Soluble salts interpretation

Material Information

Soluble salts interpretation
Series Title:
ARC-Apopka research report
Poole, R. T ( Richard Turk )
Agricultural Research Center (Apopka, Fla.)
Place of Publication:
Apopka FL
University of Florida, IFAS, Agricultural Research Center
Publication Date:
Physical Description:
4, 5 p. : ; 28 cm.


Subjects / Keywords:
Foliage plants -- Fertilizers -- Florida ( lcsh )
Soils, Salts in -- Florida ( lcsh )
Soil science ( jstor )
Fertilizers ( jstor )
Soil water ( jstor )
government publication (state, provincial, terriorial, dependent) ( marcgt )
non-fiction ( marcgt )


General Note:
Caption title.
Statement of Responsibility:
Richard T. Poole.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
71126437 ( OCLC )


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not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source

site maintained by the Florida
Cooperative Extension Service.

Copyright 2005, Board of Trustees, University
of Florida



Richard T. Poole
University of Florida, IFAS
Apopka, FL 32703 --)3
ARC-Apopka Research Report RH-81-3

Although fertilizers are formulated many ways, and applied aT varying

frequencies and concentrations by a large number of methods, there is only

one goal to supply the proper amount of fertilizer at the correct time so

that a high quality foliage plant is produced.

Symptoms of severe lack of fertilization are easily discernible. Leaves

will be chlorotic, small, and growth slow even though roots are healthy.

Inadequate nutrition is easily corrected by increasing fertilization rate or


A slight excess of fertilizer does not harm the plant significantly. A

healthy attractive schefflera can be produced with constant fertilization of

200 parts per million (ppm) nitrogen (N). The disadvantages of rates higher

than 200 ppm N are plant health, financial and environmental. Plants have

smaller root systems. The grower is spending money on fertilizer needlessly

and polluting the ground water unnecessarily. The smaller root system is

not always readily apparent to the casual observer. The vegetative above-

ground portion may appear attractive and healthy when excess fertilizer is

applied, but as the amount increases, roots of the plants grow slower and

slower and eventually root hairs shrivel and primary roots turn brown. Dur-

ing the latter stage of root death, vegetative portions manifest symptoms

caused by damaged roots such as leaves becoming small, wilting and necrotic.

Improper placement of the correct rate of fertilizer can also be detri-

mental. Surface applied fertilizer placed next to the stem of a plant can

girdle the stem. Fertilizer applied on top of plants can settle in vases

formed by leaves and produce notching or banded necrosis of young leaves. The

problem is not usually noticed until several weeks after improper application.

Soluble salts are the amount of fertilizer applied to the soil that

dissolves in water. Large amounts of some relatively insoluble (slow release)

fertilizers can be applied to plants without detrimental effects because

soluble salts remain low. The same amount of a highly soluble fertilizer might

kill the plant. When plants are irrigated properly, a film of water surrounds

the potting ingredients supporting the plant. This film contains salts nec-

essary and unnecessary for plant growth. Some of the common necessary salts

are nitrate (NO3), ammonium (NH4), calcium (Ca), sulfate (S04), magnesium (Mg),

phosphate (P04), and potassium (K).

Excess soluble saltsdamage is caused by a water deficiency in the plant.

Because of the high concentrations of salts in solution, the plant root is

unable to extract water from the solution, and the plant manifests the same

symptoms above the soil line as a plant enduring water stress.

Examination of roots is essential to determine soluble salts damage. Roots

damaged by excessive soluble salts will usually be black or dark brown and

root hairs and small feeder roots will be minimal or entirely lacking. The

root system will be small compared to above-ground plant size, or if the root

ball is large, most of the roots will be dead. Root damage caused by soluble

salts is frequently confused with root rot caused by parasitic fungi.

If high soluble salts are a problem, fertilization should be stopped and

the soil mix watered heavily to leach excess soluble salts from the root zone.

Unfortunately, excess water equals poor aeration which can further damage the

weakened roots.

Every foliage producer should have access to an instrument that determines

soluble salts. Although some governmental agencies and commercial laboratories

will determine soluble salts for the grower, a better situation is when the

nursery owns its own instrument for measuring soluble salts. These instru-

ments are referred to as Solubridges or conductivity meters and can be

purchased from suppliers of scientific instruments and some greenhouse supply

firms. These meters determine conductivity of a solution. The higher the

soluble salts, the higher the conductivity and thus, a higher reading on the

instrument. Salts concentration is measured in MHOS, which is a unit of

electrical conductance representing the conductivity of a cubic centimeter

of solution between two electrodes, one centimeter square, one centimeter


Some confusion exists concerning the quantity of MHOS as measured by

the various meters. Readings are given as micromhos, (10-6), or mhos x 10-5

or millimhos (10-3). As an example, a solution with 0.000250 MHOS, depend-

ing upon the meter used will give a reading of 250 micromhos, 25 mhos x 10-5

or .25 millimhos. All meters are actually indicating a solution of 0.00025

MHOS. When comparing soluble salts of your soil mix to a soil mix of another

grower or to a table that indicates minimum optimum and excessive soluble

salts, be certain you are comparing equivalent units.

