Grass Tetany by André Voisin

CHAPTER 10

Export of potash by pastures


Potash exported by pastures and mown swards

Before leaving the question of potassium fertilizers, solid or liquid, it would seem expedient to make some calculation of the amount of potash exported by pastures, so that dressings corresponding in quantity to those exports may be applied. As BARBIER reminds us:
"It is necessary and sufficient to restore by means of fertilizers such quantities of elements as were actually exported in the harvests or lost by some other means. There is never any point in enriching a soil indefinitely with potash by always importing more potash than is exported."
In calculating the amount of potash exported by pastures let us assume that the output per acre [hectare] 1 is as follows:
- Under ordinary continuous grazing: 175 gal. [2000 litres] milk and 223 lb. [250 kg.] meat.
- Under rational grazing: 437 gal. [5000 litres] milk and 535 1b. [600 kg.] meat.
Assume that the percentage 2 of potash (K2O) is 0-17 in milk and 0-044 in meat,3 then 219 gal. (1000 litres) milk contain 3-7 lb. (1-7 kg.) potash (K2O) and 220 lb. meat 0-37 lb. potash. On this basis the annual quantity of potash exported by an acre [hectare] of pasture is:
Ordinary continuous grazing:
( x 3-7) + ( x 0.97) = 3-93 lb.
[2 x 1-70] + [2-5 x 0-44] = 4-5 kg.
Rational grazing:
( x 3-7) + ( x 0-97) = 9-6 lb.
[5 x 1-70] + [6 x 0-44] = 11-1 kg.
Even if these figures are increased by one-third to take account of the excrement lost on the pits, on the road or in the milking shed the amount of potash exported annually by pasture managed under the two systems is 5-2 lb./acre [6 kg./ha.] and 12-8 lb./acre [14-0 kg./ha.] respectively.4 On the average 9-1 lb./acre [10 kg./ha.] K2O will probably be correct.5
The situation in mown swards is different. Assume an annual yield of 1-6 tons/acre [4 tons/ha.] hay from a good sward and 3-2 tons/acre [8 tons/ha.] from an exceptional sward, the hay containing 2-20% potash (K2O).6 This represents an annual potash export of 79-157 lb./acre [88-176 kg./ha.]. 7 Just as the yields of mown swards have too often been used as the basis of pasture-yield assessments, so also the export of potash from the former has too often been used to determine the amount of potassium fertilizer to be applied to pasture. No attention is paid to the fact that the greater part of the potash in pasture herbage (more than 85%) is rapidly returned to the soil by way of the excrement, particularly the urine. This was plainly stated by CUTHBERTSON at the 6th International Potash Institute Congress:
"In the case of pasture the greater part of the potassium is returned to the soil by the urine. The sale of animal products (including milk) does not represent any great removal of the potassium in the soil."
The "initial" applications of potash to a pasture that has never received the element, or at least not for a long time, naturally depend on the content of available potash 8 in the soil. But once this initial deficit has been made good, the problem is to know what annual "maintenance" dressings must be applied to the pasture to compensate for the annual export of potash. This is the most common and most important problem. It was seen above that the maximum amount of such dressings will be in the region of 9-1 lb./acre [10 kg./ha.].

Excessive amounts of potassium fertilizers are often advised for pastures

If the amounts exported by pastures are compared with the quantities of potassium fertilizers generally recommended the latter will be found to vary considerably from one research worker or from one book to the next: on the whole, however, they are much too high.
COIC, of the French Institute for Agricultural Research, advises the annual application to pastures of 54 to 71 lb./acre [60-80 kg.,/ha.] potash. Even this is a high figure.
GR0S, in his book on Fertilizers, advises the following annual dressings of potash for permanent pasture:
under cutting technique: 71-89 lb./acre [80-100 kg./ha.]
under extensive grazing: 45-63 lb./acre [50-70 kg./ha.]
under intensive grazing: 80-98 lb./acre [90-110 kg./ha.]
A French official review recently recommended annual potash dressings of 71-134 lb./acre [80-150 kg./ha.] for pastures.
It will be seen that in the case of temporary swards some authors have even advised annual dressings of the order of 178 lb./acre [200 kg./ha.] potash. Unfortunately the experiments or calculations on which these somewhat varied figures are based are not detailed.
Comparison of the recommended dressings of potassium fertilizers with the maximum quantities of potash exported by pastures leads to a better understanding of the findings of SJOLLEMA, the Dutch specialist in grass tetany. What he in fact established was that on farms suffering from grass tetany the amount of potash applied was twenty-six times 9 greater than the amount exported. Moreover, the amount of potassium (K) ingested by the animal was 500-600 gm. per day, or ten to twelve times its requirements. The amount of sodium ingested, on the other hand, rarely exceeded half the requirement.10

Fixation and release of potassium in the soil

Excessive and regular potash applications of this nature to pasture have resulted in an accumulation of the element in the soil of such swards and have very probably contributed to the increase in grass tetany.11
With regard to the fixation and release of soil potassium it can be said, without entering into the details of cation exchanges in the soil, that a balance is generally assumed to be established:

