Nitrogen (N)

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Nitrogen is considered the most important component for plant growth, being a core element of many plant structures and for both their internal and external metabolic processes.

There are two forms of nitrogen: organic and inorganic. Organic forms are found in the soil and need to be converted into the inorganic forms ammonium or nitrate for plant use. This process is called mineralisation, which represents about 98% of soil nitrogen.

Legume crops fix large quantities of organic nitrogen that become available to future crops as it mineralises during the season. However, as most legume crops are grown for seed, much of the nitrogen fixed is removed from the soil. As a rough rule, the nitrogen remaining after a harvested legume crop is usually less than half the requirement of the subsequent cereal crop.

As demonstrated in this nutrient removal table, nitrogen contained in residual plant material (stubble) is readily recycled into the soil.

Forms of nitrogen

Plants take up most nitrogen in the ammonium (NH4+) or nitrate (NO3-) form. Urea is the most common form of fertilizer nitrogen used in Australia, but it needs to be converted before plants can take it up. The rate of conversion depends on the same processes which control the mineralisation of organic nitrogen forms, however, urea needs special attention. Under the correct conditions of pH, temperature and moisture, urea will rapidly convert to ammonia gas and can be lost to the atmosphere if not applied correctly.

The other common form of nitrogen is ammonium (Amsul, DAPSZC®, MAPSZC®, VIGOUR). Ammonium does not have the same application restrictions as urea as it is not as subject to volatilisation (Table 1).

Table 1. Possible loss if urea is left on the soil surface

Pasture
Soil Test Level
(K - ppm or mg/kg)
Cereals
Lupins
Canola*
Pulses
Low Rainfall
Medium Rainfall
High Rainfall
>120
0
0
0
0
0#
80 - 120
P.T.
0
0
10-20
20-30
60 - 80
15-20
0
10-20
20-30
30-40
40 - 60
20-25
P.T.
20-30
30-50
40-60
25-35
20-25
30-50
40-60
50-60
Product
Autumn Application
Winter Application
After 4 weeks
After 7 weeks
After 4 weeks
After 7 weeks
Amsul
3.7
6.9
1.0
2.5
Urea
59.9
58.4
36.8
38.9
Cations
Anions
Chemical Symbol & Charge
Name
Chemical Symbol & Charge
Name
K+
Potassium
NO3-
Nitrate
NH4+
Ammonium
Cl-
Chlorine
Na+
Sodium
SO4--
Sulphate
Cu++
Copper
HPO4- -
Phosphate
Zn++
Zinc
BO3----
Boron
Mn++
Manganese
MoO4- -
Molybdenum
Ca++
Calcium
Mg++
Magnesium
Fe++
Iron
N
P
K
S
Cu
Zn
Mn
Ca
Mg
Clover Pasture
25-30
3.5
16-20
2-3
0.005
0.02
0.40
0.4
1.2

Table 1. % N lost by volatilisation from surface applied nitrogen on pH 7.0* soil (courtesy CSIRO) *NB Volatilisation of N increases when pH increases (i.e. when soils are more alkaline) It reduces when soils are more acidic.

Nitrogen in soil

The form in which nitrogen is present in the soil is also an important consideration. Urea, ammonium and nitrate all express differences in soil mobility. Urea and nitrates are very mobile in the soil and can be readily leached on lighter soils, whereas ammonium is relatively immobile in the soil and generally will not leach. As a general rule, a split application of urea is suitable in the high rainfall regions (>500mm) to minimise the loss of urea and nitrate out of the root zone, before a crop can utilize it. The ammonium nitrogen in products such as DAPSZC and MAPSZC® are generally stable until they are converted to nitrate.

Importance of nitrogen to cereals

The availability of nitrogen to cereals is critical for setting yield potential in the first six weeks of growth. It’s important that the correct amount of nitrogen for the expected yield result is available to the plants within this time period, as an inadequate nitrogen supply can result in the loss of tillers. As a general rule, each tiller aborted reduces yield by approximately 200kg/ha.

Nitrogen deficiency

Nitrogen deficiencies are usually easy to identify as nitrogen is very mobile in plants. Symptoms may include:

  • Pale colouring on the oldest leaves, which moves down the leaf blade.
  • Older leaves turning very pale and dying.
  • Tiller abortion.

Nitrogen Products

Product
% N
Urea
46.0
MAXam/Amsul
21.0
UreaPlus
37.1
NitroPlus
33.4

Compounds

Product
%Ammonium-N
DAPSZC
16.4
MAPSZC
10.6
AllStar
13.0
Vigour
5.0
Product
N
P
K
S
Ca
Mg
Cu
Zn
Mn
kg per tonne
grams per tonne
Wheat
23
3
4
1.4
0.33
0.93
5
29
40
Barley
20
2.9
4.4
1.1
0.3
1.08
3
15
11
Oats
16
3
4
1.5
0.5
1.0
3
17
40
Canola
40
6.5
9.2
9.8
4.1
4.0
4
40
40
Lupins
51
3.8
8.8
3.1
1.7
1.7
5
30
60
Chickpeas
34
3.8
8.9
1.8
1.1
1.2
7
38
34
Faba Beans
39
3.8
9.8
1.4
1.1
1.0
10
28
30
Field Peas
37
4.0
8.2
2.0
0.7
1.2
5
35
14
Hay
20
2.0
25
2.0
0.5
1.1
5
20
40
Milk
5.7
0.95
1.4
0.3
1.2
0.12
Greasy Wool
170
0.26
15.8
28.5
1.2
0.3
Sheep - Live
34
7.0
2.3
4.0
14.4
0.4

Stubble

Product
N
P
K
S
Ca
Mg
Cu
Zn
Mn
kg per tonne
grams per tonne
Wheat
17
1.8
42
2.7
Canola
18
2.4
70
4.8
Lupins
17
0.6
26
2.7