Granular fertilizers can cause higher salt concentrations in soil solutions compared to liquid fertilizers due to requiring more water to dissolve. High salt concentrations can decrease seed germination or damage seedlings. The salt index expresses the increase in osmotic pressure of a fertilizer solution relative to sodium nitrate, with higher analysis fertilizers having a lower salt index. Excessive soluble salts from sources like poor irrigation water, over-fertilization, or high salt growing media can inhibit plant growth and cause leaf tip burn or root damage. Managing salt levels involves using well-draining media, leaching salts, adjusting fertilizer and irrigation based on water quality, and maintaining adequate moisture.

1. SALT INDEX – Liquid vs Granular Fertilizers
Fluid fertilizers may produce a lower osmotic pressure in the soil solution than granular products of a similar grade. Fewer
problems generally are encountered using fluids as seed-row fertilizers when compared to granular, since less soil water is
required and salts are mainly dissolved in fluid formulations. Decreased seed germination or seedling injury is caused by
high salt concentrations in the soil solutions around germinating seeds. Also, a major concern is that some starter
components such as urea, UAN, or ammonium thiosulfate can produce free ammonia (NH3) under certain soil conditions.
Direct seed contact by NH3 could result in poor germination or seedling death. Selection of the proper starter fertilizer is
the way to minimize this occurrence.
The “salt index” (SI) of a material is expressed as the ratio of the increase in osmotic pressure of the salt solution
produced by a specific fertilizer to the osmotic pressure of the same weight of NaNO3, which is based on a relative value
of 100. Sodium nitrate was chosen as the standard because it was 100 percent water soluble and it was a commonly used
nitrogen fertilizer when the SI concept was first proposed in 1943. Higher analysis fertilizers (normally liquid) usually
have a lower SI because fewer ions of salts are placed in the soil solution per unit of plant nutrient when they dissolve.
Plants do not have mouths or teeth to “eat”, therefore they can only assimilate food via soluble nutrients, whether initially
in dry or liquid form. Since solid/dry fertilizers have only 20-30% of soluble nutrients on average, they leave 70-80% of
non-usable material and/or nutrients in the soil, creating a much higher end cost to the user, along with other issues.
Example: since Urea is so commonly used around the world as a Nitrogen additive, it tends to leave a dangerous
salt/sodium accumulation problem in the soil over time, with nowhere to go, mostly as a result of over-application, typical
of most farming operations. This will result not only in seedling injury and significantly reduced crop yields, but in
extreme cases, it will render soil unarable.
Table 1: Salt index values of fertilizer materials.
Salt Index
Material and analysis Per equal wts. of materials Per unit of nutrients*
Nitrogen/Sulfur
Ammonia, 82% N 47.1 0.572
Ammonium nitrate, 34% N 104.0 3.059
Ammonium sulfate; 21% N, 24% S 68.3 3.252
Ammonium thiosulfate, 12% N, 26% S 90.4 7.533
Urea, 46% N 74.4 1.618
UAN, 28% N (39% am. nitrate, 31% urea) 63.0 2.250
32% N (44% am. nitrate, 35% urea) 71.1 2.221
Phosphorus
APP, 10% N, 34% P205 20.0 0.455
DAP 18% N, 46% P205 29.2 0.456
MAP 11% N, 52% P205 26.7 0.405
Phosphoric acid - 54% P205 1.613**
- 72% P2O5 1.754**
Potassium
Monopotassium phosphate, 52% P205, 35% K2O 8.4 0.097
Potassium chloride, 62% K20 120.1 1.936
Potassium sulfate, 50% K20, 18% S 42.6 0.852
Pot. thiosulfate, 25% K20, 17% S 68.0 2.720
** Salt index per 100 lbs of H3P04
* One unit equals 20 lb.
(Page 1 info mostly derived from AGRONOMIC LIBRARY, published article by Dr. John J Mortvedt; spectrumanalytic.com)
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2. The presence of excessive soluble salts is perhaps the most limiting factor in the production of greenhouse crops.
Generally speaking salt accumulations result from the use of poor quality irrigation water, over fertilization or growing
media with an inherently high salt content. Although soluble salts can inhibit plant growth, when managed properly
their effects may be reduced.