  K rapidly  
  exchangeable   K fixed (not or very slowly)
K in solution     (or adsorbed to)     exchangeable at
  the surface of   present        
  the soil colloids  

In other words, the potassium applied in excess to the pasture has become "fixed" in the soil 12 and is no longer immediately available to the herbage. These stocks, however - and they may be very considerable in pasture soils - can be released again, as many experiments have shown although the factors leading up to this liberation are still not completely known.13 As BARBIER, member of the Scientific Council of the International Potash Institute, states:
"When a soil has become enriched in potassium as a result of higher potash applications than exports over a number of years, it is generally found that only a part of the potassium incorporated is present in a diffusible (exchangeable) form. The remainder, which may be 50% and more, has therefore been converted into the form of ions described as fixed or retrograde, that is, immobile. This fixation, however, is not final."
Using radio-active potassium, BARBIER studied this release of "fixed" potassium in arable soils. One of the conclusions he arrived at was:
"Old potassium soil-stocks, if present in sufficient quantity, appears capable of boosting production to a higher ceiling than a potash salt recently applied to a soil of low potash content."
It seems possible, therefore, for the potassium accumulated and "fixed" in the soil to be released in the case of pastures - unfortunately only too common - which have received excessive dressings of potash, in the form of either potassium fertilizer or liquid manure, over a period of years.

Methods of analysing exchangeable potash in the soil produce values that are too low

Another factor has contributed to this excessive application of potassium fertilizer to pastures, namely, the analytical methods available for determining exchangeable potassium which is thought to be the only available potassium. The method generally used consists in extracting soil potassium by means of a neutral normal ammonium acetate solution.14 It would appear, however, that the plant is capable 15 of "assimilating" potassium classed as non-assimilable by these methods of chemical analysis. As SCHEFFER and SCHACHTSCHABEL point out:
"Plants have the capacity to utilize non-exchangeable potassium. In the course of pot experiments very high yields could be obtained from soils from which all the exchangeable potassium had previously been eliminated.... It was noted on certain loess soils that there was no correlation between actual crop experiments and the exchangeable potassium analysis.... NEUBAUER'S method 16 of analysis using plant seedlings showed that certain soils were capable of yielding up considerable quantities of non-exchangeable potassium." 17
These current artificial methods of chemical soil analysis lead, therefore, to larger quantities of potassium fertilizers being applied than are actually required. In every case money is wasted. Where pastures are concerned, moreover, the health of the stock is greatly endangered.
In the past there has been too much analysis of the soil and not enough of the plant and the animal.18 When investigating outbreaks of illness among stock on a farm one is almost always given only soil analysis data when one begins to ask questions. Only in very rare cases can one get a herbage analysis even for the macroelements. It is essential that in future most of the money being spent at present on chemical analysis of the soil should be reserved for plant and animal analyses, which provide more reliable evidence.
When this takes place heavy dressings of potassium fertilizer will no longer be recommended, on the basis of soil analysis, for the soil of a pasture carrying herbage with more than 3% potassium (K) in the dry matter and virtually no sodium at all.

Temporary cessation of potassium application to pasture soils where there has been an accumulation of the element

Whether as a result of the confusion between mown and grazed swards, or of the imperfect analytical methods available, or of over-skilful commercial propaganda, the tendency, as has been described above, has frequently been to apply too much potassium fertilizer to pastures. One finds oneself faced, therefore, with accumulated quantities of "fixed" potassium which, when released, will help to accentuate the mineral imbalance of the herbage, thus promoting grass tetany.
Be that as it may, in the presence of such pastures all application of potash must be stopped for a certain number of years.19 Magnesium and sodium fertilizers must be applied in an attempt to reestablish the soil balance.

Use potassium fertilizers along with magnesium and sodium on pastures

Naturally there is no point in stopping the application of potassium fertilizers to pastures where the soil needs the element. What is most important is that one must not use more than is necessary. It has just been seen that the amounts of potash exported by pastures are small. Magnesium and sodium dressings, moreover, are known to cancel the depressing effect exerted by potassium fertilizer on the magnesium and sodium contents of herbage. It will be shown (Table 23) that the application of magnesium fertilizer is one of the most efficient means of preventing potassium fertilizers from encouraging hypomagnesaemia and tetany in the grazing cow. It is essential, therefore, that potassium fertilizers applied in future to pastures contain magnesium (e.g. kainit, patentkali, etc.). Potassium fertilizers, such as sylvinite or again kainit, containing sodium chloride can equally be recommended.
In cases where the potassium fertilizer contains no or only inadequate quantities of magnesium or sodium it is essential that compensatory dressings of magnesium and sodium fertilizers be applied to the pasture.

