1) Phosphorus and potassium fertilizer experiments were conducted on dairy pastures to improve knowledge of nutrient requirements. 2) For phosphorus, applying fertilizer only when soil tests were below critical levels increased pasture production, while applying above critical levels had no effect. 3) For potassium, fertilizer was rarely needed for ryegrass pastures except after removal of biomass, but could be required for clover when plant tests were below 2%. 4) Proper soil and plant testing allows for efficient fertilizer application by only applying nutrients when soil or plant levels indicate a need.

1. Results of phosphorus and
potassium experiments on Partner
Farms of the Greener Pastures 1
project
Mike Bolland, Plant Nutrition Consultant, Eaton
&
Ian Guthridge, Graham Blincow & Peter Needs,
DAFWA, Manjimup

2. Reason for doing the experiments
Continue to improve our knowledge on the
phosphorus and potassium requirements of
intensively grazed ryegrass dairy pastures.

3. P experiment
Comprised:
Nil & 3 P levels by 3 methods of application

4. P level & application method
Kg P/ha Method of P application (kg P/ha)
per year All in autumn Half in autumn & spring 1/6 autumn & after first 5
grazings
0
10 1 x 10 = 10 2 x 5 = 10 6 x 1.67 = 10
20 1 x 20 = 20 2 x 10 = 20 6 x 3.33 = 20
30 1 x 30 =30 2 x 15 = 30 6 x 5 = 30

5. Location of the 3 sites of the
phosphorus experiment
• Paddock 41, Mottershead farm, Witchcliffe
• Paddock 38, Evans farm, North Jindong
• Paddock 30, Rodwell farm, Boyanup

7. Results of the P experiment

8. Total pasture dry matter consumed 2009, Miles Mottershead,
Witchcliffe
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Nil-P
Autumn
Autumn & spring
After each grazing
Mean
Kg P/ha applied per year
Pastureconsumed(t/ha)

9. Soil test P (mg/kg) for nil-P treatment
Miles Mottershead, Witchcliffe
y = -7.1x + 14319
R² = 0.9807
0
10
20
30
40
50
60
70
80
90
2006 2007 2008 2009 2010
PBI = 75 (range 40-114)
Critical soil test P (mg/kg) = 30 (15 plots) & 35 (15 plots)
ColwellsoiltestP(mg/kg)

10. Total pasture dry matter consumed, 2007, Grant Evans,
North Jindong
0
1
2
3
4
5
6
7
0 5 10 15 20 25 30
Nil-P
Autumn
Autumn & spring
After each grazing
Mean
Kg P/ha applied per year
Pastureconsumed(t/ha)

11. Soil test P (mg/kg) for nil-P treatment
Grant Evans, North Jindong
y = -4.4x + 8888.6
R² = 0.8674
0
10
20
30
40
50
60
70
2006 2007 2008 2009 2010
PBI = 181 (range 124-249)
Critical soil test P (mg/kg) = 35 (28 plots) & 40 (2 plots)
ColwellsoiltestP(mg/kg)

12. Decline in Colwell soil test P (mg/kg per year) when no fertiliser P is
applied for a low P sorbing sand (PBI 30) and a high P sorbing gravel
sand (PBI 250)
Years since no fertiliser P applied
SoiltestP(mg/kg)whennoPapplied

13. Soil test P (mg/kg) for nil-P treatment
Victor Rodwell, Boyanup
y = -6.7x + 13480
R² = 0.9567
0
5
10
15
20
25
30
35
40
45
2006 2007 2008 2009 2010
PBI = 32 (range 19-48)
Critical soil test P (mg/kg) = 25 (14 plots) & 30 (16 plots)
ColwellsoiltestP(mg/kg)

14. Total pasture dry matter consumed 2009, Victor Rodwell, Boyanup
0
2
4
6
8
10
12
14
16
0 5 10 15 20 25 30
Nil-P
Autumn
Autumn & spring
After each grazing
Mean
Kg P/ha applied per year
Pastureconsumed(t/ha)

15. Pasture consumed at first grazing 2010, Victor Rodwell, Boyanup
0
0.5
1
1.5
2
2.5
3
0 5 10 15 20 25 30
Nil-P
Autum
Autumn & spring
After each grazing
Mean
Pasturedrymatterconsumed(t/ha)
Kg P/ha applied before first grazing in 2010

16. Published range for critical plant test values for young dried herbage of
perennial ryegrass, and plant test values obtained for mixed young annual
and Italian ryegrass dried herbage at the 3 sites of the P experiment
Element Published
critical plant
rest value
Witchcliffe N. Jindong Boyanup
P (%) 0.20-0.28 0.52, 0.49 0.42, 0.42 0.50
N (%) 3.0-3.5 4.4, 6.0 6.4, 5.1 3.8
K (%) 1.4-1.9 5.2, 5.5 4.6, 4.5 3.6
S (%) 0.18-0.22 0.36, 0.50 0.46, 0.40 0.29
Ca (%) 0.15-0.2 0.47, 0.48 0.61, 0.54 0.65
Mg (%) 0.15-0.2 0.23, 0.24 0.28, 0.25 0.22
Cu (mg/kg) 4.0-6.0 9.9, 12.3 13.2, 8.8 8.4
Zn (mg/kg) 10-15 43, 61 40, 29 25
Mn (mg/kg) 15-20 80, 97 48, 41 108
Fe (mg/kg) 40-60 104, 151 174, 155 189
B (mg/kg) 3.0-5.0 5.4, 7.2 7.6, 6.5 17.3

