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

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 detailssuper C500 C-ore

kgP/ha t fert/ha drill runs kgP/ha t fert/ha drill runs kgP/ha t fert/ha drill runs0 0 0

47 0.5 1 237 1.8 4 189 1.7 395 1.0 2 396 2.9 6 378 3.4 7210 2.2 4 792 5.9 12 693 6.2 12420 4.4 9 1188 8.8 18 1386 12.5 25

Phosphorus source details super C500 Core

component percent compositiontotal P 9.6 13.5 11.1

water soluble P 7.4 0.1 0.1citrate soluble P 1.2 6.8 0.1

calcium 23.2 17.4 9.2aluminium 0.4 11.2 11.9

iron 0.5 6.8 7.9sulphur 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 systemsfixation transport

organic net increase in soil organic pools loss in productsseed pools and OC% redistribution to camps and windrows

erosion of organic matter

inorganic reactions with soil minerals erosionreversion in fertiliser granules leachinginduced deficiencies gaseous losses

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Dissolution of homogeneous "slow release" fertilisers

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Nutrient run down [a/(t+a)]soil type (reversion) effect

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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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2014 response to 1980 treatmentsleft: 1188 kg P/ha (C500), nil, right 792 kg

P/ha (C500)

2014 response to 1980 treatmentsextreme 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?