Excessive quantities of boric acid are decidedly poisonous to plants, the action being well marked in water cultures. [Knop (1884)] found that free boric acid was poisonous in neutral food solutions when present at the rate of ·5 gram per litre, but he was not able to detect boron in the ash of the roots of the experimental plants. [Archangeli (1885)] placed seedlings of maize, white lupins, Vicia sativa and Triticum vulgare in solutions of boric acid varying in concentration from 1–·05%, with controls in spring water. In the latter case the development was normal, with 1% boric acid the plants were killed, while it was found that the weaker the solution (within the indicated limits) the stronger the root and shoot growth.

[Hotter (1890)] stated that it was known that 1/20,000 boric acid by weight was harmful to soy beans in nutritive solutions. He experimented with peas and maize, placing the seedlings first in distilled water, later in nutritive solutions. When the peas were nineteen days old they were transferred to nutritive solutions containing 1/1000–1/100,000 boric acid by weight per litre, and within three days the plants with 1/1000 showed signs of injury. Two days later all the plants showed signs of poisoning in that, even with the weakest strengths, the lower leaves were flecked with brown, especially at the edges, while with the greater strengths the lower leaves were dead and the flecking had extended to the upper leaves. In eleven days from the start the plants with 1/1000 boric acid were completely dead, while the other plants showed more or less signs of poisoning. The dry matter and ash decreased steadily with the increase in the boric acid, while the boric acid per 100,000 parts of dry matter increased steadily from 8 to 557 parts. Similar experiments were carried on with potassium borate and with borax; the results showed that, weight for weight, borax is less toxic than potassium borate, which in turn is less toxic than boric acid, while at a strength of 1/100,000 there is little to choose between the three poisons. Similar results were obtained with maize; plants treated with boric acid or potassium borate yielded about 2300 parts boric acid in 100,000 parts dry matter. The general conclusion arrived at by Hotter was that the effect is not so much that of a general poisoning as of a bleaching of parts of the leaf, mere traces of boron being harmless. The cause of injury is local inhibition of assimilation and killing of roots in stronger concentrations. Increase of the strength of boron raises the toxicity until 1/1000 practically inhibits increase in dry substance. The boron was found to be fairly evenly distributed through sound and affected organs.

[Kahlenberg and True (1896)] worked with seedlings of Lupinus albus L., limiting their experiments to those of 15–24 hours in duration. Various combinations of boron and other substances were tested. With boric acid alone 2⁄25 gram molecule per litre killed the plants, with 1⁄25 they were apparently just alive, while 1/100 and less had no injurious effect. Boromannitic acid was possibly more poisonous than the boric acid, while a combination of boric acid and cane sugar proved slightly less toxic. The short duration of these experiments limited their scope considerably, as with certain concentrations the toxic action would not become evident within the prescribed limits of time.

[Agulhon (1910 a)] worked with sterile nutrient solutions, and found that the higher strengths of boric acid hindered growth, 200 mg. boric acid per litre rendering growth impossible. He supported Hotter’s idea that the toxic action affects the roots and the formation of chlorophyll, and he stated that the plants are less green as the dose of boron increases, plants growing in doses of above 10 mg. per litre being yellowish. In other experiments he found that at 100 mg. boric acid per litre life seems impossible for the plant. The roots seem to be more adversely affected by toxic doses than do the shoots. In control plants Agulhon determined the stem/root ratio as 6, with a little boron as 7, while the ratio rose to 13 as the dose of the poison increased to 50–100 mg. boron per litre.

The Rothamsted experiments show that boric acid is definitely poisonous to barley down to a strength of 1/250,000 ([Fig. 15]), the depressing effect frequently being evident at much smaller concentrations, while peas can withstand far more of the poison, the limit of toxicity being about 1/25–1/50 thousand ([Fig. 16]). With the greater strengths of poison the lower leaves of both barley and peas are badly damaged. In barley the leaves turn yellow with big brown spots, giving the leaves a curious, mottled appearance, while with peas the poisoning seems to begin at the tip and edge of the leaves, spreading inwards, without, however, showing the large spots as in barley. So far as chemical tests go at present, it is very probable that boron is deposited in the leaves in the same way as manganese, and that this is the cause of the degeneration. As with manganese, the lower leaves are attacked first, and the trouble spreads upwards, one leaf after another being involved. These observations fit in very well with those of Hotter, and the hypothesis of direct boron poisoning gains support from the fact that in dilutions which produce stimulation of the shoot the leaves show hardly any sign of dying off, even after prolonged growth in the solutions. With barley the effects of boron can be seen in the leaves in concentrations as low as 1/2,500,000, and it may be significant that this is the point at which the depressant action of boric acid entirely ceases in many cases.

Fig. 15. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of boric acid and nutrient salts. (May 1st–June 20th, 1911.)

Fig. 16. Photograph showing the action of boric acid on pea plants in the presence of nutrient salts. (Sept. 30th–Dec. 20th, 1912.)

1. Control.
2.1/5,000boricacid.
3.1/10,000„„
4.1/25,000„„
5.1/50,000„„
6.1/100,000„„
7.1/250,000„„
8.1/500,000„„
9.1/1,000,000„„