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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
Reason for doing the experiments
Continue to improve our knowledge on the phosphorus and potassium requirements of intensively grazed ryegrass dairy pastures.
P experiment
Comprised:
Nil & 3 P levels by 3 methods of application
P level & application methodKg 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
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
Results of the P experiment
Total pasture dry matter consumed 2009, Miles Mottershead, Witchcliffe
0 5 10 15 20 25 300
1
2
3
4
5
6
7
Nil-PAutumnAutumn & springAfter each grazingMean
Kg P/ha applied per year
Past
ure
cons
umed
(t/h
a)
Soil test P (mg/kg) for nil-P treatmentMiles Mottershead, Witchcliffe
2006 2007 2008 2009 20100
10
20
30
40
50
60
70
80
90
f(x) = − 7.1 x + 14318.8R² = 0.980739299610895
PBI = 75 (range 40-114)Critical soil test P (mg/kg) = 30 (15 plots) & 35 (15 plots)
Colw
ell s
oil t
est P
(mg/
kg)
Total pasture dry matter consumed, 2007, Grant Evans, North Jindong
0 5 10 15 20 25 300
1
2
3
4
5
6
7
Nil-PAutumnAutumn & springAfter each grazingMean
Kg P/ha applied per year
Past
ure
cons
umed
(t/h
a)
Soil test P (mg/kg) for nil-P treatmentGrant Evans, North Jindong
2006 2007 2008 2009 20100
10
20
30
40
50
60
70
f(x) = − 4.4 x + 8888.6R² = 0.867383512544803
PBI = 181 (range 124-249)Critical soil test P (mg/kg) = 35 (28 plots) & 40 (2 plots)
Colw
ell s
oil t
est P
(mg/
kg)
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
Soil
test
P (m
g/kg
) whe
n no
P a
pplie
d
Soil test P (mg/kg) for nil-P treatmentVictor Rodwell, Boyanup
2006 2007 2008 2009 20100
5
10
15
20
25
30
35
40f(x) = − 6.7 x + 13480R² = 0.956734867860209
PBI = 32 (range 19-48)Critical soil test P (mg/kg) = 25 (14 plots) & 30 (16 plots)Co
lwel
l soi
l tes
t P (m
g/kg
)
Total pasture dry matter consumed 2009, Victor Rodwell, Boyanup
0 5 10 15 20 25 300
2
4
6
8
10
12
14
16
Nil-PAutumnAutumn & springAfter each grazingMean
Kg P/ha applied per year
Past
ure
cons
umed
(t/h
a)
Pasture consumed at first grazing 2010, Victor Rodwell, Boyanup
0 5 10 15 20 25 300
0.5
1
1.5
2
2.5
3
Nil-PAutumAutumn & springAfter each grazingMean
Past
ure
dry
mat
ter
cons
umed
(t/
ha)
Kg P/ha applied before first grazing in 2010
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 0.50N (%) 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
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
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
K level & application methodKg 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
Results of K experiment
Total pasture dry matter consumed 2008, Miles Mottershead, Witchcliffe
0 10 20 30 40 50 60 70 80 90 1000
2
4
6
8
10
12
Nil-KAutumnAfter each grazingMean
Kg K/ha applied per year
Past
ure
dry
matt
er c
onsu
med
(t/h
a)
Pasture dry matter consumed at the first 2 grazings in 2010, Miles Mottershead, Witchcliffe
0 10 20 30 40 50 60 70 80 90 1000
0.20.40.60.8
11.21.41.61.8
2
Nil-KAutumnAutumn & springAfter each grazingMean
Kg K/ha applied before first 2 grazings in 2010
Past
ure
dry
matt
er c
onsu
med
(t/h
a)
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
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)
General conclusions
• Apply phosphorus fertiliser when soil testing indicates it is required
• Apply potassium fertiliser when plant testing indicates it is required
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
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)
DM
resp
onse
to a
pplie
d K
(t/h
a)
Greener Pastures
Critical potassium level for clover
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)
DM
resp
onse
to a
pplie
d K
(t/h
a)
Greener Pastures
Critical potassium level for ryegrass
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 Kapplication
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
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 appliedSoil test P (mg/kg)
Rela
tive
plan
t yie
ld (%
)
Maximum yield plateau
100
50
0
95% OF THE MAXIMUM YIELD
75
25