Plants need to absorb nutrients from the soil to be able to make growth by the action of photosynthesis in the leaves. Half a dozen elements are needed in relatively large amounts and are called major or macro-nutients, whilst some others are required in much smaller quantities and are termed trace elements.
The macro-nutrients are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), sulphur (S) and magnesium (Mg), as well as carbon (C), oxygen (O) and hydrogen (H). Shortage of any one of these is very damaging to growth, but also an excess can kill. The letter in parentheses after the nutrient is the international chemical letter which will be found on fertiliser packets.
Nitrogen is needed by the plant for the formation of proteins, which are essential for growth. Although nitrogen is the main gas in the air, this is inert and plants have to obtain it from the soil as a nitrate, nitrite or ammonium compound. Nitrogen is usually restricted in the soil to the top few inches.
Phosphorus and potassium are both needed for cell division and for the ripening of fruits. Potassium is very soluble, and therefore easily lost by leaching from many soils. Phosphorus is much less soluble; in fact its very insolubility in some soils can create an artificial shortage as far as the plants are concerned.
Calcium is needed by all plants for the cell walls.
Sulphur is used in root development and as a component of proteins.
Magnesium is an essential constituent of the chlorophyll molecule, which is involved in photosynthesis.
Carbon, hydrogen and oxygen are needed in large quantities to make all organic compounds. Carbon and oxygen come from carbon dioxide in the air and are absorbed by the leaves, and hydrogen comes from water.
Trace elements are required in very small quantities; molybdenum is only needed in the plant tissues at a concentration of one part in one hundred million, and may be fatal if more than ten times this amount is present. Molybdenum is important in the use of nitrate and nitrite forms of nitrogen. Iron is essential for the manufacture of chlorophyll, boron for the uptake from the soil of calcium, and zinc, manganese and copper in the formation of enzymes and proteins.
pH and Nutrient availability
pH is a measure of the alkalinity or acidity of the soil. A neutral soil has a pH of around 6.5 to 7; above pH 7 the soil is alkaline and at pH 6 and below it is acidic. The effect of pH is to alter the way in which nutrients are dissolved in solution, and therefore the way that plants can absorb them.
Some nutrients are only available to plants between certain pH levels. Phosphorus may be plentiful in alkaline soils, but most of it is insoluble; bonemeal will not add usable phosphorus to alkaline soils, although it is a satisfactory way of adding phosphorus to acidic ones.
Certain deficiency symptoms are due to the unavailability of nutrients at certain pH levels. Many plants show a yellowing of the foliage or chlorosis on alkaline sites due to the insolubility of iron on these soils. This form of deficiency cannot be controlled by using rusty nails but is helped by giving iron as iron sequestrine, a compound which makes iron available to plants at higher pH levels. Hydrangeas will tolerate both acidic and alkaline soils, but the aluminium ions which turns the flowers of some hydrangeas blue are not available to the plant on alkaline soils.
Plants differ in their capability to extract nutrients from the soil at different pH levels and therefore some plants can only grow at certain pH levels. Others may naturally be found in areas where a certain nutrient is scarcely available; if planted in a soil where that nutrient is available they may be poisoned by their inability not to take it up – rhododendrons and calcium is one such case.