17. Conclusions from P experiment
• Only apply fertiliser P when soil test P is below
the critical value for that soil
• Applying fertiliser P when soil test P is above
the critical value for that soil has no significant
effect on pasture production
• When soil test P is well above the critical value
and no fertiliser is applied soil test P declines
gradually and it takes some time for the soil to
become P deficient for pasture production

18. K experiment
Comprised: Nil & 3 K levels by 3 methods of
application
The same K experiment was located at 2 sites:
• Paddock 41, Mottershead farm, Witchcliffe
• Paddock 50, Evans farm, North Jindong

20. K level & application method
Kg K/ha Method of K application (kg K/ha)
per year All in autumn Half in autumn & spring 1/6 autumn & after first 5
grazings
0
50 1 x 50 = 50 2 x 25 = 50 6 x 8.33 = 50
75 1 x 75 = 75 2 x 37.5 = 75 6 x 12.5 = 75
100 1 x 100 =100 2 x 50 = 100 6 x 16.67 = 150

21. Results of K experiment

22. Total pasture dry matter consumed 2008, Miles Mottershead,
Witchcliffe
0
2
4
6
8
10
12
0 10 20 30 40 50 60 70 80 90 100
Nil-K
Autumn
After each grazing
Mean
Kg K/ha applied per year
Pasturedrymatterconsumed(t/ha)

23. Pasture dry matter consumed at the first 2 grazings in 2010,
Miles Mottershead, Witchcliffe
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 10 20 30 40 50 60 70 80 90 100
Nil-K
Autumn
Autumn & spring
After each grazing
Mean
Kg K/ha applied before first 2 grazings in 2010
Pasturedrymatterconsumed(t/ha)

24. Published ranges for critical plant test vales for different elements for young tissue of perennial
ryegrass, and plant test values for the elements obtained for young tissue of mixed annual and
Italian ryegrass obtained from the K experiment at Witchcliffe for the first 2 grazings in 2010
Element Published critical plant
test values
Plant test values obtained
at Witchcliffe in 2010
K (%) 1.4-1.9 4.2, 4.8
P (%) 0.2-0.28 0.68, 0.72
N (%) 3.0-3.5 4.6, 6.2
S (%) 0.18-0.22 0.36, 0.53
Ca (%) 0.15-0.20 0.51, 0.56
Mg (%) 0.15-0.20 0.27, 0.24
Cu (mg/kg) 4-6 11, 13
Zn (mg/kg) 10-15 45, 63
Mn (mg/kg) 15-20 76, 74
Fe (mg/kg) 40-60 118, 153
B (mg/kg) 3.0-5.0 5.2, 5.2

25. Conclusions from K experiment
• Fertiliser K is usually only required for clover in
the traditional clover ryegrass pastures
• Fertiliser K is rarely required for ryegrass
pastures, except after silage or hay crops
• Apply fertiliser K when there is less than 2% K in
young clover or ryegrass herbage collected 5 cm
above the soil surface just before grazing
• Applying fertiliser K to ryegrass pastures when it
is not required will have no effect on ryegrass dry
matter production and may adversely affect cow
health (inducing magnesium deficiency in cows)

26. General conclusions
• Apply phosphorus fertiliser when soil testing
indicates it is required
• Apply potassium fertiliser when plant testing
indicates it is required

28. Phosphorus Buffering Index (PBI) of soil and corresponding
Critical Colwell soil test phosphorus
PBI
(no units)
Critical Colwell soil test phosphorus
(mg/kg)
5-10 15
11-15 20
16-30 25
31-70 30
71-140 35
141-280 40

29. Subterranean clover, top 10 cm of soil
0
1
2
3
4
5
6
0 50 100 150 200 250
Colwell soil test K (mg K/kg soil)
DMresponsetoappliedK(t/ha)
Greener
Pastures
Critical potassium level for clover

30. Annual ryegrass, top 10 cm of soil
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 50 100 150 200 250
Colwell soil test K (mg K/kg soil)
DMresponsetoappliedK(t/ha)
Greener
Pastures
Critical potassium level for ryegrass

31. Colwell soil test K measured in nil-K plots and plots receiving 100 kg
K/ha per year, K experiment, Mottershead farm, Witchcliffe
Kg K/ha
per year
Method of K
application
2007 2008 2009 2010
0 65 43 70 101
0 143 46 82 53
0 94 136 84 49
100 Autumn 127 54 93 77
100 Autumn 91 66 77 85
100 Autumn 83 155 92 41
100 Autumn & spring 111 52 119 107
100 Autumn & spring 92 81 109 58
100 Autumn & spring 188 64 83 50
100 After grazing 98 80 109 113
100 After grazing 117 96 82 57
100 After grazing 76 106 75 45

32. Relationship between percentage of the maximum (relative) plant yield and
either P level applied or soil test P for a low P sorbing sand (PBI 30) and high P
sorbing lateritic gravel sand (PBI 250)
Kg P/ha applied
Soil test P (mg/kg)
Relativeplantyield(%)
Maximum yield plateau
100
50
0
95% OF THE MAXIMUM YIELD
75
25