Grass Tetany by André Voisin

CHAPTER 7

Influence of magnesium fertilizers on the magnesium content of grass


The different fractions of magnesium in the soil

The magnesium content of the top layer of soil is not necessarily correlated with that of the parent rock, for many factors enter into play here. In particular, heavy rainfall, by promoting leaching, encourages acidity in the soil and the elimination of certain cations such as magnesium.
With many reservations, it may be said that, in general, podsolized sandy soils and lateritic soils have a low magnesium content, whereas black soils and brown soils contain more magnesium.
As in the case of other mineral elements, it is impossible to determine exactly by means of chemical analysis the amount of magnesium in the soil 1 assimilable by the plant, or, to be more exact, by a certain plant on a certain type of soil.
To facilitate study of what is known as the "dynamics" of magnesium in the soil, that is, the passage from one form to another, soil magnesium has sometimes been divided into five categories 2 only the last two of which are theoretically assimilable.
Unfortunately experience was to show that none of these categories, even including the last two, gives an exact picture of the status of the soil where magnesium is concerned. It is very difficult, therefore, to find out by chemical analysis 3 whether or not a soil will favour hypomagnesaemia and grass tetany. The magnesium content of the grass, on the other hand, gives a slightly better indication, although it is still uncertain.

The magnesium content of grass varies with the season

Study of the influence of soil magnesium and of the application of magnesium fertilizers on the magnesium content of grass is made difficult not only by the lack of suitable methods of analysing the soil and fertilizer magnesium assimilable by the grass but also by the seasonal variation in the magnesium content of the latter. In effect, the magnesium content of grass varies with the season, as illustrated in Figure. 3, established in Great Britain at the University of Durham.

Figure 3: Seasonal variations in the magnesium content of grass.

It will be seen that the magnesium content of the dry matter of grass is about 40% higher in August than in May.4 Likewise from Table 6, drawn up in Northern Ireland by TODD (Cf. Table 25), it is obvious that the magnesium content of grass can be double at the end of the summer what it was in the spring when the stock were put out to grass.5

Table 6: The magnesium content of grass varies in the course of the year

Magnesium fertilizers

After these preliminary observations on magnesium in the soil and in the grass, let us turn now to the question of magnesium fertilizers.
    There are many of these but the most common are:

  • Dolomite, a mixture of calcium carbonate and magnesium carbonate (containing about 10% total magnesium)
  • Magnesite,6 which is a magnesium carbonate (containing 26% total magnesium)
  • Magnesia, resulting from the calcination of different magnesium fertilizers, dolomite, magnesite, etc.
  • Kierserite, which is a magnesium sulphate (MgSO.K40) 7
  • Kainit, which is a natural potassium fertilizer containing magnesium sulphate 8
  • Langbeinite, natural sulphate of magnesium and potassium 9
  • Patentkali, which is more or less a double salt of potassium and magnesium sulphate. It contains about 30% magnesium sulphate (MgSO4)
  • Magnesium-nitrogen ("Stickstoffmagnesia"), which is a double sulphate of magnesium and ammonium
  • Basic slag, which contains a small quantity of assimilable magnesium
  • All the various magnesium salts, particularly magnesium chloride and magnesium sulphate.

Difficulty of determining the assimilable magnesium in a magnesium fertilizer

The analytical difficulties of determining in a magnesium fertilizer the magnesium that is assimilable by the plant are anolagous to those of determining the assimilable magnesium in the soil. In principle, the magnesium soluble in water is assimilable in toto.
But the most common magnesium fertilizer is dolomite (or magnesian limestone) and the percentage of total magnesium assimilable in dolomite can vary within considerable limits, depending on its geological origin. Moreover, the degree to which the magnesium of magnesian limestone can be assimilated by plants in general and grass in particular is also a function of the degree to which they have been pulverized, which determines the speed 10 with which dolomite takes effect.
It would appear unfortunately that the safest method of determining assimilable magnesium and the efficiency of a magnesium fertilizer is to assess the effect in practice on the magnesium content of the plant. How magnesium fertilizers affect the magnesium content of grass will be dealt with below.

