CHAPTER 5

Potassium fertilizers have a profound effect on the mineral balance of the grass

The plants listed in Table 3 were grown in a solution containing equal proportions of the four ions sodium, potassium, magnesium and calcium. This balance was greatly altered, however, in the cells of the plant tissue, and the results obtained show that in such common crop plants as maize or potatoes, potassium is accumulated by the cells of the plant to a much greater degree than the other ions, be they monovalent like sodium or bivalent like calcium and magnesium. Another remarkable fact is that in every case the cells of the roots tend to repel the sodium ion, the concentration of which is much lower than in the nutrient solution.
The result of this "avidity" of the plant cell for potassium is that fertilizer potassium is absorbed very rapidly and very heavily by grass, and not gradually and slowly like the phosphoric acid of a superphosphate or the calcium of a marl. This phenomenon is described as "luxury consumption" of potassium by the plant.
All this means that the herbage's content of potassium increases considerably and suddenly following the application of potassium fertilizer, creating or accentuating various mineral imbalances, as will be seen below.

Marked influence of potassium fertilizer on the mineral balance of grass

Table 4 below demonstrates the marked effect of potassium fertilizer dressings on the mineral balance ¹ of two common grass species, cocksfoot and ryegrass. Such dressings greatly increased the potassium content of the dry matter while at the same time lowering the content of calcium and magnesium.

Table 4: Effect of dressings of potassium fertilizers on the mineral equilibrium of two grass species

The most marked effect of the potassium fertilizer, however, was on the sodium in the grass, the content of which became very low and sometimes even non-existent, reducing in the case of cocksfoot from 0-91 to 0-04% (one-twentieth) and in the case of rye-grass from 0-65 to 0-11% (one-sixth).
The mineral imbalances are particularly obvious in cocksfoot which, as 'T HART points out, favours tetany ² much more than other grasses.³

Abnormal mineral ratios

A dressing of 2 cwt./acre [254 kg./ha.] potassium chloride is equivalent to 1 cwt./acre [127 kg./ha.] K₂0. This is more or less the quantity generally recommended for pastures ⁴ or even less. Let us take as characteristic of the mineral balances of grass the ratios:

and
		5	(in milli-equivalents) 6

It will be seen that a dressing of 2 cwt./acre [254 kg./ha.] potassium chloride causes the sodium content of the dry matter of cocksfoot to fall from 0-91 to 0-11%, that is, to an eighth, ⁷ which is considerable. The parallel increase in the potassium content, however, raises the K/Na (weight ratio) from 1-70 to 40-10, ⁸ that is, multiplies it by twenty. ⁹ At the same time the ratio (in milli-equivalents) increases from 1-00 to 3-63, which means that it is multiplied by almost four.¹⁰
When 4 cwt./acre [608 kg./ha.] potassium chloride are applied the figures become even more abnormal, the sodium content becoming almost nil (0·04%) and the K/Na ratio reaching the colossal figure of 122-5.
The objection may be raised, and rightly, that a dressing of this order is almost never applied in practice. But in the case of this "short" experiment it was a case of one single dressing, and we will see that as a result of the cumulative effects of dressings of potassium fertilizers (or liquid manure) and, due to the fact that the greater part of this potassium is returned via the urine of the grazing animal, small dressings repeated over the year produce more marked effects on the grass than one single heavy dressing annually.

By favouring white clover, potassium fertilizer alters the mineral balance of grass

The influence of potassium fertilizer on the composition of grass can vary due to the fact that this fertilizer alters the flora of the sward. The result is that, although potassium fertilizer always increases the potassium content of the grass, it does not necessarily reduce the latter's magnesium and calcium contents nor necessarily increase its ratio. In effect, potassium fertilizer favours the development of white clover, the ratio of which is lower than that of graminaceous plants (Table 20, Chapter 25) because its contents of calcium and magnesium are so obviously higher. By favouring the white clover in the sward, therefore, the potassium fertilizer applied can, in certain cases, increase the calcium and magnesium contents of the grass relatively more than it increases the potassium. The result in such cases is a lower ratio of the grass.
By contrast, nitrogenous fertilizers generally tend to depress white clover,¹¹ with the result that the normal consequence of simultaneous potassium and nitrogen dressings is a considerable increase in the ratio of grass and a slight diminution in its calcium and magnesium contents.¹²
The different aspects of these mineral imbalances in grass brought about by the application of potassium fertilizers will now be dealt with in greater detail.

1. Table 22 (Chapter 25) shows equally clearly the mineral imbalances caused by potassium fertilizer application. Cf. also Tables 4, 7, 13 and 18. *

2. See Table 24 (Chapter 28) regarding tetany triggered off by the application of sulphate of ammonia to a temporary pasture composed mainly of cocksfoot. This question of the tetany-producing nature of cocksfoot will be dealt with in discussing the influence of the flora on grass tetany (Chapter 30). *

3. In an ordinary pasture cocksfoot represents only a small fraction of the flora and is a useful species from many points of view. The danger is increased, however, if a temporary pasture composed solely of cocksfoot is grazed. It was a pasture of this nature that gave rise to the very serious outbreak of tetany at the National College of Agriculture, Grignon (France) in 1959. *

4. For the quantities of fertilizer generally recommended for pastures, see Chapter 10. *

5. For the influence of temperature and humidity on this mineral ratio, see Chapter 34. *

6. For the possible relationship between this ratio and the incidence of tetany see Chapter 26 and Table 21. The opinion is that when the exceeds 1-80 the danger of tetany is greatly increased, at least under certain defined conditions. *

7. It will be seen (Table 7, Chapter 8) that, depending on the relative rates of potassium and sodium fertilizer application, the sodium content of oats can vary from 0-04 to 1-79%, or in the proportion of 1 to 45. *

8. DE VUYST, in Belgium, has observed variations in the K/Na ratio between 5-8 and 75-9, or in the proportion of 1 to 13, for grass, in a single region.*

9. The unfavourable consequences of this high K/Na ratio for the functioning of the adrenal cortex will be discussed in Chapter 23: also how this can favour tetany.*

10. The effect of potassium dressings on other ratios or differences such as basic surplus, base : acid ratio, etc., were also studied. See B0SCH and VERDEYEN. *

11. But this is not obligatory. See Part XIII "Effect of nitrogenous fertilizers on grassland flora", pp. 213-40 of my book Better Grassland Sward. *

12. This question of white clover will be discussed again in studying the influence of the flora on grass tetany (Chapter 30). *