ARCHIVE: Fertiliser Manual (RB209)

Calculation of the crop nitrogen requirement (CRN) for field vegetable crops

Note: Values for the variables and parameters in these equations are tio be found in the table in Appendix 10.

Where sufficient data are available the nitrogen fertiliser recommendations are based on a three-step framework:

  • Size of the crop – the size, frame, or weight of the crop needed to provide optimal economic production.
  • Nitrogen uptake – the optimum nitrogen uptake associated with a crop of that size.
  • Supply of Nitrogen – based on the nitrogen supply from the soil within rooting depth including the potential nitrogen mineralised from soil organic matter.

1) SIZE OF CROP (Expressed as t/ha Dry wt Yield)

Dry wt Yield = (Fresh wt yield * dm% / 100) / DW-HI

Fresh wt Yield (t/ha) – i.e. the yield of marketable produce removed expected to be removed from the field in commercial practice. Data based on field experiments or expert opinion for well grown crops.
dm % – dry matter % of marketable produce for optimally fertilised crops.
DW-HI – proportion of the whole crop that is taken for market expressed on a dry wt basis. – in onions 81% (0.81) of the green crop is bulb. For sprouts only 26% (0.26) of the crop grown is produced as sprouts.

2) N UPTAKE IN OPTIMALLY GROWN CROP (kg/ha)

Total N Uptake (kg/ha) = Dry wt Yield t/ha  x  N% x  10

N% – This is the estimated nitrogen concentration of an optimally fertilised whole plant at harvest. Generally %N declines as yields increase. For small changes in yield if yield increases by 10% it could be assumed that nitrogen uptake increases by the same amount. For bigger changes the increases in nitrogen uptake will be less than the increase in yield and the formulae below should be used.

%NCrit = a(1+be–0.26W)

Where a and b are parameters controlling the shape of the curve. W = total dry matter yield t/ha.

The data have been derived from experiments – similar relationships have been used to define critical N concentrations for EU-Rotate N and WELL_N models.

3) SUPPLY OF NITROGEN AND CALCULATION OF CROP NITROGEN REQUIREMENT (CNR)

CRN = (NUptake – MineralisedN + SoilMinN x Rdepth/90) FertRec

Mineralised N – amount of nitrogen released from soil organic matter by mineralisation during the cropping period. This is normally between planting and harvesting dates but for some crops such as lettuce and onions where early nitrogen supply is important a shorter period has been chosen see table below.

The calculations are based on mineralisation within the WELL_N model (i.e 0.7 kg/ha @ 15.9 Degree C scaled for Wellesbourne temperature). When temperature is less than 4 Degrees mineralisation is assumed to be negligible.


RDepth – Rooting Depth, generally related to the Dry matter yield of the crop. Roots 10cm deep per tonne of dry matter yield – exceptions such as onions and leeks. Maximum rooting depth assumed not to be more than 90 cm. There will be circumstances on fertile soils where rooting is much deeper so potential nitrogen supply is larger than assumed in these tables.
SoilMinN – Soil mineral nitrogen to 90 cm depth.
Fert Rec – Fertiliser recovery assumed to be 60%.

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