What will it take to restore soil fertility?
A series of nutrient trials conducted in high rainfall cropping zones in 2016 is showing high levels of nutrient removal with high yields. Lead researcher on the project, Dr Malcolm McCaskill, Soil Research Scientist with DEDJTR Victoria, shares a preview of the results with Agronomy Community members and asks: what will it take to restore soil fertility for 2017?
Download this article
We have seen some eye popping nutrient removals recently in a new joint research project between GRDC, Agriculture Victoria, Southern Farming Systems, the MacKillop Farm Management Group and SARDI.
In some cases, the full replacement of these losses would require over half a tonne per hectare of urea and a quarter of a tonne per hectare of muriate of potash.
The project is examining the responses of wheat and canola to various nutrients and includes an ‘all nutrients’ and a ‘nil nutrients’ comparison.
As part of our work, we sent samples of grain and stubble from the all and nil treatments to the Nutrient Advantage laboratory for analysis.
This exercise showed us that nutrient quantities in grain are highly variable.
We believe it is worth collecting grain samples at harvest and arranging nutrient balance analysis. Relying on standard values may lead to over or under fertilisation.
The following information is a selection of the data, showing nutrient replacement budgets for the all and nil treatments.
In estimating nutrient replacement, we have used literature values to estimate the proportion of stubble nutrient lost during burning, but all other values are from field measurements.
Inverleigh, Victoria
This site was highly fertile before sowing. A wheat yield of 9.8 t/ha at 13.6% protein was achieved in the all treatment, compared with 9.6 t/ha at 10.9% protein for the nil treatment.
Nutrient replacement was estimated at 254 kg/ha of nitrogen, 26 kg/ha of phosphorus and 114 kg/ha of potassium for the all treatment.
Due to the site’s high soil fertility, it would not be necessary to replace all this nutrient through fertiliser in the following year. Nevertheless, soil mineral nitrogen to a depth of 60 cm declined from 390 kg/ha of nitrogen prior to sowing to 163 kg/ha of nitrogen after harvest on the all treatment and to 109 kg/ha of nitrogen on the nil treatment.
Table 1: Nutrient replacement budgets following wheat at Inverleigh, Victoria, 2016
|
Crop component |
Dry matter (t/ha) |
Nutrient (kg/ha) |
|||||
|
N |
P |
K |
S |
Cu |
Zn |
||
|
“All” treatment |
|||||||
|
Grain |
9.8 |
203 |
24 |
36 |
13 |
0.055 |
0.155 |
|
Stubble |
10.8 |
62 |
5 |
196 |
11 |
0.057 |
0.039 |
|
Replacement if burned |
254 |
26 |
114 |
20 |
0.077 |
0.171 |
|
|
“Nil” treatment |
|||||||
|
Grain |
9.6 |
159 |
22 |
33 |
10 |
0.098 |
0.108 |
|
Stubble |
10.3 |
37 |
3 |
111 |
6 |
0.043 |
0.037 |
|
Replacement if burned |
189 |
23 |
77 |
14 |
0.115 |
0.123 |
|
Note: All = 133 kg/ha of N, 50 kg/ha of P, 50 kg/ha of K, 20 kg/ha of S, 1.1 kg/ha Zn and 2 kg/ha of Cu. Nil = 30 kg/ha of N. Wheat grain yield is shown at 12% reference moisture, and stubbles at 100°C drying.
Bool Lagoon, South Australia
The canola trial at Bool Lagoon showed significant (about double) yield responses in the all treatment compared with the nil treatment.
The site was inundated for over four weeks, restricting nutrient uptake. A final nitrogen application was used in the all treatment shortly after the inundation finished.
Replacement nutrients for the all treatment were estimated at 103 kg/ha of nitrogen, 15 kg/ha of phosphorus and 13 kg/ha of sulphur. These nutrients would need to be replaced for the 2017 growing season.
Replacement potassium was not recommended because no potassium responses were observed at the site and the soil showed high levels of available potassium (> 1000 mg/kg).
Table 2: Nutrient replacement budgets following canola at Bool Lagoon, SA, 2016
|
Crop component |
Dry matter (t/ha) |
Nutrient (kg/ha) |
|||||
|
N |
P |
K |
S |
Cu |
Zn |
||
|
“All” treatment |
|||||||
|
Grain |
1.9 |
63 |
13 |
17 |
6 |
0.219 |
0.060 |
|
Stubble |
5.0 |
49 |
6 |
73 |
11 |
0.005 |
0.027 |
|
Replacement if burned |
103 |
15 |
46 |
13 |
0.221 |
0.071 |
|
|
“Nil” treatment |
|||||||
|
Grain |
1.0 |
27 |
6 |
8 |
2 |
0.003 |
0.022 |
|
Stubble |
2.2 |
11 |
2 |
32 |
7 |
0.002 |
0.014 |
|
Replacement if burned |
36 |
7 |
21 |
7 |
0.004 |
0.027 |
|
Note: All = 251 kg/ha of N, 50 kg/ha of P, 50 kg/ha of K, 20 kg/ha of S, 1.1 kg/ha of Zn and 2 kg/ha of Cu. Nil = 30 kg/ha of N. Canola grain yield is shown at 6% reference moisture, and stubbles at 100°C drying.
