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Starter and residual P give strong early vigour in the north

Aerial photograph of starter and deep P trial site plots

This season, starter phosphorus (P) is producing strong early crop vigour. Deep P placement effects and the response to starter P are being documented at a long term monitoring site at Terry Hie Hie, in far north New South Wales.  

The trial site compares plots with different rates of P applied at 5 cm or 25 cm depth in July 2014. In 2017, half of the treatment plots received starter P. The other half are relying on the 2014P applications.

Both visual comparison and NDVI assessment reveals better early crop development in the plots with starter P. As well as the starter effect, the higher P rates applied in 2014 are also showing a benefit to early crop vigour (see graph).

Chart of early crop response to starter P

NDVI scores from young wheat plants.

Young plants have smaller, shallower root systems. The purpose of starter P is to provide a small amount of the crops’ P requirement close to seedlings.

Crops tend to take up most of their P from soil reserves, rather than P fertiliser in the year it’s applied. In this case it looks like the higher background P from the 2014 application could be slightly assisting P uptake in the 2017 starter P plots.

2014 P application depth did not have an effect on early crop NDVI readings. Results for the two depths are combined in the graph. It makes sense that depth of fertiliser application might not make a difference in early crop development, because the roots may not be accessing much of the deep placed P yet.

As the crop grows, P deeper in the soil becomes much more important than the small amount of starter P. Deep P placement can be of great value when the topsoil is too dry for active roots.

The crop rotation at the trial site follows the host farm’s normal practice. This is the first year of monitoring with a promising crop. In 2015 the paddock was fallow. In 2016, a promising chickpea crop failed due to waterlogging and/or herbicide residue damage.

Monitoring of the 2017 crop continues to see how these early starter P effects impact wheat yield. To understand the long term effects of a one off application of deep P monitoring of the trial site continues through to 2019.


Photo: Pete Formann. NDVI analysis: Bruce Haigh.


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