by André Voisin
CHAPTER 6
Antagonism between potassium and magnesium in the soil and in the plant
Indirect magnesium deficiency in the plant as a result of the antagonistic action of potassium
One consequence of the "luxury consumption" of potassium by the plant is that it helps to create in the latter either a latent or an apparent deficiency of magnesium. As SCHEFFER and WELTE state: "Increased absorption of potassium by the plant represses magnesium." HORST, for example, has observed 1 that the application of potassium fertilizers causes the potassium content 2 of the dry matter of grass to rise from 1-60 - 1-89 to 3-08 - 3-30%, while the content of magnesium drops from 0-21 - 0-29 to 0-09 - 0-14%.3
It must be emphasized, however, that magnesium deficiency in the plant
is not necessarily due to a low content of magnesium in the soil.
It can be caused by an excess 4
of antagonistic ions.5As early as twenty years ago, for example, it was noted that on
soils relatively rich in magnesium fruit trees suffered from magnesium
deficiency following the application of heavy dressings of potassium
fertilizers. But without symptoms of magnesium deficiency being
manifested it has likewise been observed over the years that heavy
dressings of potassium fertilizers reduce the magnesium (and
calcium) content of the straw and grains of wheat. Here is an
example of a common fact: the impoverishment of the plant with regard
to a mineral element well in advance of deficiency symptoms being
obvious.Continued application of potassium fertilizers progressively exhausts the magnesium in the soil
Apart from the direct and immediate effect of potassium fertilizers on magnesium, the potassium of the fertilizer has another effect, slow and cumulative, which helps to impoverish the soil with regard to magnesium: in effect, the potassium "drives back" into the soil the exchangeable magnesium which is washed away by rainwater. As DIDIER BERTRAND writes: "A soil with apparently the right content of magnesium for vegetables may lead to a deficiency of this element consequent upon large-scale application of potassium fertilizers followed by abundant rainfall. What happens in effect is that the magnesium is veritably leached out of the soil."
As a result the continued application of potassium fertilizers
can lead in the long run to an increasingly accentuated impoverishment
of the soil with regard to magnesium. It is possible, therefore, where
pastures are concerned, not to see any immediate effect from potassium
fertilizer dressings on the magnesium metabolism of the
stock,6
provided the soil still has sufficient reserves of assimilable
magnesium.Magnesium deficiency in plants has multiplied in recent years
Excessive and repeated dressings of potassium fertilizers cause magnesium deficiencies in plants,7 particularly grasses, and in recent years these deficiencies have been becoming more and more common. Even those concerned with the potash industry have been compelled to admit it. One scientist very closely connected with this sphere, SCHMITT, President of the Association of German Agricultural Research and Experimental Stations, wrote in 1959: "In recent years more and more magnesium deficiencies have been observed among our cultivated plants.... In view of this situation it appears that, to maintain the fertility of our soils, it is time that we paid much more attention than we have done hitherto to the question of magnesium fertilizers."
WELTE of the German Potash Syndicate Research Centre
at Buntehof writes of "the more and more frequent observation of symptoms
of magnesium deficiency in cultivated plants".At Kiel University SCHRÖDER states:
"In the years that have followed the Second World War symptoms of
magnesium deficiency have been observed more and more in plants and
since 1954 these observations have multiplied alarmingly."What is the main cause of this increase in magnesium
deficiency in plants? - SCHMITT states categorically: "The reason
is because about the 1920s losses of magnesium from the soil due to crop
harvesting and leaching were compensated, without anyone being aware of
it, by dressings of potassium fertilizers containing
magnesium."8SCHRÖDER is likewise of the opinion
that one of the reasons for this increase in magnesium deficiency
symptoms in plants in recent years is the use of purified potassium
fertilizer instead of kainit. He recalls that before the First
World War, in 1910, 46% of the potash used in Germany was in the form of
kainit, whereas in 1938 the percentage was no more than 10% and in 1957
no more than 6%. The percentage of magnesium in kainit (6%) is almost
half the percentage of potassium (8-12%), whereas the purified potassium
fertilizers generally used today contain almost no magnesium.This use of purified potassium fertilizers unfortunately
not only contributes to the occurrence of magnesium deficiencies in
plants but, by accentuating mineral imbalances in grass, also favours
grass tetany in the stock. The development of grass tetany is one,
among many, of the obvious consequences of the suppression
of potassium fertilizers containing magnesium and their replacement
by so-called concentrated and purified potassium fertilizers.Excessive dressings of potassium fertilizer give rise to magnesium deficiencies and lower the yield of grass
Interesting studies have been carried out to see to what extent the appearance of magnesium deficiency symptoms coincides with a drop in yield when progressively increasing dressings of potassium fertilizer are applied. In the case of grass an investigation by WALSH was described at the "Potash Symposium" organized in 1954 by the International Potash Institute (Table 5).
