Determining soil nutrient levels a must after the dry of 2015

Author: | Date: 19 Feb 2016

Man standing in a crop field on a sunny day facing the camera

Dr Mike McLaughlin, a GRDC Southern Regional Panel member, says having soils tested to assess their P status could save growers significant input costs this year.

Grain growers who experienced extremely dry seasonal conditions in 2015 are advised to determine their soils’ phosphorus (P) status ahead of this year’s sowing programs.

In areas where rainfall was minimal during the 2015 growing season, the removal of P in the grain of low-yielding crops was likely to have been low. This also applies to failed cereal crops that were cut for hay.

Therefore, lower than normal maintenance rates of fertiliser P will in some instances be sufficient this coming season. In some cases, application of fertiliser P may not be necessary, if soil P tests are in the optimal or high range.

Having soils tested to assess their P status could save growers significant input costs, says Grains Research and Development Corporation (GRDC) Southern Regional Panel member Dr Mike McLaughlin.

“Fertiliser investment decisions should start with knowing how much P is in those soils where growers are planning to sow a crop,” said Dr McLaughlin, a soil fertility and crop nutrition scientist with the University of Adelaide and CSIRO.

Dr McLaughlin said rates of required fertiliser P normally varied with soil P status, soil type, target grain yields, climatic conditions and method of P fertiliser application.

In neutral and acidic soils, a significant amount of P fertiliser applied to 2015 crops may still be effective for the 2016 crop due to reduced and slower soil P fixation occurring in very dry soil conditions experienced last year.

Where crops failed in 2015 and soil P status is at the maintenance phase, 2016 P application rates of up to 5 kilograms of P/hectare should be considered.

On soils in the “build up phase”, growers who have been experiencing dry conditions and low yields are advised to apply a level of P that can be afforded.

Normally, a major proportion of P taken up by crops is derived from the soil reserves. Only about 20% is derived from freshly-applied P fertiliser. Hence soil testing is an important way to gauge potential P supply to crops.

The residual effectiveness of fertiliser P applied in 2015 for crops in 2016 is likely to be low on calcareous soils and above average on neutral and acidic soils.

Growers planning to use P fertiliser are encouraged to do so by drilling the fertiliser with or below the seed, as this is the most efficient method of application. Broadcasting P is known to be inefficient.

Dr McLaughlin said P must be accessed by the plant in the first six weeks of growth. “That means it must be applied at sowing – it’s not feasible to apply P to the growing crop.”

Dr McLaughlin advises that it would also be worthwhile assessing the status of other plant nutrients by testing soil before sowing or plant analysis during crop growth this year. Targeting fertiliser inputs to the most deficient soils on the farm is likely to give the greatest return on investment.

More information on phosphorus management is contained this link to the GRDC Crop Nutrition Fact Sheet

Other useful resources can be found at:

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Mike McLaughlin, GRDC Southern Panel

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