The need to reduce tillage is a widely accepted goal for farming to improve soil health and sustainability of agroecosystems. A new research paper from Rodale compares the effects of the different strategies employed by Organic and Non-Organic farming systems on soil health.
To investigate this, data was collected from a long term field experiment in Pennsylvania that was set up in 1981, comparing three management systems, namely an agrochemical based maize-soybean system (CNV), a low input legume-based Organic system (LEG), and a manure based Organic system (MNR). The experimental site was on a silt loam soil with a subhumid climate (12.4 degrees Celsius mean temp and 1105mm annual precipitation). The CNV system was a simple Maize-Soybean rotation with a winter cover crop of cereal rye and fertilser and pesticide inputs. The LEG system was a four course rotation of maize, soybean, wheat and oats with fertility supplied through the use of forage legumes, with red clover undersown into oats and hairy vetch with cereal rye used as an over winter cover crop. The MNR system was a much longer, diverse rotation consisting of maize, soybean, small grains and 2-3 years of cocksfoot and lucerne, and maize silage, with composted manure added to the oats and the maize silage. As with the LEG system, hairy vetch and rye were used as winter cover crops.
Ten years after no-till was introduced in 2008, the CNV herbicide based system did not affect soil organic matter (SOM), cation exchange capacity (CEC), Active carbon or extractable protein content, a measure of organic nitrogen (N) which can be easily converted to plant available N. Potentially mineralisable carbon (PMC) which is carbon respired by microorganisms was increased under the CNV system, which was also found to be higher in surface compaction.
The LEG and MNR systems were managed through cover cropping and the use of a roller crimper, where soya and maize were planted in spring following termination of an autumn planted cover. There were very few discernable changes in soil health parameters' apart from an increase in the PMC from the MNR system. Comprehensive assessment of soil health (CASH) didn't generally differ between the CNV and LEG systems but was higher in the longer rotation MNR system.
In summary, the diversified cropping and organic matter inputs in Organic systems seem more relevant for soil health than does reduced tillage, whilst additional management practices beyond CNV no-till may be required to realise the full benefits and potential of a agrochemical based systems that eliminate tillage.
Applied to our own context, with conventional no-till regen systems that rely on herbicides but that also include diverse cropping, there may be greater positive effects on soil health. I think it does make sense that no-till could lead to greater soil density and compacting, and the study shows that organic manure and diversified cropping can alone maintain and build SOM and has at least as bigger impact as no tillage. SOM results are heavily affected by the depth of sampling and whilst not explicitly stated it appears sampling in the present study was done to 20cm (8in) being able to take account of gains in SOM deeper in the soil profile. There wasn't any evidence that no-till under the two organic systems or the conventional system had an effect on the SOM %. Reducing tillage in organic systems can be considered an important aim for improving soil health but results of this study and others suggest it should not be considered the "holy grail", in the context of well managed rotations that include diverse cropping including fertility building leys and regular organic manure additions.
The most important thing as organic farmers you should now do, if you haven't already begun, is implement a systematic soil sampling regime to create evidence of the benefits your farming system has for soil health and SOM. The carbon footprinting exercise has revealed just how crucial this data is in offsetting your carbon emissions from your farming enterprises.
A tale of two systems Does reducing tillage affect soil health Conv vs Org.pdf