Salinity refers to and is used as a measure of the quantity of solutes contained in water. It is the term commonly applied to salts of the sea and to soluble inorganic substances contained in water from alkaline land.
Less frequently it is used in con- nection with the solutes in ground waters from humid regions. In this book it is used to denote all soluble inorganic constitu- ents of natural waters. For example, so-called "hardness of water is due to salinity caused by the presence of calcium and magnesium bicarbonates. Likewise, sodium carbonate used for ''softening," to overcome hardness, is a class of salinity. Total salt content, as well as the character of the salts, affects the quality of water for hydroponic use.
The physical prop- erties of solutions vary with their saline content. This salinity is quoted either as a weight in volume measurement, that is, by grains or milligrams per gallon or liter, or by weight ex- pressed as the number of parts of solutes per million of water. Because the physical property of the solution, when expressed as an osmotic force, cannot exceed certain limits without harm- ing the crops, it follows that the greater the natural salinity of the water the less leeway there is for the addition of nutrients.
Furthermore, the elements should be present in their proper percentage relationships, and those used in large quantities con- stitute a correspondingly large part of the total concentration of the nutrient solution. If elements which are not needed, or are needed in small quantities, are already present in large amounts in the water, you must add correspondingly larger amounts of those nutrients which the plants need in quantity. Thus, the original salinity of the water determines how much chemical plant food can be added before the solution becomes too concentrated. Furthermore, the character of the salinity determines in what form and how much of each nutrient must be added to obtain the correct proportions between the ele- ments in the solution.
The effect which the character of a water's solutes has on its salinity is well illustrated by a comparison between ground water and sea water. Ocean waters are too saline even when diluted thirty-five times; that is, to a concentration of about 1,000 parts per million, which is equal to that obtained by add- ing one pound of nutrient salts to 125 gallons of water. This is the recommended concentration for the hydroponic solution when pure water is used. It will not necessarily render ground waters unfit for use. The difference lies in the fact that ocean salinity is caused chiefly by sodium chloride while that of ground water is caused by mixtures of carbonates, bicarbonates, sulfates, and chlorides of calcium and sodium. Sodium chloride has a much smaller molecular weight than the salts of ground waters. Consequently, it has more molecules per a given per- centage of salinity and is capable of exerting greater osmotic pressure.
The point at which salinity becomes too high is not defi- nitely known, but information from various hydroponicums indicates that it lies within the range from 1,500 to 2,500 parts per million. The potential osmotic pressure of solutions in this range may not be too high but the non-essential elements con- stitute so large a proportion of the solutes that abnormal com- position of the plant may result. This would cause curtailment of plant growth. When pure water is used, plants may grow quite well in solutions of greater concentration than 2,500 parts per million. Usually, however, pure water will not be used. And experience has shown that you should be very cautious in using water containing such large quantities of solutes that addition of nutrients will bring the solution up to the 2,500 mark. The best growing conditions seem to be obtained when chemicals are added to water containing solutes so that the total concentration is between 1,000 and 1,500 parts per million.
Effects of Climate
Elements not absorbed by the plants increase in concentra- tion as the water is used up. The time required for the solu- tion to reach too high a concentration depends upon the original salinity of the water and the rate at which it is used. Rate of use is in turn influenced by rate of growth and char- acter of environment. Therefore, the effect of salinity on the properties and fitness of a solution for crop production will vary with climatic conditions. A given water may give satisfactory results in one locality but not in another.
Types of Water
Classification of waters according to the properties imparted by their solutes is necessarily quite arbitrary. However, some clear idea of hydroponic technique may be obtained by arranging .them in the following categories: 1. Saline waters well-suited for use. 2. High-saline waters that can never be used. 3. Low-saline waters containing toxic elements which may or may not respond to corrective treatment. Comparatively few non-saline waters contain substances which render them permanently unfit for hydroponic use. Some may, however, require treatment before they are suitable. For example, some spring and ground waters contain harmful quantities of sulfides. These can be rendered harmless by allowing the water to stand in shallow basins with its surface exposed to air before being used. In another harmful class of substances are the borates, which render water permanently unfit for hydroponic use since there is no practicable way of re- moving them. Boron is needed only in very small quantities by the plant, so water containing even traces of this element must be watched carefully. Various saline waters and some mineral springs contain toxic concentrations of manganese. Unless contaminated by industrial waste products, natural water usually does not contain toxic concentrations of salts of 68 Wale* the heavy metals: copper, zinc, nickel, and cobalt. There is some danger, however, that water passing through copper and zinc fittings may dissolve harmful amounts of these elements. It is doubtful whether any ground water contains too much iron but, if so, it can be rendered harmless by aeration or by the addition of hydroxides. These chemicals precipitate the iron out of solution. Aluminum occurs in large quantities in certain springs and in acid waters. Toxic concentrations of this substance can be treated in the same way as those of iron. Acid waters should always be examined thoroughly before any attempt is made to use them. Sulfuric acid is usually the cause of the acidity and can be corrected by the addition of lime. Occasionally, how- ever, hydrochloric acid is the offender and any attempt to cor- rect this condition results in the formation of chlorides even more undesirable than the sulfates. City water supplies are usually treated with disinfectants, of which chlorine is the most common. The effect of such sub- stances on water for hydroponic use has not been studied but it is improbable that they would prove harmful to plants. Some materials not toxic to plants may still prove undesir- able. For example, selenium in certain forms may be absorbed by the plant without harm but crops containing this element may be harmful when fed to animals. Fluorine is another element in this category. Nothing is known about its effect or humans when eaten as part of the plant. But the fact that very small amounts of it in drinking water cause discoloration of the teeth shows the need for positive knowledge as to fluorine's role in plant nutrition, for the chances are that more of this ele- ment will be contained in crops grown by hydroponics than in those produced by agriculture. Most waters contain from 100 to 1,000 parts per million of dissolved material. The average for well-water used in hydro- ponicums now operating exceeds 500. This is a higher figure than that reached in most run-off and drainage water from land. At the same time these ground waters, particularly those orgi- nating in and draining arid lands, are usually higher in solutes than are lakes and rivers. Rivers draining agricultural land average well over 200 parts per million. But there are many mountain streams, lakes, and rivers that average considerably less than 100. A complete analysis need not be taken of water containing 100 parts per million, or less, provided it is not unfit for drinking and irrigation. When salinity exceeds 200 parts per million, have the water analyzed.