This is an excellent solution for barley plants, giving good and healthy growth. While peas grew very well in it, they showed some slight signs of over-nutrition. A weaker solution is being tested which gives very good results. Peas grow very strongly in it and it also seems to be sufficiently concentrated to allow barley to carry on its growth long enough for the purposes of experiment. The solution is as follows:
| Sodium nitrate | ·5 gram |
| Potassium nitrate | ·2 „ |
| Potassium di-hydrogen phosphate | ·1 „ |
| Calcium sulphate | ·1 „ |
| Magnesium sulphate | ·1 „ |
| Sodium chloride | ·1 „ |
| Ferric chloride | ·04 „ |
| Distilled water | to make up 1 litre. |
The latter solution was made up so that the quantity of phosphoric acid and potash approximated more or less to the amount of those substances found by analysis in an extract made from a good soil.
The experiments are usually carried on for periods varying from 4–10 weeks, six weeks being the average time. Careful notes are made during growth and eventually the plants are removed from the solutions, the roots are washed in clean water to remove adherent food salts, and then the plants are dried and weighed either separately or in sets. In order to reduce the error due to the individuality of the plants, five, ten or even twenty similar sets are grown in each experimental series, the mean dry weight being taken finally. Also the same experiment is repeated several times before any definite conclusions are drawn.
Another method of water cultures is used by some investigators, in which the experiments only last for a few hours or days, usually 24–48 hours. While such experiments may not be without value for determining the broader outlines of toxic poisoning, they fail to show the finer details. The effect of certain strengths of poison is not always immediate. Too great concentrations kill the plant at once, too weak solutions fail to have any appreciable immediate action and so appear indifferent. Between the two extremes there exists a range of concentrations of which the effect varies with the plant’s growth. A solution may be of such a nature and strength that at first growth is seriously checked, though later on some recovery may be made, while it is also possible that a concentration which is apparently indifferent at first may prove more or less toxic or stimulant at a later date, according to circumstances. Consequently too much stress must not be laid upon the results of the short time experiments with regard to the ultimate effect of a poison upon a particular plant.
An examination of the various experimental methods shows that while no one of them is ideal, yet each of them has a definite contribution to make to the investigation of toxic and stimulant substances. Each method aids in the elucidation of the problem from a different standpoint, and the combination of the results obtained gives one a clearer picture of the truth than could be obtained by one method alone. Water cultures, with their exactitude of quantitative control lead on by way of sand cultures to pot cultures, and these to field experiments in which the control is largely lost, but in which the practical application is brought to the front.