27 February 1999



Available nitrogen and sulphur is slipping down the soil profile. We ask how soon the spreader should be out this spring.

ITS your call. There are two schools of thought on which way to go with fertiliser top dressings.

One argues that you shouldnt set too much store by what the neighbours think of those yellow-looking fields, while the other advocates up to 50kgN/ha for starters, and soon.

In between, there is agreement on where the priorities should lie – oilseed rape first with late-drilled and stressed cereals with poor root development getting the spreader next.

Soil sampling shows that on many sites available nutrient has been diluted and pushed down the soil profile by continuing wet weather at the turn of the year. This has produced losses of available nitrogen greater than in most years while the effects on sulphur availability have been balanced out to some extent.

Dr Ian Richards, of Levington Agriculture, which is monitoring soil mineral S and N for The Sulphur Institute and Hydro Agri, says deposition of sulphur from the air and breakdown of soil organic matter has left available S at about the same in February as it was in September when monitoring for SulphurCheck started. However, he warns that averaging masks changes taking place over the winter.

Changes in soil-available S between September and February varied from a gain of 54kgS/ha on a clay loam to a loss of 37kgS/ha on a sand. He expects yield benefits in responsive crops such as oilseed rape or grass cut for silage where the amount of available S in the 0-90cm soil profile is found to be less than around 50kgS/ha.

In the Levington survey, this applies to four of the 12 sites, unless there are further changes before the next check in March. "In most cases, application of around 15kgS/ha (38kgSO3/ha) will meet the needs of the crops," says Dr Richards.

Extra yield

"A common response to that would be 0.5-0.75t/ha extra yield on sites with borderline S deficiency. In East Anglia or other rape growing areas that might be worth £75-80 in response to a fertiliser cost of £3-4."

On severely deficient sites yield response from oilseed rape has gone as high as 2t/ha but that is exceptional.

Soil mineral nitrogen on the same monitored sites showed net losses at all sites between September and February of 2-89kgN/ha, with an average of 40kgN/ha. "Although comparisons are difficult to make, these losses are probably greater than in most years which could affect fertiliser recommendations," says Dr Richards.

However, he puts himself squarely in the camp that argues against rushing on to cereals with early N. Even very sick looking cereal crops in late February or early March can recover to give full yield potential, he argues.

"Any adjustment in N rates can wait until the main application and the temptation to make up for losses by early application should be avoided; yield is unlikely to benefit and there is still the risk of losing what is applied."

Richard Martin, of Terra Nitrogen, takes an alternative stance, particularly for backward or late-drilled cereal crops which may not be best placed to take up available nitrogen.

"Crops that were drilled late last autumn, or went into poor seedbeds, are showing poorer root structures than normal for this time of year," points out Mr Martin. "This will exacerbate the problem and increase the urgency for early top dressing.

In terms of priority, oilseed rape should be top-dressed first, followed by wheat with poor root development, second and subsequent wheats, and then winter barley. A soil with a nitrogen index of 0 at this time of year should receive 40kg/ha of nitrogen (1 bag/acre of Nitram) in late February or early March as an initial top dressing."

Ironically, the Terra Nitrogen recommendations are based on monitoring of 18 sites, also carried out by Levington Agriculture. Mr Martin says this monitoring mostly on Midlands or eastern England sites with sandy or clay loams shows only two sites had sufficient nitrogen levels at the end of January to see crops through the early growth stages. The concentration of the nitrogen below 60cm is also important because it may be beyond the reach of crops with poor root development.

"Mineralisation of organic nitrogen has been accelerated due to the warmth in the soil," he explains. "High levels of rainfall have then increased the amount of leaching that would normally occur."

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

Texture Sept Oct Nov Dec Jan Feb Change Sept Oct Nov Dec Jan Feb Change

Devon Sandy loam 117 105 48 36 49 34 -83 61 52 47 43 43 46 -15

Devon Sandy loam 38 57 33 41 26 22 -15 38 35 37 36 34 44 6

Devon Sandy loam 49 32 60 45 59 47 -2 97 91 95 98 86 108 11

Devon Clay loam 86 403 126 135 124 79 -7 73 94 90 119 96 127 54

Lincs Sandy loam 84 110 118 44 38 33 -51 77 183 89 50 57 68 -9

Lincs Sandy loam 96 44 114 42 42 54 -42 412 330 321 295 212 444 32

Lincs Sand 133 133 90 90 147 59 -74 94 115 81 72 64 57 -37

Lincs Loamy sand 82 98 76 149 48 34 -48 67 76 84 537 40 55 -13

Suffolk Sandy loam 84 91 53 61 47 30 -55 45 47 50 53 55 45 0

Suffolk Sandy loam 24 25 14 19 19 14 -10 107 98 83 99 92 96 -11

Suffolk Sandy silt loam 112 123 56 49 28 23 -89 67 68 60 66 63 64 -4

Suffolk Sand 24 21 10 12 18 13 -11 43 27 26 28 31 31 -12

Average 77 105 66 60 54 37 -40 98 101 89 125 73 99 1