Orchard soil characterisation

This study aimed to explore the role of different soil properties in relation to avocado orchard yield. Data from this study will also be incorporated into the Avovantage project looking at on-orchard practices that can help reduce the risk of fungal rots. The analysis relating to fruit quality is not included in this report but will rather be included in the
Avovantage reporting channels.

Key variables relating to soil physical, biological and chemical composition were successfully collected from groups of high performing orchards (>15t/ha) and lower yielding orchards in the Bay of Plenty region. Results of this study provide a valuable comparison to growers wishing to test their own soil to see what variable may be influenced to potentially improve yields.

Nutrient testing on soil, leaf, fruit skin and flesh was conducted by Hill Laboratories. Commercially available biological soil tests from Hill’s Laboratories, Linnaeus and Soil Foodweb were used to assess soil biology and a visual soil assessment system was used to assess soil physical properties. A comparison of soil biology results from Soil Foodweb and Linnaeus highlighted that common variable
results did not agree with each other. The different tests use different methodologies but further investigation is required to understand why these different tests are providing different results.

Aspects of soil physical, biological and chemical composition all showed importance in classifying whether an orchard was high yielding or not. None of the leaf nutrient results showed a correlation with yield classification, but the sample size is small relative to most nutrient studies. Some of the correlations observed were contrary to what is expected to contribute to a high performing soil. It may
be that there are common management practices among high performing orchards that negatively impact soil characteristics. Therefore, the correlations seen within the soil variables may be more related to the management practice rather than the high yields common to these orchards.

Higher levels of iron, low levels of aluminium, and high C/N ratio were chemical components of soil that were important predictors of yield and correlated with the group of orchards achieving over 15t/ha. Of the biological variables measured, higher ciliates, lower flagellate protozoa communities, higher dry weight, lower total bacteria and lower gram-negative bacteria correlated with higher yielding orchards. Several of these biological correlations are counter to what might be expected of a highly productive soil.

Soil worm counts was the only physical soil observation to correlate with orchard yield classification with lower worm counts present on higher yielding orchards. Again, this is counter to what is expected in a highly productive soil.

A combination of the Hill Laboratories chemical variables and the Soil Foodweb biological variables showed the highest classification accuracy of 79%.

The variables collected provide a valuable benchmark from high producing orchards in different regions for growers to compare their own soils to. This will support decision making on whether soil amendments or changes to an orchards management may be the best avenue to pursue to improve yields. The results also highlight that there might be improvements that can be made to the soils of high yielding orchards that may further enhance yields.