Magnesium fertilizers increase the magnesium content of grass

Good-quality magnesium fertilizers enable the magnesium content of grass to be increased where this is inadequate either due to the nature of the soil or to the fact that potassium fertilizers have been applied in excess.11
Thanks to regular dressings of magnesian limestone (2 tons/acre [5000 kg./ha.] in three consecutive years), REITH was able to double the magnesium (Mg) content of grass, which increased from about 0-18 to 0-38% in the dry matter.
WALSHE has claimed that a dressing of 10 cwt./acre [1270 kg./ha.] magnesium sulphate raised the magnesium content in the dry matter of a temporary pasture of rye-grass and white clover (at the beginning of April) from 0-17 to 0-26%. In another temporary pasture the application of 325-3 lb./acre [360 kg./ha.] magnesium (Mg), half in the form of magnesium sulphate and half in the form of calcined magnesite,12 increased the content of magnesium in the dry matter from 0-21 - 0-22 to 0-29 - 0-32%.
BARTLETT got a similar result from applying one single dressing of 25 cwt./acre [3180 kg./ha] calcined magnesite containing 87% magnesite (MgO). The content of magnesium (Mg) in the dry matter of the grass rose from 0-17 to 0-34% (see Table 24 in Chapter 28).
SMYTH applied one dressing of 325-3 lb./acre [360 kg./ha.] magnesium (Mg), half in the form of magnesium sulphate and half in the form of calcined magnesite. The content of magnesium in the dry matter increased from 0-14 to 0-21%. (see Table 23, p. 157).
In these four instances the application of magnesium fertilizers allowed the magnesium content to be increased above 0-20% in the dry matter, where it had been below this figure prior to the application. It will be seen (Figure 16, Chapter 26) that when the magnesium content in the dry matter of grass is above 0-20% there is much less risk of tetany.
The occasion will present itself later in this work to examine other effects of magnesium fertilizers on the magnesium content and more generally on the mineral balance of grass (see Chapter 11, Table 13, among others).

Persistent effect of magnesium fertilizers

Another problem is to establish the persistence 13 of the effect of a magnesium fertilizer on the magnesium content of grass and on the magnesium in the blood serum of the grazing animal. At the Veterinary Research Centre, Weybridge (Great Britain), Mrs. ALLCROFT has attempted to compare the effects of a single application of calcined magnesite and magnesian limestone:
by determining their effect on the magnesium content of the grass and in the blood serum of the grazing animal.
by assessing the effect these fertilizers were still exercising four years later.

These experiments will be discussed in Chapter 28, and the reader is referred to Figure. 17, where it will be seen that four years later, relative to the grass in the control plot, the calcined magnesite and magnesian limestone were still doubling the magnesium content of the grass. This content is a little higher (0-24 - 0-27%), however, with the calcined magnesite than with the magnesian limestone (0-17 - 0-20%).14

Magnesium deficiency in plants is favoured by a soil pH either too low or too high.

These experiments of Mrs. ALLCROFT'S were conducted on relatively acid soils in which magnesium carbonate is relatively effective and in which, moreover, if the acidity is very marked, the calcium carbonate of the limestone can strengthen the effect of the magnesium.
But the use of magnesian limestone presents difficulties and even dangers on soils where the pH is too high, either naturally or as a result of excessive marling or liming. This is explained by the dual antagonism of the magnesium ions towards the hydrogen and calcium ions. In effect, the calcium ion applied to an acid soil neutralizes part of the excess hydrogen ions which are antagonists of magnesium ions. This neutralization, therefore, promotes the absorption of magnesium by the plant. But if this dressing of limestone is applied to a soil where the pH is already relatively high the calcium, which is also an antagonist of magnesium under certain conditions, will reduce the absorption of magnesium by the plant. Here, once again, therefore, is the great law of Nature, the law of balance and optimum: not enough and too much calcium in the soil will impair the absorption of magnesium by the plant.
This effect is particularly marked on sandy soils, as shown in Figure. 4 illustrating the gravity of magnesium deficiency symptoms in oats (a Gramineae) as a function of pH. These deficiencies are the expression of the extent to which the magnesium in the soil is assimilable . Under these conditions the maximum assimilability for this soil can be seen to lie between 4-9 and 5-3.15

Figure 4: A soil pH that is too low or too high favours magnesium deficiency in plants.

At higher or lower pH levels this assimilability is reduced and symptoms of deficiency make their appearance in the oats in both cases.
It will now be more understandable, therefore, that it is preferable in the case of a soil with a relatively high pH 16 to use calcined magnesia and better still to use magnesium sulphates despite their high cost. What happens is that the magnesian limestone raises the pH still higher and accentuates the antagonistic effect of the excess calcium ions present on the magnesium ions.
Of course, magnesium fertilizer dressings not only increase the magnesium content of the grass; they alter its whole mineral equilibrium, reducing, for example, its content of calcium and potassium. In some very rare instances it has been established that excessively high rates of magnesium application can give rise to potassium deficiencies in plants. 




