Boron Dynamics and Sources

Farmers, in Brazil as elsewhere around the world, are under increasing pressure to improve their sustainability, fertilizer use efficiency, and of course their productivity. With these different pressures it is more important than ever to look at all factors that affect crop growth and yield.

Micronutrients matter

For field crops to reach their full potential it is important to consider more than just nitrogen, potassium, and phosphorus (NPK). Micronutrients are also essential, including boron (B). Over recent years, the incidence of micronutrient deficiencies has increased. In Brazil, up to 60% of savannah soils are believed to be deficient in boron. This is, in part, a result of intensive cropping, loss of topsoil by erosion, loss of micronutrients through leaching, liming of acid soils, decreased use of farmyard manure in favour of chemical fertilizers, increased purity of chemical fertilizers, and use of marginal lands for crop production.

Boron in the soil

Boron is found in the soil solution, mainly as boric acid (H₃BO₃). This is also the form that it is absorbed by plant roots. But boron availability and take-up by plants is influenced by different factors including the soil pH, the soil moisture level, and levels of calcium and nitrogen in the soil. In addition, excess rainfall or irrigation can leach boron from the soil. Boron is introduced to the soil by the action of organic matter breaking down in the soil. The texture of the soil also influences the availability of boron. In Brazilian soils, especially in sandy soils with low levels of organic matter, the lack of access to this element becomes even more pronounced.

Factors affecting boron availability

When adding boron to the soil, the nature of the soil is important. For boron deficient soils with high clay or organic matter content, or for soils that have been limed, it is recommended that a higher dose of boron is applied. This is due to boron adsorption in the soil which locks the boron away from the plant roots.

The pH of the soil also impacts boron availability. Between pH 5 and 7, the ideal range for crops, boron remains largely in the undissociated, and therefore available, boric acid form. Above this, between pH 6 and 9, boron becomes adsorbed by iron and aluminum oxides. In soils that have been limed, or with a higher pH, boron can bind to carbonate, precipitate in the form of calcium borate, or be adsorbed into calcium carbonate, becoming unavailable to plants.

Generally in Brazilian soils, it is organic matter which provides the majority of boron required by plants. The soil conditions which favor organic matter decomposition, making boron available to plants, are high temperatures, soil moisture, microbial activity, and soil aeration.

Soil water can become a limiting factor for boron uptake by plants. Boron needs to be in the soil solution. In dry conditions, there is less moisture in the soil, and so plant roots cannot absorb the boron. In addition, dry conditions decrease the mineralization of organic matter, so reducing the boron availability further.

Under high precipitation the risk is leaching, especially in sandy soils where boron can be leached away from the plant roots, a process that is more significant in soils that have not been limed.

Boron within plants

Once boron has been absorbed by the roots, it’s movement within the plant is unidirectional within the xylem, moving from the plant’s roots to the shoots. In the phloem, which provides transport from the leaves to the rest of the plant, boron is considered to be practically immobile. This means that boron deficiency symptoms appear in young tissue and at growth points.

Role of boron in plants

Boron’s is key in a diverse range of plant functions, including:

• plasma membrane stability
• nucleic acid metabolism
• movement of sugars
• auxin regulation
• tissue differentiation
• root elongation
• synthesis of phenolic compounds
• biological nitrogen fixation, and
• pollen tube growth.

Plants exhibit boron deficiency symptoms in new tissues and apical meristems, causing a reduction in plant size, leaf deformation, and even death of the terminal bud, leading to a reduced yield in many crops.

Boron toxicity

Plants are also susceptible to surplus boron, which can cause boron toxicity. This is more common in arid regions; where the soil’s source material is marine in origin; or where there has been excess boron application through either soil or foliar fertilizers. Symptoms of boron toxicity include mottled chlorosis, and later, necrotic spots on the edges of older leaves, where there is greater transpiration.

