SCIENCE IN FARMING

New Zealand Herald, Volume LXIX, Issue 21296, 24 September 1932, Page 19

SCIENCE IN FARMING

SOIL REACTION i 'use of fertilisers EFFECT ON ACIDITY HYDROGEN lON CONCEPTION , BY Q.R.H. The rapid expansion of the synthetic nitrogen industry has resulted in renewed interest in the question of the. effect of nitrogenous fertilisers oh soil acidity. This is quite natural, for one of the important criteria of any fertiliser is its residual effect on the soil. As far back as 1881 it was recognised that applications of artificial nitrogenous fertilisers could cause changes in the reaction of the soil, but it is only of re- ■ cent years that this phenomenon has been properly understood; previous to this the J mechanism of soil reaction was but imperfectly known. It is, however much more easily understandable in the light of modem advances in the knowledge of soil acidity, especially with regard to the hvdrogen ion conception of acidity, and also the buffer capacity and base exchange properties of the soil. Buffer Capacity This question of buffer capacity is of some importance, and an understanding of it will help to explain the inconsistent results which have been obtained by dU-

ferent investigators in the residual effects of different fertilisers. By buffer capacity of soils is meant the ability of a soil to resist a change in its reaction on the addition of a base, such as lime, or of an acid or acid-forming fertiliser. Of course, the presence of free base, such as. free lime, will neutralise any acid that may be formed, and_ so prevent an increase in the acid reaction, but in actual practice the amount of free lime is seldom sufficient to be of any practical value, and the ability of a particular soil to resist change in reaction is almost entirely a function of its buffer capacity. . . . % • A further example may help to explain this phenomenon. Assume a soil with a certain reaction (or pH value), and adi to this a certain amount of acid. Under these circumstances one would expect the reaction to become more acid. Actually the change that does take place is always considerably less than calculated, and in some cases there is practically no change at all. It is difficult in a semi-technical article to explain properly the reason for this "buffer effect," suffice to say it is a physical-chemical phenomenon due to certain constituents of the soilA Varying Quantity Buffer capacity varies widely in different soils, being, as a rule, least in sandy soils and greatest in clay soils. I his means that the addition of equivalent amounts of acid to a sandy soil and a clav soil will cause the sandy soil to acquire a more acid reaction than the clay soil, that is, of course, assuming that both had the same reaction at the start. Of course, the amount of acid produced bv a certain weight of, say, sulphate of ammonia, will-be the same whatever the type of soil, but the acid produced will cause a smaller change in reaction in a soil of high buffer capacity than in a soil of lower buffer capacity Obviously the same reasoning will hold true for the addition of lime. A greater amount of lime will be required to change the reaction of a clay soil than would be required for a sandy soil or a loam. As a general rule, whether it is understood or not, the main^ reason for hming is to change the reaction of an acid soil nearer to that pH value at which growth for a particular crop is at a maximum. | Therefore, with two soils of the same reaction but of different buffer capacity, the one of lower buffer capacity will require less lime to give , the desired change m reaction than will the one of higher buffer capacity. Effect on Experiments This understanding of buffer capacity also serves to explain the inconsistent results which have been obtained in the past by experimenters with ammonium sulphate. In one case an investigator treats a plot of land with this fertiliser and reports that no acidity developed, while another experimenter reports that the addition of the same amount of sulphate of ammonia caused the development of an acid reaction. In both cases, of course, the amount of acid formed was the same, but in the first case the soil was highly buffered and the change in reaction, was, as a result, only slight, whereas, in the second case, the soil, being of lower buffer capacity, reacted acid to a 81 1f t one d wants to take advantage of the