Table 5: Influence of potassium fertilizer dressings on the potassium : magnesium ratio in grass
WALSH concluded that under the conditions of his experiment any
dressing of magnesium chloride above 2 cwt./acre [254 kg./ha.] was
useless, because above this level yields started to drop. For this
optimum dressing of potassium chloride the content of potassium in
the dry matter of the grass was 2-99%. If the
dressings were increased further the grass continued to "gorge"
itself on potassium while its content of magnesium diminished
progressively. Deficiency symptoms in the grass became more and more
marked,9
to the extent that the yield fell. It will be seen that the K/Mg
ratio can be multiplied by ten thanks to the potassium fertilizer applied.
The health of the animal, and not the health of the grass, is the judge of
optimum potassium fertilizer dressings
The above experiment was expertly conducted from the
botanical point of view, the aim being to ascertain what maximum
dressing of potassium fertilizer gave rise to magnesium deficiencies
in grass. What is of interest to the grassland farmer,
however, is the maximum economic quantity of potassium fertilizer he
can apply to his pastures without causing harm to his stock. Once again
it must be stressed that it is not simply a question of producing the
maximum possible quantity of grass, but of producing the maximum
quantity of grass consistent with healthy stock. It is the health of
the animal and not that of the grass that determines the
optimum dressing of potassium fertilizer.
Attempts have been made to escape from this dangerous
situation by stating, but without any proof, that magnesium deficiency
symptoms, or indeed any deficiency symptoms, are obvious in the plant
before the animal consuming it is in danger of suffering from magnesium
deficiency. It is more than ten years, however, since WATSON
pointed out that hypomagnesaemia and grass tetany occurred in pastures
where there was no evidence of the herbage being deficient in
magnesium.
By the same logic, it could be said that one should only
become concerned with the cobalt and
iodine 10
contents of the soil and grass when symptoms of deficiency
in these elements make their appearance in the grass. Up till now
little perceptible effect of soil cobalt and iodine has been observed
on the yield and health of the plant. Nevertheless, if the cobalt
content of the grass is too low, the grazing animal will fall victim
to pernicious anaemia; and if the iodine content of the grass is
inadequate the thyroid of the animal will be seriously upset. The
grass is healthy, and one might think it would contain sufficient
cobalt or iodine, just as one imagines it contains sufficient
magnesium 11
because it presents no external symptoms of being deficient
in this element. The animal grazing it, however, will not be healthy.
| Home | Ag. Library | Health Library | Sovereignty Library |
- See Table 18 (Chapter 23) concerning this same series of
observations. *
- Cf. the data and comments of SEEKLES.
*
- See Table 22 (Chapter 27), among others, where the
application of potassium fertilizer causes the magnesium content of the
dry matter of grass to fall from 0-16 to 0-13%.
*
- CUTHBERTSON, Director of the Rowett Research
Institute, states: "Since the magnesium ion has the lowest speed of
diffusion of the various cations of the soil, its mobility suffers the
most on increasing the potassium content of the soil solution.
This is particularly the case with heavy additions of potassium, since
only few magnesium ions oppose the many very mobile potassium ions."
*
- Note that this antagonism, according to MULDER,
does not take place in Aspergillus niger, which makes the use of this
micro-organism to determine assimilable magnesium in the soil
problematical. For the methods of analysing
soluble or exchangeable magnesium in the soil See DIDIER BERTRAND
and A. JACOB. *
- The more so as this "conditioning" of the soil by
potassium fertilizer is accompanied by a "conditioning" of the animal
following prolonged and repeated consumption of grass unbalanced in
composition which causes the deterioration of certain fundamental
organs (for the "feeding story" of the animal see Chapter 33).
*
- This deficiency of magnesium upsets to a greater
or lesser degree the synthesis of chlorophyll. The result is chlorosis,
the leaves becoming paler in colour and then yellow. In general, the
symptoms appear first on the tip of the leaf and in the oldest leaves.
They are not always easy to distinguish from those caused by a
deficiency of other elements. *
- It should be noted that although the manufacturers
of potassium fertilizers have admitted the responsibility of potash for
magnesium deficiency in plants, they have tried to prove that potash
in no way favours grass tetany (see pp. 159-60). *
- These inverse variations of magnesium and
potassium can take place within very wide limits. WERNER has
observed variations in the content of the dry matter of grass of
the order of:
0-05 - 0-77% magnesium (Mg)
1-46 - 6-8% potassium (K)
In general, the potassium maximum corresponds with the magnesium minimum. If this is so, the K/Mg ratio (in milli-equivalents) varies from 0-58 to 41-8 or from 1 to 72.
Cf. also Tables 4, 5, 8, 9, 13, 18 and 22. * - The same will be said regarding sodium (Chapter 8),
which does not cause deficiencies in plants but gives rise to very
serious deficiencies in animals. *
- More correctly, that the magnesium is well balanced in relation to the other mineral elements. *