FLORIDA AGRICULTURAL EXTENSION SERVICE
UNIVERSITY OF FLORIDA
INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES
Vegetable Crops Department
November 9, 1959
Considerable interest has developed in the work of Dr. C. M. Geraldson in adopt- ,
ing the principle of INTENSITY AND BALANCE of plant nutrients in the soil as guides "
for the fertilization of vegetable crops, Dr. Geraldson's work, although quite ad-
vanced and very promising, is not yet being recommended for general use. County
Agents, who have cooperated in this work, are encouraged to continue to do so. Other
agents are advised to keep informed on this approach to fertilization, but not to
attempt to use it until more information becomes available.
Balance refers to the percentages or ratio of the nutrient elements (Ca, K, Mg,
Na, NH44, NO3, C1, S04) in the soil solution. Balance, in other words, is a measure
of the relative proportions of the various fertilizer elements desirable for satis-
factory crop response and not a measure of total amounts of those elements. As an
example, in soil solution, calcium should be over 20 percent of the total soluble
salts for the production of good quality tomatoes.
Intensity is a measure of concentration (amount) of the total soluble salts in
a soil solution. An intensity determination gives a reading of amount of fertilizer
as against balance which gives a reading of ratio of the various fertilizer elements.
One, two or more balance determinations during the growing season may be needed
to properly estimate nutrient fertilizer needs of a given crop. Caution: All de-
tails necessary for proper interpretation of a balance determination have not been
worked out as yet. Until more data becomes available, a balance determination
should be used as only one of many tools in helping to carry out a satisfactory
vegetable fertilizer program. Intensity determination may be needed weekly, bi-
weekly or after leaching rains to keep a check on the total fertilizer status of a
The analysis to determine balance of nutrients is complicated and must be run
in a well-equipped laboratory. On the other hand, intensity determinations are
quite simple and can be made in the County Agent's Office. The equipment needed is
simple and not too expensive.
(1) A Solu-Bridge* (Model RD-15, with short cell) Approx. Cost o80.00.
* Listing of a specific trade name does not constitute an endorsement of this equip-
ment over others capable of performing the same operations.
COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS, STATE OF FLORIDA. COLLEGE OF AGRICULTURE.
UNIVERSITY OF FLORIDA, U. 5. DEPARTMENT OF AGRICULTURE. AND BOARDS OF COUNTY COMMISSIONERS, COOPERATING
(2) 50 ml. beakers )
(3) 200 ml. beakers or equiv. glasses
(4) Glass stirring rods
METHODS OF ANALYSIS
Steps in making the determination are:
A. Soil Moisture determinations:
(1) Obtain a moisture holding capacity determination of the soil
from the Extension Soils Laboratory. Make all necessary arrange-*
ments with the Extension Soils' Specialist before sending samples
to this laboratory for analysis. For the determination, take the
soil sample when soil moisture is optimum for plant growth. One
determination will suffice for all soils with the same physical
B. Taking Solu-Bridge Readings:
(1) Get a representative composite soil sample (the depth and area
of sampling should be representative of the effective root zone).
(2) Air-dry the soil and screen to remove large clods.
(3) Measure 50 mls. of soil by pouring air-dry soil into a 50 ml.
beaker and scraping off excess with thin-edged ruler.
(4) Pour the measured 50 mis. of soil into a 200 ml. beaker.
(5) Pour ,100 ml. of distilled orde-ionized water (substitute clean
rain'water) over the soil.
(6) Stir vigorously for 1 minute.
S:(7) Allow to stand for 30 minutes.
(8) Stir vigorously again for 10 to 15 seconds.
(9) Take and record reading from Solu-Bridge directly in soil-
water mixture. (Be sure to read carefully instructions on
use of Solu-Bridge).
INTERPRETION OF ANALYSIS
After a soil moisture determination for a specific soil is made and a good
Solu-Bridge Reading is taken, the results can be interpreted from Table I or-from---
Figure I. The tabular interpretation is simplest but it does not present the
complete picture as shown in Figure I. Both methods of interpretation will be ex-
A. From Table I:
(1) Compare the Solu-Bridge reading taken with those given opposite
the appropriate soil moisture percentage. Low, optimum and ex-
cessive ranges are explained in the legend at the bottom of the
(2) For soil moisture levels not listed in the table, interpolate by
proportioning the difference between the two nearest soil mois-
B. From Figure I:
S(1) Find the point on the Solu-Bridge line (Vertical line on left
side) corresponding to the reading taken on the sample from the
Solu-Bridge. Follow this line across until it intersects the
line corresponding to the -soil moisture percentage of the soil
Example Assume a soil with 25% moisture and a Solu-Bridge
reading of 50. The two points intersect on the
chart at a point marked (X). The intensity of
the nutrients in the soil used in this example
falls in the optimum range for most crops.
(2) The chart, also, illustrates the relative sensitivity of .some of
the vegetable crops to soluble salt injury,
a. The range below the 1000 PPM line on the chart corresponds
to the low intensity level of Table I. This range is gen-
erally inadequate in nutrient intensity for best growth of
most vegetable crops.
b. The range between 1000 PPM and the 4000 PPM lines corresponds
to the optimum level of Table I. Intensity levels in this
range are generally optimum for most vegetable crops.
c. The range between the 4000 PPM and 15,000 PPM lines corresponds
to the excessive level of Table I. Most vegetable crops may
be injured from these excessively high salt concentrations.
The vegetables are placed in the chart to illustrate rela-
tive sensitivity of the various crops to salt injury. Beans
are relatively more sensitive than celery and celery more than
Each vegetable crop is placed on chart to indicate the level of nutrient inten-
sity which may reduce yield of that crop by 50%. Squash growing in a soil solution
containing slightly over 6000 PPM salts in the soil solution may yield 50% less
than expected as a result of salt injury. Similarly, cauliflower yields may be
reduced by 50% when salt concentration of the soil solution is about 10,000 PPM.
NOTE: To calculate PPM soluble salts obtain factor from
Factor Line on chart which corresponds to the soil
moisture percentage used.
Example: Assume a soil with 12.5% soil moisture taken at
level optimum for plant growth. Assume a Solu-
Bridge Reading of 120.
(a) Factor at 12.5% moisture = 50
(b) Factor x Solu-Bridge Reading = PPM Soluble Salts.
50 x 120 = 6,000. PPM Soluble Salts.
This is complicated and hard to simplify. If you need further explanation,
Remember these interpretations are made at optimum field moisture. As the
soil becomes drier in the field, concentration of soluble salts goes upllI
4 TABLE I
INTENSITY LEVELS SOLU-BRIDGE READING)
Soil Moisture Low (a) Optimum (b) Excessive (c)
5.0% below 13 13-54 54 above
6.0 14 14-60 60
7.0 15 15-63 63
8.0 16 16-67 67
9.0 i 17 17-70 70
10.0 ~ 18 18-74 74
11.0 19 19-77 77
12.0 20 20-80 80
16.0 o' 22 22-86 86
20.0 23 23-92 92
25.0 25 25-100 100
50.0 33 33-130 130
100.0 43 43-170 170
150.0 50 50-200 200
(a) Low-Concentration is too low for most plants and fertilizer is
(b) Optimum Best range for growth of most plants.
(c) Excessive Most sensitive plants (beans, celery, radish) may be
F. S. Jamison Kr
Vegetable Crops Specialist
Associate Vegetable Crops Specialist
Mason E. Marvel
Assistant Vegetable Crops Specialist
FIGRE I--FACTORS (for conversion to PPM Soluble Salts)