CHAPTER 8

Sodium fertilizers and the potassium- sodium balance in grass

"It is surprising to find in accepted plant physiology and agronomy text books no mention of the antagonism that exists between potassium and sodium ions. These works limit themselves to discussion of the antagonism of other cations, such as potassium-calcium and potassium-magnesium."

Table 7, for example, shows that plant yield remains more or less identical when:

	100 parts potassium and no sodium, or
	20 parts potassium and 80 parts sodium are applied. The composition of the plant varies tremendously, however, despite the constancy of the yield.

There is no correlation between yield and biological quality of grass

The sodium question appears in a very different light if, instead of only considering actual plant yield, the composition of the grass and its influence thereby on the production and health of the animal are taken into account.
It appears to be necessary (Chapter 23) for grass never to contain less than 0-25% sodium in the dry matter, and it is even desirable for the content to be 0-50% if lower milk production and surreptitious impairment of the animal's health are to be avoided. It is likewise desirable for the ponderal potassium/sodium (K/Na) ratio not to exceed 5 (five), 8 (eight) being the absolute maximum.²
To return to Table 7. The yields are equal for respective dressings of:

1.	100 parts potassium, no sodium;
2.	40 parts potassium and 60 parts sodium.

In the first case, however, the sodium content of the plant is more or less nil (0-04%) and the K/Na ratio colossal (74-0). A food of this nature could only cause grievous harm to the health of the animal, not to mention its production. In the second case, the plant's sodium content (0-76%) is more than satisfactory, and the K/Na ratio is excellent (2-3). This would be a sound foodstuff.
This question of biological quality in the plant has received far too little attention,³ observations and effort having been concentrated on yield, be it yield of the animal's grass or of Man's grain. Unfortunately, as Table 7 shows, there is no correlation at all ⁴ between yield and biological quality.

Excessive dressings of potassium fertilizers dangerously reduce the sodium content of the grass

This same Table 7 provides good confirmation of the considerable depressing effect exerted by potassium fertilizer on the sodium content of grass. It has been seen elsewhere ⁵ that increasing rates of potassium application can reduce ⁶ the sodium content of grass to very low levels (0-04% in the dry matter) and raise the ponderal potassium : sodium ratio (K/Na) to extremely high levels above 50. In other words, as a result of excessive dressings of potassium, the sodium content is much too low and the K/Na ratio much too high. This, as has been stated, can have dire consequences for the health of the animal ⁷ and contribute towards its being attacked by grass tetany (see Chapter 23).
The opportunity will be given below to examine data compiled by various authors (see Tables 26 and 27in Chapter 23), revealing that extreme sodium deficiency in grass has unfortunately become commonplace in all countries due to excessive quantities of potassium fertilizers or liquid manure being applied.⁸

Sodium fertilizers

To compensate for this considerable diminution in the sodium content of grass, fertilizers containing sodium must be applied. The following are the most common:

1.	sylvinite,9 which is a crude potash salt containing 30-50% sodium chloride;
2.	rhenania phosphate obtained by calcination and containing sodium; 10
3.	sodium chloride
4.	sodium nitrate, synthetic or natural.11

The effect of dressings of sodium chloride and sodium nitrate on the composition of grass will now be examined in greater detail.

Effect of applying sodium chloride on the mineral equilibrium of grass

Sodium chloride, or cooking salt, is the fertilizer most commonly employed to increase the sodium content of grass.¹²

Table 8 shows the comparative effects of sodium chloride and potassium chloride on the grass of a temporary pasture

In the first place it is obvious that the yields obtained with the two forms of fertilizer are practically the same.¹³ The potassium chloride, however, reduces the sodium content, causing it to fall from the almost adequate level of 0-29% to the low level of 0-15%, almost half. As the potassium content increases at the same time, the K/Na ratio is multiplied by 2-5, jumping from 7-7 to 18-7, which is a highly dangerous figure.

Effects of sodium chloride on soil structure

Because sodium ions are very hydrated they exercise a dispersion effect on heavy soils. Where the application of large quantities of sodium chloride is prolonged, therefore, the result is a modification of the structure of heavy soils, which crust easily in dry weather. In light soils, on the other hand, sodium ions can improve the water regime.
These effects of sodium have been observed in striking manner in soils which naturally or as the result of flooding or irrigation with salt water are extremely rich in sodium. The few rare results available, however, are far from being uniform. ADAMS, in Britain, for example, is of the opinion that sodium causes no visible harm to the structure of soils used by him in his experiments with sugar beet.¹⁴ It must not be forgotten, moreover, that potassium salts have likewise a tendency to impair soil structure.¹⁵
So far as is known, however, the problem fundamental to the present issue has not yet been studied, namely, the effect of applying sodium chloride to soils containing too much potassium. Attention has been focused almost exclusively on the deterioration of soils following the application of enormous quantities of sodium in the form of salted irrigation water. On the one hand, the dressings of sodium thus applied were abnormally high, and on the other, the soils involved did not contain an excess of potassium.¹⁶ Briefly the task in future will be to study the effect exercised on the physical characteristics of the soil by fertilizer sodium IN THE PRESENCE of an excess of its antagonist, potassium.

Sodium nitrate can improve the mineral balance of grass

Another method of applying sodium to the soil is to use a common fertilizer, sodium nitrate. Table 9 shows that, provided that the rates of potassium application are not too high, the use of sodium nitrate in place of nitrate of lime leads to a considerable increase in the sodium content of ryegrass, without the potassium content being appreciably altered.

Table 9: Effect of potassium fertilizers applied with calcium nitrate or sodium nitrate on the composition of rye-grass

In effect, where dressings of potassium fertilizers are low, rye-grass is found to contain:

(a)	with sodium nitrate, 0-64% sodium (Na) 17 and a potassium/sodium weight ratio (K/Na) of 5-63;
(b)	with calcium nitrate, 0-18% sodium (Na) and a potassium/sodium (K/Na) weight ratio of 18-40.

