1 March 1996


By Robert Harris

HIGH sugar impurities are not just a processors problem. They affect the growers profits, too.

Despite this many sugar beet growers take little notice of sugar impurities when they read their grower return form. After all they are neither penalised for a high figure nor paid a bonus for keeping it low.

But there are hidden costs, says British Sugars head of agricultural research and development, Mike Armstrong. High levels often reflect excessive nitrogen fertiliser application, which costs UK growers between £1m and £1.5m a year.

"There are three main non-sugars – amino-N, potassium and sodium," he explains. "These reduce our processing capability by preventing some of the sugar from crystallising. Instead, it stays in solution and ends up as molasses. Impurities also affect sugar colour, reducing the value of the final product."

Unsure of cause

He admits BS is unsure what affects potassium and sodium levels. But they are closely linked to the amount of amino-N, so growers taking steps to cut that impurity will often reduce the other two.

Amino-N is measured as mg/100g of sugar. Soil type, agronomy and seasonal factors like drought and virus yellows all affect the level, so seasonal averages vary. As a guide a target of 100mg/100g is achievable in most years, he says.

"We consistently achieve a lower figure in trials but I appreciate growers find it difficult to give their crops the same amount of attention. But the best growers are in that sort of area."

Matching nitrogen to crop need is the key to low amino-N levels, so amount and timing are vital, says Dr Armstrong. Growers should apply N in two splits, the first going on at drilling and the second at the two-leaf stage if needed.

"For the first dressing a maximum of 30-40kg/ha (24-32 units/acre) should be applied. More can stop seeds germinating, leading to herbicide timing problems and chemical damage. It also minimises leaching – the crop is only taking N up slowly at this time.

"The second dressing needs to go on much earlier than many growers realise. The longer you leave it, the more adverse the effect on yield and amino-N. Beet will still take up the nitrogen but is less able to use it."

Soil type, previous cropping and organic manures dictate the total level of N needed. On sands and sandy loams without organic manures, BS recommends a maximum of 120kg/ha (96 units/acre) applied as a 30:90 split for maximum yield.

Silts and clays need 60-90kg/ha (48-90 units/acre) depending on fertility. "If the previous crop was brassicas or something similar you would be looking at the lower end."

Sugar beet grown on fen peats needs just 30kg/ha at drilling, he advises. "In four out of five years you could get away with nothing. These soils are extremely fertile, yet growers regularly put on at least 60kg/ha."

These recommendations are checked each year at 10-12 representative sites across the country, he explains. "We can be very precise in our recommendations."

But organic manures alter the picture completely, he warns. "About a third of the crop is treated with manure each year. Most is treated with FYM, and N should be reduced by 30kg/ha."

Cut poultry manure

A quarter receives poultry manure, which concerns Dr Armstrong. "It is extremely fertile. It is just like applying bagged N. From a processors point of view I would prefer growers not to use it."

Too much depresses root yield and sugar content but raises impurities. But it is extremely difficult to apply accurately, he warns. "We found a quarter to half-inch layer on the soil equated to 55t/ha. But for maximum yield and quality just 6t/ha is adequate. That is only a dusting."

Apart from tailoring N use, growers should follow good practice to keep crops healthy to ensure they can use that N, says Dr Armstrong.

"Evidence shows that the higher the yield of beet, the lower the impurities. The best growers producing well over 55t/ha adjusted yield regularly achieve amino-N levels around 80 across all soil types. Reducing yield to 45t/ha doubles that figure." &#42

&#8226 High levels hit processing.

&#8226 Target of 100mg/100g in root sugar "achievable".

&#8226 Split N and apply early.

&#8226 Allow for soil type, previous crop and organic manures.

&#8226 Analyse muck for nutrients.

&#8226 Use poultry manure sparingly.

&#8226 Aim for maximum yield.

Although British Sugar wont pay growers for lower impurities, it says they should aim to cut them for a host of other reasons.