Boron is an essential plant micronutrient. It is used by plants during cell division and is required for development of tissue near the tips of shoots and roots. It is also required for the growth of the pollen tube during flower pollination and thus fruit and seed production. Boron is thought to increase nectar production by flowers so it is important for attracting pollinating insects. It is also required for good cell structure and as a result, the tissue of boron deficient plants often breaks down prematurely resulting in brown flecks, necrotic spots, cracking and corky areas in fruit and tubers. Excess boron inhibits seed germination.
A wide variety of factors can influence the availability of phosphorus including:
- pH ‑ boron is less available in moderately alkaline soil and more available in acidic soil.
- Water ‑ high rainfall and excess irrigation can leach boron from the soil.
- Organic matter ‑ organic matter provides boron as it breaks down and also improves the ability of soil to hold boron.
- Moisture ‑ low levels of soil moisture inhibit the uptake of boron.
- Calcium ‑ some research indicates that excess calcium can inhibit the uptake of boron by plants.
- Nitrogen ‑ severe nitrogen deficiency may reduce a plant's ability to uptake boron.
Ideally, for healthy and productive soil you should aim for a boron concentration of 0.5‑4 mg/kg. Some laboratories may only be able to detect concentrations as low as 4 mg/kg and while this is sufficient for determining whether soil has an excess, such information cannot tell you whether there is a soil deficiency. If you suspect your soil does not have enough boron, make sure you check that the laboratory you chose can perform a test with a low enough detection limit (the test should measure down to 0.5 mg/kg or lower). If the concentration of boron in your soil is greater than 4500 mg/kg it would be a good idea to have your home grown food tested for boron and then seek expert advice about any action you might want to take.
Boron is easily leached from soil so is most likely to be deficient in sandy soils located in areas of high rainfall. A lack of organic matter in the soil will also increase the likelihood of plant boron deficiencies.
Symptoms of Boron Deficiency
The symptoms of boron deficiency can vary greatly between different species but often new leaves will be discoloured ‑ usually they will be a reddish colour though they may just be chlorotic. Often new leaves will develop necrotic spots and will be wrinkled. Frequently, the buds at the ends of stems (apical buds) will die. In severe cases, older leaves may become irregularly shaped. Root necrosis may result in reduced growth of the whole plant. Plants affected by boron deficiency will most likely fail to set seed regardless of how many flowers they produce though flower production is frequently reduced.
Treating Boron Deficiency
As with any plant problem, the first thing you should do is check that the pH is appropriate for the plant ‑ usually close to neutral. Boron is most available when the soil pH is between 5.5 and 7 and when the pH is 10.5 or greater. Provided the pH is between 4 and 7.5 though, sufficient boron should be uptaken by most plants.
You should also add lots of organic matter to your soil and check that you are only watering when the soil is dry or nearly dry (depending on the water requirements of your plants).
If symptoms persist, it would be a good idea to have your soil tested for boron, calcium and nitrogen at the very least. If nitrogen levels are low, take steps to boost the level of nitrogen in your soil. Likewise, if calcium levels are excessive, cease any practices that may be contributing to the high levels (this includes the addition of calcium containing fertilisers and other soil amendments such as gypsum, limestone and dolomite).
If symptoms still persist and the soil test revealed low levels of boron, then you can apply boron containing soil amendments. Adding even more organic matter is always a good idea but by this point you may require an inorganic source of boron such as a foliar spray (eg polybor), boric acid or borax.
Boron toxicity most often occurs either as a result of excess boron containing fertiliser use or because the soil is naturally high in boron. Soil in areas that were previously under sea water are more likely to be naturally high in boron. Irrigation water may also be a source of boron but soil may build up high levels of boron if it is contaminated with wastes from surface mining, fly ash or industrial chemicals.
Symptoms of boron toxicity initially consist of chlorosis along the margins of leaves that progresses towards the veins. When boron toxicity is severe, leaves may blacken and then die between the veins.
Treating Boron Toxicity
As indicated above, boron is most available when the soil pH is between 5.5 and 7 and when the pH is 10.5 or greater. It is still fairly easily absorbed between pH 4 and 7.5 though and at pH 10 and above. As such, if your soil is very alkaline, boron toxicity is much more likely and plants are also likely to experience nutrient deficiencies as well, which means that plants are under greater stress. So the first step to treating boron toxicity is to make sure that the soil pH is close to neutral. While your plants may still uptake a similar amount of boron at pH 7, they will at least also be able to take up other nutrients more easily and this will help to strengthen them.
Once the soil pH has been neutralised if required, it would be a good idea to have the soil tested to determine both the boron and calcium concentrations. If the soil definitely contains excess boron but it also lacks calcium then applications of calcium may help to reduce the amount of boron being uptaken by the affected plants.
Once excess soil boron has been diagnosed, take some time to learn the source of the problem. If contamination (perhaps through irrigation with contaminated water) is the cause, take steps to prevent any further contamination and then monitor the soil boron level over a number of months. This may be enough to rectify the problem. If not, it may be possible to leach the excess boron from the soil but this is laborious, expensive, may result in other nutrient imbalances and may not result in a permanent improvement.
If you cannot correct soil boron levels or it is impractical to do so, it would be a good idea to limit your selection of plants to species that are tolerant of higher boron levels. In a garden situation you might consider building raised beds and purchasing healthy, nutrient balanced soil. You could also build your own soil overtime (to fill raised beds with or to use as top soil) through composting etc. but of course you'll need to make sure that you don't use organic waste grown on your property otherwise the compost will end up high in boron too. Purchasing soil is the quickest option but of course it is more expensive. If you have a large property, growing tolerant crops is the most suitable solution.