How to manage soil fertility

28 August 2015

Pasture agronomy consultant Jim Virgona, based at Wagga Wagga, NSW has a mantra when it comes to pastures: you can’t manage what you don’t measure.

He told producers at the MLA-sponsored Grassland Society of NSW’s conference at Goulburn in July that managing fertiliser in a grazing system should be underpinned by soil tests to guide efficient input decisions.

“There are a wide variety of soil conditions in dryland pastures across the higher rainfall zone of south-east Australia, but far too many are deficient in phosphorus (P),” Jim said.

“Pastures need a critical level of P for the legume component to fix nitrogen and be productive.”

Livestock producers often face a challenge in targeting inputs due to a lack of paddock by paddock production data, so Jim advocates for collecting data on a paddock, rather than property, basis.

He takes a page out of the grain industry’s manual when he talks about the need to develop precision agriculture for pasture management, such as considering fertiliser rates at sub-paddock levels to manage soil variability.

“The P soil test results from 464 paddocks we sampled in 2014 were extremely variable, from acute deficiency right through to excess,” Jim said.

“The wide distribution of soil test values – commonly on a single property – suggests that individual paddocks must be monitored if management of soil P is to be targeted and efficient.”

Jim’s key principles for collecting and using paddock data are:

  1. Test: Take an initial comprehensive soil test which includes at least soil phosphorus (Colwell), Phosphorus Buffering Index (PBI – to calculate the critical phosphorus value for each paddock), pH and exchangeable. This might cost up to $50, but most of the in depth analyses should not need to be repeated for 5-10 years.
  2. Test again: Take simple annual soil samples (only around $15/test) to provide a representative and repeatable indication of soil P status.
  3. Test properly: Avoid sampling in sheep camps, steep sections, waterlogged areas etc. Take at least 30 samples at 10cm depths. Sample at approximately the same time every year, ideally at least four months after fertilising (Jim samples his clients’ paddocks in September/October). Send samples to an accredited lab.
  4. Don’t rely on history: Fertiliser history is not a reliable indicator of current soil fertility. For example, nutrient loss could occur through changes to grazing management which influences animal camping behaviour.

A final tip: If you want to take managing soil test data and fertiliser records a step further, consider using a mapping program or developing a Geographical Information System (GIS) for your property. Jim has developed a GIS for clients’ properties, and describes it as a system that allows data (e.g. P status) to be associated with spatial features such as paddocks and soil sampling transacts. He generates maps each year with fertiliser rates for each paddock, as a guide for contractors.


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