The constitution of the nutritive solution is important, and it is becoming more and more evident that different plants have different optima in this respect. For several years a solution of medium strength was used, containing the following: Potassium nitrate 1·0 gram Magnesium sulphate ·5 „ Sodium chloride ·5 „ Calcium sulphate ·5 „ Potassium di-hydrogen phosphate ·5 „ Ferric chloride ·04 „ Distilled water to make up 1 litre. 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.