CHAPTER 11

Influence of nitrogenous fertilizers on the organic and mineral balance of herbage

	The amino-acid composition of the crude protein 4 is altered.
	The nitrogenous substances as a whole, for a given physiological age of grass become more readily reducible to ammonia 5 in the course of digestion (see Table 20); this effect becomes more marked, the shorter the interval between the application of the nitrogenous fertilizer and the grazing of the herbage.

When no magnesium or potassium fertilizers are applied, yield and magnesium content increase in the following order:

	sulphate of ammonia
	nitrate of ammonia
	sodium nitrate
	sulphate of ammonia + chalk.

More or less the same order of effect from the four types of nitrogenous fertilizer is maintained where potassium fertilizer is applied. The latter very significantly reduced the magnesium content of the crop. The application of magnesium fertilizer increases both yield and magnesium content of the crop in every case. but the fundamental point that emerges is that this application of magnesium, i.e. adequately increasing the magnesium content of the soil, eliminates the differences in effect on magnesium content between the different nitrogenous fertilizers although the difference in yield due to these fertilizers persist.¹¹ This is a further illustration of the fact that variations in the yield and biological quality of plants do not necessarily follow a parallel course.

The use of sulphate of ammonia on pastures is only permissible if the soil contains sufficient available calcium and magnesium

In other words, the beneficial effect of nitrate ions compared with sulphate ions on the magnesium content of the plant is particularly obvious where the soil is deficient in magnesium.¹² This means that the unfavourable effect of sulphate of ammonia diminishes, and may even disappear if the pH is higher and/or the soil richer in magnesium. Fig. 17 also demonstrates that even on a slightly acid soil sulphate of ammonia reduces the magnesium content of the herbage only very slightly. Two very important practical conclusions from the grass tetany point of view may therefore be reached:
1. On a soil acid and/or relatively low in available magnesium there is the danger that sulphate of ammonia will accentuate the magnesium deficiency of the herbage and favour grass tetany (a situation, moreover, that frequently arises in practice).¹³
2. If the necessary dressings of calcium ¹⁴ and/or magnesium are applied to the soil, sulphate of ammonia neither reduces the magnesium content of the herbage nor increases the risk of tetany.
It must not be forgotten, moreover, that sulphate of ammonia can cause the magnesium content of herbage to vary not only by virtue of direct action but also by modifying the flora.¹⁵ HEMINGWAY, for example found that on a heavy loamy soil with a pH of 6-5, sulphate of ammonia was able in the first year to increase very slightly ¹⁶ the magnesium content of the grass ¹⁷ but ultimately eradicated clover, which is richer in magnesium than the grasses (see Tables 25 and 26), with the result that after a few years of sulphate of ammonia application the magnesium content of the herbage was reduced.

The animal grazing a young sward rich in protein excretes urine of very high nitrogen content

Sight must not be lost of the fact that if the herbage affects the grazing animal, the animal likewise affects the herbage. The potassium in grass affects the cow, but the result is a urine richer in potassium (Table 18), which in its turn will alter the composition of the grass. A similar phenomenon is manifested in the case of nitrogenous fertilizers. As was stated at the beginning of this chapter, fertilizer nitrogen increases the crude protein content of the herbage, but, as SJOLLEMA found, cows grazing herbage very rich in crude protein excrete in their urine quantities of nitrogen which may be as much as three times what they excrete when fed a normal ration. In addition, the nitrogenous composition of such urine is markedly altered.¹⁸
In other words, a large part of the nitrogen applied in the form of fertilizer will be rapidly returned via the excrement, particularly the urine. How will this urine, very rich in nitrogenous substances of a different nature and composition, act on the composition of the herbage and subsequently on the animal? No reply is possible as yet because of our very imperfect knowledge of the exact composition of the urine of a cow grazing a very young sward. It seems probable, however, that a urine much richer in nitrogen than normal will alter the composition of the nitrogenous fraction of the herbage. It is not impossible, moreover, that certain nitrogenous substances present in this urine of special composition may help to give rise ¹⁹ in very young, rapidly growing grass, to the development of tetanigenic substances.²⁰
Finally, let it be noted that this immediate return to the soil of a high proportion of the nitrogen in herbage of very high nitrogen content will not lead, as in the case of potassium, to an accumulation of nitrogen in the soil. As soon as ammonia is converted to the nitrate form it is either absorbed by the herbage or washed away by rain.

Antagonism of nitrogenous fertilizers and copper

The statistical data published by different research workers show that grass giving rise to grass tetany is abnormally low in copper. It is still impossible to state with certainty whether this copper deficiency plays a part in grass tetany, although this does seem to be probable. It might be wise, therefore, to recall that nitrogenous fertilizers,²¹ by their slow, cumulative effect produce a deficiency of copper, resulting probably from the formation of complex compounds of ammonium and copper ions.
To counteract this effect of nitrogenous fertilizers it seems essential ²² that the necessary dressings of copper fertilizers ²³ should be applied in future to pastures when nitrogenous fertilizers are being used.²⁴

1. See Grass Productivity (pp. 60-1) on the common and serious confusion of nitrogen and protein. *

2. See Better Grassland Sward (pp. 213-54). *

3. See Table 24, where the application of nitrogenous fertilizer depressed the white clover but nevertheless considerably increased the crude protein content of the herbage *

