Where has all the phosphorous gone?
Long term effects of alternative phosphorus sources
on soil fertility
Bill Bowden, WMG
Craig Scanlan, DAFWA, Northam
February 2015
Background to the talk
• Rediscovering the site
• What the old trial was all about:
– Sources
– Peak P
– P budgets and residual value
• What current measurements can we show:
– Chemical fertility: P, OC%, CEC, N
– Physical: Wetting, compaction, bio-ploughs, water use
– Biology: disease, weeds
Location
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Location
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Location
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What the old trial was about?
– Alternative sources of P to orthophosphates (Super, DAP etc)
• Water soluble phosphates
– The commercial fertilisers
• Less soluble phosphates
–
–
–
–
The raw minerals – A, B and C grade rock Ps
Calcined rock P
Reactive rock P
Lime super and partially acidulated super
– Peak P – how real is it?
• Christmas Island story
• Duchess, Mt Weld etc
– P budgets and residual value
• Where does the P go
• Changes in P availability
• Changes in soil fertility
80Ba6 – Sources of P on pale grey sand
• History of site
– Pre trial – not well documented
• Semi-cleared 1973 and super and sub-clover spread into regenerating
scrub
• Occasional grazing by sheep
• Site cleared (200 metres by 100 metres) in 1980
– Trial (Bolland et al, 1987, AJEA, 27:647-56)
• P treatments top-dressed in May 1980 and incorporated with tines
• 1980-1986 crop sequence:- Illyarrie lupin, Seaton Park sub-clover, Pitman
serradella, Illyarrie lupin, volunteer pasture, volunteer pasture, cross plot
Jacup wheat, Mortlock oats, Forrest barley
• Basal muriate of potash and gypsum applied at 100 kg/ha in sowing years
(1980-83 and 1986) and 30 kg/ha MnSO4 in lupin years.
80Ba6 treatment details
super
C500
C-ore
kgP/ha t fert/ha drill runs kgP/ha t fert/ha drill runs kgP/ha t fert/ha drill runs
0
0
0
47
0.5
1
237
1.8
4
189
1.7
3
95
1.0
2
396
2.9
6
378
3.4
7
210
2.2
4
792
5.9
12
693
6.2
12
420
4.4
9
1188
8.8
18
1386
12.5
25
component
total P
water soluble P
citrate soluble P
calcium
aluminium
iron
sulphur
Phosphorus source details
super
C500
Core
percent composition
9.6
13.5
11.1
7.4
0.1
0.1
1.2
6.8
0.1
23.2
17.4
9.2
0.4
11.2
11.9
0.5
6.8
7.9
11.2
0.2
0.2
Residual effects of fertilisers
Change soil fertility:
–
–
–
–
–
–
Carry forward of nutrient
Build up of soil organic matter
Build up of soil nitrogen pools
Induced deficiencies
Non-wetting soils
Weeds and diseases
Table 1. Processes which reduce nutrient availability in farming systems
fixation
transport
organic net increase in soil organic pools loss in products
seed pools and OC%
redistribution to camps and windrows
erosion of organic matter
inorganic reactions with soil minerals
reversion in fertiliser granules
induced deficiencies
erosion
leaching
gaseous losses
Dissolution of homogeneous "slow release"
fertilisers
Nutrient run down [a/(t+a)]
soil type (reversion) effect
0.1
0.80
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0.60
4
8
0.40
16
0.20
1.00
residual nutrient
cumulated dissolution
2
1.00
2
1.20
1.20
4
0.80
16
0.60
64
0.40
0.20
256
32
0.00
0.00
0
10
20
30
40
0
10
20
64
time
time
30
40
1024
P availability of low solubility sources
= dissolution rate (see key) by residual
This assumes no level effects or feedbacks of soil on dissolution
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1.00
0.1
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2
0.80
P availability
4
0.60
8
0.40
16
0.20
32
0.00
64
0
10
20
time
30
40
I
2014 response to 1980 treatments
left: 1188 kg P/ha (C500), nil, right 792 kg P/ha (C500)
2014 response to 1980 treatments
extreme left: 420 kg P/ha (super), centre 1388 kg P/ha
(Core), extreme right 792 kg P/ha (C500)
2015 - Watch this space
• Take soil profile samples to complete the P budget
• Bioassay with a wheat crop
– Responses to N, P, NP and deep cultivation
– Test strip diagnostics
– Risks with management of residues, weeds and non-wetting
• Investigate soil compaction and bio-plough effects
• Use soil bank and current samples to follow changes
in soil fertility
Three key messages
• Poorly soluble P sources can have long term
residual effects
• Poor, long term volunteer pasture lets soil OC
levels build up
• Watch this space for interesting long term soil
fertility effects
Acknowledgements
Thanks go to the 1980 instigators of the trial:
Mike Baker and Mike Bolland.
To the landowners, David Frankland, Geoff
Pearson.
To the funders, NACC, and to the sponsors,
the West Midlands Group and DAFWA
Questions?