Forest Fertilization: Two Topics

1. Forest Fertilization: Two Topics Roderick Negrave PhD, RPF, PAg Research Section Head, Coast Area MNRO, Nanaimo

2. You Will Hear About: • Spring versus fall applications of urea • Nitrogen alone compared to N plus P fertilizer applications in western redcedar

3. Timing of Urea Application What is the issue? • Volatilization and leaching losses of N • Reduced recover of N by crop trees • Less efficient use of fertilizer product by the crop • Increased possibility of environmental contamination

4. Urea in the Soil • Plants cannot absorb urea directly • Urea decomposes to ammonium and then nitrate under appropriate conditions • Unfavourable condition will promote volatilization of N-based gasses (leaching) • Fertilizer-N best applied during periods of rapid fine root growth (spring and fall)

5. General Findings • Studies have been conducted on pines and Douglas-fir on the BC Coast, Interior, US Northwest and Scandinavia • Season of urea application has not been studied for western hemlock nor for western redcedar • Studies of Coastal Douglas-fir response are likely most useful for our discussion

6. Applicable Findings • Differences in urea-N uptake were found between spring and fall applications • Fall applications of urea resulted in greater foliar N concentrations • Foliar concentrations were examined but not growth • Less conclusive results in BC Interior

7. Why? • Warmer and drier spring weather conditions promote volatilization of N • Cooler and moister fall weather conditions may promote leaching of N as nitrate • However, tree roots are present in sufficient volume to assimilate nitrate before is washes from sites

8. We Can Conclude... • Nitrogen assimilation by stands is likely greater with fall urea applications on the Coast • This difference may decrease as sites become more continental • Effects of seasonal application on growth are uncertain

9. Western Redcedar: N Versus N+P Fertilizer Applications • Hw responds well to N+P: What about Cw? • Phosphorus is a pivotal plant nutrient and constituent of cell contents and meristems • Broadly, P found at about 20% of N concentrations • Long-term fertilizer responses (>50 yrs) associated with P applications

10. Sources of Information • Weetman et al (2 x 1989): • One-year of small plot data plus screening trials • Seven-year old plantation on CH site • Harrington and Devine (2009, 2011): • Thinning and fertilization of a low-site Cw stand on the Olympic Peninsula • Fertilizer applied at 1 and 13 years • Complicated my K, S and Ca application in first fertilization • Followed to 25 years

11. Weetman et al.One-Year Height Growth (cm) N0P0 33 (24) N100P0 39 (20) N100P100 42 (18) N200P0 45 (21) N200P100 50 (20) N300P0 43 (19) N300P100 53 (19)

12. Weetman et al. (cont.) • One-year trial suggests Cw response to P likely varies with N application • Three-year plot and single-tree screening trials confirmed that Cw does respond to N+P additions • Significance of difference between N vs. N+P not confirmed but suggested

13. Harrington and Devine • Volume (Thinned): N: 266.9 (22.7)a N+P: 283.9 (11.5)a • Basal Area (Thinned): N: 41.8 (2.9)a N+P: 44.3 (1.9)a

14. Devine and Harrington • BA PAI (13 – 25 years, thinned): N+P > N • Height PAI (13 – 25 years, thinned): N+P >N

15. Considerations • Cost of fertilizer-P product • Cost of additional turns to deliver added product i.e. 300 kg/h of N versus 300 kg N plus 100 kg/ha of P • Value of added increment: Marginal return complicated by time and N x P interaction

16. Conclusion • There is added growth in N+P compared to N-only applications • This difference may accelerate over time • Whether additional growth will pay for additional P is likely fairly complex due to complexity of P response with time