DEC 1 -954,
Department of Soils Mimeograph Report 55-3
A RAPID METHOD FOR EETERINING TOTAL DISSOLVED SALTS IN IRRIGATION WATER.
Seton N. Edson and F. B. Smith
A simple and rapid method for measuring total soluble salts in irrigation
water using a hydrometer type instrument called a salimeter is described. The
development and use of the salimeter are discussed* The only equipment needed
to test for soluble salts in water are a salimeter, a centigrade thermometer,
and a 10 quart bucket. Tables for correcting for temperature and evaluating
the quality of irrigation water are given.
University of Florida
Agricultural Experiment Station
Willard M. Fifield, Director
December 1, 195
A RAPID METHOD FOR 7ETERRINING TOTAL T~lt SOLVrD SALTS IN IRVIGlTION WA TI,
Most plants take up their supply of water and essential minerals mainly from
the soil solution. When the concentration of soluble salts in the soil solution
is stronger than that in the plant, turgidity is lost because of the movement of
water from the plant into the more concentrated soil solution. Because of the
upset water balance, high concentration of soluble salts in the soil solution may
result in poor seed germination or stunted growth of plants. In some areas the
cause of harmful levels of soluble salts in the soil may be irrigation water.
Damage to plant growth because of high salt content of irrigation water may
take place where intensified farming is practiced. These areas may be found in
coastal regions and in the arid west. In the humid southeast, poor quality
irrigation water occurs in specific areas where there has been encroachment of
sea water or the water from deep wells contains a high mineral content,
Laboratory methods presently employed to determine salt content of water do
not lend themselves to field use. A simple method of measuring the total dissolved
salts in irrigation water would be of large practical value. A method utilizing
a simple hydrometer, hereafter referred to as a salimeter, was developed. This
paper describes the new salimeter and gives instructions for its use in the field.
DESCRIPTION OF THE S.ALIMRTRR
Commercial types of the common hydrometer are not sensitive enough to measure
salt concentrations in water in the range below 0.4 percent of total dissolved
salts. A laboratory model based on theoretical calculations was found to be
sensitive in the range of salt concentrations normally found in irrigation waters.
The relation between stem volume and bulb volume determines the range of any
hydrometer. For a hydfometer floating in a uniform liquid Downs (2) found the
following formula applicable:
d2 dl (v/V + 1) (A)
Where dl and d2 are densities corresponding to the highest and lowest graduation
marks on the. stem, respectively; V is the submerged volume of the bulb when the
density is d2, and v is the volume of the stem between d2 and dl marks. The range
of the salimeter is directly proportional to the ratio v/V. However, for a given
value of v, the sensitivity can be increased by reducing the diameter of the stem.
The stem/bulb ratio value at 200C. for 0.4 percent salt solution is obtained
by substituting in Formula At
1.0009 0.9982 (v/V + 1), and v/V = 0.0027
With submerged volume of bulb (V) 135 ml., the submerged volume (v) of the
stem was calculated from formula (4) as follows:
v 0.3645 ml.
With an outside diameter (D) of 0.32L5 cm., the length (L) of the stem was obtainer
by the following calculations
L (4) (o.36h5) / (3.166) (o,3l2h)2
The salimeter was constructed* according to the following specifications:
Outside diameter of stem 3.25 mmu
Length of stem from dl to d2 h4. cm.
Overall length of stem = 10.0 cm.
Overall length of salimeter = 27.0 cm.
Length of bulb 17,0 cm,
Displacement of bulb = 135.0 mle
Bisplacement of stem (dl d2) 0.3650 ml.
The distance from dl to d2 was divided into l equal parts, reading from sero,
041 percent, 0.2 percent, 0.3 percent, and 0.4 percent of total dissolved salts.
