P-saturated Ochre: Performance as a Fertiliser and Environmental Acceptability

P-saturated Ochre:Performance as a Fertiliser and Environmental Acceptability K.E. Dobbie, K.V. Heal and K.A. SmithSchool of GeoSciences, University of Edinburgh

Introduction • P-saturated ochre prepared by repeatedly mixing with H2PO4- in solution until no further P was removed • P-saturated ochre (available P = 0.94 g kg-1) then added to low-P soil • Pot experiments • grass and barley grown in soil mixed with varying amounts of P-saturated ochre (Jul-Oct 2002) • spruce and birch seedling trial (Mar-Nov 2003) • Field experiment • acid grassland and barley field trial (Mar-Sep 2003)

Experimental design – pot experiment • P treatments (5 replicates)

Experimental design – field experiment • barley and acid grassland (4 replicates) – seedling experiment • pot experiment with one year old spruce and birch seedlings (5 replicates)

Analysis • Soil analysis (at start and end of experiment) • total and available P • total metals • (Al, As (end only), Cd (start only), Cr, Cu, Fe, Mn, Ni, Pb, Zn) • pH • Plant analysis • biomass • barley/grass – above ground (stems, heads) and below ground (roots – pot experiment only) • seedlings – tree volume at start and end of experiment • total P, metals

Results … Total P concentration in soil c b a,b a a a • Total P increases with increasing amounts of ochre

Available P concentration in soil • CP and O(1) not significantly different at start • O(1) > CP at end →less depletion of available P in ochre treatments →more P available for future crops P-saturated ochre acts as a slow release fertiliser

Metal concentrations in soil • Al, Cr, Ni, Zn – no significant differences between treatments • Cu, Pb – no significant differences between ochre treatments or between CP and O(1) • Fe, Mn – increases with increasing ochre concentration but no significant difference between CP and O(1) • As – v. low < 2 mg kg-1 dry soil • Cd – below limits of detection

Are soil metal concentrations potentially hazardous? Maximum permissible concentrations of PTEa in soil after application of sewage sludge to agricultural land (MAFF, 1998) • As, Cd, Cr, Cu, Pb, Zn < guideline values • Ni ~ guideline values (but not significantly different from CO)

Soil pH • pH increases with increasing ochre content (ochre pH = 8.7) → treatments O(1), O(2) & O(5) are sig. higher than untreated soil • NV = 8 % CaO equivalent → 40 t ochre ha-1 is equivalent of 3.2 t CaO ha-1

Plant response to available P in soil • CP treatment • lower yield than ochre treatments but not sig. • Ochre treatments • no differences in yield despite initial range of soil available P conc. • O(0.5) > CP • P released from ochre during growing season Critical soil P content above which increasing P does not significantly increase yield → no adverse effects on crop production using P-saturated ochre as a fertiliser

Elemental concentrations in vegetation • P concentration in plants increases with increasing soil ochre concentration • no significant difference in P concentration between plants grown in conventional fertiliser and those grown in the equivalent amount of P-saturated ochre • No significant difference in Cd and Pb concentrations in barley grain between plants grown in conventional fertiliser and those grown in the equivalent amount of P-saturated ochre

Are plant metal concentrations potentially hazardous? • European Commission regulation No. 466/2001 • limits for Cd & Pb in foodstuffs of 0.2 mg kg-1 wet weight • Barley grain from ochre amended soils in field • 0.01 ± 0.01 mg Cd kg-1 • 0.09 ± 0.07 mg Pb kg-1  below regulation values  not significantly different from CP grain no adverse effect on plant metal concentrations

Conclusions • P-saturated ochre can be used as a P fertiliser • increases the total and available soil P • no adverse effects → on crop yield → on soil metal concentrations → on plant metal concentrations • Other advantages • less frequent application required • reduced lime requirement • replacement for valuable natural sources of P • waste product being reused

Publication in Soil Use and Management Dobbie KE, Heal KV & Smith KA (in press). Assessing the performance as a fertiliser and the environmental acceptability of phosphorus-saturated ochre. Acknowledgements Andy Gray, Rab Howard, John Mormon, Graham Walker

P-saturated Ochre: Performance as a Fertiliser and Environmental Acceptability