Soil fertility decline
Soil fertility decline occurs when the quantities of nutrients removed from the soil in harvested products exceed the quantities of nutrients being applied.
In this situation, the nutrient requirements of the crop are met from soil reserves until these reserves cannot meet crop demands. This results in a reduction of plant growth and yield.
Nutrients may be removed from the soil by:
- growing crops
- soil erosion
Nitrogen can also be lost from the soil as a gas by the process of denitrification.
Organic matter plays a key role in maintaining soil fertility. It holds nitrogen and sulfur in organic forms and other essential nutrients such as potassium and calcium.
The loss of organic matter mainly occurs through continuous cropping with stubble removal or burning, and is accelerated by frequent tillage.
In all agricultural systems, nutrients are removed over time in harvested products, such as grain. Off-site losses of nutrients can also occur through soil erosion, runoff, leaching and burning of crop residues.
The usual management response to nutrient removal or loss is to apply fertiliser.
Significant areas of cultivated land in Queensland are now unable to produce economic crop yields and high protein grains without the use of fertilisers.
In some rain-fed cropping systems such as grains, minimum/zero tillage with stubble retention has resulted in nutrient stratification (particularly phosphorus and potassium) in the surface soil (0–10cm).
Limited in-season rainfall often causes crops to meet their water (and nutrient) requirements from the subsoil. This has resulted in subsoil depletion of nutrients even though the surface soil may have adequate levels due to redistribution via crop residues.
Managing soil fertility decline
Fertility management aims to maintain soil organic matter, soil structure, soil nutrient status and satisfactory soil pH.
This can be achieved in cropping systems by:
- including pasture phases and leguminous crops
- including the addition of soil amendments or fertilisers
- employing a reduced tillage system
- retaining crop residues on site.
However, managing nutrient stratification in reduced till/stubble retained systems may require strategic tillage to redistribute surface soil nutrients into the subsoil and/or deeper placement of phosphorus and potassium fertilisers.
Best management practices such as fertiliser application rates are often adopted by growers but due to the lack of underpinning research, these practices are sometimes not well defined.