The need for fertilisers

Press, 15 September 1976, Page 14

The need for fertilisers

Why do we need artificial fertilisers? Well, the short answer is that because the average garden constantly uses the same land, which is expected to produce life year after year after year, the nourishment taken out of it must be replaced.

Fertilisers are the most simple and practical way

to maintain soil fertility but they should be looked upon only as supplementary and complementary to the addition of organic matter.

Green crops dug in; compost added to the land; occasional spelling and fertiliser all add up to successful gardening. But it must be emphasised that the use of fertilisers without some knowledge of the various types can be wasteful and, under some circum-

stances, harmful. Some help to grow leaf; others assist fruit or vegetable cropping; some make root crops soggy and some have the opposite effects. But it is not really difficult to know the fertilisers and their function. The basic facts are simple. There are three main fertilising elements needed for healthy growth — nitrogen, phosphate and potash.

NITROGEN: Sometimes called the growth accelerator, nitrogen promotes strong, fast growth, producing big leaves and stalks. It is tremendous for stimulating quick salad crops such as lettuce, cabbage,

etc., as well as many of the leafy indoor plants such as monsteria or philodendron. However, nitrogen can be a trap, for, while it stimulates rapid, dark green leaves, it also tends to make plants very soft and sappy and possibly more prone to disease. It can also stifle the fruiting and flowering habits of plants.

Like everything else, too much of a good thing can be just as bad as not enough. Plants with nitrogen starvation have yellow leaves and stunted growth. Nitrogen fertilisers include blood and bone (8-12 per cent nitrogen), sulphate of am-

monia (21 per cent nitrogen). PHOSPHATE: In the simplest terms phosphate stimulates root development and increases the healthy function of the leaves. Most root crops such as carrots, potatoes, beetroot, need good amounts ot it to produce their best yields.

Phosphate is generally important in all New Zealand soils. In fact most of our gardens need an annual dressing at the start of each season of about 100-200 g (two-three good handsful) per square metre.

Plants with phosphate starvation have inter-vei-nal yellowing. They need

feeding with phosphate fertilisers which include bone dust (9-11 per cent P) and superphosphate (79 per cent P).

POTASH: Potash is a most important element for flowering and fruiting plants. It improves quality and flavour and helps regulate soft, rank growth produced by overdoses of nitrogen.

Plants with potash starvation tend to have burnt edges to their leaves. Potash fertilisers include sulphate of potash (40 per cent K) and nuriate potash (48 per cent K). These three major plant foods are needed in varying proportions and the secret of feeding plants

correctly is more easily obtained by using a com-mercially-made single fertiliser preparation of N.P.K. mixture. Other soil minerals required by plants are obtained in much smaller quantities than the main one with the possible exception of calcium, magnesium and sulphur but these secondary elements are often supplied indirectly with the application of other fertilisers.

The need for other minor soil nutrients including iron, copper, zinc, manganese, boron, chlorine and molybdenum is essentially needed only in microscopic amounts. Most soils contain these elements in sufficient

quantities — although growth responses have been obtained from treatments of liquid seaweed preparations and other compound micro-nutri-ents.

The simplest means of feeding plants well is to maintain regular, adequate dressings of compound fertilisers containing N.P.K. Adequate, means about 4-5 handsful of N.P.K. mixture to each square metre before planting each crop. Often composts and other humus materials can supply valuable plant foods which have the advantage over fertilisers in that they are longer lasting, non-burning and usually well balanced.