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Agricultural Research & Education Center
IFAS, University of Florida
Bradenton AREC Research Report GC1981-14. November 1981
ARTIFICIAL SOIL H1IXES1
Alexander A. Csizinszky
Growth media are substrates in which seeds are germinated and/or plants
are grown. Artificial soil mixes are media made up by mixing natural or artificial
ingredients in ratios which are not usually found in nature. Artificial soil
mixes are sometimes called soilless media (8, 12).
The importance of growth media for vegetable transplant producers. The
producers of vegetable transplants probably devote more time and effort to
prepare growing media and to maintain proper moisture and fertilizer levels
for plant growth than other operations. In Florida the cost of growing media
amounted to 17% of the total production costs for the containerized vegetable
transplant industry and was the second highest item after labor (9). For
transplants, the media provide the mechanical base for anchorage and support
and serve as a source of supply for water and nutrients and permit the diffusion
of oxygen to plant roots. There are several premixed media on the market for
immediate use, or the ingredients are available to make up the media according
to individual requirements and ideas (8, 10, 12).
I. Desirable characteristics of growth media.
Whether a commercially prepared mix is purchased or a mix is formulated
by the grower from various ingredients, the media should have the following
1. Free from harmful soil pests: pathogens, insects, nematodes and
2. Free from pesticide residues, especially herbicides.
3. Provide good aeration and water drainage.
4. Adequate exchange capacity and good water retention to retain
and supply nutrients necessary for plant growth.
5. Low total soluble salt (TSS) content and should not contain
excessive amounts of any one nutrient.
6. Lightweight and have a relatively constant volume, wet or dry.
7. Available in uniform quality at low cost.
8. Chemically uniform, relatively inert and should not deteriorate
9. Provide good mechanical support to hold plants in position.
There are physical and chemical considerations when selecting an artificial
mix or the ingredients to make up the mix. The physical considerations deal
with the maintenance of adequate water and air for plant growth and enough
Presented at the Mid-Florida Bedding Plant Short Course, Orlando, FL Aug. 25, 1981
absorption sites for retaining nutrients in the medium. Chemical considerations
deal with the proper concentration and balance of essential nutrients.
II. Physical properties of soil mix components. The most important physical
properties of the ingredients for the preparation of artificial soil mixes are
the bulk density (dry and wet), water holding capacity (IJHC), also called
water retention capacity, and cation exchange capacity (CEC), and particle size.
A Bulk density refers to the unit weight of the medium and is expressed as
lb/ft gm/cm or kg/m (Table 1). It is important for the ease of handling,
ability of the medium to hold the plant upright, provision of aeration, rate of
water infiltration and workability of the mix.
In general, the more small mineral particles (e.g. clay) the greater the bulk
density, higher the nutrient and water retention capacity of the medium. Porosity
of particles contribute to air space (as with vermiculite).
B. Uater holding capacity (UHC) or water retention capacity determines the
amount of water held by the mix after free drainage has occurred. tHC influences
the frequency of watering and the amount of water needed at each watering (1, 2,
12). HHC is expressed as percent dry weight or percent volume of the mix. A
HHC of 20-50% by volume or 40-100% by weight is satisfactory for vegetable
seedlings (others like a 30-60% WHC by volume). A procedure is descirbed in
detail on page 9 for the determination of airspace and water holding capacity in
soils and in mixes.
C. Cation exchange capacity (CEC), refers to the ability of soil particles
to attract and retain nutrients against leaching effects of water and release
them for plant growth. Cation exchange capacity is measured by the number of
units held by a given quantity of soil and is expressed in milliequivalent per
100 g (meg/100 g) of mix. A range of 10-40 meg/100 g on dry weight, or 20-50
meg/100 cc is satisfactory for good plant growth. Anions, which have a negative
charge (e.g. nitrate, NO3) are not attracted or retained to a significant degree
on the soil particles.
III. Chemical Properties
The most important chemical properties of the artificial soil mixes and
ingredients are: the pH, total soluble salt content and the amount of the various
plant nutrients present in the media.
