Advantages of zero till proven from decades of study

The papers are "Changes in soil water storage with no-tillage and crop residue retention on a Vertisol: Impact on productivity and profitability over a 50 year period" and "Changes in soil organic carbon and nitrogen after 47 years with different tillage, stubble and fertiliser management in a Vertisol of north-eastern Australia". The former article was published in Soil & Tillage Research (2019) Volume 194 and the latter in Soil Research (2020) Volume 58.

Senior author of both papers was Kathryn Page, School of Agriculture and Food Sciences, University of Qld. Site of the 50 year study was Hermitage Research Station, near Warwick Qld, with mean annual rainfall of 685mm, about 60 per cent received during December to March. Many other prominent researchers have been involved in the study which was established by Qld DPI researchers John Marley and Jack Littler who set out to explore the impact of tillage, stubble and nitrogen rates on crop yield. No till was not common in 1969.

No-till (NT) has proven to assist in reducing erosion, lowering fuel costs, conserving soil moisture and improving soil physical, chemical and biological characteristics. Improvements in soil are often driven by greater soil organic matter accumulation, as measured by soil organic carbon (SOC) in NT compared to conventional tillage (CT).

NT and stubble retention (SR) combined with nitrogen fertiliser resulted in highest SOC (by up to 2.8pc) and total soil nitrogen (STN) stocks (up to 31.7pc) in the 0-0.1 m layer relative to CT, stubble burnt, (SB) and no nitrogen (N) fertiliser.

However, declines in SOC (up to 20pc) and total soil N (up to 25pc) occurred in all treatments over the 50 years research, indicating changes in practices were unable to prevent loss of soil organic matter over time, regardless of farming system. However, NT with stubble retained and N fertiliser reduced the rate of N and SOC losses compared to tilled and stubble removed treatments. Soil analysis highlighted that crop residues significantly contributed to SOC stocks and that their contribution is increasing over time.

Over the research period (1969-2018), NT and stubble retention resulted in 54mm more soil water storage in the top 1.5m of the soil. However, nitrate build-up during fallow period was generally lower with SR, prevented wheat from fully capitalising on increased soil water storage.

While crop yield was greater under NT verses CT and more so under NT+SR verses CT+SR (particularly in years where in-crop rainfall was greater than 300mm), nitrogen fertiliser was needed to maximise yields. While 90kg/ha N resulted in best yields, 30kg/ha N resulted in highest gross margin returns. Nematode tolerant wheat varieties had also been critical for high yields under NT+SR farming. However note that this trial mainly grew wheat year after year, not necessarily good practice .

Other research indicates that as NT farming proceeds in time, loss of SOC declines and maybe even reaches an equilibrium. The world's oldest trial, the UK Rothamsted Broadbalk wheat experiment assessing winter wheat production for 175 years (since 1843) on land cropped for many centuries prior to 1843, is especially noteworthy.

Wheat in rotation treated with farm yard manure (35t/ha per annum) plus added nitrogen (96-144 kg/ha) on average yields just under the best yielding NPK fertiliser treated crops. Wheat with farm yard manure plus N and NPK fertiliser in rotation (first wheat crop in rotation) yields around 4.0 t/ha more grain compared to continuous wheat with FYM only and NPK (144 kg N/ha).

SOM in the plough layer (0-23cm), has more than doubled on the FYM treatments, with increases greatest in the early years. Mineral nitrogen fertiliser has enhanced SOM a little, probably because of increased returns of organic matter in crop roots and above ground residues.

Next week: Northern Australia pasture management strategies valuable for NSW.