Date on Master's Thesis/Doctoral Dissertation
College of Arts and Sciences
Soon after the atomic theory was established, it was found that the plant uses as sources of food not only the air and water, but also different constituents of the soil, dissolved in the soil moisture—the mineral nutrients. Ashes of different burned plants were analyzed; they were found to vary with the soil in which the plants were grown as well as with the species of the plant and the special organ from which the ashes were obtained. A great number of minerals was found in the ashes. The question arose soon, which of these elements is essential to the nutrition and growth of plants. Different experimental methods were evolved to solve this problem. It is possible to decide if any of the mineral nutrients is really essential to the plant only by growing the plant in an artificial medium, from which we are sure the element in question is absent. Soil extracts were used as artificial culture media; ash of plants dissolved in water was used as such. But the most satisfactory results were obtained by water cultures and pure sand cultures. Each one of the two last mentioned methods has its advantages and disadvantages. In sand cultures we are never sure that the sand itself does not contain any traces of soluble minerals, and it is hard to keep constantly the same moisture conditions in all cultures; but at the same time, sand cultures approach closer the natural physical conditions in the soil. Water cultures are much easier to prepare and to control; all plants are sure to be presented the same amount of the same nutritive solution, and there are no interactions between constituents of the nutritive solution and any inert matter. It was established by using different artificial media that the elements needed for the normal nutrition and growth of plants are carbon, oxygen, hydrogen, calcium, potassium, phosphorus, magnesium, sulphur, nitrogen, and iron. The plant gets its carbon from the carbon dioxide of the air and it obtains oxygen from the air. Oxygen and hydrogen, combined as water, are absorbed by the roots. Iron is needed only in later stages with the forming and functioning of the chlorophyll. All the other elements must be presented to the plant in the nutritive solution. Water culture nutritive solutions made up in different ways were used. In such water culture solutions it is possible to get fully grown plants, bearing normal seeds, and this fact was actually demonstrated many times with different plants. The earlier workers, as Knop, Sachs, Pfeffer, and the others, generally used water solutions made up with four, five, six, or even more salts. The procedure has been simplified in recent investigations, and now most nutritive solutions are made up of three main salts. Livingston (1919) has shown that there are six possible combinations of the elements necessary for normal plant growth that can be made in pairing them in three salts. The first possible combination is calcium nitrate, monopotassium phosphate, and magnesium sulphate. This is the one mostly used in contemporary experimentations, and these three salts were also used in the present work. The salt combinations mentioned above may be regarded as final and incapable of further simplification. The greatest amount of work done till now with water cultures has consisted in growing the plants in different nutritive solutions containing all the essential elements. But it must clearly be understood that the complete nutritive solution is itself a physiologically balanced solution, which contains different elements acting differently upon the plant and one upon the other. The action of a balanced solution is such that the toxic effects that each salt would exert, if it alone were present, is counteracted or antagonized by the other salts present in the solution. Simultaneously with the simplification of the composition of the nutritive solution, other improvements were being made. Among these refinements of technique has been (1) the use of purer salts and purer distilled water, (2) the testing a much greater number of plants so as to minimize the effects of variability, and (3) the controlling more strictly the different environmental conditions, in the nutritive solution itself as well as in the serial surroundings of the plants. The results obtained are of course of increasing value, and they show more and more the actual meaning of the matters investigated. It is believed that the time is ripe now to investigate into the fundamental properties of these nutritive solutions, to get at the finer details of the function of each one of their constituent salts. With this general aim in mind the present work was undertaken. Each one of the three salts constituting the nutritive solution was investigated separately, and its influence upon growth of tops and roots was studied. These three salts were then tried in different mixtures in pairs, and their influence was again investigated. The results obtained from these mixtures were finally compared with those obtained for the single salts. It is hoped that the present work will help us in obtaining a deeper insight into the function of water culture solutions, and will help in understanding better the nutritive requirements of the plant and the function of each one of these nutritive substances separately. It is also hoped that this work will serve as a sound basis for further investigations in the same direction.
Carmin, Joseph 1895-, "The growth of wheat seedlings in solutions containing monopotassium phosphate, calcium nitrate, and magnesium sulphate, singly and in pairs." (1924). Electronic Theses and Dissertations. Paper 209.