A. The desirable pH range (or acidity and alkalinity) for vegetable transplant
production is from 6.0 to 6.8. The pH of medium can be measured by a pH meter
or checked by indicators. Soil mix pH may be adjusted to the desired level by the
addition of liming materials, e.g. dolomite (CaCO3 and g1qC03), limestone (CaC03),
hydrated lime (Ca(OH)2), burned lime (CaO) and gypsum (CaSO ) (5, 6, 10). Liming
materials differ in their capacity to change soil pH. FurtHermore the rate of
reaction at which the soil pH is changed depends on the particle size of the
For example: 100 lb of limestone has the same effect in changing soil pH as
86 lb of dolomite, or 82 lb of hydrated lime or 64 lb of burned lime (6). In
small scale experiments (13), regular dolomite, at an equivalent rate of 3000 Ib/A,
increased soil pH from the initial pH 4.2 to 5.G in 1 day and to pl 6.5 in 8 days.
Medium size (60 to 100 mesh) dolomite increased pH from 4.2 to 4.7 in 1 day to
pH 5.0 in 8 days. Very fine (200 mesh) dolomite increased pH from 4.2 to 6.4
in 1 day and to pH 7.0 in 8 days.
The proper p1l of the media is important for the availability of plant
nutrients. A low pH (5.5) in organic soil mixes, potassium, calcium, copper
and molybdenum are less available than at higher pH's. In a low oH (high acid)
mix, the solubility of aluminum and fluoride are increases. Both of these
elements are toxic to plants. At high pH (above pH 7.2) in organic mixes
the availability of phosphorous, boron, manganese and, to a lesser degree,
magnesium and copper are reduced.
In mineral soil types (sand) and soil mixes, low pH affects the availability
of phosphorous, calcium, magnesium and molybdenum. At pH 7.2 as above, manganese
and iron are less available.
When selecting a fertilizer it is important to remember that ammonical
fertilizers (except ammonium nitrate lime) and urea have an acidifying effect
on soil pH, while calcium nitrate and potsssium chloride, superphosphate and
calcium sulphate (gypsum) are neutral fertilizer materials (6).
B. Total Soluble Salt (TSS) Content.
The effect of salts in the soil and in the soil water is that with increasing
salt concentration, the water is less available for the seeds and to the plant
roots. Availability of water is especially critical under hot, dry conditions.
For example, a soil containing 1000 ppm salt at 40% moisture content will contain
2000 ppm salt at 20% soil moisture. High salt content in the soil mixes can
also be toxic to the plants. Excess salt in soil mixes may be the result of too
much fertilizer application or the excessive amount of one or more ingredients in
the fertilizer mix. Therefore, both the intensity and balance (I & B) of the
plant nutrients in the soil mix and in the ingredients from which the soil mix is
made up, have to be measured to prevent or to rectify salt problems (10).
Soil salt content is determined by measuring the electrical conductivity
(EC) of the soil saturated extract on a Solu-Bridqe at 250C. The Solu-Bridge
is calibrated to read specific conductance of the soil solution from 0.1 to
10 mho x 10-3, or from 13 to 1000 mhos x 10" for the older models. To convert
the ECe 10-3 into ppm TSS, the reading is multiplied by 700 and by a moisture
factor. The moisture factor used is 2.0 for sand and sandy soils, 1.2 for
organic soils and soil mixes, 1.5 for 1/2 sand and 1/2 peat mixes and 1.0
for water. If the Solu-Bridge is calibrated for ECe 10"5 mhos, then the reading
is multiplied by 7 and by the appropriate moisture factor.
In general, TSS content fo up to 1400 ppm can be tolerated by most of the
vegetable seedlings, provided the ions in the mix are balanced. Below 450 ppm
TSS level is considered to be low in nutrients for a 1/2 sand, 1/2 peat mix.
The TSS content of irrigation water is also very important. The permissible
range for water salinity is from 525 to 1400 ppm.
Irrigation water, with a TSS concentration higher that 1400 ppm will cause
salinity problems in the growth medium, The symptoms of high soluble salt levels
in the medium are: marginal leaf burn, wilting, stunting, yellowing of new
growth and root burn.
How to avoid high salt problems?
