Executive summary
Australian soils commonly exhibit multiple constraints to plant productivity. Six farming system groups identified problem soils that exhibit multiple constraints to plant production. These soils were treated with amendments that address the identified constraints to quantify the benefit of amelioration and determine the mode of action of the observed benefit.
Dispersion associated with sodicity was common in most soils studied. The addition of organic amendments can influence microaggregate flocculation as measured by decreased turbidity however the effectiveness was much less than gypsum. Gypsum and S treatments also have the potential to increase soil EC above the thresholds for healthy plant growth.
Plant growth enhanced the effectiveness of subsoil organic matter amendment’s influence on soil structure, with benefits increasing with time (>12 months). However structural benefits from organic matter alone did not translate into increased plant growth.
Organic amendments have the advantage of simultaneously increasing N, P, S and K nutrition with chicken manure pellets providing greater nutrition compared to wheat straw pellets.
Plant performance was improved in soils exhibiting multiple constraints with the addition of amendments that provided N and P to the subsoil. Nutrition, as N and/or P, from organic material or synthetic sources applied to the subsoil increased yield as much or more than manure alone. Evidence of plant/microbe interaction in P cycling existed. Available P in the soil at the end of experimentation was greater in the presence of plants compared to where no plants were grown. Surface applied manure and fertilising with N and/or P in the subsurface layer provided biological benefit by increasing abundance of genes associated with C, P and N cycling.
The improved nutrition in the subsurface layers provides biological benefit to the soil system that may carry over to subsequent crops. Field investigation of the longer-term benefit of adequate subsurface/subsoil nutrition to plant performance and plant/soil biology interaction is warranted.