By Damian Jones, ICC
Soil Ameliorants Trial
As the soil on the trial block is prone to slaking if overworked, this can become an issue when trying to establish canola, particularly when watering up. Calcium, either gypsum or lime, can assist in improving soil structure and hence reduce soil crusting and slaking. Gypsum is widely used as it is available locally, saving transport costs, does not aﬀect soil pH and is relatively soluble compared with lime.
In 2014, rates of gypsum, lime, gypsum/lime and PAM (polyacrylamide, which acts in a similar manner to calcium in that it helps aggregate the soil particles) were applied to the soil surface (which was crusting after pre-irrigation) and incorporated by sowing. Plant counts were taken as representing the success of the ameliorants in reducing soil crusting.
The 2014 results showed an improvement in establishment where gypsum was used at 3 t/ha over all other treatments. All other treatments except for the PAM were an improvement over the control.
To test if there was a lag in soil structure improvement from lime application, or if the gypsum treatment was only shortterm, the site was spread with GT50 canola at approximately 3.5 kg/ha with 120 kg DAP/ha and watered up.
Statistical analysis showed that there was no diﬀerence between any of the treatments, including the control. Establishment was aided by rainfall, which kept the surface moist and prevented a hard crust forming post watering up. The 2016 results are similar to those seen in 2015. No treatment provided an increased establishment rate. So apart from the results in 2014 when the treatments were applied, there has been no eﬀect. In 2014, gypsum applied at 3.0 t/ha gave the best establishment, followed by all other treatments which showed an improvement over the control (no ameliorant applied).
Stubble Incorporation Trial
Work by Clive Kirkby, a CSIRO scientist, suggests that to improve our soil carbon levels, stubbles have to be rapidly broken down by soil microbes rather than slowly decomposing on the soil surface. The risk is that by incorporating stubbles, if they do not break down quickly, they can be an issue with trash ﬂow when trying to sow into them.
Fortunately we have irrigation to provide the necessary moisture for the microbes to breakdown the stubbles. However stubble breakdown also requires enough nutrients for the microbes, in particular Nitrogen, Phosphorus, Calcium and Sulphur.
Clive’s rule of thumb is for every tonne of stubble, there needs to be 5 kg N and 3 kg P or 35 kg N/ha and 18 kg P/ha for the site in 2014.
However it was noted that shortly after emergence, the trial site appeared N deﬁcient, and topdressing with 100 kg urea/ ha quickly overcame the deﬁciency symptoms.
With this in mind, the treatments for the 2015 trial used higher rates of N in order to reduce the potential for N deﬁciency. This seemed to work at double the rate (ie 70 kg N/ha) and also showed an added beneﬁt in that a more vigorous crop was more competitive with the resistant ryegrass. Soil tests taken in March 2016 saw small increases in soil carbon on the plots with the added N but it could not be attributed to the incorporated stubble yet.
The incorporation was repeated in 2016 in two parts:
- A replicated trial comparing incorporated stubble and fertiliser and burning. These treatments then had diﬀering N budgets post sowing.
- Demonstration plots with diﬀerent stubble and N treatments
Prior to pre-irrigation in early April, the stubble, assumed to be 11 t/ha of stubble following an 8 t/ha barley grain yield, was either burnt, left standing or multi-disced and the following pre-sowing N treatments were applied (bold treatments were replicated):
- No added fertiliser and stubble incorporated
- Stubble burnt, no added fertiliser applied
- 56 kg N/ha as urea plus 32 kg P/ha as triple super, stubble standing
- 56 kg N/ha as urea plus 32 kg P/ha as triple super, stubble incorporated
- 112 kg N/ha as urea plus 32 kg P/ha as triple super, stubble standing
- 112 kg N/ha as urea plus 32 kg P/ha as triple super, stubble incorporated
- 150 kg N/ha as urea plus 32 kg P/ha as triple super, stubble incorporated
- Manure applied at 10 t/ha, stubble incorporated.
The trial/demonstration site was then pre-irrigated.
Canola was spread at 3.5 kg/ha with 120 kg DAP/ha on April 18th and watered up. The incorporated stubble provided no issue with water ﬂow.
Establishment was similar across all treatments.
The replicated trial treatment (incorporated stubble plus 112 kg N/ha and burn) plots were split in two and topdressed on June 16th with either 70 or 100 kg N/ha. All other plots received 100 kg N/ha. A second topdressing of 60 kg N/ha was applied on July 29th. This resulted in an N budget of either 215 or 245 kg N/ha. The trial was direct harvested on November 29th.
Vigour was based on a score of 0 – 9, where 9 is most vigorous. No harvest results are presented for the 150 N treatment as the plots were completely ﬂat on the ground and unharvestable.
Analysis of Stubble Treatments
Analysis of N Budget
Overall analysis of the trial
LSD: Yield = 0.53 t/ha, Oil = 1.34%
What does it mean?
The results can be interpreted in several ways, depending on what you see as the aim of retaining stubble in the cropping system; it is to avoid burning and still be able to establish canola with as few hassles as possible, or is it to utilise the stubble to build up soil carbon to produce a better structured soil?
Purely from canola agronomy perspective, the replicated trial comparing burning to stubble incorporated with 112 kg N/ha plus 32 kg P/ha showed burning the stubble gave signiﬁcantly greater yield and oil content and therefore a better $/ha return.
Looking at the demonstration results, and keep in mind they are only demonstration plots and the results may be simply due to chance, burning still gave the best yield. Looking at the ﬁgures, I would suggest the early N rates in excess of 56 kg N/ha pre-sowing, “overcooked” the canola and produced excessive vegetative growth, which then resulted in lodging that interfered with harvest.
By spreading the seed rather than sowing, we avoided the issue of trash ﬂow. We had suﬃcient stubble cover and niches for the seed to ﬁnd places to lodge and when watered up, establish suﬃcient plant numbers for a crop. The standing stubble does not present the soil microbes with a feast that requires large amounts of N and potentially results in N tie up post sowing. In our situation, the stubble height was no more than 100 mm and so there was no impact from light interception that can interfere with canola establishment in taller stubbles.
But if you were looking to try and build soil carbon, then incorporation is essential, and the cost and eﬀect on the crop are secondary to your aim (though you would probably like it to have minimal impact). The 2015 trial saw a 0.3% increase in organic carbon (beware, soil OC levels from soil tests can vary greatly even when taken from the same spot and so I cannot claim the increase is all due to the treatment) where the stubble was incorporated with N and P over the control (no added fertiliser). Now 0.3 % sounds like a trivial amount, but the maths demonstrates that it represents an increase in OC of 3300 kg/ha. So raising soil OC levels by simply retaining stubble on the soil surface is not going to happen and it is going to require considerable inputs to get there.
The stubble project – maintaining profitable farming systems in Victoria and Tasmania with retained stubble (project number BWD00024) is funded by the GRDC.
A printer-friendly version of this research report is available here.