Solubility of Solubor in Liquid Fertilizers

Solubility of Solubor in liquid fertilizers

  • Boron deficiencies in crops can be conveniently corrected by soil applications of boron sources included in liquid or granular N, NP, and NPK fertilizers.
  • Solubor® is an excellent boron source for inclusion in liquid fertilizers because it is readily soluble and does not react with other components of these fertilizers.
  • Solubor is very soluble in urea-ammonium nitrate (UAN) solutions and in 10-34-0 and 7-21-7 liquid fertilizers, ranging from 1.5 - 8.0% Solubor in stable solutions.
  • The maximum Solubor solubility in these liquid fertilizers is greater than that needed to provide the recommended boron rates at the usual NPK rates.
The recommended soil application rate of boron generally ranges from 0.25 to 2.0 lbs. of B/acre. Because it is difficult to separately apply these low rates of boron fertilizers to soil, granular or fluid NPK fertilizers generally are used as carriers of boron.

Such application assures more uniform application of boron to the soil and also eliminates the cost of separate applications. Soluble boron sources, such as Solubor, may be easily mixed with fluid fertilizers to provide the recommended boron rates for crops.

Solubility studies of Solubor in various liquid fertilizers
Studies were conducted to determine the maximum solubility of Solubor in typical liquid fertilizers applied in the U.S. These are UAN solutions (28 and 32% N) and 10-34-0 ammonium polyphosphate solution. Also included was a 7-21-7 fluid fertilizer, based on 10-34-0, because it is widely used as a starter fertilizer.

Salt-out temperatures of all fluid fertilizers at various Solubor contents were determined because of potential salt-out problems with storage of boron-containing fertilizer solutions. Solubor may be easily mixed with fluid fertilizers to provide the recommended boron rates for crops.

Solubor may be easily mixed with fluid fertilizers to provide the recommended boron rates for crops.


Results in Table 1 summarize the maximum solubility of Solubor in these fertilizers. Highest solubility was in 10-34-0 fertilizer. Inclusion of KCl in the 7-21-7 fertilizer reduced the maximum Solubor solubility, due to the higher salt content of this fertilizer. The maximum solubility was lower in the UAN solution with 32% N than 28% N for the same reason.

The salt-out temperatures of all liquid fertilizers containing the maximum percentages of Solubor are lower than those during the growing season. This suggests that there should be no problems with shortterm storage of NPK fluid fertilizers containing Solubor during that time. It was found that potassium salts crystallized out of the 7-21-7 fertilizer at temperatures below 35°F, but salts did not precipitate until the temperature reached -4°F. Solubility of Solubor increased only slightly with increased temperature to about 75°F.

Based on these results, the maximum rates of boron that can be applied at usual N, NP, or NPK rates with these liquid fertilizers (100 - 250 lbs. of product/acre) range from 0.3 to 4.0 lbs. of B/acre (Table 2). Since the recommended application rates of boron usually range from 0.25 - 2.0 lbs./acre, such boron rates can be conveniently applied by inclusion of Solubor in these liquid fertilizers. It is important to dissolve the correct amount of Solubor to provide the desired rate of boron at the intended liquid fertilizer application rate.


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