Borax

Borax is a hydrated sodium borate, refined from mined borate deposits, that supplies boron at around 11%. Other boron fertilizers such as more concentrated refined borates exist, but borax remains a common and economical source for soil and spray use.

Chemical composition

Sodium tetraborate decahydrate

Boron content

About 11% boron

Other boron sources

More concentrated refined borates are also used

Solubility

Soluble in water

Borax supplies boron, taken up by plants as borate. Boron is mobile in soil and readily leached, especially on sandy soils under high rainfall, so deficiency is common there; conversely, its low availability at high pH can also limit uptake. This mobility contributes to the difficulty of keeping boron in the narrow sufficient range.

• Boron is supplied as readily available borate
• Boron is mobile and prone to leaching on sandy soils
• Availability can also be reduced at high soil pH

Borax is used to correct or prevent boron deficiency in responsive crops such as oilseed rape, sugar beet, brassicas, and some fruit and legume crops. Because the safe range is narrow, it is applied at low, even rates, often blended into bulk fertilizer or applied as a dilute foliar spray for precise dosing.

• Responsive crops include oilseed rape, sugar beet, and brassicas
• Even distribution at low rates is essential to avoid toxicity
• Blending into bulk fertilizer or foliar spraying helps dose precisely

The narrow gap between too little and too much boron is the central risk: uneven spreading or an excess rate can cause boron toxicity, which harms crops and can persist in soil. Crop sensitivity to boron also varies widely, so a rate that helps one crop may injure a more sensitive one in rotation.