Many details of the sand and soil culture methods have been published by various investigators, e.g. [Hiltner] gives accounts of sand cultures, while the various publications issued from Rothamsted deal largely with the soil experiments. As this is the case, and as all crucial experiments have always been and must always be done in water cultures, it is only necessary to give here full details of these.
The great essential for success in water culture work is strict attention to detail. Cleanliness of apparatus and purity of reagents are absolutely indispensable, as the failure of a set of cultures can often be traced to a slight irregularity in one of these two directions. Purity of distilled water is perhaps the greatest essential of all. Plant roots are extraordinarily sensitive to the presence of small traces of deleterious matter in the distilled water, especially when they are grown in the absence of food salts. Ordinary commercial distilled water is generally useless as the steam frequently passes through tubes and chambers which get incrusted with various impurities, metallic and otherwise, of which slight traces get into the distilled water. [Loew (1891)] showed that water which contained slight traces of copper, lead or zinc derived from distilling apparatus exercised a toxic influence which was not evident in glass distilled water. This poisonous effect was removed by filtering through carbon dust or flowers of sulphur. Apparently only about the first 25 litres of distilled water were toxic, in the later distillate the deleterious substance was not evident.
The best water to use is that distilled in a jena glass still, the steam being passed through a jena glass condenser. For work on a large scale, however, it is impossible to get a sufficient supply of such water, while the danger of breakage is very great. Experiments at Rothamsted were made to find a metallic still that would supply pure water. While silver salts are very injurious to plant growth it was found that water that had been in contact with pure metallic silver had no harmful action. Consequently a still was constructed in which the cooling dome and the gutters were made of pure silver without any alloy, so placed that the steam impinged upon the silver dome, condensed into the silver gutter and was carried off by a glass tube into the receptacle. Such water proved perfectly satisfactory so long as any necessary repairs to the still were made with pure silver, but a toxic action set in directly ordinary solder was employed. More recently a new tinned copper still has been employed with good results, but this is somewhat dangerous for general purposes, as in the event of the tin wearing off in any place, copper poisoning sets in at once. The water is always filtered through a good layer of charcoal as a final precaution against impurity.
In the Rothamsted experiments no attempt is made to carry on the cultures under sterile conditions. Bottles of 600 c.c. capacity are used, after being thoroughly cleaned by prolonged boiling (about four hours) followed by washing and rinsing. The bottles are filled with nutritive solution and the appropriate dose of poison, carefully labelled and covered with thick brown paper coats to exclude the light from the roots and to prevent the growth of unicellular green algae. The corks to fit the bottles are either used brand new or, if old, are sterilised in the autoclave to avoid any germ contamination from previous experiments. Lack of care in this respect leads to diseased conditions due to the growth of fungi and harmful bacteria. Two holes are bored in each cork, one to admit air, the other to hold the plant, and the cork is cut into two pieces through the latter hole.
The seeds of the experimental plants are “graded,” weighed so that they only vary within certain limits, e.g. barley may be ·05–·06 gm., peas ·3–·35 gm., buckwheat ·02–·03 gm. In this way a more uniform crop is obtained. Great care is needed in selecting the seeds, the purest strain possible being obtained in each case. With barley it has always proved possible to get a pure pedigree strain, originally raised from a single ear. In this way much of the difficulty due to the great individuality of the plants is overcome, though that is a factor that must always be recognised and reckoned with. The seeds are sown in damp sawdust—clean deal sawdust, sifted and mixed up with water into a nice crumbly mass—and as soon as they have germinated and the plantlets are big enough to handle they are put into the culture solutions. Barley plants are inserted in the corks with the aid of a little cotton wool (non-absorbent) to support them, care being taken to keep the seed above the level of the water, though it is below the cork. With peas it is impossible to get a satisfactory crop if the seed is below the cork, as the plant is very prone to bacterial and fungal infection in its early stages, and damp cotyledons are fatal for this reason. Consequently the mouths of the bottles are covered with stout cartridge paper, the pea root being inserted through a hole in the paper, so that the root is in the liquid while the cotyledons rest on the surface. As soon as sufficient growth has been made the papers are replaced by corks, the remnants of the seeds still being kept on top in the air. Other plants are treated according to their individual needs and mode of germination ([Fig. 1]).
Fig. 1. Diagrammatic sketches showing methods of setting up water cultures.
| A. | a. | Seedling of cereal. |
| b. | Cork bored with two holes, and cut into two pieces through one hole. | |
| c. | Food solution. | |
| B. | a. | Pea seedling. |
| b. | Paper shield which supports the seedling. | |
| c. | Brown paper cover over bottle of food solution. |
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. |