* Phipps and Bird Company, Richmond, Virgini&
Calibration was conducted in a NaCI solution at 20CC, S.T.P. The difference in
weight of the commonly dissolved salts found in ground waters, not to exceed 4000
p.p.m. (0.i%) in solution, is small and may be disregarded. towns (2) showed that
the changes in density with changes in temperature of soil suspensions up to five
percent in water may be assumed to be the same as for pure water. The salimeter
was tested against NaC1 and 1gSO solutions and no measurable differences between
dl and d2 on the stem were found at similar temperatures. The difference in
coefficients of expansion of pure water and 0.4 percent NaCI solutions at 120C.
and 25C. amounts to only 0.0002 grams per ml., or 0.02 percent (200 p.p.m.) of
total dissolved salts. This amounts to a reading well under the sensitivity of
the salimeter. However, it is necessary to correct for temperature* This is
especially necessary because of the sensitivity of the salimeter.
Using the density tables of Chen and Hua (1), the essential temperature correc-
tion factors for the salimeter were calculated. Table 1. Temperature corrections
were rounded to the fourth decimal place for percent and p.p.m. of total dissolved
Correction Factors to Add or Substract From the Salimeter Readings for Specific
Temperature of Absolute density Difference from Equivalent correction for
irrigation of pure water, density at 200C. salimeter up to 0.4% (L000
water p.p.m.) total dissolved silts.
(oc.) (gms/ml.) (gms/ml.) (percent) (p-pFmO)
5 1.0000 0.0018 -0.22 -2200
7 0.9999 0.0017 -0.21 -2100
9 0.9998 0.0016 -0.20 -2000
11 0.9996 O.014 -0.17 -1700
13 0.9994 0.0012 -o.14 -1400
15 0.9991 0.0009 -0.10 -1000
17 0.9988 0.0006 -0.07 700
19 0.9984 0.0002 -0.02 200
20 0.9982 0.0000 0.00 0000
M 0,9980 0.0002 +0,02
23 0.9976 0.0006 +0.07 + 700
25 0.9971 0.0011 +0.13 +1200
27 0.9965 0.0017 +0.21 +2100
29 0.9960 0.0022 +0.27 +2700
30 0.9997 0.0025 ..+0.31 +3100
A classification of irrigation water adapted from 'ilcox (6), McGeorge (3),
Westgate (5), and the U.S.D.A. Salinity Laboratory, Riverside, California (4) is
given in Table 2* It is assumed here that the quality of irrigation water depends
more upon the amount of dissolved salt than upon the kind of salt. This is probably
true for Florida conditions where the type of salt does not vary greatly and the
soils are light in texture. 'There the proportion of sodium in the irrigation water
is high and the soils are heavy textured, the kind of salt in the water is also
very important in determining the quality of irrigation water.
Table For Evaluating the Quality of Irrigation Water
salts, Relation of Salt Content to Plant Tolerance
0 to 500 Excellent quality irrigation water-All plants.
500 to 1000 Fair quality irrigation water-Moderate tolerant
1000 to 2000 Poor quality irrigation water-Strongly salt tolerant
2000 to 3000 Very poor quality irrigation water-Toxic to all bit a
few salt tolerant
DIRECTIONS FOR USE OF THE ALI~MTF
A 10 quart pail is a suitable container for the water to be tested. The
salimeter is placed in the water and read. The temperature of the water near the
bulb of the salimeter is taken immediately. A correction factor, Table 1, is
applied to the salimeter reading to give pPp.m. of total soluble salts. This value
referred to Table 2 shows the quality of the irrigation water.
1. Chen, C. T# and M. Huao Mechanical Analysis of Soils by Means of a Common
Hydrometer. Soil Sci. 64: 389-392. 1947.
2. Downs, Ro G. Use of the Hydrometer for Mechanical Analysis of Soilse Jour.
Council Sci. and Ind* Res. (Australia) 17(3)0 197-207. 194.
3. McGeorge, W. T. Interpretation of Water Analysis. Ariz. Agri. Ext. Ciro No.
4. U. So Regional Salinity Laboratory. Diagnosis and Improvement of Saline and
Alkali Soils. Agriculture Handbook No. 60. U.S.D.A. Regional Laboratory,
Riverside, Calif. 1954.
5. Westgate, P. J. Effects of Soluble Salts on Vegetable Production at Sanford,
Fla. Proceedings Fla* State Horto Soc. pp 116-123. 1950.
6, Wilcox, L. V. The Quality of Water for Irrigation Use. U.S.D.A. Tech. Bul.
No0 962. 1948.
Soils 12/1/54 100 copies