Mystery solved by soil tests

The Brokenshire family planted new lucerne pasture on their Mount Compass dairy farm on land cleared of a blue gum plantation several years earlier.

The pasture initially established well but distinct strips of stronger lucerne growth were apparent from the first spring.

The stronger growth coincided with the old blue gum tree lines, with much weaker impoverished lucerne growth in the former inter-rows, with the strips much more noticeable on the sandiest sections. 

The Brokenshires were surprised by the variation in new lucerne growth as the paddock had received good rates of super, potash and trace element fertilisers.

DairySA arranged for soil testing of topsoils (0-10cm) but found no significant difference in topsoil pH or nutrient levels between the good and bad strips. It was only when deep soil samples (40-50cm) were taken that much higher phosphorus levels in the old tree lines were identified.

As a result, the Brokenshires have a better understanding of the underlying reason for the strips of better lucerne growth and will focus on robust phosphorus fertiliser rates.

Tim Orr manages the Burnfoot beef grazing farm at Myponga and also saw marked variation in lucerne on sandy ground previously under blue gums.

Plant tissue tests proved a good way to identify nutrient deficiencies in this 18-month-old lucerne stand and Mr Orr will be focusing on fertilisers that contain more potassium as a result, even considering chicken litter as a way to deliver a better balance of nutrients in a less leachable form for his sandy pasture soils.

Objective nutrient testing of plant tissue and soils at different depths gives farmers better insights into pasture nutrient requirements, enabling more cost effective use of fertilisers.