Recently I have received many questions about alternative forms of weed control, and if nutrients might be a possible means of control, specifically boron.
This is not an area where I have personal experience, and I am not personally familiar with how to manage boron applications to produce this effect. Test for yourself, with eyes wide open, and please let me know. I would love to learn more.
From what I have been able to read, it seems that boric acid and sodium borate can be used as an effective means of killing weeds. While the information I have been able to find is not particularly clear, it seems effective control is solution concentration dependent rather than quantity per area dependent.
Recommended rates I have been able to uncover are for either three ounces of boric acid or four ounces of sodium borate per gallon of water. Typically, boric acid contains 17% boron, and sodium borate usually contains 10% boron, so these recommended application rates don’t equal the same quantity of applied boron on a per-acre basis.
We need to be aware of the quantity of boron being applied on a per-acre basis, and the boron sensitivity of the crops we are growing. In our agronomic recommendations, based on soil analysis and plant sap analysis we often recommend between one and three pounds of actual boron per acre per year. The rate varies with the crop, soil levels of boron, and annual rainfall. In low organic matter soils, ten inches of rainfall can leach about one-half pound of boron per acre. Thus, if you get forty inches of rainfall per year, you need to add two pounds of boron annually just to replace what the rainfall removed. As organic matter increases, and soils anion exchange capacity increases, less boron is leached through the soil profile.
If we follow the recommended concentration rates, and apply 20 gallons of solution per acre, with each gallon containing four ounces of 10% boron, this application will give us eight ounces of actual boron per acre. This rate is well within the range of what is routinely applied as a soil amendment or nutrition source of boron on many soils and crops. This type of application would also supply much more uniform soil distribution than a broadcast application of pellets with some distance between the pellets as occurs with such small application rates.
Obviously, this application is non-selective, and should not be applied directly on crop plants.
With the exception of a very few boron sensitive crops, I do not expect that boron toxicity to the crop is nearly the danger that it is sometimes made out to be. Boron toxicity in most plants is simply a calcium deficiency. In cases where excessive boron was applied in the past, a foliar application of calcium will snap a crop out of boron toxicity in a matter of days, even when tissue analysis levels are ten times higher than desired values.
What these experiences suggest to me, is that using boron salts as an herbicide is likely to produce the biggest effect on calcium deficient soils and that soils with adequate or generous calcium may require stronger application rates to produce the same effect. Of course, crop sensitivity to the boron application will also depend on soil calcium levels.
It is important to mention that using boron as a form of weed control is specifically prohibited under USDA NOP rules for organically certified producers. It can be used as a nutrient source with restrictions, but not as an herbicide.
If you would like more information on the toxicity of boron in the environment, the National Pesticide Information Center link provides very thorough and useful information.