2 February 1996


Maize growers are applying too much bag fertiliser to their crop. Jessica Buss reports

TOO much bag fertiliser applied to maize crops is wasteful, costly, could damage the environment, and may even affect crop quality.

This is the message from Wolverhampton-based ADAS soil scientist Ken Smith.

He cites a MAFF and Fertiliser Manufacturers Association survey that shows an average of 81kg/ha (65 units/acre) of nitrogen was applied to forage maize crops in 1994 – despite applications of some manure or slurry to 89% of these crops (see table 1).

"Bag fertiliser is one of the major variable costs for growing forage maize but this could be reduced or removed in many cases," he says.

"Overuse of fertiliser has partly occurred because it is too cheap – although prices have risen recently, it is still cheaper than 10 years ago. For example in 1985 ammonium nitrate cost £125-144/t, and triple super phosphate £150-175/t.

However, producers cannot afford to continue to undervalue the nutrients in manure.

"Figures vary from year to year but generally, the survey shows that very similar rates of bag nitrogen, phosphate and potash are applied whether maize crops receive manure or not. This is a major waste of nutrients.

"The basis of any fertiliser recommendation system must start with soil analysis for phosphate, potash, magnesium and lime. Fertiliser recommendations can then be based on soil nutrient status, to satisfy crop requirements under normal conditions."

He suggests that previous crop yield information may also help to guide phosphate and potash needs. In this way nutrients removed by the crop can be replaced on a "rotational basis", over perhaps a four- to five-year period. But most or all of phosphate and potash needs can be supplied by slurry and muck.

As for nitrogen, crop requirements are more difficult to assess, says Mr Smith, for soil analysis cannot always be relied on.

This calls for a tactical N approach to assess the need for nitrogen applications to maize crops, based on the existing mineral supply in the soil. But such a tactical N system has yet to be developed for maize.

Currently Mr Smith recommends assessing the nitrogen content of manures or slurry on-farm before spreading (see table 2).

"There is equipment available for use on-farm to test slurry solids and nitrogen content," he says. "These slurry nitrogen meters seem to be reliable providing the sample tested represents material to be spread.

"Then a sensible target rate of nitrogen from manure can be planned before drilling. After which a tactical N approach would allow confirmation of manure nitrogen contribution predicted on the basis of currently published MAFF advice; either way, the crop could be top-dressed with bag nitrogen after drilling if necessary, he says.

"This has important potential benefits – it is environmentally friendly for it limits nitrogen losses, it saves fertiliser inputs and may even improve crop quality."

He cautions that crop quality may be reduced by bag fertiliser – resulting in a crop that looks healthy but produces masses of herbage, delaying crop maturity and reducing energy yields.

Crop yield is driven mainly by solar energy once soil conditions, moisture supply and nutrient requirements are satisfied, he adds. It appears that a crop grown on ground with no history of manure application could require 80kg/ha (65 units/acre) of N, with modern early maturing varieties.

"However there is still a need to re-evaluate nitrogen needs generally and, in relation to organic manure applications," he says.

"Organic manures can be used to supply a major proportion of the crop requirement for nitrogen, with the last 30-40% if necessary from bagged fertiliser, where greater precision is possible. A few kg/ha N supply above or below crop N needy are unlikely to affect yield. But the development of a tactical N approach would allow even greater confidence in this strategy."

There is little evidence of any yield response to P or K fertilisers under UK conditions, he says. Although placement of soluble phosphate fertiliser such as di-ammonium phosphate (DAP) has been known to produce a visual crop response, such response is less likely where manure dressings have been applied recently. &#42

Crop nutrient requirements



Nitrogen80 (65)


(P index 2) 40 (30)


(K index 2)120 (100)

Nutrient supply of typical

application of slurry, 50m cu/ha (4500 gal/acre) at 6% solids



mineral N)