SoilMate Issues
Soil testing for deep nitrogen – use and misuse
Chris Dowling,
Back Paddock Company
Measurement of deep soil nitrogen can be a double-edged sword. Good predictions of fertilizer
nitrogen rates can be made if the information gained from the soil analysis is appropriately
interpreted and considers other factors that influence plant availability of deep soil nitrogen. If
these other factors are not considered, nitrogen rates predicted from deep soil tests can be
inaccurate.
To be valuable the nitrogen measured in samples from deeper soil layers must fit some important
criteria;
 The N must reside somewhere in the root zone for the entire crop growth season.
 The N must remain in a layer of soil with active root uptake.
 The soil layer where the N resides must be unaffected by soil properties that restrict the
availability of the N or inhibit root exploration
 The concentration in all soil layers must be adequate to meet crop demand whiles roots
are active in that layer.
Failure to meet these criteria or to recognise and adjust for the effects of these limitations can
severely devalue measurements of deep soil N.
Soil N in different forms
Crop available nitrogen is present in soils in three main forms; organic N, ammonium-N and
nitrate-N. Collectively ammonium-N and nitrate-N are called mineral nitrogen. Over a full season,
the nitrogen available to plants usually consists of 2 – 5% organic N, 10 – 40% from fresh organic
matter (legume residues and/or manures) with the remainder from mineral nitrogen and fertilizer.
Organic forms must be mineralised to the mineral nitrogen forms before they are available to
plants.
Organic N and ammonium –N are quite immobile in soils so the highest concentrations of these
forms are generally in the surface layers of soils. Shallow soil sampling (0 – 10 cm or 0 – 15 cm) is
usually adequate to describe the soil content of these forms.
Nitrate-N is very mobile in soils so it can be found at varying concentrations through the soil
profile. Its concentration and location within the profile are influenced by coincidence of warm
temperatures and moist soils (which favour mineralisation), the presence or absence of crop
uptake and the net soil water balance. Nitrate-N is the prime target of deep soil N testing.
Phone: 1800 557 166 • Email: [email protected] • Web: www.backpaddock.com.au
Figure 1 Pools and flows of N in a soil N cycle
Plant availability of soil N pools
Organic N is not immediately available to plants so the amount of it likely to be available for crop
growth is usually estimated indirectly from measures of soil organic carbon and history of legumes
in the rotation. Organic N is frequently the main source of soil N for crops grown on lighter soils,
particularly following ley pastures and where major mineralisation events occur during autumn
and spring. Mineralisation rates up to 2 kg/ha/day are commonly measured in the temperate
climatic areas of the Australian wheatbelt.
Mineral N is more commonly the main source of soil N in continuously cropped sub-tropical areas.
In these areas mineralisation is predominantly during the summer “wet season” and crop yield is
up to 60 % reliant on fallow stored moisture. N mineralised during accumulation of fallow
moisture is sequentially moved down the soil profile as it fills with moisture. Consideration of
deep soil N is essential for good nutrient management decisions in these situations.
Why consider deep N?
Surface soil samples do not always reflect the quantities and location of available soil N in deeper
soil layers.
Deep N is of most value in soils where crops have a large reliance on the mineral N present in the
soil at or just after sowing. These are commonly characterised by higher clay content and
dependence on fallow stored moisture for reliable crop yield. In these circumstances up to 80 % of
the crop N requirement is present in the soil as mineral N before the crop is sown. In contrast,
deep sandy soils may have as little as 10 % so the most soil N taken up by crops is obtained from
nitrogen that is mineralised during crop growth. Because of this deep soil N is of little value for
determining crop N requirements in lighter soil types.
Phone: 1800 557 166 • Email: [email protected] • Web: www.backpaddock.com.au
Figure 2 Three soil profiles with 60 kg/ha of nitrate –N ( assuming a bulk density of 1)
How to calculate deep soil N
Soil test concentrations of mineral nitrogen (mg/kg) can be converted to a weight of nitrogen per
unit area (kg/ha) quite accurately with information on the depths at which samples were taken
and the bulk densities of the soil layers sampled.
