Temporal Trends in Soil-Test P and P Recovery in Grassland Soils

1. UNIVERSITY of LIMERICK O L L S C O I L L U I M N I G H Temporal Trends in Soil-Test P and P Recovery in Grassland Soils Dr Michael Herlihy1 and Dr Juliet McCarthy1,2 1 Teagasc, Johnstown Castle Research Centre, Wexford 2 Department of Life Sciences, University of Limerick, Limerick The Irish Fertiliser Manufacturers’ Association

2. Outline of Presentation • Background • Introduction • Results: • P balance • Soil-test P • P recovery • Summary and Conclusions

3. Background • agronomic importance • environmental importance • trends in fertiliser P use and soil Morgan P values

4. Fertiliser Use Courtesy of Soils Laboratory, Johnstown Castle

5. Introduction • Long-term trend: fertiliser P vs. soil-test P • Trends in Morgan P: build-up or depletion • Soil-test P changes in relation to P balance • P balance = P inputs – P offtakes • Potential for enrichment or depletion of soil-test P • P recovery

6. P balance trial • Component of larger trial: Fertiliser P requirements in relation to soil P status for grass production on representative soils • Objective: examine trends in P balance, soil-test P • 32 sites (8 x 4); low and high index groups; 4 yrs • Fertiliser inputs: P0, P20, P40, once-off P60 kg ha-1 P • Herbage and soil samples • Paper: Irish Journal of Agricultural and Food Research • Focus: soils 14 and 30

7. Location of soil series

8. Classification and characteristics of soil series

9. Results: P balance

10. Effect of fertiliser P input on P balance †Sum of 4 years (kg ha-1)

11. Effects of soil series on P balance †Sum of 4 years (kg ha-1)

12. Cumulative P balance† for soils 14 and 30 †kg ha-1

13. Results: Morgan P • Changes in P balance affect trends in soil-test P

14. Effects of fertiliser P on Morgan P (a) low index group (Spring)

15. Effects of fertiliser P on Morgan P (b) high index group

16. Soil 30 mg l-1 P mg l-1 P Trends in Morgan P: low index group Soil 14

17. Soil 30 mg l-1 P mg l-1 P Trends in Morgan P: high index group Soil 14

18. Results: Morgan P • Soil-test P decline esp. P0 • Largest decline: higher index • Change on P40 plots ns - annual fluctuations, inputs ~ offtakes • P20 and P60 Morgan P comparable after 3 years • Decline: 30 > 14

19. Unit change ratio • Change P balance : unit change Morgan P • P0 plots • Low index: 79:1 (soil 14); 28:1 (soil 30) • High: 29:1 (soil 14) and 12:1 (soil 30) • Variation with index and P input • Effects of soil chemistry and texture

20. (b) high index group % % % Recovery - P60 (a) low index group

21. Results: P Recovery • Highest on soil 30 • Highest in low index group • Increased with time … 4 years • May underestimate recoveries attainable over a longer period

22. Summary of results • Changes in P balance and soil-test P affected by: • Soil series • Fertiliser P input • Initial soil-P status (index) • High offtakes reduced soil P rapidly: large decline in 4 yrs • Depletion of soil P, fertiliser-P recovery varied widely between contrasting soils • Clonroche loam 14 vs. light limestone Baggotstown 30

23. Conclusions • Cannot assume direct relationships between inputs and offtakes in different soils • Effects of soil series, fertiliser-P inputs, P index on change in P balance, soil-test P and recovery • Large reductions in soil-test P in 4 yrs with no fertiliser-P input … • Cut vs. grazed swards: offtakes, duration • Regular soil testing essential

24. Future Work • Differences in soil-test P changes, recoveries between soils … causes?

25. UNIVERSITY of LIMERICK O L L S C O I L L U I M N I G H Acknowledgements • IFMA • Research Office, UL • Farmers The Irish Fertiliser Manufacturers’ Association