1. Check ingredients and/or soil mix and irrigation water before planting
seeds and seedlings.
2. Avoid the use of chloride (Cl), sodium (Na), and sulfate (S04) containing
fertilizers to reduce initial salt input into the mix.
3. Use good quality water.
4. Provide good draingae to remove salt.
5. Use a lightweight mix.
6. Do not let the growth medium dry out.
Frequent, light irrigations in which no water passes through the medium,
favor soluble salt accumulation from fertilizer and well water. Heavy watering
at least once per two weeks will remove excess salts or reduces the probability
of salt build up.
Nutrient content or artificial soil mixes.
host artificial soil mixes contain various amounts of fertilizers and are
ready to use. Growers who decide to make up soil mixes from ingredients have
to add the essential major and minor (macro and micro) nutrients to the mix.
The ingredients for 3 commonly used mixes are listed on Table 2.
The basis of many nutrient formulae are the nutrient solutions developed by
Arnon and Hoagland in California (Table 3). The nutrients for the solutions may
be obtained from commercially available chemicals. To prevent the build up of
salts in the growth medium, Hoagland solutions are applied at 1/2 or 1/4 strength.
lost of the ready mix soilless type growth media and ingredients, e.g. vermi-
culite and perlite, are thought to be free of plant pests. It is also believed
that peat originating from the northern bogs is relatively disease free. Soil
mixes containing sand and local peat, however, need to be treated to reduce or
eliminate plant diseases. (7)
Soil mixes may be treated by heat or by chemicals. Each method has its
advantages and disadvantages. Heat treatment by steam or dry heat requires
generating equipment and covering materials. Chemical treatment requires injectors
or drenching equipment and covering materials to prevent the escape of toxic fumes.
Steam heat at 180-2120F is usually applied for 1 hr or longer, and will
kill insects, nematodes and plant pathogens in the vegetative stage. It is also
effective against weeds. The soil mix has to be moistened to field capacity
before steaming and may be used for planting as soon as it has cooled.
Among the chemicals, methyle bromide-chloropicrin combinations have a
broad spectrum against plant pests. The commercially available IC-33 (66% methyl
bromide and 33% chloropicrin) at a rate of 2-3 lb/100 cu feet of mix has to be
applied for 24-48 hrs at above 50"F. The soil moisture has to be slightly below
field capacity and the medium to be treated should not be deeper than 14 inches.
After treatment the medium should be aerated from 14 to 16 days before using
1. Botacchi, A. C. 1980. Soil aeration don't guess. Connecticut
Grower Newsletter 101:1-3. Sept. 1980.
2. Bruce, R. R., J. E. Pallas, Jr., L. A. Harper, and J. B. Jones. 1980.
Water and nutrient element regulation prescription in nonsoil media for
greenhouse crop production. Commun. in Soil Science and Plant Analysis
3. Conover, C. A., and R. T. Poble. 1977. Characteristics of selected peats.
Florida Foliage Grower 14(7). July 1977.
4. Hohlt, H. E. and F. D. Schales. 1979. Vegetable transplants. The Vege-
table Growers News 36(6).
5. Keisling, T. C. and J. A. Lipe. 1981. Comparison of post-plant liming
methods for quick response in potting media. Commun. in Soil Science
and Plant Analysis 12:453-460.
6. Lorenz, 0. A. and D. N. Maynard. 1980. Knott's Handbook for vegetable
growers. 2nd en. John Wiley and Sons, New York. 390 pp.
7. Marlowe, G. A., Jr. 1976. Vegetable transplant production. Vegetable Crops
Extension Report VC1-1976.
8. Mastalerz, J. W. 1977. The greenhouse environment. John Wiley & Sons,
New York. 629 pp.
9. Miller, M. N. and C. N. Smith. 1980. The containerized vegetable trans-
plant industry. Economic Information Report #129. Food and Resource
Economics Dept., Univ. of Fla., IFAS.
10. The U. C. system for producing healthy container-grown plants, K. F. Baker
(ed.). Calif. Agr. Exp. Sta. Ext. Serv. Manual #23. Berkeley, CA.
