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

Introduction P-saturated ochre prepared by repeatedly mixing with H 2 PO 4 - 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)

P treatments (5 replicates) Experimental design – pot experiment

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

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 Analysis

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

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

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

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 Are plant metal concentrations potentially hazardous?

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 Conclusions Other advantages –less frequent application required –reduced lime requirement –replacement for valuable natural sources of P –waste product being reused

Acknowledgements Andy Gray, Rab Howard, John Mormon, Graham Walker 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.