"COVER crops - a game changer."
This was the banner headline of a media release from Hort Innovation on March 3.
It goes on to say: "Cover crops are one of the most useful tools for managing intensive vegetable growing soils."
"New research (Optimising cover cropping for the Australian vegetable industry project) by Hort Innovation found that integrating cover crops into the growing of vegetables improves soil health and on-farm productivity."
I have to say I had a bad attack of déjà vu when I read that - progressive farmers have been doing this for years.
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It reminded me of the carbon dioxide story I've referred to in this column on several occasions - small scale crop producers were using it years ahead of the others.
Having said that, being sceptical is an easy trap to fall into. It's easy to say something works - it's more difficult to say how well it works. The new research mentioned above removes that difficulty.
Looking at on-farm productivity first, this was measured in a field experiment in Cowra (NSW). Plots were rectangles measuring one by 10 metres.
There were four blocks/replicates, each containing six plots (five treatments and a control).
The commercial crop was spinach, and the five treatments were cover crops of oats, "mix", pea, mustard or radish (the "mix" was of oats and peas).
The cover crops were grown for 14 weeks, then sprayed with herbicide.
The next step was mulching with rubber, followed two weeks later by incorporation in the soil.
The spinach crop was then sown and seven weeks later yields were estimated from 1m sq biomass cuts. Fresh and dry weights were then recorded.
The results are frustrating.
There were statistically significant increases related to the mustard and radish cover crops of 2.75 and 3.63t/ha respectively, when compared with the control, but I read and reread and couldn't find what the actual control tonnage was.
I checked on the web and you can expect about 20-30t/ha so a 2.75 tonne increase is about 10 per cent but it would have been much better to know what the actual yield was.
Being sceptical is an easy trap to fall into. It's easy to say something works - it's more difficult to say how well it works.
The reference is here (www.agronomyaustraliaproceedings.org) so you can check for yourself.
This lapse is explained by the title of the paper: "The effect of cover crops on physical, chemical and microbial properties of a sandy loam soil"- then as an afterthought "and baby leaf spinach yield".
The authors are Adam Harber, Gordon Rogers and Daniel Tan. You have to have a science degree to understand the first three.
For example: "In ratio UniFrac analysis, the model takes into consideration a genomic mean, muting the operational units (OTUs) that are close to the mean and accelerating the OTUs that are more abundant than the mean."
The conclusion is in plain English: "Adding cover crops residues to the soil may improve soil structure, water holding capacity and organic carbon."
Tasmanian grower Deon Gibson, Premium Fresh farm manager is quoted as saying: "We've never had such healthy-looking carrots. There are no nematodes, the crops have beautiful, green, healthy tops and they're in free-draining soil. And in terms of cultivation, the soil breaks down very easily and has plenty of organic material and worms."
Try it yourself on your farm (or in your garden).
- Dr Mike Walker welcomes feedback and questions. E-mail him: firstname.lastname@example.org
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