Phytotoxicity, boron

Boron in the soil and water exists either as boric acid or as the borate ion. Although the levels generally present pose no problems to animal life, in many areas, present levels of boron in water supplies can be deleterious to agricultural crops. Boron is a typical case in which there exists a small difference between a deficiency level and a toxicity level in the water used for agricultural purposes. Boron phytotoxicity can occur with levels above 2 ppm, and deficiency can occur for many plant species if the levels are below 0.5-1.0 ppm of B. 

Aitken, R.L. and L.C. Bell. 1985. Plant uptake and phytotoxicity of boron in Australian fly ashes. Plant Soil 84 245-257. 

Thus, B(0H)3 is a Lewis add rather than a Bronsted acid (see Chapter 1). Because boron adsorbs most effectively in the pH 8 to 9 range on A1 and Fe oxides and silicate minerals, its availability is generally low in coarse-textured, acid-leached soils and in calcareous soils. Deficiency in add soils is the result of boron depletion by leaching, while deficiency in calcareous soils is caused by strong adsorption and predpitation as relatively insoluble Ca borate salts. In contrast, B toxicity is most commonly found in alkaline soils of arid regions these soils often contain high levels of Na which forms quite soluble borate salts. A lack of rainfall allows soluble borate to accumulate to phytotoxic levels. 

The phytotoxic action of boron compounds was recognised as early as the end of the last century (Peligot, 1876), but they were not applied as total and selective herbicides until the 1920s (Crafts and Raynor, 1936). 

The phytotoxic actions of rubidium mostly affect the transportation of substances in the xylan (Zornoza and Carpona 1996). In order to prevent excessive amounts of rubidium in plant tissues, these authors proposed an increase in the content of potassium, manganese and boron in the soil solution, because of the known antagonism of these elements towards rubidium. Young, growing plants or parts of plants are extremely rubidium-rich and accumulate this element like most other macro, trace and ultratrace elements (Angelow 1994, Wyttenbach et al. 1995). The toxicity of rubidium in plants is low, and essentially unknown. 

These results indicate a striking increase in boron uptake with increasing available boron content in the soil and they suggest that increases in the boron contents of plant leaves of the order of tenfold can be tolerated without adverse effects. It appears that the high toxicity of boron for plants is due to the ease with which this element enters the plant and is translocated within it [131] rather than to any innate phytotoxic property. Since this element appears to be translocated along with water within the plant, there appears to be no device by which it can be excluded at the root surface. 

Domestic soot usually contains relatively high levels of water-soluble boron (around 500 ppm) and this is normally allowed by wise gardeners to leach in the rain for some weeks before it is used as a fertiliser in order to take the heat out of it. It is probably the case that this heat, or factor producing phytotoxic effects, is largely the content of water-soluble boron. 

Among essential trace elements, boron is unusual in that the toxicity problems it produces are normally not persistent, because of the high mobility of this element combined as borate. On the other hand, this very property can produce phytotoxic problems which are very acute. 

In general, the results of these experiments suggest that, while application of municipal compost at rates up to 100 tonnes/ha can lead to substantial contamination of the soil with boron, copper and zinc and to significant effects on the uptake of these elements by a number of vegetable crops, a phytotoxic... 

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