If one soil sample is taken to depth, the amount of nitrogen is calculated from:
SMn (kg/ha) = STV x D x BD
where
SMn = soil mineral N
STV= soil mineral N concentration (nitrate-N + ammonium N) (mg/kg) from the soil analysis
D = number of increments of 10 cm represented by the STV
BD = soil bulk density (g/cm3)
For example if the STV was 8mg/kg, sampling depth was 60 cm and the bulk density of the whole
60 cm layer was 1.5 for then the available N would be:
STV
= 8 x 6 x 1.5
= 72 kg/ha
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This calculation is based on the cubic volumes of soil in one hectare to a depth of 10 cm (i.e. 1000
m3), the relationship between soil weight and volume (i.e. bulk density) and weight scale
equivalence.
For instance for a soil with BD of 1.4 g/cm3,1000m3 weighs 1,400,000 kg so if the concentration of
mineral nitrogen in the soil was 5 mg/kg there would be the equivalent of 7 kg/ha of nitrogen per
10 cm.
Soil bulk density values are generally in the range of 0.9 to 1.7 but vary considerable between soils
and soil depths. Inaccuracy in measurements or estimates of BD values can result in large errors
when calculating the amount of N available to crops.
Although slightly more difficult to sample, sampling of segments down the soil profile can provide
a more accurate measure of available nitrogen. The total amount of mineral nitrogen in the
profile is calculated by adding the values for each depth:
SMn (kg/ha) = (STV1 x D1 x BD1) + (STV2 x D2 x BD2) + (STV3 x D3 x BD3) + …….
where 1, 2, 3, …. represent each sampling depth
Whether all of this N is available for crop uptake and equivalent to freshly applied N in the surface
soil is dependent on two factors;


the presence of factors that limit the uptake of N
distribution of the nitrogen through the profile. The amount in the total profile may be
adequate but low availability in a particular soil layer may limit crop growth.
For these reasons it more informative to calculate the availability based on STVs for several soil
layers, such as 0 - 10 cm, 10 - 30 cm, 30 – 60 cm. This ensures N availability is better related to
crop requirements during early, mid and late growth stages. Segmentation of the profile, together
with analysis for other chemical factors that can affect root growth and nutrient uptake, provides
a better picture of the true availability of N in the whole soil profile.
Deep nitrogen is not always available
Root growth and nutrient extraction in many subsoils are reduced by the presence of chemical,
physical and biological limitations. In general interactions between these constraints and
availability of deep soil N are poorly described. Where subsoil constraints exist the availability to
plants of deep soil N may be overstated.
Comparisons at harvest of root density and extension in unaffected soils with soils with subsoil
constraints provides an indication of the effect of the constraint on nitrogen availability.
Phone: 1800 557 166 • Email: [email protected] • Web: www.backpaddock.com.au
Figure 3 Root zone constraints can limit nutrient availability (Rengasamy and Gupta 2002)
Using deep soil nitrogen to predict fertilizer rates
Deep soil N is usually used to calculate fertiliser N requirements using a nitrogen budget. For
instance:
Nf (kg/ha) = (Gn/NTE) – OMn –LMn- SMn
Where
Nf = fertilizer nitrogen requirement
Gn = grain N requirement (kg/ha)
NTE = efficiency with which N is taken from the soil and incorporated into grain
OMn = estimate of N mineralised from soil organic matter (kg/ha)
LMn = estimate of the N mineralised from fresh legume of manure (kg/ha)
SMn = soil mineral N (kg/ha)
Many computer programs and/or competent advisers provide sound estimates of these
parameters. Obviously SMn and OMn are calculated from soil test measurements.
These principals can also be applied on a sequential basis to check likely availability of N at critical
growth stages of a crop. With information on crop N uptake requirements at key developmental
stages and root development at those same stages, demand for nitrogen can be compared to
likely plant uptake. Timing of fertilizer nitrogen can then be planned to coincide with periods
when plant demand exceeds soil supply.
Rengasamy, P and Gupta V V S R 2002. Rootzone soil constraints: an overview, 17 th WCSS,
Thailand.
About the author:
Dr Chris Dowling is a Technical Services Manager at Back Paddock Company, based in Brisbane,
Queensland.
Email: [email protected]
Phone: 0407 692 251.
Phone: 1800 557 166 • Email: [email protected] • Web: www.backpaddock.com.au