Another problem that leads to confusion is the method of determining

soluble salts. There are three common methods. Soluble salts can be deter-

mined by the 2:1 dry weight method. Two units by weight of water are mixed

with one unit by weight of dry soil and the conductivity of the solution

determined. Another method utilizes two volumes of water mixed with one

volume of soil. This is probably the most common method. A third method,

called the saturated paste extraction method involves moistening the soil

until the soil surface has a moist glistening appearance. The solution is

then extracted from the soil and conductivity determined. The various

methods each have advantages and disadvantages. Again when comparing readings,

the grower must know the methods of extraction, and must readings

from one extraction method with readings of a different method. Comparisons

of soluble salts readings between two labs are frequently meaningless because

of the variation in methods. A comparison of the 3 methods is found in Table 1.

Several other conditions produce necrotic leaves that appear similar to

soluble salts damage. Excess levels of boron and fluorine will produce

necrotic leaf edges on some plants. An excess or deficiency of water can

produce symptoms similar to soluble saltsdamage. Low temperatures will cause

tip necrosis and browning of leaves. In almost all situations a review of

the environmental condition will reveal the cause of the problem.

Table 2 contains micromhos/cm for lightweight mixes which can be used

as a guide for soluble saltsconcentration.

CAUTION. Soil mix, plant type, environmental conditions and water regime

greatly influence the effect of soluble salts on plant growth. The time soil

sample was taken in relation to fertilization program is also important. Palms

exposed to full sun and watered weekly will suffer damage at a much lower

concentration of soluble salts than a dieffenbachia watered 3 times weekly and

exposed to 1,000 foot candles maximum. Plant grade and soluble saltsconcentra-

tions of some foliage plants are presented in Tables 3 and 4. Examination of

these tables showsthat some plantsPhilodendron, Boston Fern and Dieffenbachia,

respond satisfactorily to a wide range of soluble salts, but Aphelandra and

palm have a comparatively limited range.

Table 1. Conductivity (micromhos/cm) of 2 media supplied with varying
fertilizer levels as determined by 3 extraction methods.

3 Peat/l sand (vol/vol)
Extraction Method
Volume Weight Saturated
grams 20-20-20/liter 2 water/i soil 4 water/i soil paste




2 Peat/I sand/i shavings (vol/vol)
Extraction Method
Volume Weight Saturated
grams 20-20-20/liter 2 water/I soil 4 water/I soil paste
0 85 104 515
5 860 925 3000
10 1580 1850 5230
15 2370 2560 11230

Table 2. Interpretation of Conductivity Meter (micromhos/cm) for light-
weight mixes. This table should be used as a general guide only.
Varying conditions will cause different responses by plants to soluble
salts levels.

Volume Weight Saturated
2 water/l soil 4 water/l soil paste

Low 100 200 500
Med-Low 400 600 1200
Optimum 600 1000 2000
Med-High 1000 1500 2750
High 1500 2500 3500

and soluble salts levels.

Chrysalidocarpus lutescens

Plant Gradel


Soil: 3 peat: 1 sand (vol)
Extraction method : 2 water: 1 soil (vol)

Philodendron scandens oxycardium (ground beds)

Plant Grade Micromhos/cm
1.7 140
3.3 1,000
4.1 1,300
4.8 1,600
4.8 1,700
4.9 1,800

Soil: 4 sand: 1 peat (vol)
Extraction method: Saturated paste

Plants were rated on a scale of 1 to 5 where 1 = poor quality, 3 = salable and
5 = excellent quality plant.

Table 3. Plant growth

Table 4. Plant growth and soluble salts levels.

Plant Gradel

Philodendron scandens oxycardium

Soil: 2 peat: 1 bark: 1 shavings (vol)
Extraction method: 2 water: 1 soil (vol)

Boston Fern Nephrolepis exaltata 'Bostoniensis'

Plant Gradel


Soil: 2 peat: 1 bark: 1 shavings (vol)
Extraction method: 2 water: 1 soil (vol)

Dieffenbachia maculata 'Exotica'

Plant Gradel


Soil: 2 peat: 1 bark: 1 shavings (vol)
Extraction method: 2 water: 1 soil (vol)

Table 4. (continued)

Aphelandra squarrosa

Plant Gradel




Soil: 2 peat: 1 bark: 1 shavings (vol)
Extraction method: 2 water: 1 soil (vol)

Plants were rated on a scale of 1 to 5 where 1 = poor quality, 3 = salable and
5 = excellent quality plant.