CROPS GROWN after lucerne in a paddock are likely to be supplied with nitrogen for three to five years, but nitrogen release may be slower than expected.
Mark Peoples of CSIRO Plant Industry says this compares with a nitrogen supply of only one or two years to crops following an annual legume pasture such as a subclover.
"The relative size of the nitrogen benefit obtained after lucerne will depend upon a combination of the density and productivity of the lucerne, and the duration of the pasture phase," he says. "Apart from supplying crops with nitrogen, lucerne can also aid weed control and improve the porosity and structure of soil.
"In research near Temora in southern NSW, where there was a physically restrictive layer below the soil surface, lucerne and another perennial pasture plant, phalaris, were grown in rotation with annual crops. Researchers found that not only did more lucerne roots penetrate deeper into the subsoil than phalaris or canol a, but the channels (macropores) they made were also wider.
More effective water-holding capability
"The effect of these structural differences was highlighted by water infiltration measurements conducted. The large macropores present after the lucerne pasture improved permeability of the subsoil. With water being applied at 20 mm per hour, it took six minutes after a canola crop for the soil to become waterlogged, 19 minutes after phalaris, and 61 minutes after lucerne.
"Wheat roots have been seen in and around old lucerne root channels at the trial site and there was enhanced Water extraction by wheat sown following lucerne."
Dr Peoples says that while lucerne is an efficient nitrogen producer, farmers should not assume that all of the nitrogen it produces will become immediately available to crops following the pasture phase.
Drier soils affect soil organism activity, N release
"Because lucerne is verY efficient at extracting water in the soil, the soil is much drier following lucerne than after an annual pasture," he said. "These drier conditions reduce the activity of soil microbes which break down plant residues, and so it takes longer for the nitrogen to be released from these residues.
"As well, lucerne residues in the soil are likely to be more resistant to decomposition than residues from annual pasture species."
Program 4 Contact: Dr Mark Peoples 02 6246 5244