Exercise 3. Effects of Soil Fertility and Soil Water Storage Capacity on Productivity

Problem: You wish to know the extent that you might increase yields through frequent additions of commercial fertilizer and possible interactions with soil water storage capacity.

Solution: For a given environment, modify soil fertility (FR), linked or unlinked to quantum efficiency (alpha). At the same time, vary soil water storage capacity (ASW) for each FR rank. Plot relationships between FR and Maximum Mean Annual Increment (MAIx); determine slope and r² of the regression equations to evaluate relative importance of the two soil properties. Insert maximum mean annual increments (MAIx) and maximum Leaf Area Indices (LAIx) in output box on Sensitivity spreadsheet: ‘Sensitivity Soil Fert & H₂0’.

1. Open spreadsheet ‘WindRiver Ps me’ and make sure that the ‘alpha’ is set = 0.05 (molC/mol photon) and that the linkage to FR, denoted by FN_{0 (zero)} is not operational.

2. Return to Sensitivity Soil Fert & H₂0 spreadsheet and type in the following:

Sites	FR	ASW	Max LAI	Peak MAI
WindRiver Ps me	0.2	100
WindRiver Ps me	0.2	200
WindRiver Ps me	0.2	300
WindRiver Ps me	0.3	100
WindRiver Ps me	0.3	200
WindRiver Ps me	0.3	300
WindRiver Ps me	Etc. to 1	Repeat to end

4. Push run button (calculator symbol) in lower left hand corner of page.

5. Make charts: 1) FR x-axis, Peak MAI y-axis, 2) ASW x-axis, Peak MAI y-axis.

6. Add trend lines to charts and select options that produce equation with r².

7. Compare slopes and r² values of two regressions to determine relative importance of soil fertility and soil water storage.

8. Repeat the above sequence after returning to spreadsheet ‘WindRiver Ps me’ and linking FR to alpha by setting maximum alpha =0.07 and activating fNo =0.3.

9. For comparison with another site, move down ‘Sensitivity’ spreadsheet and run analysis for ponderosa pine stands growing at Bariloche, Argentina.