MAXIMISING soil nitrogen in Mallee soils can bring big benefits by boosting yields without increasing fertiliser costs.
In a trial across seven Mallee properties involving 38 sites, the nitrogen carryover for the last three seasons has been measured at a range of grain legume and canola sites, before being compared to neighbouring intensive cereal paddocks.
Vetch has consistently resulted in the highest soil nitrogen, often contributing more than $100 a hectare when compared to the equivalent urea application cost.
The Mallee Sustainable Farming Mallee Challenge Project, funded by Caring for our Country and SA MDB NRM community grants, aims to help farmers place a value on their break crop options in terms of nitrogen input.
While legume crops are often risky to grow and may not always break even due to drought, frost and hot northerly winds, their rotational value increases when N contribution, as well as grass and disease control, is taken into account.
While these results are not from replicated trial plots, and are therefore subject to paddock variations, they are from real farming experience, and are mostly comparing crops from the same or neighbouring paddocks. While numbers may vary between seasons, soil types and situations, there appears to be some consistent trends and interesting outcomes worth paying attention to. This project has just presented soil N results from the top 50 centimetres on loamy soils, as many of these have high subsoil constraints that tend to distort the result with historically accumulated unavailable N. On the sandy soils, however, 0-80cm comparisons have been made.
As there is always natural N mineralisation each year from the soil organic matter, the value of the extra pulse N has been estimated by measuring the soil N in the following autumn, and then subtracting the N level produced from a neighbouring area with a history of multiple cereal crops.
Vetch consistently produced between 57 kilograms a hectare and 85kg/ha more N than cereal paddocks across six sites and two seasons, with an average N value of $88/ha. Peas have been the next best, producing an average of 37kg/ha more N with a value of $48 contribution over two sites. Lentils, lupins and chickpeas each showed very low levels of extra soil N contribution at sites last season.
The trial found that brown manuring vetch reduced N fixation.
At two different farms last season, vetch left growing to maturity produced an average of 31kg/ha more N than vetch that was brown manured. It would appear from all the sites that even where vetch was reaped, heavily stocked or cut for hay, it still produced far more nitrogen by actively growing through spring, than was taken out the gate in produce. While brown manuring vetch can be an important tool for stopping grass seed set, if good control has already been achieved through an early grass selective spray, it may be far more beneficial to let vetch grow though to maturity.
Results indicated that N carryover continued in the second year.
At one Pinnaroo farm, nitrogen levels were measured after both a cereal/canola sequence and a vetch/canola sequence. The paddock where the vetch preceded the canola had 28kg/ha more N than the canola after cereal, even after yielding 0.6t/ha higher. This is certainly consistent with the rule of thumb that 20-30 per cent of the N contribution of a legume crop will become available in the second year.
Other break crops studied included canola and oaten hay crops.
Over three sites last season, nitrogen after canola was very similar to that after cereal; however, another site resulted in 28kg/ha less N. At Pinnaroo a 4.75t/ha oaten hay crop resulted in 30kg/ha less N than after a nearby cereal crop.
* Full report in Stock Journal, October 9, 2014 issue.
