The majority of researchers and farmers have recognized the importance of boron in agriculture. Boron is necessary for proper plant nutrition. And, boron-deficiency can significantly affect crop yields. This is especially true in Brazil where most soils have low or medium levels of boron.

Types of borates

Although its clear crops need boron, there is still debate as to what sources of boron are best in agriculture. There are basically two classifications of borates:

1. Those where the original source material has gone through a refinement process. Unsurprisingly, these are known as refined borates. For example, boric acid, sodium octaborate, borax decahydrate, borax pentahydrate, and anhydrous borax.
2. Those borates which have had no refinement process. These are called mineral or unrefined borates. Examples include hydroboracite, colemanite, and ulexite.

Borate solubility by type

The more soluble a product is, the more boron is available to the plant. The solubility of borates depends on the source material and the interaction of the boron with sodium (Na), calcium (Ca), and magnesium (Mg). The more Mg and Ca a borate has the less soluble this mineral will be.
 
Most of the borate fertilizers used in agriculture, come from the following boron ores:

Hydroboracite (CaMgB₆O₁₁6H₂O)
Calcium and magnesium borate which is practically insoluble in water (solubility of 0.8 g/L at 20° C)
Colemanite (Ca₂B₆O₁₁5H₂O)
Calcium borate which has low water solubility (4.7 g/L at 20° C)

Ulexite (NaCaB₅O₉8H₂O)
Calcium and sodium borate which has partial solubility in water (10.9 g/L at 20° C)

Kernite (Na₂B4O₇4H₂O)
Sodium borate, which is water soluble (19.0 g/L at 20° C)
Tincal, also known as borax, (Na₂B₄O₇10H₂O)
Sodium borate which is water soluble (26.5 g/L at 20° C)

Refined borates such as boric acid (H₃BO₃) and disodium octaborate tetrahydrate (Na₂B₈O₁₃.4H₂O) are highly soluble in water having solubility of 47.2 g/L and 97.0 g/L at 20° C, respectively. Because of this, they are widely used to produce liquid fertilizers for foliar fertilization and fertigation.

What is borax?

Many people think that there is only one type of borax. But, in addition to several other products, U.S. Borax produces three different forms:

1. Borax decahydrate (Na₂B₄O₇ • 10H₂O): About 11.4% boron
2. Borax pentahydrate (Na₂B₄O₇ • 5H₂O): About 15% boron
3. Anhydrous borax (Na₂B₄O₇): About 21.3% boron

 
The water solubility of each type of borax will depend upon the degree of hydration. Because borax decahydrate has a greater solubility in water and a lower percentage of boron, it is not widely used in agriculture. Rather it is mainly used in a variety of industrial manufacturing end uses, including as a laundry detergent booster.
 
On the other hand, borax pentahydrate (sold commercially by U.S. Borax as Granubor^® and Fertibor^®) and anhydrous borax are very efficient sources for use as fertilizers.
 

Borates in the field

When it comes to borated soil fertilization, growers should be concerned about using efficient boron sources capable of releasing 100% of the present boron and making boron available when plants really need the nutrient.
 
It was with this in mind that U.S. Borax created Granubor, which is capable of releasing 100% of boron in order to meet the nutritional uptake of crops. In the graph below we can see that Granubor is able to release boron in order to meet the demand of this nutrient for corn crop.
 

Continue to part two of the series: Borate research results

Resources

• Boron: Why quality matters
• The value of refined borates
• Boron solubility
• Borates mineral solubility in water (PDF)