30 January 1999

weighing up the early bag

With nitrogen prices likely to stay low, the temptation to use early N is greater than ever – but some say hold off with that first bag. David Millar reports.

PEACE of mind is the main justification for getting out with the fertiliser spreader whenever conditions allow.

Fertiliser prices are sitting nicely around the £95-98/t mark for UK-manufactured ammonium N so the early bag will stay part of February routine for many growers who see their soils at field capacity for water and high nitrogen leaching risk on lighter soils.

Indeed, for those growers with second cereals or light soils, the early bag is still the best route to take, provided they can get a tractor and fertiliser spreader on their fields.

Not all advisers agree, however, on the value of early nitrogen. Ian Richards, of Levington Agriculture, accepts that farmers do take the opportunity to put on the early nitrogen even although the majority of crops can wait quite satisfactorily for top dressing.

Levington is supplying Crops regularly with the data for SulphurCheck (see table) which includes sampling for nitrogen as well as sulphur on 12 varied sites in Devon, Lincolnshire and Suffolk. Although the latest available sampling results show losses of both mineral S and N from the 0-90cm soil layer over the autumn, mineralisation of organic matter led to some temporary increases, and on one loamy sand site in Lincolnshire there were significant increases in both sulphur and nitrogen, particularly in the 30-60cm and 60-90cm soil layers.

Leaving this unusual site aside, average losses between September and December were 25kgN/ha and 20kgS/ha from the 0-90cm soil layer. However, yet another Lincolnshire site lost 54kgN/ha and 117kgS/ha. Dr Richards points out these early results confirm the quite large changes – both positive and negative – in soil mineral S and N that can take place in autumn and early winter.

Now the Ministry of Agriculture has dropped its funding of ADAS nitrogen leaching sampling over winter, the Levington checks supported by The Sulphur Institute and Hydro Agri will provide one of the best guides for growers planning to top dress this spring. Fuller nitrogen and sulphur advice from Levington Agriculture will be published in the 27 February issue of Crops.

Along with other commercial organisations, ADAS continues to offer soil sampling for nitrogen, particularly aimed at growers using organic manures. ADAS soil scientist Brian Chambers is a supporter of the early bag. "In most circumstances I would go for the early bag – and for a second wheat almost certainly," he says.

"For first cereals, which are generally first wheats, I think it is a matter of judging it on the crops growth and development. If the crops look well forward with plenty of N there is no need to go with that early bag because there should be enough to see them through to early March or April."

However, Dr Chambers points out it is often worth putting 40kgN/ha on winter wheats or oilseed rape in mid-February because there is always the risk of the main top dressing being delayed by weather or ground conditions. "Generally this will be enough to see these crops through to the end of March. If the crops looked yellow in the autumn there may be enough justification for 60kgN/ha."

Dr Jim Lewis of Hydro Agri urges caution on soils that may have been waterlogged with the result that wheat root systems will not be operating as efficiently in taking up nitrogen as they might. "I would prefer to see fields dry out a bit before throwing nitrogen on," he says.

Even on lighter soils, which are free draining, there is a risk that applied N will move out of the profile rapidly. If soils are waterlogged there is also some volatilisation (loss to the atmosphere) of N when it is put on.

Dr Lewis does not favour hitting crops showing yellowing symptoms with 60kgN/ha in one hit. Growers should consider instead a couple of split applications before their main top dressing if they were concerned about giving such crops a boost.

However, provided fields can bear the traffic, many oilseed rape crops are growing well enough to justify early nitrogen because their root systems are strong and will be able to cope.

&#8226 In NVZs, growers should not be applying nitrogen before 1 February.


Soil mineral N 0-90cm kgN/ha Soil mineral S 0-90cm kgS/ha

County Soil texture Clay% Sept Oct Nov Dec Change Sept Oct Nov Dec Change

Devon Sandy loam 13 117 105 48 36 -80 61 52 47 43 -18

Devon Sandy loam 12 38 57 33 41 4 38 35 37 36 -2

Devon Sandy loam 17 49 32 60 45 -4 97 91 95 98 1

Devon Clay loam 23 86 403 126 135 49 73 94 90 119 46

Lincs Sandy loam 15 84 110 118 44 -39 77 183 89 50 -27

Lincs Sandy loam 17 96 44 114 42 -54 412 330 321 295 -117

Lincs Sand 5 133 133 90 90 -43 94 115 81 72 -22

Lincs Loamy sand 7 82 98 76 149 68 67 76 84 537 470

Suffolk Sandy loam 9 84 91 53 61 -24 45 47 50 53 8

Suffolk Sandy loam 16 24 25 14 19 -6 107 98 83 99 -8

Suffolk Sandy silt loam 14 112 123 56 49 -63 67 68 60 66 -2

Suffolk Sand 5 24 21 10 12 -11 43 27 26 28 -15

Average 77 105 66 60 -17 98 101 89 125 26