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Validation of an enzyme-based soil alteration index for testing soil health in local apple orchard soils
Objectives and Rationale
Enzyme-based soil alteration index, Al3, may prove to be effective for quantifying soil health in local apple orchards (Meyer et al., 2014a). This hypothesis was tested by evaluating AI3 in relation to (1) compost/mulch treatments, (2) ARD/fumigation treatments, and (3) different cover crop treatments, and to soil/tree data
Methods
Compost/mulch treatments were applied at Oak Valley (Grabouw), ARD/fumigation treatments at Vergelegen (Elgin), and different cover crop treatments at Vyeboom Boerdery. AI3 indices were generated using the formula of Puglisi et al. (2006). All trials followed a statistical design.
Key Results
AI3 indices were generally better in the top- than in the subsoil layers, at the respected trial sites. Indices were generally better under mulching than bare soil at the Oak Valley trial site. In accordance with AI3 indices, mulch-compost combinations performed better than the unmulched control. AI3 indices were generally better one year post-fumigation (sterile soils) compared to ARD plots and AI3 was also able to differentiate between the different rootstock genotypes response post-fumigation. Positive correlations between AI3 and soil parameters, notably soil C, as well as tree parameters, was mainly observed at the Vyeboom cover crop trial.
Key Conclusions of Discussion
After testing, it can be concluded that the AI3 was able to:
- Reflect gradients in mineralizable substrates across a transect of different soil layers in apple orchards (Oak Valley, Vergelegen and Vyeboom trials).
- Distinguish between mulched and unmulched treatments, confirming wider experience that mulching favours microbial decomposition processes in the soil relative to bare soil conditions (Oak Valley trial).
- Distinguish between mulch-compost treatments combinations (Oak Valley trial).
- Confirming the “recovery” of soil health status of Apple Replant Diseased soil after soil fumigation (Vergelegen trial).
- Differentiate between the capacities of various apple rootstock genotypes to support rhizosphere-inhabiting organisms and susceptibility of rootstocks to soilborne pathogens in ARD soils subjected to fumigation (Vergelegen trial).
- Predict soil nutrient status in apple orchards, e.g. being consistent with soil C availability (Vyeboom trial).
- Predict apple tree performance, e.g. being consistent with stem circumference, tree height, cane mass and yield mass (Vyeboom trial).
- Forewarn heavy metal contamination in apple orchard soils, e.g. Cu (Vyeboom trail).
The findings presented in this report are thus proof that the AI3 index is a suitable method for quantifying soil health in Western Cape apple orchards, under the range of conditions set out in this report. The method is more sensitive (accurate), rapid and cheaper than current traditional chemical tools, which will help achieve the twin objective of making the biological quality of agricultural soils better ‘measurable’ and ‘manageable’.
Further validation of the AI3 is strongly recommended for wider application in Western Cape deciduous fruit orchards. Longer-term soil and tree effects of soil management should also be included in subsequent research since current findings are limited to more short-term effects.
For Final Report, please contact:
anita@hortgro.co.za