TOP-DRESSING

Hawke's Bay Tribune, Volume XVIII, Issue 288, 21 November 1928, Page 9

TOP-DRESSING

Effects of Over-supply of Phosphate

Necessity for More Nitrogen

(By “SUNDOWNER.”)

It is an acknowledged fact that many substances which are excellent foods when taken in moderate quantities become harmful, and in some instances poisonous, when taken in excess. In fact this applies to almost everything which is consumed by human beings and animals. It is also a fact that the more nourishing the food the more harmful it is if taken to excess.

What applies to animal life must similarly apply to vegetable life, for it is accepted that all life is the same in essence, and is governed by the same fundamental laws.

It is not surprising then, to learn that the most valuable manures can be supplied to the land in such quantity that they are actually harmful to the plants which they are intended to benefit. What the plant wants, as does the animal, is a balanced diet, especially of the main food elements, nitrogen, phosphorus and potash.

OVER-SUPPLY OF PHOSPHATE

In several articles published in the past the writer has stressed the fact that to supply too heavy a dressing of superphosphate to a thin, run-out pasture, not only means a waste of the phosphate in excess of that which the weak, scattered plants cau absorb. but is actually detrimental to those plants which receive this oversupply. A confirmation of this opinion is contained in reports of experiments undertaken during the past twelve months at South Kensington, England, where a study of the effect of phosphate on plant growth is in progress.

EXPERIMENTAL RESULTS WITH PHOSPHATE.

“Seven different concentrations of phosphate have been taken,” says the report, “and the plants are grown in sand. The phosphate is found to influence growth in three main directions. The effect on tillering is marked, and there is an increase in total growth. But after the maximum leaf area is reached some curious things happen. The more phosphate there is present the more rapidly does the leaf surface die off; in other words, phosphate seems to speed up growth in the early part of the season, and then to speed vp ripening.

“The manner of growth varied considerably with the amount of phosphate given, and at a certain concentration there was a period of a fortnight, at the time when the maximum leaf area had developed, when the plants made no growth at all. With higher amounts of phosphate, this period was less; with lower amounts it did not occur and the plants continued to grow throughout the season.’’

Many interesting effects of phosphate on growth were studied, toe numerous to mention here. Among them, however, was the curious result of transferring plants at an early stage from phosphate soluti ms tn solutions containing no phos hate. They grew better than when they were left all their lives with a supply of phosphate to draw on.

BEST MANURES CAN BE OVER SUPPLIED.

These experiments conducted on a small scale in specially washed sand, and where the supply of phosphate is purposely overbalanced, are of course not parallel to conditions to be found in the field, but they do point the fact that even the best manures can be oversupplied for the plant’s health and each soil requires special study through a close observance of the grass growth in response to various kinds of manure.

PRACTICAL MANURING EXPERIENCE.

From practical experience the writer advises the application to a thin ,weedly pasture of a light dressing of superphosphate—say, one cwt. per acre—at the end of winter or as early in the spring as possible, followed by half a hundredweight of sulphate of ammonia in the autumn. These quantities may be increased as the sward thickens up until perhaps 3cwt. of super and lewt. of nitrogenous manure is being applied annually or every second year.

SUPER BEST AS SPRING DRESSING.

The phosphate is recommended for early spring application because, besides increasing the bulk of herbage through promoting tillering and root development, it has a very important effect on the development of a good sale of closer which, of course, is most affected in the spring. Further it is in its early growth when grass most needs the assistance and this time, of course, is in the spring. If distributed in the autumn, superphosphate, which is very readily soluble in water loses much of its fertilising qualities before the spring growth wants it, besides which a certain amount is lost through leaching into the soil below the root depth, a certain amount converts into an insoluble form when in contact with other soil minerals and acids, and a certain amount is washed off the surface into gullies and streams where it is lost.

SUPER’S EFFECT ON NUTRITIV7 VALUE.

Besides the effects enumerated above which superphosphate has on grasses, its chief effect is that by stimulating the chlorophyll, or greencolouring matter of plants, it enables them to extract more nourishment from the sunlight. In the absence of abundant sunlight superphosphate loses half its virtue as a manure, hence the spring application, when it can be used by the plant through the sunny season, is the right one for superphosphate. Nitrogenous manures, on the other hand, are most valuable in sunless countries and in sunless seasons. Their chief effect is that they encourage strong growth under these conditions, hence in the North Island of New Zealand, especially, they are of more value in encouraging abundant winter growth than they would be in our usually bright summers.

USE OF NITROGENOUS MANURES ADVISED.

Many farmers who are now confirmed believers in the use of superphosphate fot top-dressing will not use sulphate of ammonia because they have tried it as a spring dressing and it has failed to show results. If these same men would try it as an autumn dressing they would rapidly change their opinion as to its value.

Where clovers of various sorts are naturally very much in evidence in the pasture, nitrogent may not be so absolutely necessary, for these ligumes are gatherers of nitrogen from the air and help to keep up the supply of nitrogen in the soil. Further, much more nitrogen is fixed in the soil in a sunny than in a sunless climate.

INDICATIONS OF NITROGEN EXHAUSTION.

In spite of these advantages of native clovers and sunlight which New Zealand enjoys, there is distinct evidence that the supply of nitrogen in our soil is becoming greatly reduced, if not exhausted This is indicated by the class of grass and consequently the class of stock our land now carries compared with even twenty years ago. The country which at that time successfully carried Lincoln sheep and Shorthorn cattle now carries much less exacting breeds. The change has been so slow that it has not been noticed by some, nevertheless it is going on all the titre,’ and we should now apply nitrogen and phosphate to check this decline and. if possible, bring the land back to its original fertility.