Calcium.— Lack of this element in plant nutrition is first indicated by a more or less marked yellowing of the young leaves due to a reduction in number and size of the chloroplasts though the chlorophyll itself may be normal. The starch made by the chloroplasts is converted with difficulty into sugar. This retardation is probably due to the fact that in calcium starvation the production of the starch transforming enzyme (diastase) is greatly reduced.
Magnesium.— While this element does not appear to be of such fundamental importance to the vegetable cell it is nevertheless absolutely required, especially during the periods of flowering and fruiting. A very small amount may often suffice to meet the requirements of growth up to the flowering period, but at this time a decided lack of magnesium results in the failure to set fruit or its failure to mature. With a more decided lack normal vegetation soon ceases, proleptic shoots develop with shortened sterns, small, distorted, yellowish leaves crowded together in rosettes somewhat resembling potash starvation except in color.
It appears to be necessary that magnesium and calcium always be associated in plant cells except in some of the lower classes of plants. The ratio of one part of magnesium to three of calcium is the most favorable in many cases though this is quite variable for different species. In a lame number of cases an excess of magnesium over calcium results in greatly retarded growth and an early death of the cells.
Both magnesium and calcium oxides (lime) play a secondary part in the soil by combining with the acids set free by root action, fermentations and other biological and chemical processes. If these were not at once combined the root hairs and fowling roots would be injured or and the plant would suffer from general starvation if not actual poisoning.
Sulphur.— This element usually absorbed by plants in the form of sulphates is one of the absolute requirements of all organisms. It enters into the structure of protoplasm and the more highly organized nitrogenous foods, proteids, required as the dirwt food of growing cells. Its abnormal reduction results in the cessation of proteid manufacture and consequently causes proteid starvation. Growth ceases and death follows unless the element in some available form is supplied.
Other elements and compounds are often important to the plant though not required. Silica for example is found very commonly deposited in the cell walls of plants, especially in the epodermal cells of cereals and grasses. Its presence stiffens the stems and leaves and in a measure protects them from biting and sucking insects and other small animals. Aside from decreased resistance to such attack a plant grows as well without silica as with it.
Chlorine.— This element in very small quantity appears to be necessary to the vigorous growth of plants, especially during the formation of flowers and fruit. In excess it is usually injurious except to plants adapted to soils rich in chlorides, such as sugar beets, asparagus, celery. etc. It is an interesting fact that sugar beets and celery are rendered much more resistant to spot diseases caused by Ceroospora when well supplied with sodium chloride, than without it. Why this is so has not been determined.
Iron.— A small amount of this element is necessary to the formation of chlorophyll. An insufficient amount of iron is indicated very quickly by the formation of chlorotic or white leaves which green on the addition to the soil of a soluble iron salt, such as iron sulphate. Often the •failure of a plant to obtain iron is due to the death of the root hairs and feeding roots from various causes, alkali or acid, excessive moisture or dryness, insect or fungous attack, etc. In such cases, however, there are indications of general starvation as well as of.lack of iron, and the diseased con dition can be cured only by removing the cause of the death of the roots.
Atmospheric Conditions.— The close co ordination between the conditions affecting the plant in the soil and atmosphere is at once apparent. In most of our ordinary plants, water, with various materials in solution, is absorbed by the roots from the soil and passes up through the stem to the leaves and other parts above ground. From these there is a continual loss of moisture, regulated in part by the plant, but depending largely upon whether or not conditions of the atmosphere, light, etc., favor evaporation, or transpiration as it is more correctly termed where living plants are under consideration.