Effects of Boron on Plant and Soil Health
Getting Familiar with Boron

Boron (B)  is one of the essential nutrients for plants. B is required in small amounts, but the requirements vary depending on the plant species and cultivar. Tubers and legumes typically consume the most B, while cereals consume the least. Boron requirements are highest during reproduction. 

The majority of B is found in the plant’s cell wall, but it’s also involved in the metabolism of nucleic acids and carbohydrates, protein synthesis, phosphorus and nitrogen metabolism, hormonal regulation, and maintaining proper membrane permeability. Boron-tolerant species/cultivars can efflux boron from their cells and reduce cellular B concentrations. Species producing sugar alcohols such as sorbitol and mannitol can transport B through the phloem due to increased solubility, increasing plant growth.
Boron has been shown to reduce the effects of drought on agricultural plants. Similarly, B applications can alleviate salt stress when plants are boron deficient. B reduces the adverse effects of excess salinity by maintaining cell wall elasticity and recovering K+ levels. B applications can also improve carbohydrate metabolism and transport, which is critical during periods of salt stress.

Addressing Boron Toxicity in the Plant

Proper calcium nutrition can prevent excess B uptake by maintaining membrane permeability and ion transport. Excess B can be removed from agricultural soils by leaching it out with irrigation. In alkaline soils, the addition of acid to the irrigation water will make B more mobile. However, this can leach other beneficial nutrients from the soil and requires a lot of water. Adjusting soil pH with lime can be a more feasible solution. At pH 7, B in the soil is mostly found as boric acid, which is bioavailable to plants. But at pH 10, it’s primarily found as a borate anion that adsorbs to clay minerals, making it less available for plants. Another method to reduce B uptake is the addition of organic matter to the soil. Although this can cause an increase in B retention in the soil, it has been shown to reduce B uptake in the plant by providing binding sites for boron.

Source: Shireen et al., 2018

Figure 1. Plant functions induced by the uptake of boron. The uptake of boron by plants can be induced by the presence of boric acid or borax. The uptake of boron can also be induced by the presence of other chemicals, such as nitrate or phosphate ions.

Problems Associated with Boron

It can be challenging to manage B in agricultural settings because the range between deficiency and toxicity can be narrow. B is typically found as boric acid, borate, or a borosilicate mineral. It is most available to plants as boric acid or borate. The bioavailability of boron and the amount of B adsorbed by the soil depend on soil pH, soil type, temperature, and moisture. Boron is mobile, especially in acidic soils, and poor management practices have led to reduced B concentrations in many agricultural soils. Irrigation of agricultural soils promotes boron leaching, especially when the irrigation water is acidic. Abiotic factors such as extreme temperatures, drought, salinity, and heavy metal contamination can increase the sensitivity of plants to B toxicity or deficiency.

Boron deficiency leads to inhibited root development and nutrient uptake. Root morphology changes lead to inhibited absorption and transport of nutrients, which causes chlorosis in young shoots. Boron deficiency affects plant metabolism and photosynthesis, causing decreased yields.

Excess B in plants can be seen as chlorotic and/or necrotic patches found on mature leaves. Boron stress causes nutritional imbalances, decreases the photosynthetic rate, increases lipid peroxidation, and increases membrane permeability. Furthermore, excess B and salinity stress are often found together in arid and semi-arid areas. The combined stress of salinity and B toxicity results in more inhibition in the plant than individual salinity or B stress.

Further Reading

Brdar-Jokanović, M. (2020). Boron toxicity and deficiency in agricultural plants. International Journal of Molecular Sciences, 21(4), 1424. https://doi.org/10.3390/ijms21041424

Garcia-Sanchez, F., Simon-Grao, S., Martinez-Nicolas, J. J., Alfosea-Simon, M., Liu, C., Chatzissavvidis, C., Perez-Perez, J. G., and Camara-Zapata, J. M. (2020). Multiple stresses occurring with boron toxicity and deficiency in plants. Journal of Hazardous Materials.

Hua, T., Zhang, R., Sun, H., and Liu, C. (2020). Alleviation of boron toxicity in plants: Mechanisms and approaches. Critical Reviews in Environmental Science and Technology.

Importance of boron in plant growth. Mosaic Crop Nutrition. https://www.cropnutrition.com/resource-library/importance-of-boron-in-plant-growth

Shireen, F., Nawaz, M. A., Chen, C., Zhang, Q., Zheng, Z., Sohail, H., Sun, J., Cao, H., Huang, Y., and Bie, Z. (2018). Boron: Functions and approaches to enhance its availability in plants for sustainable agriculture. International Journal of Molecular Sciences.