(b) Effect of soluble zinc salts in the presence of nutrients.

[Krauch (1882)] grew various plants in the presence of nutrient solutions and quantities of zinc sulphate varying from ·1 to ·8 gm. per litre (= 1/10,000 to 8/10,000). Barley proved to be very sensitive, even to the weakest strength of the poison, as the plants soon showed reddish flecks, while all were dead within six weeks, the control plants without zinc remaining quite healthy. Certain grasses took longer to kill than barley, those with ·4 gm. zinc sulphate per litre dying in about seven weeks, while 13 weeks elapsed before the others were killed. Even after this length of time the plants with ·1 gm. zinc sulphate per litre still survived, although in a very sickly condition. With willow, again, even ·1 gm. zinc sulphate per litre made the plants very sickly after four weeks, growth being weak, the leaves yellow, and the roots brownish. In this case the solutions were renewed, but the plants treated with zinc compounds were dead within eight weeks from the start, the controls being very healthy.

The next year (1883) [Storp] repeated these experiments made by Krauch and corroborated his results fully. Barley and grasses (timothy and others) grown in solutions of zinc sulphate, both with and without nutrients, soon lost their green colour and became covered with rusty brown flecks, the barley dying within 14 days, and the grasses soon after. With willow, too, the toxic action was again manifested.

[True and Gies (1903)] showed that the addition of calcium salts in appropriate concentrations reduced the toxicity of zinc salts considerably, a result similar to that which they obtained for copper.

Recent experiments at [Rothamsted] have shown that zinc sulphate is very toxic to barley, though the plant is able to make some slight amount of growth even in the presence of a solution of the anhydrous salt ZnSO4 as strong as 1/5000, rapid improvement occurring as the concentration decreases to 1/2,500,000 or less ([Fig. 6]). On the whole the higher strengths of zinc sulphate are less poisonous to peas than they are to barley. At a concentration of 1 in 1⁄4 or 1 in 1⁄2 million in different experiments the growth was nearly as good as with the control plants, though it consistently lagged a little way behind until a dilution of 1/10,000,000 was reached (Figs. [7] and [8]). Incidentally it is very striking to see the desperate efforts that badly poisoned pea plants make to reproduce themselves. Growth of the roots is nearly always checked in advance of that of the shoots, probably on account of the contact of the roots with the poison. In the greater strengths of such poisons as zinc and copper sulphate root growth is checked from the outset, but usually a very little shoot growth is made, and one frequently obtains ridiculous little plants about an inch high bearing unhappy and diminutive flowers, which are occasionally replaced by equally unhappy and miniature fruits. The same thing has also been noticed when unsuccessful attempts have been made to introduce spinach as a test plant for water cultures.

Fig. 6. Curve showing the mean value of the dry weights of ten series of barley plants grown in the presence of anhydrous zinc sulphate and nutrient salts. (March 2nd–May 8th, 1911.)

Fig. 7. Photograph showing the action of anhydrous zinc sulphate on pea plants in the presence of nutrient salts. (Sept. 30th–Dec. 20th, 1912.)

1. Control.
2.1/5,000zincsulphate.
3.1/10,000„„
4.1/50,000„„
5.1/100,000„„
6.1/250,000„„
7.1/500,000„„
8.1/1,000,000„„
9.1/2,500,000„„