Tarrington, Victoria
The all treatment resulted in a high yielding canola crop (5.4 t/ha) on the well-drained site at Tarrington near Hamilton last year. The nil treatment yielded 2.4 t/ha.
We estimated replacement nutrients for the all treatment at 247 kg/ha of nitrogen, 28 kg/ha of phosphorus, 73 kg/ha of potassium and 44 kg/ha of sulphur.
Post-harvest mineral nitrogen samples (0-60 cm) showed soil nitrogen levels had been depleted to 88 kg/ha of nitrogen beneath the all treatment, compared with 380 kg/ha of nitrogen before sowing.
Unless the harvested nutrients are replaced in 2017, crop yields may be similar to the nil treatment of 2016.
Table 3: Nutrient replacement budgets following canola at Tarrington, Victoria, 2016
|
Crop component |
Dry matter (t/ha) |
Nutrient (kg/ha) |
|||||
|
N |
P |
K |
S |
Cu |
Zn |
||
|
“All” treatment |
|||||||
|
Grain |
5.4 |
176 |
26 |
36 |
19 |
0.247 |
0.174 |
|
Stubble |
10.8 |
87 |
5 |
92 |
37 |
0.005 |
0.076 |
|
Replacement if burned |
247 |
28 |
73 |
44 |
0.249 |
0.205 |
|
|
“Nil” treatment |
|||||||
|
Grain |
2.4 |
78 |
10 |
15 |
8 |
0.161 |
0.075 |
|
Stubble |
7.4 |
39 |
2 |
71 |
21 |
0.004 |
0.027 |
|
Replacement if burned |
109 |
11 |
44 |
23 |
0.162 |
0.086 |
|
Note: All = 215 kg/ha of N, 50 kg/ha of P, 50 kg/ha of K, 20 kg/ha of S, 1.1. kg/ha of Zn and 2 kg/ha of Cu. Nil = 30 kg/ha of N. Canola grain yield is shown at 6% reference moisture, and stubbles at 100°C drying.
Rutherglen, Victoria
Prolonged waterlogging followed by rapid drying of the soil during grain fill led to low canola yields, a low harvest index and small grain sizes in the Rutherglen trial.
Large amounts of nitrogen from the all treatment at this site are thought to have been leached or denitrified. Soil testing to a depth of one metre post-harvest showed there was only 29 kg/ha more mineral nitrogen under the all treatment than the nil treatment. Most of this was in the 60-80 cm layer.
Where there is a risk of nitrogen being leached through the profile, the safest place for nitrogen is in the plant and applied early.
Table 4: Nutrient replacement budgets following canola at Rutherglen, Victoria, 2016
|
Crop component |
Dry matter (t/ha) |
Nutrient (kg/ha) |
|||||
|
N |
P |
K |
S |
Cu |
Zn |
||
|
“All” treatment |
|||||||
|
Grain |
0.7 |
23.0 |
3.9 |
5.3 |
1.8 |
0.008 |
0.017 |
|
Stubble |
3.1 |
23.1 |
2.2 |
53.4 |
4.7 |
0.002 |
0.027 |
|
Replacement if burned |
41.9 |
4.9 |
26.7 |
5.0 |
0.009 |
0.028 |
|
|
“Nil” treatment |
|||||||
|
Grain |
0.2 |
4.5 |
1.4 |
1.5 |
0.4 |
0.009 |
0.004 |
|
Stubble |
0.7 |
2.1 |
0.3 |
7.7 |
1.1 |
0.001 |
0.003 |
|
Replacement if burned |
6.2 |
1.5 |
4.6 |
1.1 |
0.009 |
0.005 |
|
Note: All = 396 kg/ha of N, 50 kg/ha of P, 50 kg/ha of K, 20 kg/ha of S, 1.1 kg/ha of Zn and 2 kg/ha of Cu. Nil = 30 kg/ha of N. Canola grain yield is shown at 6% reference moisture, and stubbles at 100°C drying.
Summary
A focus on restoring fertility will be essential for many growers with high yielding crops in 2016.
Even in low yielding crops, like the Rutherglen site affected by waterlogging, there may have been significant leaching losses depleting mobile nutrients such as nitrogen and sulphur, or nitrogen losses through denitrification.
Across our two wheat trials, nutrient replacement was estimated at 22 kg N/tonne grain, 3 kg P/t and 8 kg K/t. This includes an allowance for nutrient losses during stubble burning.
Across our four canola trials, nutrient replacement was estimated as 42 kg N/t grain, 6 kg P/t and 20 kg K/t.
Nutrient removal rates (kg/t) in our study varied almost two-fold, so grain testing is advisable to understand your balance. If that is not possible, arrange soil testing to quantify nutrient depletion.
Want to know more?
For any additional information, feel free to send me an email at Malcolm.McCaskill@ecodev.vic.gov.au.
Acknowledgements
Funding for this project is from GRDC, Agriculture Victoria and IPNI. The research team includes Amanda Pearce (SARDI, Struan), Aaron Vague (Southern Farming Systems, Inverleigh), Brendan Christy (Agriculture Victoria, Rutherglen), Rob Norton (IPNI, Horsham), and Penny Riffkin (Agriculture Victoria, Hamilton).