Improving soybean yield by 9% with compost tea and humic acid

A 50-50 mix of humic acid and compost tea was applied to soybeans to test effects on yield and soil. This tea is based on a wide range of locally-sourced soil biology and is ‘brewed’ on farm. The tea was injected as an inoculant to boost fungal growth into the soil. The only extra costs are minor inputs into the ‘brew’ and one pass-over of a tractor.

The soybean yields in the treated areas averaged 9% higher than those in the controls. This equated to an estimated $190 increase in Gross Margin/ha. Effects of better drainage and increase in soil biology were recorded in photographs. Observations indicated a slow spreading of the fungi into the nearby rows of the untreated area.

Trial details

The paddock had grown sugar cane for the previous four years and soybeans are used as a break-crop. The farm uses trash blanket, minimum till and controlled traffic practices, all part of regenerative agriculture. Controlled-traffic farming uses computers and satellites to steer machinery on designated tracks to minimise compaction.

Application required one pass-over by a tractor with coulters and equipped with a 1,000-litre pod to hold the tea. The tea is applied at low pressure through pipes behind the coulters into the soil at about 10 cm deep. (For details see the Gladstone Conservation Council’s article on Regenerative Agriculture)

• Site 1 — The compost tea was applied on 6 November 2020. Soybeans were planted on 12 January 2021 following normal practices and rates. The variety was Kuranda. The area had been fertilised with 150 kg/ha GF402 (7N:6P:32K:2S).
• Site 2 — The compost tea was applied on 23 December 2020. Soybeans were planted on 12 January 2021, using the same practices, rates, and fertiliser.

Observations

Following an irrigation of 45 mm of water on 17th February, a visual inspection of possible effects of fungi was made six hours after the irrigation on 18th February.

As shown above, 6 hours after irrigation there is free water (light patch) in photo 1 and none in photo 2

Water present indicates poor soil drainage. Roots require water and oxygen to live and be active. Seeing water in the soil like the Control indicates lack of oxygen. If water logging is short term, say a few hours, it will restrict root function but cause little or no permanent damage. If it is days or weeks, it will kill roots. Lack of oxygen encourages the spread of root diseases like Phytophthora.

A second inspection was made the next day on 19 February, 105 days after applying the first compost tea. Both times 8 holes were dug in the control and 8 in the treated areas.

As shown above after 30 hours after irrigation, once free water has drained, it leaves behind soil with good air.

Soil-free areas on the shovels in photos 3 and 4 above, the sticky bits between the white lines show the depth of soil that is well drained. They show an increase in depth of well-drained soil from 10 to 30 cm. The soil below the 10 cm in photo 3 is still saturated, tight and too wet. Not shown in photo 4 is the crumbly, aerated structure for more than 30 cm. There were signs of very fine crumbles showing that the biology was active. (Too fine to be visible on these photos.)

Photo 5 taken at 26 April shows samples of weeds with significant soil adhering to roots. This adherence of soil to roots is a good indicator of biological activity in the rhizosphere.

Soil from the treated and control areas was then photographed under a microscope. Photo 6 shows grains of sand from the control soil with very clean sand particles. Photo 7 shows grains from the treated soil with dark stains and less light being reflected indicating the presence of humic compounds. The increased soil adhesion to roots and production of humic compounds on the sand grains indicate an increase in soil biological activity and organic carbon.

Yield and Increase in Gross Margin/ha

Yield samples were taken on 21 May, 150 days after application of the tea. Treated and control were hand-sampled with 8 replications, each 2m of bed, with 2 rows per bed. Samples from treated areas showed a 9.1% yield gain over those from untreated areas.

A 9% increase on 3.5 t/ha yield equates to 0.32 tonnes soybeans. At $750/t this equates to an increase of income of $240/ha. The cost of tea was $20/ha and that of application about $30/ha, giving an increase in Gross Margin of $190/ha.

Under good management, the fungi will not need to be re-introduced in following years.

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