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Summer Rainfall Triggers Nitrogen Windfall

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Sustainable Industries

This year’s wet summer has provided a nitrogen windfall for many broad acre farmers in the Wheatbelt. The massive rainfall events of late January and February, where falls of more than 100 millimetres were recorded, has triggered a revision of the Nutrient Use Efficient project.  The revision studies the impact that massive summer rainfall events on soil nutrients, with a focus on nitrogen. The Nutrient Use Efficiency Project held workshops with the 20 farmers involved in the project during early March to discuss the results of their early soil testing.

During these workshops a groundswell of interest emerged among the growers around: what impact would the massive summer rainfall events have on the nitrogen levels in the soil; did mineralisation occur or was all of the nitrogen leached down the profile? Previous research has indicated a two-inch rainfall event can leach soluble nutrients out of the immediate root zone.  However 200 to 500 grams of nitrogen per hectare per day can be generated through mineralisation in wet soil after significant summer rainfall. Initial soil testing was done in December on 120 sites over 20 farms using a soil corer to a depth of 50 centimetres. These sites were then re-tested in May to compare the pre and post rainfall soil test results.

Mineralisation occurs when soil bacteria convert nitrogen oxides and other nitrogen sources into nitrate and ammonium forms, which are plant available. Results from the re-testing of the sites found there was a significant amount of mineralisation of nitrogen across most of the sites after the summer rainfall events. Some sites however, mainly those with sandy soils, showed a definite leaching of nitrogen down the profile with some nitrogen leaching lower than the 50 centimetre sampling depth.  Some of these sites then received a top up of mineralised nitrogen in the top layer.  Other sites lost nitrogen due to late spraying of weeds.  Weed growth on unsprayed sites was stimulated by the summer rain which then drained much of the available nitrogen within the soil. The majority of sites however, did show an increase in the nitrogen levels with at least 2 sites obtaining more than 100kg/ha of nitrogen from mineralisation.

The aim of the Nutrient Use Efficiency Project is to give farmers more information about their soils before they start planning their fertiliser program. These results have reinforced the importance of farmers testing nutrition levels as close to seeding as possible to achieve a more accurate understanding of crop requirements. The Avon, Dale and Mortlock Rivers contribute over ⅔ of the nitrogen and over ⅓ of the phosphorous load into the Swan River.  With fertiliser costs at around 25% of budget for cropping programs, capacity to reduce application without adversely effecting yield makes strong economic sense.

The Nutrient Use Efficiency Project continues to work with the current group of participants, currently collecting of tissue samples and testing for microbiological activity within the soil. Grain testing will also be carried out at the same sites in the lead up to harvest. The Nutrient Use Efficiency Project is funded by the Australian Government’s National Landcare Program and the WA-based Department of Biodiversity, Conservation and Attractions - Rivers and Estuaries Division.