Do not apply potassium fertilizer to pastures in winter

Despite these measures it is essential that in the case of pastures, contrary to arable soils, potassium fertilizer is never applied in winter. By this means the plant is prevented from saturating itself in spring, the critical time for tetany, with potash and succumbing to "luxury consumption" with its attendant grave consequences. The atmospheric effects of the winter season have been seen to be able to "release", that is to render available, a fraction of soil potassium that was present in the unavailable form. The recommendation is consequently to apply potash at the end of spring or beginning of summer after two rotations or one cut.20
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Notes



[Click on asterisk (*) at the end of a note to return
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  1.  
 See VOISIN, Grass Productivity.
    
 According to W0HL and BARBIER. *



    

    


  2.  
To get the amounts of potassium (K) multiply by 0-82. *


    

    


  3.  
Losses due to leaching by rain-water are very low for potash, which, as 
will be stated below, is "fixed" by the soil. *

 
    

    


  4.  
CARPENTIER quotes figures of the same order for the export of potash by 
pastures. *




    

    


  5.  
GARAUDEAUX, of the Alsace Potash Mines Research Service, puts forward
       various, in the author's opinion unconvincing, arguments in an 
attempt to increase these figures. *



    

    


  6. 
Or 1.80% potassium (K) in the hay, i.e. 2-1% potassium (K) in the dry 
matter (hay with 85% dry matter). *


    

    


  7. 
It would appear reasonable where the sward is grazed and a hay crop taken 
one year out of two to base one's calculations on an annual potash export 
of 45 lb./acre [50 kg./ha.] *



    

    


  8.  
The indication provided by soil analysis, as will be seen below, is not 
always correct.  The flora is often a valuable indicator, the presence of 
Nardus stricta pointing to a very probable lack of available potash in the 
soil (see Grass Productivity (pp. 278-80) and Better Grassland Sward 
(p. 203). *


    

    


  9. 
If the actual export figure under normal continuous grazing which, as was 
seen above, is 4.1 lb./acre [4-5 kg./ha.] potash (K2O) 
per annum 
is multiplied by 26, the answer is 104 lb./acre [117 kg./ha.], a figure 
which has just been seen to be exceeded by the recommended dressings of 
potash for pastures. In addition, liquid manure from pigs
made the rate of potassium application even higher on the farms studied 
by SJOLLEMA. *
       

    

    


  10. 
Cf. Chapter 26. *
       

    

    


  11. 
A very recent article in the Farmer and Stock-breeder states: "The 
quantity of potash required in the case of grazing is generally very 
small. However it is common to see large quantities of potassium 
fertilizers being used under these conditions. It is
not surprising that, on these pastures, the losses due to tetany are 
severe." *
       

    

    


  12. 
A small fraction perhaps having been removed by the water. *


    

    


  13. 
Some research workers, as CUTHBERTSON, are of the opinion that transition
from the unavailable to the available form of soil potassium generally 
takes place in winter as a result of the effects of weathering. He 
therefore advises applying potassium to pastures not in winter but after 
the first cut or two grazing rotations. This will also
be recommended. *


       

    

    


  14. 
Sodium acetate is used in MORGAN'S method and appears to give even less 
accurate results. *


       

    

    


  15. 
This capacity depends both on the nature of the plant and on the nature of 
the soil. *


    

    


  16. 
On some loams the ammonium acetate method shows 76 p.p.m.
exhangeable potassium in the soil. Within 18 days rye seedlings 
(NEUBAUER'S method of analysis) assimilate 264 p.p.m. potassium in this 
soil. It must be acknowledged, therefore, that these seedlings have 
extracted 264 - 76 = 188 p.p.m. of potassium classified as nonexchangeable 
by the analytical methods. *
       

    

    


  17. 
Reporting on various experiments, GARAUDEAUX, of the Alsace Potash
Mines Research Service, writes: "No correlation was found between the 
potassium contents (K) of the forage crops and the contents of exhangeable 
potassium in the soils." *
       

    

    


  18. 
Regarding the importance of analysing the animal and what has been termed 
"raising the status of chemical analysis" see Soil, Grass and Cancer for 
the copper content of the liver, for example, (p. 40) and the manganese 
content of the ovary (p.69). Analyses of this nature provide the best 
indication of the content of available copper and available manganese in 
the soil the animal is grazing.
    
On the same subject see also the author's paper: "Healthy Soil, Healthy 
Forage, Healthy Animals". *
       

    

    


  19. 
In Germany NAUMANN, Director of the Research Services of the Chamber of
Agriculture, Bonn, considers it essential to stop applying potassium 
fertilizer completely for several years when pasture soils contain more 
than 30 mg. potash per 100 gm. He also advises stopping liquid manure 
application when the soil of a pasture exceeds this content of potash.
 *
       

    

    


  20. 
The difference in effect on grass tetany between winter and summer 
applications of potassium fertilizer has been clearly noted by PATERSON,
 a British farmer. When he applied potash to his grass in winter instead 
of summer, tetany developed in his herds. After some years he returned to 
his old practice of spreading the potash in summer after a silage cut. The 
tetany obviously diminished. *