27 February 1999


New research suggests that its time to reconsider placed fertilisers for wide row crops. Lucy Stephenson reports.

ITS official: yield and quality are improved by placed fertiliser, according to the latest MAFF-sponsored research. Placing fertiliser in bands of about half the row width reduces the total application with no yield loss.

Research shows that starter fertiliser placed under the seed at drilling or around the roots of transplants can increase yield and quality. In both cases nitrogen losses to the environment are reduced.

Finding out where and when roots develop will help to maximise nitrogen efficiency, according to Dr Clive Rahn of the Horticulture Research International.

"Roots of wide row crops only explore a small proportion of the soil even after several weeks of growth. Theres more broadcasting of fertiliser going on than there should be, especially for base dressings. The nitrogen needs to be put in the root zone," says Dr Rahn.

But there are practical difficulties with targeting fertiliser. Its easier to broadcast. Targeting could be marginally more expensive: because its generally more precise using liquid fertiliser, farmers may have to use a contractor to inject it.

Banding fertiliser doesnt enhance growth, but it does effectively place it where root growth is expected in the first four or five weeks. Yields of short season brassicas, onions, maize, lettuce, potatoes and sugar beet are still maintained, and the input cost is cut.

"Fertiliser is applied at the same rate but to only half of the soil area. With a later top dressing broadcast at normal rate, total fertiliser usage for the season is cut by up to a quarter," explains Dr Rahn.

Starter fertiliser goes further than banding. It places the fertiliser near the young roots and helps improve establishment. An 8:24 ammonium:phosphate mix at 9ml per metre of row, placed 25mm below the seed delivers 20 to 30kg/ha nitrogen and 60 to 90kg/ha of phosphate.

Just 40kg of starter gives a yield equivalent to having used 100kg of broadcast fertiliser, says Dr Rahn. Crops tested by HRI include carrots, lettuce, maize, onions, and parsnips. Using a starter fertiliser followed by a conventional broadcast fertiliser gives growth and yield consistently above broadcast alone.

"Its so startlingly effective for these crops that you cant get the same yield even if you put tons and tons of broadcast on," says Dr Rahn. "With onions weve seen an 8% increase in marketable yield. The quality increases too, with a 31% increase in bulbs over 55mm." Quality benefits are seen in other wide row crops as well, and theres no evidence of increased fanging in parsnips or carrots, he says.

Starter fertilisers can increase fertiliser recovery by up to 50% in shorter season crops such as maize, onions, and lettuce. There are two reasons for this. Firstly, starter fertiliser is used more effectively than broadcast. Secondly, the plant is bigger and so uses the top dressing more efficiently.

Dr Rahn cautions though that too much placed fertiliser can have a negative effect on the crop. Leeks are especially sensitive, showing reduced root growth, and potato tubers can scorch. Some seeds wont germinate in conditions with high conductivities. This applies particularly on light soils where conditions are drier.

The research also found that starter fertilisers dont give an additional beneficial effect in longer season crops. This applies to brassicas generally and Brussels sprouts in particular: because the root system is very branched it finds fertiliser wherever it is in the soil.


Once in the right place, its the concentration of nitrogen thats important. The optimum level is not the same for all crops, and faster-growing crops have lower critical level. For example, the optimum level of banded nitrogen given to cauliflowers depends on when they are grown.

Cauliflowers planted in mid-May grow much more slowly than those planted in the warmer conditions of mid-July. Their root volume is three times smaller 26 days after planting. To compensate, May cauliflowers need five times more fertiliser than July cauliflowers.

Targeting is more important for crops with only a short growing season because they cant waste time looking for nutrients. Nutrient supply is optimised by meeting early demand with targeted fertiliser, and satisfying the later demand with a top dressing, says Dr Rahn. It may be eight weeks before the soil is fully explored by the roots.

Appropriate rates of mineral fertiliser for the top dressing could be calculated from mid-season soil tests, but models such as Fertiplan, Sundial or WellN can also be used, taking into account factors such as irrigation, says Dr Rahn.

He recommends that decisions about the amount of top dressing to apply should be guided by measuring crop size and soil mineral nitrogen. Use of fertiliser models such as WellN, NCycle, and Fertiplan, he says, can be used to interpret these measures and other factors such as the effect of irrigation.

No With Starter Starter

Maize 47% 79%

Onions 40% 60%

Lettuce 40% 50%

No Starter (t/ha) With Starter (t/ha) Benefit (%)

Bulb onion Bulbs >55mm 18.3 23.9 31

Early carrot Target grade 25-30mm 19.4 25.3 30

Lettuce (drilled) Iceberg quality 19.2 27.4 43

Lettuce (transplanted) Iceberg @ 47 days 0 24.3

Iceberg @ 57 days 26.5 42.9 62

Parsnip Marketable roots 17.6 20.3 15

Source: Mr David Stone, HRI Wellesbourne (MAFF/HDC funding)

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