Boron

• Availability / solubility of boron is pH dependent, boron deficiency is a problem on alkaline soils (Soil pH >7.0).
• Boron is easily leached from the soil
• The best way to establish the soils boron status is either through soil / plant analysis.
• Soil organic matter has a key role to play in trace element supply
• Boron deficiency is common on light free draining soils
• In mineral soils a content of 1mg/l is satisfactory & 0.5mg/l is deficient
• Crop boron requirement can be satisfied either as a soil/liquid fertiliser or a foliar spray.
• Boronated compounds containing 0.22 – 0.33% B applied at 1,250kg/ha supply equivalent of 2.7 – 4.0kg/ha of B, respectively.

BORON (Supplied by P Parle,1998)

Boron (B) is essential for plant growth and there is some evidence to suggest that it has a role in animal nutrition (Nielsen, 1986). Boron is one of the less abundant elements with an average value of ca. 10 mg/kg in the earth’s crust. The B concentration in rocks ranges from 5 mg/kg in basalts to 100 mg/kg in shales. Total B values for Irish soils fall within the range 20-100 mg/kg, however, the total content is of little value for assessing availability. The water-soluble fraction represents the B which is readily available to plants, and this fraction can be quite variable depending on soil texture and reaction. The water-soluble B concentrations in Irish soils normally fall within the range 0.1-4.0 mg/kg with a mean value of the order of 0.75 mg/kg.

Some factors affecting availability

Liming

Liming reduces the availability of boron. This reduction in availability following liming arises from several reactions or processes (a) the adsorption of B on freshly precipitated Al (OH)₃, which accounts for most of the B adsorption, (b) the formation of insoluble meta-calcium borates and (c) in the case of peat soils the reduction in availability is ascribed to the formation of boric acid esters with higher alcohols.

Organic matter
The greater availability of B in surface soils compared to subsurface soils is related to relative organic matter levels and in humid regions, where soluble salts are leached out, available B is largely held in the organic matter. Organic matter can assist in reducing B loss from soils and at the same time hold the element in an available form. Although organic matter does influence B availability it is greatly conditioned by such factors as moisture content of soil, the pH status and the degree of decomposition of the organic matter.

Soil moisture
Boron availability has been shown to decrease under dry conditions and B deficiency is associated with dry summers. This can partly be explained by the plant’s inability to utilise soil boron due to lack of moisture in the root zone. When the surface dries out, roots are forced to explore deeper horizons and B availability has been shown to decrease with depth.

Soil texture
It is self evident that there is a greater movement of nutrients in sandy soils than those of heavier texture. Light-textured soils contain less available B than do the heavy-textured soils and B deficiency is more common in them.

Relationships with other elements

Nitrogen
Nitrogenous fertilisers as such do not seem to affect B availability but where B soil concentrations are low, the increase in growth resulting from the use of N would predispose towards deficiency.

Control measures

Apply boron based on crop requirement. Please refer to Teagasc The-Green-Book