Boron mitigates aluminum toxicity

Tropical soils present limitations to the growth and establishment of crops, and this is mainly due to the effects of soil acidity. Among the acidity components, aluminum is one of the main limiting factors for crop productivity. In addition to aluminum’s phytotoxic effects, when in the soil solution it also decreases cell division, and the ability of roots to elongate, producing thicker, deformed, and brittle roots. Aluminum also interferes with DNA synthesis, decreases cell division and replication, alters plasma membrane permeability, and creates changes in the functions of mitochondria and the Golgi complex, reducing plant growth and development.

It has been proposed that boron can be used to mitigate these negative effects thanks to its role in maintaining membranes and cell walls. Boron also increases the formation of proteins and pectic compounds close to the roots, increasing the plant’s tolerance to toxic aluminum levels. Indeed, research has shown that citrus roots grown in an aluminum solution were 69% larger when boron was added to the solution. Further research has also shown that under high aluminum conditions, boron stimulates the antioxidant system, especially the enzymes catalase, peroxidase, ascorbate peroxidase, and glutathione reductase, which reduce the effects of aluminum on the roots, and its concentration in leaves and roots.

Soil analysis

Soil analysis is important to assess the availability of nutrients in the soil. While the use of soil analysis is still restricted, it is increasing and contributing to significant yield increases. Soil analysis shows that boron content in soil typically varies between 7 and 80 mg/dm³, only between 0.1 and 3 mg/dm³ is available to the plants due to the dynamics of boron in the soil plant system.

Table 1: Boron classification in Brazilian soils

Within Brazil, up to 90% of the soil in the Brazilian savannah and in São Paulo (SP) is classified as average-to-low in boron (see table 1). Most recommendation manuals stipulate that adequate boron content in Brazilian soil for most crops is close to 0.6 mg/dm³, except for the southern states where the level is lower due to the higher organic content in the soil (see table 2). However, in a 2016 competition for the soybean yield, it was shown that areas with yields above 78 sacks/ha had boron content in the 0-20 and 20-40 cm layers that were above 0.8 and 0.7 mg/dm³, respectively. This highlights the importance of boron for reaching higher yields, and has encouraged more producers to apply boron fertilizer products.

Table 2: Recommended boron content for soils in Brazil

Boron fertilizer sources

The majority of boron containing fertilizers have guaranteed levels close to 10% boron. However, as mentioned earlier, the form of the product will vary the availability of the boron. The main boron sources are: ulexite, colemanite, hydroboracite, and kernite. The solubility and stability of these sources is indicated in figure 1 below. In addition to the intrinsic variation between these groups, there is also variation within these groups of rocks. The manufacturing process can impact the solubility of the product in the field, and therefore the availability of boron to plants. So, while different products may have similar boron content, in terms of boron availability to the plant, there can be big differences.

Figure 1: Solubility and stability of boron sources.

If we look at ulexite, for example, there are two types of manufacturing process, using either calcination or acidulation. Calcination uses high temperatures to dry the raw material, aiming to increase the boron levels in the fertilizer through dehydration. While the boron concentration of these fertilizers in Brazil is typically higher, at around 12-15%, the solubility is generally lower, meaning the boron is not so readily available to plants.

ICL’s Produbor 10 technology uses natural (non-calcined) ulexite, which undergoes partial acidulation. Produbor has 10% boron content, and it is 90% soluble in water. The granules are uniform (2-4 mm) and have a high degree of hardness. Practically this means there is less dust, less segregation, better fertilizer distribution, and improved operational performance, making Produbor 10 and excellent management option for boron correction in soil.

In soils which are also deficient in sulfur, Sulfurgran B-Max can be applied. With formula’s containing 1% or 2% boron, and high concentration of sulfur, Sulfurgran B-Max provides a gradual supply of boron and sulfur to plants, improving fertilizer efficiency and reducing leaching losses. The net result from trials in Minas Gerais, is that Sulfurgran B-Max 2% can increase the yield of soybean crops by up to 21% (see table 3).

Table 3; Effect of sulfur and boron on soybean yield in Minas Gerais state