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Measuring Soluble Salts and pH with the Pour-Through Method

John Ruter, Nursery Crops Research, The University of Georgia
Mel Garber, Extension Horticulturist, The University of Georgia

Fact Sheet H-93-015. Ornamental Horticulture Facts. The Cooperative Extension Service. The University of Georgia, Athens, GA 30602 U.S.A.

Very few nursery operators regularly monitor the nutritional status of their container-grown plants. Nursery producers have been slow to accept this technology for the following reasons: 1) lack of uniformity in testing procedures, 2) limited experience by nursery operators in the area of applying results, and 3) cost of testing procedures or equipment. Two simple tests that can be performed at the nursery are pH and soluble salts measurements. Reliable instruments are now available in a price range that most any nursery operator can afford.

The University of Georgia Soil Testing and Plant Analysis Lab in Athens uses the saturation extraction method developed at Michigan State University. Another method, which is being used in research trials at the Coastal Plain Experiment Station, has been used in operational situations is the Virginia Tech Extraction Method (VTEM) or pour-through method. The advantage of VTEM over other procedures includes 1) short time period required for sample extraction, (2 soluble salt and pH analysis can be made in the field, 3) no container medium is handled, 4) no specialized equipment is required for sample extraction, and 5) there is no chance of rupturing slow-release fertilizer materials, which could cause false readings. The results of the VTEM method should be compared to previous results before full-scale implementation.

VTEM Pour-Through Procedure

  1. The container to be tested is placed on a pvc ring or other suitable material to elevate the bottom of the container above the collection vessel. The collection vessel should be wide enough to collect all leachate (an 8" saucer works well for one gallon containers). (See Figure 1.)
  2. Distilled water is added to the surface of the container medium such that approximately 50 ml (5 oz) of leachate is accumulated in the collection vessel. Typically, 150 ml (15 oz) of distilled water per one-gallon container is sufficient for most media. For bedding plants, use 5 to 10 ml per cell. With plug trays, try 200 ml (20 oz) per tray. Five minutes will be sufficient time for leachate to drain from the container medium for collection. Uniform media moisture content is important; therefore, all samples should be collected when the medium is near its maximum water-holding capacity (approximately 2 hours after irrigation). A minimum of three containers from each block of plants should be tested.
salts-1 salts-2
salts-3 img

The ranges of pH and soluble salts levels found in Table 1 should be used as guidelines only. Factors to be considered include 1) different species hae different nutritional requirements, 2) stage of crop growth, 3) time of year, 4) fertilization (liquid feed versus controlled release) and irrigation program, 5) growing medium, and 6) other environmental factors. Media should be tested at least every two weeks to determine if adequate nutrient levels are being maintained. Since the soluble salt level gives an indication of the concentration of total salts and not individual elements, nursery operators should have individual nutrient concentrations checked every four to six weeks. A growth medium that tests in the low range will generally not have sufficient levles of nutrients to support good growth. Plants on a constant liquid feed program can be grown at levels in the acceptable range for the VTEM. Soluble salt levels of 3.00 dS/m will generally result in decreased plant quality and injury in young plants and seedlings.

The electrical conductivity of a solution is affected by temperature. Many conductivity meters are calibrated to make readings at 25 degrees C (77 degrees F). Some conductivity meters have automatic temperature adjustments while others can be manually adjusted after measuring the solution temperature. If temperature compensation is not avialable on your machine, then the solution must be at 25 degrees C for an accurate reading. The following table of corrections factors (Table 2) can also be used if the solution temperature is known.

Growers of nursery crops should make every attempt to regularly monitor the pH and soluble salt levels in their growing media. Development of this knowledge will allow growers to make educated decisions regarding fertility and leaching requirements before problems arise.

Fact Sheet H-93-015

The University of Georgia and Ft. Valley State College, the U.S. Department of Agriculture and counties of the state cooperating. The Cooperative Extension Service offers educational programs, assistance and materials to all people without regard to race, color, national origin, age, sex or disability.

An Equal Opportunity Employer/Affirmative Action Organization Committed to a Diverse Work Force

Issued in furtherance of Cooperative Extension work, Acts of May 18 and June 30, 1914, The University of Georgia College of Agricultural and Environmental Sciences and the U.S. Department of Agriculture cooperating.

Gale A. Buchanan, Dean and Director

Table 1. Interpretation of soluble salt and pH measurements by extraction method.

Method

Soluble salt level

pH

Electrical conductivity (dS/m)*

VTEM

Sensitive crops (liquid feed)

0.50-0.75

Nursery crops (liquid feed)

5.2-6.2

0.75-1.50

Nursery crops (controlled-
release)

0.20-1.00

Saturated Extract Method (Nursery crops)

Low

0.00-0.74

Acceptable

0.75-1.49

Optimum

5.8-6.8

1.50-2.24

High

2.25-3.49

Very High

3.50+

Saturated Extract Method (Greenhouse crops)

Low

0.00-0.75

Acceptable

0.75-2.00

Optimum

5.6-5.8

2.00-3.50

High

3.50-5.00

Very High

5.0+

* or (mMhos/cm)

Table 2. Temperature Correction Factors for Conductivity Meters
Solution Temperature

C

F

Factor*

10

50.0

1.411

13

55.4

1.309

16

60.8

1.218

20

68.0

1.112

22

71.6

1.064

25

77.0

1.000

28

82.4

0.943

32

89.6

0.873

35

95.0

0.829

28

100.2

0.788

*Measured EC (dS/m) x Temperature Correction Factor = EC at 25 degrees C

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Last updated on Tuesday, July 09, 2002 at 10:15 AM
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