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Notes



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  1.  The classical method of determining the magnesium 
in the soil "available" to the plant is to shake the soil with a solution 
of neutral salt, the nature, concentration, etc., of which vary greatly 
according to the author and the country. The
so-called SCHACHTSCHABEL method is widely used in Germany. *



    

    



  2. 
 (1) Total magnesium, soluble in hydrofluoric acid.
    
 (2) Fixed magnesium, soluble in strong hydrochloric acid.
    
 (3) Easily accessible magnesium, soluble in dilute 
                hydrochloric acid.
    
 (4) Exchangeable magnesium, extracted by a solution of 
                barium chloride and triethanolamine.
    
 (5)  Magnesium available to the plant, extracted by exchange 
             with a calcium chloride solution.
    
       This illustrates the complexity and all the uncertainties of 
the problem.
*


    

    


  3.  SCHACHTSCHABEL'S method may be able to establish the 
minimum content of assimilable magnesium in the soil below which symptoms 
of magnesium deficiency are manifested in the plants. This minimum varies, 
moreover, with the nature of the soil. But the magnesium content of the 
plant is far from being well correlated with the content of assimilable 
magnesium in the soil as indicated by this method. It has been found, 
for example, in the case of potatoes on a certain type of soil, 
that the content of magnesium in the dry matter of the leaves can vary 
between one and three times (0-20 - 0-60%) for the
 same soil content of assimilable magnesium (3 mg. in 100 mg. soil). Thus, 
SCHRÖDER, at the University of Kiel, has concluded that 
too much must not be expected from the assimilable magnesium analysis 
of the soil and that, in every case, this analysis is quite incapable 
of indicating what quantities of magnesium fertilizer should be applied: 
which, from the practical point of view, is unfortunately the fundamental
question. *
              

    

    


  4.  See also the seasonal curve of this variation in 
Ireland. *


    

    


  5.  Another factor capable of altering the magnesium 
content of grass is the external temperature. In effect, under stipulated 
conditions the magnesium content of the grass is lower at low temperature. 
'T HART found that grass with a magnesium
content of 0-23%. (in the dry matter) at 21° C. 
contained only 0-16% when 
the temperature was only 4° C. But there are other factors involved, for 
we see from Table 6 (p. 30) that the magnesium content of grass is higher 
in December than in July. *


    

    


  6.  The name can likewise designate a natural 
magnesium silicate, better known as "meerchaum". Calcined magnesite is 
used to make fire-bricks. *



    

    


  7.  There are also "Epsom salts", which are magnesium 
sulphate with seven molecules of water. *

       


    

    


  8.  The average formula is
       MgSO4/KC1.3H2O.
*



    

    


  9.  For the exact magnesium contents of these 
fertilizers see COOKE. *


    

    


  10.  At best the speed of action of dolomite is rather 
poor. Also it appears that the use of ammonium nitrate on a dolomite 
base is of interest to maintain the magnesium content of soils already 
supplied with the element but does not allow a satisfactory
state of the soil with regard to magnesium to be re-established where 
the soil is deficient in the element. *
 

    

    


  11.  See Table 13 (Chapter 11), where the application of 
magnesium fertilizer (magnesium chloride) multiplies the magnesium 
content of green oats by 4 despite heavy dressings of potassium 
fertilizers. *



    

    


  12.  Mixing these two magnesium fertilizers offers the 
following advantages: the magnesium sulphate, being rapidly soluble, 
acts immediately, while the calcined magnesite has a persistent effect. 
*


    

    


  13.  A rapid effect may also be desirable, in which case 
soluble fertilizers, such as the various magnesium sulphates, are 
indicated. *


    

    


  14.  It will be seen that the difference in the effect of 
the two fertilizers on the blood serum of the cow is the same 
(see Chapter 28, Figure 18).  *
 

    

    


  15.  This optimum pH for the absorption of magnesium 
depends on the nature of the soil, the other fertilizers applied, the 
nature of the plant, etc. *


    

    


  16.  It should be borne in mind that this high pH can 
contribute towards manganese and copper deficiencies. It will be seen 
(Chapter 26) that grass causing tetany generally seems to be low in 
manganese and copper, but there is no clear explanation available
as to how these deficiencies aid the development of grass tetany. 
*



 



         


















 

 
TABLE 6 

       
  The magnesium content of grass varies in the course of the year





  
 Percentage magnesium (Mg) in the




        Date
 dry matter of the grass



 1st April
       0-126



 20th May
       0-146



 17th June
       0-210
 


 16th July
       0-181



 12th August
       0-211



9th September
      0-245



7th October
      0-270



18th December
     0-230



    
       
N.B. Cf. Figure 3 and Table 25 (Chapter 30).

From TODD  *