very high nitrogen content of these synthetic fertilisers, the right and proper thing to do is to add at the same time the correct amount of lime to neutralise the acid that will be formed. On the Other hand, it is often desirable to.make the soil more acid to get the best results - from those crops which thrive in an acid medium, and under these / conditions no lime should be used, and the fertiliser i selected should be one which can develop considerable acidity. Nitrate of soda, calcium nitrate or cyanamide should not be used, as they do not react acid, bm. on the contrary, they tend to produce an alkaline condition. Dispensing with Lime Instead of using lime to neutralise the acidity due to the decomposition of the acid-forming fertiliser, one of these alka-line-producing type can be used, and, as a matter of fact, they are tach equivalent to three times their weight of carbonate of lime. Thus a mixture of two parts of nitrate of soda and one part of sulphate of ammonia does not develop either an acid or an alkaline reaction, does not require the use of any lime, and does not alter the reaction of the soil to which it is applied. Such a mixture contains the nitrogen from the nitrate of soda as well as from the sulphate of ammonia. The reaction of the soil at the time the fertiliser is added should also be taken into consideration, as this determines the extent to which the nitrogen is rendered available to the plant. Am- \ monium sulphate gives the best results when the soil is decidedly alkaline (pH 7.5), urea with a neutral soil of pli *.u, and' calcium cyanamide and nitrate of soda with distinctly acid soils of pH 4.0 to pH 5.0. With soils more acid than pH 5.5 nitrate nitrogen is a better plant food than ammoniacal nitrogen, and accordingly fertilisers like nitrate of soda or calcium nitrate will give the best results, whereas with soils less acid than pH 5.5 both types of nitrogen are of equal value. This refers to average farming conditions, and does not apply where advantage of the acid-forming properties is to bp taken, such as, for example, where sulphate of ammonia is used to increase the acidity o? the soil when pota- - toes are to be grown.

PASTURE CONTROL / GRAZING AND MANURING EFFECT ON GRASSES ENGLISH OBSERVATIONS As stems develop in grass the quality and digestibility of the pasture begin to decline. The best pastures are those which retain a leafy type of herbage for the longest time. | Grassland management is neither simple nor easy. Much prominence has been given in recent years to the question of intensive complete manuring and rotational grazing. The early results have been highly successful, but where the system has been in operation for a number of years and careful observation has been ' made, changes are noted and difficulties experienced that cannot be disregarded. The nature of the grass changes with the manuring and with close grazing. At first all the grasses would appear to respond to higher manuring; perennial ryegrass is usual predominant in early summer and bent grass (Agrostis) in late summer and autumn, says Mr. William Lawson, Director of Agriculture for West Sussex, in the British Journal of Agriculture. Payability of Ryegrass In practice the greater palatability of ryegrass results in the ready consumption of this grass in early summer, whereas the autumn growth of bent is less readily eaten and it consequently increases in vigour and in amount. Under such condii tions the perennial ryegrass appears to ; get weaker and to give place to the less i desirable Agrostis. This is a tendency that should be counteracted. Different ! types of pastures will be affected differi ently and will require a type of management suited to the particular conditions. Mr. Lawson's observations have been : mainly made on good old pastures in the . south of England, and from them it would appear that applications of nitrogenous manures to pastures should be confined to , early spring, and that the first growth should not be too closely grazed. In the interest of the pasture close grazing should be specially aimed at in the autumn, and I if the autumn grasses are well eaten off the ryegrass will increase during winter , and will be ready to produce bulky and nutritious grass early in tho following

summer. Preparation for Spring. 1 Preparation for an early spring bite, he suggests, should commence in August or September with a close grazing, a 1 period corresponding with February or 1 March in New Zealand, and if followed in late February or early March (August or September in New Zealand) witn a dressing of complete artificial manures, r maintenance of quality and quantity [ should be economically possible. ? Much has been said as to the increased stock-carrying capacity of the land after 1 intensive manuring. On good land this may have disadvantages, said Mr. Law- ' son. Parasitic and other diseases are fc spread on the pastures, and the greater ; the' number of stock on a given area the J more likely is infection to be spread from -animal to animal.

Sheep in particular suffer in this respect, and whereas mixed grazing with cattle and sheep will mitigate the likelihood of trouble, the possibility of disease spreading more rapidly under intensive stocking conditions can never be wholly disregarded.