The importance of salts for plant nutrition had already been highlighted by
J. v. Liebig and Carl
Sprengel. A. Fr. J. Wiegmann and A. L. Polstorff confirmed their findings in 1842. However, the
question of the type (quality) of the salts remained open, as the composition of a plant's ash
content does not indicate whether a specific element detected in the plant is actually essential for
its survival or whether it was merely taken up as a non- essential substance.
This problem was solved when the Würzburg plant physiologist J. v. Sachs
(1832–1897)
rediscovered the method of hydroponics. This technique allowed for the preparation of precisely
defined nutrient solutions and the study of the effect of each cation and anion on plant growth. Even
earlier experiments by J. Woodward (1665–1728) in London had shown that plants grew better in
river water than in rainwater and that growth improved when the water had absorbed dissolved
substances from the soil.
The first practical synthetic nutrient solution was developed by J. v. Sachs in collaboration
with the
chemist J. A. Stöckhardt. It contained per 1000 ml of water:
1 g potassium nitrate,
0.5 g calcium sulfate,
0.4 g magnesium sulfate,
0.5 g calcium phosphate,
and a trace of iron(III) chloride (trace elements were unintentionally and unknowingly
supplied as
impurities).
Sachs recognized the importance of iron through experiments with iron-free nutrient
solutions. In
1882 he wrote:
“…After some time, however, when the third or fourth leaf of our
experimental plants unfolds, a
disease becomes apparent: the new leaves developing from then on remain completely white,
producing no chlorophyll, and microscopic examination shows that no chlorophyll grains are
present in the protoplasm of such colorless leaves. This proves that our nutrient mixture was still
lacking something; from earlier observations by Gris we know that the disease of our plants, so-
called chlorosis, is due to iron deficiency... It is enough to introduce a small amount of a soluble
iron salt into the water absorbed by the roots… to see the previously completely white leaves
turn
green… These experiments clearly demonstrate that iron is necessary for the formation of
chlorophyll, but not whether iron itself is a component of the green pigment.”
In the context of these experiments, Sachs recognized the importance of root hairs
for the uptake
of dissolved salts.
Around the same time (1861), J. A. L. W. Knop developed the nutrient
solution named after him,
which is still widely used today. Per 1000 ml of water:
1 g calcium nitrate,
0.25 g magnesium sulfate,
0.25 g potassium dihydrogen phosphate,
0.25 g potassium nitrate,
and a trace of iron sulfate.
The experiments showed that the cations potassium, calcium, magnesium, and small amounts
of
iron(II) (or iron(III)), as well as the anions sulfate, hydrogen phosphate (or phosphate), and nitrate,
are essential for plant growth and survival. In addition, oxygen, carbon dioxide, and hydrogen are
obtained from the air and water, respectively (respiration, photosynthesis). The absence of one of
these elements cannot be compensated for by an excess of another (chemically similar) element.
For example, potassium can be replaced neither by lithium, sodium, nor rubidium. Similarly
useless are atmospheric nitrogen, metallic potassium, or elemental sulfur. Only the respective ions
are required.
Sachs and Knop are considered the fathers of modern hydroponics. With
this method, they
simultaneously created a tool for proving the essentiality of nutrients.
