Phosphorus (P)

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The addition of phosphorus fertilizer (plus other nutrients) has become essential for agriculture in Western Australia to remain sustainable. It’s a known fact that WA soils are some of the poorest in the world when it comes to phosphorus, however, our native plants have evolved and adapted to survive in these conditions.

Introduced plants, including agricultural crop and pasture species, have no such adaptations. As a result, they will not produce to their maximum potential or even survive without the addition of phosphorus in the form of fertilizer.

Role of phosphorus in plants

Phosphorus is essential to the health of plants. This mineral is a major component of the genetic material DNA, plus it’s involved in many of the biochemical processes critical for plant metabolism, such as photosynthesis and energy production. Its critical role in early growth is demonstrated by the lack of root growth seen in phosphorus deficient plants.

As demonstrated in this nutrient removal table, relatively small amounts of phosphorus are removed in grain.

Phosphorus in soils

Phosphorus is very mobile once within the plant, yet immobile in the soil. Once phosphorus from water-soluble fertilizers is dissolved into the soil solution, it quickly becomes ‘bound’ by iron, aluminium and calcium particles, rendering it less available to the plants for which it was intended. For this reason, crops sown with phosphorus-containing fertilizer are placed as close to the seed as possible to enable seedling uptake before adsorption into the soil particles takes place.

Phosphorus deficiency

Given the major role phosphorus plays in plant metabolism, a deficiency will affect all aspects of growth. Plants lacking phosphorus will appear stunted, often with a reddish colouration on the stems. They will also have dark green leaves and dying leaf tips.

Preventing phosphorus deficiency

To prevent crop or pasture deficiencies, apply adequate rates of phosphorus-containing fertilizer either at seeding or prior to germination. An adequate rate can be established from a soil analysis, in conjunction with tissue testing.

Correcting phosphorus deficiency

Correcting a deficiency in an established crop is very difficult. This is due to the immobility of applied phosphorus, which would be bound by soil before getting into the root zone.

Supplying adequate phosphorus to the plant can be difficult in some soil types and some seasons. Recent research has highlighted the problem in soils with high phosphorus buffering index (PBI), particularly during dry seasons where the surface of the soil dries out early in the plants' life.

To find out more about preventing or correcting a phosphorus deficiency, speak to your local Area Manager.

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
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