The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 218

soluble N (TSN) under wheat (18 kg SON-N ha −1 ) and pasture (28 kg
SON-N ha −1 ). Our data (Table 4.2) show, for 'snapshot' samples from a
wide range of soil types (0-30 cm), a KCl-extractable SON content of
20-30 kg N ha −1 and a surprisingly constant ratio between mineral N and
SON in arable soils, with SON comprising ~40-50% TSN. In a range of
soils on an organic farm, SON in the 0-25 cm layer accounted for 80%
of TSN and ranged from 24 to 46 kg SON-N ha −1 , increasing with the
number of previous years under grass-clover ley (data not shown). Clearly,
SON is a significant pool within agricultural soils, but how much is leached?
Table 4.3 shows some data recently collected from the Broadbalk
Continuous Wheat Experiment at Rothamsted: DON in samples collected

Table 4.2. Mineral nitrogen (SMN) and soluble organic nitrogen (SON) pools extracted in
2 M KCl from the 0-30 cm layer of 12 soils from a range of sites in England under arable cropping
in spring 1999. Values in parentheses are standard errors of the mean of three replicates.

Bulk density
(g cm − 3 )

SMN
(kg N ha − 1 )

SON
(kg N ha − 1 ) Soil type Previous crop

Sandy loam Winter wheat 1.01 26 (6) 29 (8)

Silty loam Winter wheat 1.44 35 (3) 33 (3)

Sandy clay loam Winter rape 1.42 37 (3) 22 (4)

Sandy clay loam Winter wheat 1.27 81 (3) 30 (1)

Silty clay loam Celery 1.31 26 (1) 34 (1)

Clay loams (two soils) Winter wheat 1.13-1.41 25-32 (2) 24-28 (3)

Clay loam Field peas 1.31 21 (3) 33 (1)

Clays (four soils) Winter wheat 0.97-1.36 27-33 (6) 23-33 (3)

Table 4.3. Mineral N and soluble organic N (SON) in a 2 M KCl extract from the 0-75 cm soil
layer (samples collected on 7 December 1998), and dissolved organic N (DON) in the drainage
waters collected from tile drains at 65 cm during September-November 1998, on the Broadbalk
Experiment. Values in parentheses are standard errors of three replicates. There were no
replicates of drainage water samples.

N applied (kg ha − 1 year − 1 )

0 144 288 240 as FYM

Source Analysis N content (kg ha − 1 )

Soil extract: Mineral N 19.3 (1.3) 35.9 (1.3) 49.3 (2.2) 82.9 (3.0)

DON 46.9 (8.9) 23.7 (8.7) 55.3 (11.7) 60.5 (11.9)

Drainage water: Mineral N 9.9 (9.0) 6.3 (9.0) 29.0 (19.0) 52.0 (19.0)

DON 1.2 (9.0) 1.1 (9.0) 2.5 (19.0) 7.0 (19.0)