic, basic (alkaline), or neutral. For
example, calcium nitrate is considered chemically neutral when dissolved in water
as Ca2+ and 2NO3 ions are formed with charges that balance each other. Other
fertilizers are chemically acidic or basic in solution and thus have an immediate
effect on the root medium solution pH. However, when plant roots absorb the
different nutrient ions, root medium pH is changed again. For example, when the
nitrate from calcium nitrate is absorbed, the roots excrete OH' into the root
medium to maintain the charge balance. This results in an increase in pH. Thus,
chemically neutral calcium nitrate is considered physiologically basic (alkaline)
in a root medium. Other changes occur in the soil due to microbes. For example,
microbes may nitrify ammonium ions (NH4 basic) releasing hydrogen ions (H
Fortunately, one does not have to be a chemist to determine the effect of
fertilizers on root medium pH. Fertilizer labels should indicate the "potential"
acidity or basicity of the fertilizer. Potential acidity is expressed as the
calcium carbonate equivalent to neutralize a ton of fertilizer. Most common
fertilizers used in the bedding plant industry are acidic due to the use of urea
or ammonium compounds. For example, the potential acidity of a 20-20-20
commercial fertilizer was listed as 550. This means that the use of a ton of
this fertilizer would take the equivalent of 550 pounds of calcium carbonate to
neutralize the acidity produced. The higher the potential acidity, the more
acidifying the fertilizer will be.
Potassium nitrate, sodium nitrate and calcium nitrate are the three most common
fertilizers that have a basic reaction in the root medium. Basic fertilizers
should be labeled with a calcium carbonate equivalent. For example, the calcium
carbonate equivalent of calcium nitrate is 400. If a ton of calcium nitrate was
used to fertilize a crop, it would have a basic reaction in the root medium
equivalent to applying 400 pounds of calcium carbonate. Superphosphate,
potassium chloride and potassium sulfate are considered neutral fertilizers.
Fertility programs are generally chosen to prevent changes in root medium pH
during the crop cycle. They can also be used to change root medium pH once the
crop is planted. However the changes are usually small (in the range of 0.5 pH
unit) and occur over a period of many weeks. If immediate changes in pH are
required, or a change in pH of greater than 0.5 unit, then hydrated lime or iron
sulfate solutions may have to be used. Use of both materials require precautions
To raise root medium pH, growers have successfully mixed 1 pound of hydrated lime
to 100 gallons of water, the mixture is allowed to settle, and the clear solution
applied to the root medium. When applied over plant material, the plants had to
be misted to rinse off the hydrated lime solution to prevent any damage. Some
plants may be sensitive so test a small area or a few plants before treating a
large area. The solution is corrosive so it shouldn't come in contact with metal
or skin. The hydrated lime may also cause the conversion of ammonium into
ammonia gas, or displace ammonium from exchange sites into the soil solution
causing injury to roots. If high levels of ammonium or other salts are present
in the root medium, it would not be wise to use a hydrated lime solution.
To lower the pH of root media during the crop cycle, growers have used 1 to 2.5
pounds of iron sulfate dissolved in 100 gallons of water. Iron sulfate will
burn most tissue so it should be applied to the root medium only. As above, be
sure to test a small area before attempting to treat you entire greenhouse or
planting. Recheck the pH in a week and reapply if necessary. Iron sulfate will
raise soluble salt levels in the root medium and may release toxic levels of
minor elements from exchange sites or from compounds which are insoluble at
higher root medium pHs.
Of course these adjusting methods are not the ideal way to handle pH.
Consideration of the root medium substrate, medium amendments, fertilization
program and irrigation water quality in advance of planting should prevent the
necessity for these corrections. The importance of testing all root media well
in advance of use and after an incubation period can not be stressed enough.
This allows for adjustments to be made and stabilization of reactions before the
crop is planted. Once the crop is planted, weekly graphing of root medium pH
will allow the use of acidic or basic fertilizers to fine tune the medium pH if
necessary, and avoid dangers associated with use of hydrated lime and iron