Productivity gains from biologically active soil initiated through biochar activated compost in an avocado orchard

Avocado production is a growing industry in the Manjimup area. As it is not a native species, the avocado cultivation encounters a number of challenges, including soil and climatic conditions very different to their native habitat. Local production difficulties include infestation by Phytophthora cinnamomi (exacerbated by waterlogging conditions and resultant low oxygen levels which are detrimental to avocado root health) and biennial bearing of fruit.

To quote Doug Pow, the farmer who undertook the trial, “I’ve seen avocados growing on a volcano and I’d like to replicate, as best I can, those andosol soils, because avocadoes have a very high requirement for oxygen in the soil and that is a very tall order in a soil which is of a very high bulk density, like our clays.”

This project established two rows of 36 trees, the rows 9 metres apart – the rows running in a north-south direction on a north-facing slope. Soil type varied from loam over clay, loamy gravel over clay and gravel over deep clay down the gradient. Each soil type had three replicates of varying applications of biochar incorporated into the planting zone: 20%, 10% and 5% v/v in the top 60cm of soil, with four trees in each treatment percentage per soil type.

Row 22 was ridged and ripped, as per normal orchard practice, and rotary hoed in preparation for planting. Trees in the Control row (Row 22) were planted on the 11th November 2014 according to industry best practice.

A road grader cut a trench and ripped the base of Row 21 – four metres wide and 600mm deep. The top 300mm was rolled to one side, the 300mm subsoil to the other. Biochar incorporation calculations: 4 x 4.5m x 3.5 wide x .45m deep = 28.35m3 for a 4-tree block. 20% = 5.67m3, 10% = 2.84m3 and 5% = 1.42m3. This was incorporated using a front-end loader, tipping trailer and rotary hoe. Half the calculated amount of biochar was spread in each of the 4-tree blocks, covered with the subsoil and rotary hoed to incorporate. The remaining biochar was then spread, top soil returned and also rotary hoed. The surface was hoed again in preparation for planting, staking and fitted with trunk guards. Biochar incorporation, as per application schedule, took place two days prior to trees being planted on 21st November 2014 (Row 21).

Alternate pairs of trees of common soil incorporation rate, including the control 0%, had a topical application of mulch – either a mulch comprised of chunky wood chips, old sawdust and chicken bedding litter (manure and sawdust) which was blended at a ratio of 1:1:1, or the same material with biochar added (ie biologically *activated biochar) with a ratio of 1:1:1:0.5 and spread at a rate of approximately 100l/year. Mulch was blended and applied in December 2014 and topped up annually. Biochar was sourced from Simcoa Pty Ltd, predominantly jarrah feedstock pyrolysised at high temperatures.

* the term “activated” is used to mean “charged with organo-mineral nutrients and inoculated with living soil fauna.”

Benchmark soil testing was somewhat compromised as 55kg of limesand was applied to each row by hand (2/10/2014). In the planting hole of Row 22 under each tree the following was applied: 500gm single superphosphate, 60gm MgSO4, 30gm MnSO4, 10gm CuSO4 (11/11/2014). This was repeated with trees planted in Row 21 (21/11/2014). All trees had 60gm of Organibor (slow release boron) applied (24/11/2014) and 25gm of urea spread around the trees (27/11/2014) with a foliar application of Bortrac 150 sprayed (8/12/2014). Soil sampling which occurred 16/12/2014 would have been influenced by granular fertilisers and potentially foliar run-off however the data remains valuable for its comparative purposes.

Testing of compost and activated compost occurred in 2015. Biochar analytical data was sourced from a previous analysis of Simcoa’s biochar. Monitoring of tree growth was recorded photographically regularly throughout the trial with biometric data recorded in September 2016. Tissue testing occurred annually and sampling for comprehensive end-of-trial assessments of soil and biological activity occurred in 2017.

On-going management of the treatment row and the control row was according to industry best practice ie fertilisation and irrigation.

Doug Pow maintained meticulous records of fertiliser, herbicide and pesticide (wingless grasshoppers) application. Monthly photographic monitoring of tree growth enabled tracking of performance.

By the end of the first year it was clearly evident that trees, irrespective of biochar rate, were growing at a faster rate, with greater biomass, than the control row which was performing at an acceptable industry standard.

Based on his fruit counts, the land manager anticipates that around October 2018 he will pick half a bin from the control row and from the biochar row he will pick two bins.

Doug Pow says, “Every tree I plant from now on, I would plant in biochar – that’s lesson number one in flashing lights.

“It’s interesting that we have tested 5% to 20% biochar, which is a 400% increase in biochar proportion in the soil, but the trees look exactly the same. So, the chances are 20% is not too much and 5% is not too little. We were trying to have too little by trialling 5% and too much by 20%, but it looks like we underestimated the top and overestimated the bottom.

“So, we need to continue the experiment with a lot less biochar and see what happens. The real test will be in fruit volume, fruit quality and fruit size and I guess the biggest test of all will be if it affects biennial bearing. It’s ability to help the tree survive attack by Phytophthora cinnamomi will also be very interesting.”

The land manager has already taken up this practice by planting a further 4ha of avocadoes in 10% biochar.  He will also continue to monitor the trial site over time.

From anecdotal evidence, 20 farmers have since adopted this practice on new plantings of avocadoes, citrus, olives, apples and stonefruit.  Doug Pow has been very generous with his time and knowledge, which has contributed to the adoption of this practice.