4. See the influence of nitrogenous fertilizer on the amino acid composition of Italian ryegrass in Soil, Grass and Cancer (Table 3, p. 3). *

5. And to hydrogen sulphide (see Fig. 7). *

6. It is impossible to fix a dangerous maximum nitrate content for grass in view of the many factors entering into play to accentuate or diminish the toxic effects of nitrates. According to BECKER'S research work the dangers of a nitrate excess appear to be limited. *

7. Measured with a potassium chloride solution. *

8. As always this is only a very general rule, to be altered according to the soil. *

9. A higher pH rapidly gives rise to manganese deficiencies, not to mention its many other effects. *

10. Between the magnesium ions (Mg++) and the ammonium ions (NH₄+) there is an antagonism that is not direct but indirect. In very simplified terms it may be said that when the ammonium ion is adsorbed by the roots there is an exchange of ammonium and hydrogen (H+) ions. The latter will therefore exert an antagonistic influence on the absorption of magnesium by the plant. The more acid the soil and the greater the quantities of nitrogenous fertilizer with ammonium ions (such as ammonium sulphate or ammonium nitrate) applied, the more marked this antagonistic effect. *

11. To cite one single example: the magnesium contents of green oats are more or less identical (0-25 and 0-24%) whether sulphate of ammonia is applied alone or in conjunction with chalk in cases where no potassium fertilizer is applied but where magnesium fertilizer is used. Under these conditions the application of chalk along with sulphate of ammonia increased the yield from 55 to 82. *

12. As MULDER states, the difference in effect on plant magnesium between nitrates and sulphates is smaller, the higher the MgO : N ratio in the fertilizers applied.
    According to 'T HART's data nitrogenous fertilizer appears rather to tend to increase the magnesium content of the grass very slightly.*

13. For the effect of sulphate of ammonia on hypomagnesaemia see Chapter 28, Tables 20 and 24 and Fig. 18. ENDER made the observation that hypomagnesaemia was more marked where sulphate of ammonia and potassium sulphate were applied than in the case of equivalent dressings of nitrogen and potash in the form of ammonium nitrate and potassium chloride. In addition, under the conditions of the experiment, cases of tetany occurred with the first combination of fertilizers, whereas there were no cases with the second. It appears, therefore, that the application of large quantities of sulphate ions may, under certain conditions, promote hypomagnesaemia and tetany (see Fig. 7).
    It must be remembered that the sulphur content of grass is increased by dressings of ammonium sulphate. ENDER, for example, found that the sulphur content of the dry matter of grass was 0-188% when ammonium nitrate was applied and rose to 0-312%, i.e. 65% more, when sulphate of ammonia was applied.
    In another connexion it appears that the sulphate ion content of the blood serum of cows suffering from grass tetany is increased. In view of this correlation some authors have thought that the sulphate of ammonia contributes to the tetany not only by reducing the magnesium but also by raising the sulphur content of the grass. *

14. Note that there is a "reciprocal" effect between the calcium fertilizer and the sulphate of ammonia. Excessively high rates of chalk or marl application to a pasture give rise to various serious deficiencies, both in the herbage and in the stock. One of these is manganese deficiency, the manganese ions in the soil becoming unavailable because the pH is too high.
    The application of fertilizers containing manganese has relatively little effect, because the conditions existing in the soil rapidly render the element unavailable. The most effective method, therefore, is to apply sulphate of ammonia, which, by lowering the pH, will re-establish the availability of part of the total manganese in the soil. *

15. For the influence of nitrogenous fertilizers on the flora see Better Grassland Sward (pp. 213-54). *

16. The Gramineae in this experiment contained about 0 - 11% magnesium in the dry matter, a very low and very dangerous content. In the first year sulphate of ammonia application raised this mean figure to 0-13%. *

17. TODD, however, observed that on certain soils the application of sulphate of ammonia brought about an increase in the magnesium content of the herbage at the beginning of the grazing season but this effect could be the reverse during the remainder of the season. *

18. In cows eating young grass very rich in crude protein:
    
    The percentage urea nitrogen in relation to total nitrogen increases considerably.
    The benzoic acid content can be multiplied by four.
    
        It is interesting to note that the histamine content of the urine of an animal receiving a ration low in magnesium may be multiplied by 7 (seven). *

19. 'T HART, at the 7th International Grassland Congress, quite rightly remarked in the course of the discussion that: "The return of urine by the grazing cow might be a more important cause of tetany than the application of nitrogenous or potassium mineral fertilizers to the pasture." *

20. For the possible presence in young grass of a particular tetanigenic substance, see Chapter 25. *

21. It was seen in Soil, Grass and Cancer (pp. 35-7) that this copper deficiency manifests itself in the grazing animal in a lower copper content in the blood serum and liver. *

22. Even if the low copper content of grass does not promote tetany, this dressing of copper in conjunction with nitrogenous fertilizers is obligatory. Copper deficiencies give rise to many metabolic disorders in the grazing animal, the most common being sterility. *

23. Each year 6 lb./acre [7 kg./ha.] approximately copper sulphate or the equivalent thereof in the form of other copper fertilizers. *

24. Having recognized this danger, certain European firms producing chemicals now manufacture nitrogenous fertilizers containing copper. The enlightened attitude of these manufacturers can only be applauded. *