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Mineral Nutrients

Several mineral nutrients taken up from the soil are imperative for a plant's survival. Without them the plant dies. Among these minerals is a group of elements necessary in so small amounts that its members are called trace elements.

The importance of mineral nutrients for plant nutrition has been pointed out already by J. v. LIEBIG and KARL SPRENGEL. A. FR. J. WIEGMANN and A. L. POLSTORFF confirmed their findings in 1842. The question of the nature of the mineral nutrients remained unanswered since the composition of a plant's ashes does not show whether a certain element found is actually necessary for the survival or whether it is merely a roughage. The problem was solved when the plant physiologist J. v. SACHS (1832-1897) rediscovered the hydroculture technique (hydroponic). It allows to compose exactly defined nutrient solutions and to study the effects of every cation and anion on the growth of the plant. Earlier experiments of J. WOODWARD (1665-1728) had shown that plant grow better in water from a river than in rain water and that growth was promoted after the water had taken up solutes from the soil. The first useable synthetic nutrient solution was produced by J. v. SACHS together with the chemist J. A. STÖCKHARDT. It contains

1g KNO₃, 0.5g CaSO₄, 0.4g MgSO₄ x 7 H₂O, 0.5g CaHPO₄ and a trace of FeCl₃ per 1000 ml water.

He recognized the importance of iron in experiments with iron-free nutrient solutions. In 1882, he wrote:

".....But after some time when the third or fourth leaf of our experimental plant unfolds, the symptoms of an illness become apparent: the leaves that begin to unfold from now on are completely white and produce no chlorophyll. The microscopic analysis shows that no chlorophyll grains exist in the protoplasm of such colourless leaves. This now is the proof that something was missing in our nutrient mixture; we know from earlier observations by GRIS that the illness of our plant, the so-called chlorosis is caused by the lack of iron.....it is sufficient to add a small amount of a soluble iron salt to the water that the roots take up,....to let the previously completely white leaves become green......This observation proves very obviously that iron is necessary for the production of chlorophyll though it does not show whether the iron is actually a component of the green colour itself."

These experiments let SACHS understand the importance of the root hairs for the uptake of solute nutrients. At about the same time (1861), J. A. L. W. KNOP developed the nutrient solution still used very often that is called after him:

1g Ca(NO₃)₂, 0.25g MgSO₄ x 7 H₂O, 0.25g KH₂PO₄, 0.25g KNO₃ and a trace of FeSO₄ per 1000 ml water.

The experiments showed that the cations K⁺, Ca²⁺, Mg²⁺ and small amounts of Fe²⁺ or Fe³⁺, as well as the anions SO₄^2-, H₂PO^4- (or H₃PO₄) and NO^3- are essential for the growth and survival of the plants. Oxygen, carbon dioxide and hydrogen that are taken up from the air or the water (respiration, photosynthesis) are also imperative. The lack of one of these elements cannot be made up for by the surplus of another, chemically closely related one. Potassium, for example, cannot be replaced by lithium, sodium or rubidium. Atmospheric nitrogen, metallic potassium and elementary sulphur are just as useless. Only the respective ions are necessary. The problem of nitrogen use has been touched by H. HELLRIEGEL and WILFARTH in 1886 at the meeting of natural scientists in Berlin (cited from SACHS 1887 according to a story of the Kölnische Zeitung, 1886):

"Buckwheat, rape, mustard, sugar beets, oat and potatoes can take up their complete nitrogen from nitric acid or its compounds. If these plants are fed with nitrogen in the form of ammonia, then they can use it only as far as it is transformed into nitric acid by micro-organisms of the soil. Peas, lupines, seradella, vetches and clover in contrast do not depend on nitrogen bound in the soil but are able to take up nitrogen from the air; they do not use the bound forms but the free nitrogen of the air. These plants live and use the free nitrogen with the help of bacteria that form so-called nodules at their roots."

Certain other ions can be taken up by some plants without being used. Halophytes, for example, take up Na⁺ only because they have a stronger resistance against it than other plants. They have thus opened up an ecological niche for themselves. Silicon (SiO₂) is found in the ashes of horsetail and in the shoots of grasses sometimes even in considerable amounts. But it is not essential. Only diatoms and some other algae need it for the production of their shells. Some marine algae (especially brown algae) accumulate iodine but nothing is known about its significance. The average share that the single mineral elements have in the dry weight of plants is:

NO₃^-: 1- 3%, K⁺: 0.3- 6%, Ca²⁺: 0.1- 3.5%, HPO₄^2-: 0.05- 1%, Mg²⁺: 0.05- 0.7%, SO₂^2-: 0.05- 1.5%.

When in the 20^th century the demands to the purity of chemicals grew, it did become apparent that plants need a number of additional elements , the so-called trace elements for their nutrition. R. D. HOAGLAND (1884- 1949) developed a solution of trace elements he called the A-Z solution, 1 ml of which is added to one of the standard nutrient solutions (for example the nutrient solution of KNOP):

0.5g LiCl, 1g CuSO₄ x H₂O, 1g ZnSO₄, 11g H₃BO₃, 1g Al₂(SO₄)₃, 0.5g SnCl₂ x 2 H₂O, 7g MnCl₂ x 4 H₂O, 1g NiSO₄ x 6 H₂O, 1g Co(NO₃)₂ x 6 H₂O, 0.5g KI, 1g TiO₂, 0.5g KBr in 18 l water.

To today's mind, the trace elements boron, copper, manganese, zinc and molybdenum are necessary for the plant's normal nutrition, too. If the other components of HOAGLAND's solution are really - and especially for all plant species - required is unknown. There are hints that certain algae need Co²⁺ for the synthesis of vitamin B₁₂. The lack of certain elements leads to characteristic symptoms. The deficiency of Fe²⁺, Mn²⁺ and molybdenum causes a brightening of the leaves (chlorosis, originating from a loss of chlorophyll). A zinc deficiency induces the stunted growth of leaves, the lack of boron acid leads to heart blight in sugar beets suggesting an effect of boron on meristematic tissues.

The significance of the respective mineral components for the plant metabolism can be found in the table below.