In the first instance from the point of view of both absolute sodium content and K/Na ratio the grass is such that it will maintain perfectly healthy animals. This is not the case at all, however, in the second instance.

Two mineral ratios that can favour tetany do not necessarily vary in the same way

Variations in the ratio (in milli-equivalents) are also shown in Table 9. It will be seen later (Table 21,) that different research workers are of the opinion that when a ratio of this kind is high, and particularly when it exceeds 1-80, the risk of grass tetany is very much greater. This is correct under some but not under all conditions. It will also be seen that a high K/Na ratio, particularly one higher than 8- 00, by upsetting the adrenal cortex (see Chapter 23), can likewise favour tetany.
Comparison of the development of the K/Na and ratios where sodium nitrate or calcium nitrate are applied aids our understanding of the irregular results obtained concerning the influence of the ratio of the grass on the incidence of tetany. In effect, when small quantities of potassium fertilizer are applied it is obvious from Table 9 that:

(a)	In the case of the sodium nitrate dressing the is 2-12, which is above the danger level of 1-80, but the K/Na ratio is only 5-63, which is below the danger level of 8-00.
(b)	In the case of the calcium nitrate dressing the position is reversed, the ratio being 1- 63, which is below the danger level of 1-80, while the K/Na ratio is very high (18-40) and far above the danger level of 8-00.

In other words, K/Na and ratios, a high figure for which favours tetany, do not necessarily vary in the same way under the influence of a certain fertilizer.
Depending on which of the two ratios is taken into account, it can be stated that sodium (or calcium) nitrate acts in favour of or to the detriment of grass tetany. It is easily understood, therefore, that although such ratios may be valid under certain conditions, they can lead to erroneous conclusions under conditions that are not comparable.¹⁸

1. Apart, however, from sugar beet. For the role of sodium in plant physiology see the work of LEHR among others. *

2. This ratio of 8 for a sodium content of 0-25% (Na) means a content of 2-00% potassium (K) in the dry matter. In the desirable instance where grass contains 0-50% sodium a K/Na ratio of 8 would necessitate a potassium (K) content of 4-00%, which is much too high. This is why, in a case of this kind, it is desirable not to exceed a ratio of 5, which corresponds with a potassium content of 2-5%, which is almost a maximum. *

3. It is more than disturbing to read in the very recent and noteworthy book Nutrition and Plant metabolism (202, p. 188) by MENGEL of the Justus-Liebig University: "Research into the influence of the content of different mineral elements in foodstuffs on the human and animal organism is still in its infancy." In other words, since the discoveries made by LIEBIG one hundred years ago, much work has been devoted to studying the influence of fertilizer dressings on plant yield but little attention has been paid to their influence on animal (and human) health. *

4. Table 13 (Chapter 11) provides another example: a combination of fertilizers can increase the yield by 50% without raising the magnesium content. It has already been pointed out in Chapter 6 that the cobalt and iodine contents of grass are not correlated with the latter's yield but they nevertheless exert a profound influence on its biological quality.*

5. See Chapter 5 and Table 4. Cf. also Table 18 and Table 22. *

6. All the observers have confirmed this depressant effect of potassium fertilizers on the sodium content of plants. WALSH encountered the phenomenon in the case of cereals (wheat, barley, etc.), but, in the case of beet leaves, he found that under the conditions of his experiment very high dressings of potassium fertilizers could increase the sodium content. SHEPHERD, however, got a contrary result.
   See also recent studies by HUFFAKER using radio-active sodium and showing how an excess of potassium in the nutrient solution reduced the absorption of sodium by the plant. *

7. For example, reduce its fertility, not to mention other consequences which will be discussed in Chapter 23. *

8. For the effects of liquid manure see Chapter 9 and Table 11. *

9. BOSCH has noted that by applying 71 lb./acre [80 kg./ha.] potash (K₂O) in the form of sylvinite to a pasture low in sodium (Na) the sodium content in the dry matter is increased from 0-13 to 0-17%. There was also an increase in the potassium content (K) from 2-92 to 3-36%, but, notwithstanding, the K/Na ratio fell from 22-4 to 19-8. He observes that this improvement is not sufficient and that the biological quality of the grass remains very poor.
   Note that the crude salt of potash, kainit, contains sodium as well as magnesium. *

10. BEHRENS' studies with a phosphate of this nature containing radio-active sodium confirm that the sodium of this fertilizer is well absorbed by the plant. *

11. Nitrate of soda from Chile. Note that it is not impossible to use other sodium salts such as sulphate, carbonate, etc. *

12. It is understood that the use of sodium chloride as a sodium fertilizer will be extended only when the price is brought more within reach and not increased by a tax of any kind. *

13. Cf. Table 7. *

14. ADAMS considers that on most of the soils in Great Britain sodium increases the sugar yield more than potassium. As BOITEAU observed, one may therefore wonder whether it is the sodium, the chloride or the nitrate that destroys soil structure. *

15. It appears, however, that generally speaking the air and water permeability of the soil is diminished to a greater extent by sodium than by potassium ions. *

16. It is interesting to note that conversely on soils containing too much sodium the best remedy is to apply heavy dressings of potassium fertilizers (Chapter 24). *

17. At the 8th International Grassland Congress in 1960 LEHR reported that the application of Chile sodium nitrate (for the first cut) enabled the sodium content (Na) of the dry matter of grass to be increased from 0 - 06 to 0 -42%. *

18. It will also be seen in Chapter 15 that the antagonism existing between calcium and magnesium modifies the effect of the ratio of the grass on tetany. *