SUPPRESSION OF WEEDS.

New Zealand Herald, Volume LXVII, Issue 20519, 21 March 1930, Page 7

SUPPRESSION OF WEEDS.

EFFECT OF FERTILISERS.

SMOTHERING CROPS AIDED.

The most usual way in which manures favour tlie suppression of weeds is by enabling good smothering crops to be grown. A well-measured dressing of nitrogenous manure will often enable a good plant so to fill out and cover the ground that few weeds can make headway below it.

The' "scorching" action of most soluble fertilisers when left in contact with the leaves of crops, observed, for example when topdressings of nitrate of soda or sulphate of ammonia have lodged on the tops of mangel or sugar beet, has also been turned to account in combating weeds. The effect consists in the abstraction of water from the leaf cells by the strong solution which is formed when the fertiliser dissolves in a trace of moisture on the surface of the leaf. If a sufficient leaf area is damaged in this way the plant is killed. A special grade of finely divided kainit is used for this purpose, says the British Journal of Agriculture. It is best applied as a dust cloud while the leaves are covered with dew and hot weather is likely to follow. This can be done even when the weed is growing in corn, for the upright waxy leaves of the cereals suffer little permanent damage from the treatment.

A strong solution of sulphate oi ammonia has been used to some extent for the same purpose, but applied as a liquid spray in bright weather. A further case is the utilisation of the caustic properties of calcium cyanamide. The bad effect of this manure on germinating seedlings, guarded against as far as the crop is concerned by applying it a few days before the sowing, no doubt eliminates a number of weeds whose seeds happen to be at a vulnerable stage. Moreover, the burning e.ffect of this fertiliser on foliage is used against charlock and other weeds in corn by broadcasting dusty cyanamide, by means of a "blower" or dry sprayer, under the same conditions as are required for kainit.. In addition to damaging weeds these substances also exert a good measure of their usual fertilising effect, which may in itself justify their application.