Deep banding of phosphorus (P) and potassium (K) fertilisers has shown promising yield responses in some settings, but not in others. Deep-banding trials across Queensland and New South Wales have shown that banding rarely reduces crop yield, but it doesn’t always lead to profitable yield increases.
Some of the variability in response to deep P and K is explained by the background levels of those nutrients present in soils. It seems that the lower the background level of P and K in the soil the more likely it is a yield response will be achieved with deep banded fertiliser Soil nutrient testing in the deeper soil (10-30 cm) is important before investing in deep banding. Critical subsoil P and K soil test values are being refined in the current research.
The greatest responses to deep P have been achieved where nitrogen supply is sufficient to achieve a higher yield target and P applications are split between:
- deep P applied as far in advance of sowing as is practical
- starter P at rates appropriate for the crop row spacing and soil moisture conditions at sowing.
Starter P clearly remains important.
Researchers are now working on getting a better understanding of how plant roots acquire nutrients from deep bands in drier soils, and how these drier conditions affect fertiliser responses. In drier soils, the banded nutrient is not able to efficiently diffuse from the band to the active root zone, and this seems to restrict crop uptake in some cases.
At this stage, it’s thought that for a crop to successfully access banded fertiliser the fertiliser band location needs to coincide with a zone of active root growth. The crop’s active root zone is influenced by seasonal moisture conditions and can vary considerably both between and within a season.
The top 30cm of the soil profile contributes the most P and K to crops. The 10-30 cm zone is important because there are still lots of roots but the soil remains moist for longer. Placing bands deeper than 30cm is probably not worth the extra difficulty and cost of putting them there, as root numbers start to decline and uptake is less efficient.
Different crops have different root architecture that affects their ability to exploit fertiliser bands. Fibrous rooted grass crops are more likely to access banded nutrients compared to coarse rooted legumes because of the higher root density. Proximity of crop rows and fertiliser bands may, therefore, be more important in coarse rooted crops. Preliminary research work suggests crop variety may also influence how effectively the plant can utilise fertiliser in a band.
The cost of a deep-P application may not be fully recovered in the first crop. Yields and returns in subsequent crops may also respond to a single application of deep-P. Most of the applied P will remain available for several years on soils with a phosphorus buffering index (PBI) of less than 150.
This article was prepared in response to discussions between growers, advisors and researchers at the More Profit from Crop Nutrition Roadshows 2016.
extensionAUS Root systems that respond to banded fertilisers
Photo courtesy of the GRDC.