Physiological responses to subsoil manuring in crop species across high and medium rainfall regions

Poorly-structured dense clay subsoils are widespread in dryland cropping regions in south-eastern Australia. They limit nutrient and water uptake from the subsoil which can limit grain yields. Recent research has shown that ameliorating subsoils with organic amendments can improve yields on these soils but the mechanisms underpinning this response remain unclear. Thus, different subsoil amelioration practices were imposed once at the high- and medium-rainfall sites followed with different crop sequences from 2018 to 2020. Both sites had poorly-structured sodic clay subsoils. The four experimental treatments assessed were 1) unamended control, 2) deep-banded gypsum, 3) chicken manure pellets applied to the topsoil, and (4) deep-banded in the subsoil. Compared to the control, both manure treatments significantly (p < 0.05) increased greenness in the upper leaves (SPAD readings), the green leaf area, and green leaf duration, and thereby delayed canopy senescence. The manures generally increased leaf N concentrations at anthesis, and at mid grain fill in all cereal crops, but not in faba bean. The manures also increased photosynthetic rates and instantaneous transpiration efficiencies (ITEs) in upper leaves of both cereal and faba bean crops, during grain or pod fill, but not in very dry years. Only one site and year combination, i.e. Tatyoon in 2018, showed an increase in grain yield. Despite the favourable effects of manures on crop canopies, unfavourable seasonal (rainfall) conditions were considered to be the major factor that limited yield increases by manured crops. Manuring in the medium-rainfall regions is likely ineffective in terms of crop productivity improvement.