11. Waters, U. E., J. NeSmith, C. N. Geraldson and S. S. Woltz. 1972. The
interpretation of soluble salt tests and soil analysis by different
procedures. Florida Flower Grower 9(6), June 1972.
12. White, J. W. 1966. Growing media, watering and fertilization. In
J. W. Mastalerz (ed.). Bedding plants. Pennsylvania State University
13. Woltz, S. S. 1976. Speed of reaction of finely ground dolomite limestone.
Bradenton AREC Research Report GC1976-14.
Method for determination of Air Space and !ater Holding Capacity in
Measuring cup or measuring cylinder, masking tane, marker per, bucket,
the container in which the plants will be grown, the medium to be tested and
1. Measure the volume of the pot. Tape the holes outside at the bottom of
the pot. Fill the pot with water to the expected soil line. Lark this
line with the pen. Pour the water from the pot to the measuring cup and
count the number of cups of water held by the pot. This is the total
volume of the pot.
2. Dry the inside of the pot, but do not remove the tape. Fill the pot with
the soil mix and pack it as it would be done when potting a plant.
3. Using the measuring cup, wet the medium. Count the number of cups of
(including fractions) water it takes to saturate the medium, i.e. until
a thin film of free water appears at the soil line. The total amount of
water added is the total porosity, the pore space of the medium which can
be occupied by water or air.
Oz. of water required to saturate the medium 100
% of Porosity = Total volume of the pot in oz.
4. After the medium is saturated, remove the tape from the holes while
holding the pot over the bucket. Allow the pot to drain until no more
water comes out. Measure the amount of water collected in the bucket.
This amount of drained water is equivalent to the air space in the
% Air space = Oz. of drained water x 100
Total volume of the pot in oz.
5. The difference between the amount of water applied to saturate the
medium and the amount of water drained is the water holding capacity of
% of water holding capacity = % Porosity % Air space.
TABLE 1. SOME PHYSICAL AND CHEkICAL PROPERTIES OF AMENDMENTS AND ARTIFICIAL SOIL MIXES*
BULK DENSITY WATER RETENTION CAPACITY AFTER EXCHANGE
DRY WET VOLUME WEIGHT POROSITY DRAINAGE CAPACITY
MATERIAL LB/FT % % VOLUME % MEQ/100 g pH
BARK, FIR, FINE
PEAT 1OSS, SPHAGiUM
COMPILEDD FROM REFERENCES 3, u. and 10.
TABLE 2. INGREDIENTS FOR CORNELL PEAT LITE AND U. C. SOIL MIX C
(50% SAND: 50% PEAT MOSS. v/v)*
CORNELL PEAT LITE MIX U. C. MIX
INGREDIENT A B C
SPHAGNUM PEAT MOSS, SHREDDED
VERMICULITE #2, 3 or 4
PERLITE, HORT. GRADE
POTASSIUM NITRATE **
TRACE ELEMENTS (FTE 503)
WETTING AGENT (NONIONIC)
PER CUBIC YARD
11 bu 11 bu
*COMPILED FROM REFERENCE 6, 8, and 10.
**OR CALCIU.t NITRATE, 1 LB. IN OTHER FORMULAS, 5-10-5 ANALYSIS
FERTILIZER IS USED AT A RATE OF 2-16 LB PER CU YD.
TABLE 3. CONCENTRATION OF SELECTED NUTRIENTS (ppm) IN FULL STRENGTH
NUTRIENT HOAGLAND #1 HOAGLAND #2
N, as N03- 210 196
N, as NH4+ 0 14
P, as P04--- 31 31
K+ 234 234
Ca++ 200 160
Mg++ 48 48
S, as S04-- 64 64
8 0.1 0.5
Cu 0.0014 0.02
Fe 0.6 0.6
Mn 0.1 0.5
Mo 0.016 0.01
Zn 0.01 0.05
*FROM: E. J. HEWITT. 1966. SAND AND WATER CULTURE METHODS USED IN THE
STUDY OF PLANT NUTRITION. 2nd ED. COMMONWEALTH AGRICULTURAL
BUREAUX, FARNHAM ROYAL, BUCKS, ENGLAND.