Salt Injury to Plants
Plant injury resulting from excessive soluble salts may first occur as a mild chlorosis of the foliage, later progressing to a
necrosis of leaf tips and margins. This type of injury is largely attributed to the mobility of soluble salts within the plant.
As these salts are rapidly translocated throughout the plant, they accumulate at the leaf tips and margins. Once the salts
reach a toxic level they cause the characteristic "burn" associated with excessive salts.
Roots may also be injured by the presence of soluble salts. This often predisposes the plant to a wide range of root
diseases (i.e., phythium, fusarium, etc.). Extreme injury may also interfere with water uptake and result in excessive
wilting of the plant. It is extremely important to inspect the root systems of plants on a regular basis in order to monitor
the effects of soluble salts.
Irrigation Water
Irrigation water is a major contributor of soluble salts to the growing medium. These occur primarily as salts of Na, Ca
and Mg, although others may be present.
Soluble salts in irrigation water are measured in terms of electrical conductivity (EC). The higher the salt content, the
greater the EC. Generally, EC values exceeding 2.0 millimhos/cc are considered detrimental to plant growth. Water
quality should be monitored on a frequent basis in order to avoid potential problems from soluble salts.
Fertilizers
Fertilizers are forms of salts and therefore contribute to the total soluble salt content of the growing medium.
Depending on the inherent salt content of the irrigation water used, fertility levels must be adjusted to avoid salt
accumulations.
Fertilizers are often classified by the amount of total salts they contain. This "salt index" can be used to determine the
amount of salts contributed to the growing medium. Table 1 presents the salt index of a number of commonly used
fertilizers.
Table 1. Relative salt index for several fertilizers.
Fertilizer Salt index
Sodium nitrate 100
Potassium chloride 116
Ammonium nitrate 105
Urea 75
Potassium nitrate 74
Ammonium sulfate 69
Calcium nitrate 53
Magnesium sulfate 44
Diammonium phosphate 34
Concentrated superphosphate 10
Gypsum 5
(Sodium nitrate was arbitrarily set at 100. The lower the index value, the smaller the contribution the fertilizer makes to the level of soluble salts)
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3. Growing Media
Growing media can be formulated from a variety of components. These include peat, perlite, vermiculite, pine bark and
others. Generally speaking these materials do not contain excessive quantities of soluble salts. However it is important
to monitor the quality of media components carefully.
In some cases it is necessary to thoroughly leach a medium before using it. This is particularly important for seed
germination and other forms of propagation. Leaching may be accomplished by running water through individual pots or
trays prior to planting or by leaching the entire volume of bulk medium.
For a quantitative evaluation of this process the electrical conductivity of the leachate may be evaluated. When the EC is
less than 2.0 millimhos the medium is free of excessive salts.
Managing Soluble Salts
Managing soluble salts involves an integrated approach to production. This includes the type of growing medium used,
irrigation frequency, water quality, type of fertilizer(s), fertility regime and plant tolerance.
Growing media should contain a substantial quantity of large pores to facilitate good drainage. Media with these
characteristics are easily leached and reduce the potential for the accumulation of soluble salts. When irrigating this
medium it is important to apply enough water to allow sufficient quantities to leach through the container.
Approximately 15-20% more water than the container can hold should be applied at each irrigation if the salt hazard is
high. Water pressure must be adjusted to avoid overflow.
Since the concentration of soluble salts in plant tissues increases as moisture levels decrease, it is important to monitor
the water content of the growing medium. In the presence of excessive soluble salts, growing media should not be
allowed to dry out. Maintaining adequate moisture levels can be difficult in porous growing media and requires careful
attention.
Providing adequate fertility is important in maintaining optimum plant growth. However, if fertility levels are too high,
injury from soluble salts may occur. Determining the amount of nutrients to use must be based on the quality of
irrigation water as well as the fertilizer's salt index. Generally most fertility regimes used for the production of potted
greenhouse crops are between 150 and 350 ppm (N). Higher levels of fertility create a much greater potential for
injury from soluble salts.
(Page 2/3 info mostly derived from Texas Greenhouse Management Textbook)
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