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’.


The Sensitivity analysis for Douglas-fir growing at Wind River, WA showed that soil fertility, when unlinked to quantum efficiency was the most important variable constraining growth (and maximum leaf area index). The slope of the FR regression was ~2.8 compared to near 0 with H2O, r2’s also were higher with variation in FR (0.61) than with H2O (0.24).


When soil fertility was linked with alpha, the r2 of the relationship between FR and Peak MAI increased to 0.7 at Wind River, WA.


Ponderosa pine growing in Bariloche, Argentina showed more sensitivity with H20 storage than did Douglas-fir at Wind River, WA.  For a five-fold increase in FR and H20, however, the  r2 with FR was still higher (0.51 vs. 0.43).


Linking alpha to FR increased the r2 from 0.51 to 0.84.


Note: To compare slopes, the data need to be normalized. This can be accomplished by multiplying the soil water storage capacity values by 10-3 or the FR values by 103.