RF Brennan A, MDA Bolland B and JW Bowden C. Abstract. Introduction. Material and methods

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1 Post canola syndrome: burning swathed canola stubbles enables potassium deficiency and induced molybdenum deficiency and aluminium toxicity to be expressed in the next cereal crop RF Brennan A, MDA Bolland B and JW Bowden C A Department of Agriculture, 444 Albany Hwy, Albany, WA 6330, Australia. B Department of Agriculture, PO Box 1231, Bunbury, WA 6231, Australia. C Department of Agriculture, PO Box 483, Northam, WA 6401, Australia. Abstract Nutrient omission experiments determined the cause of waves of good and bad growth in wheat and barley crops after swathed canola crops on acidified sandy soils in south-western Australia. Potassium deficiency, and molybdenum deficiency and aluminium toxicity induced by soil acidification, were identified as major problems. Burning canola swaths increased bicarbonate-extractable soil potassium by mg/kg, and as measured in 0.01 M CaCl 2, increased soil ph by of a ph unit and decreased aluminium extracted from soil by 1 6 mg/kg. Root lesion nematodes were not identified as a problem. Keywords: molybdenum, potassium, windrowing, stubbles. Introduction This study was undertaken to determine the cause of waves of good and poor growth that appeared in wheat (spring wheat, Triticum aestivum L.) and barley (Hordeum vulgare L.) crops grown after swathed canola (rape, Brassica napus L.) crops on acidified formally marginally acidic to neutral sandy soils in south Western Australia. To prevent reductions in seed (grain) yields, canola crops are swathed and grain is subsequently harvested from the swaths when the grain is mature. Usually, 6 8 metre wide strips of canola biomass is cut and concentrated into metre wide rows (swaths). Hence, about 80% of the crop biomass is concentrated into swaths that make up 20% of the original cropping area. When the swath is harvested for grain, most of the material that passes through the harvester falls back onto about where the original swath was located. Before the start of the next growing season, the swaths are usually burnt. When the next crop emerges, usually a cereal crop, waves of good growth and poor growth soon become evident. The good growth occurs on top of the old burnt swath rows, and the poor growth occurs on the area of the previous canola crop cut to make the swath. Therefore, about 20% of the cereal crop shows good growth, and 80% shows poor growth. Possible reasons for this phenomenon included nutrient element deficiencies, and/or a liming effect of the burnt canola stubble in the swaths reducing problems caused by acidification of the soils, and root lesion nematodes. We did nutrient omission experiments to assess if nutrient element deficiencies and soil acidity reduced growth of the next wheat or barley crops after swathed canola crops when the swath was burnt after harvesting canola grain. The plots were laid out across several adjacent burnt swaths and areas that were cut to make the swath of the canola in the previous year. Two types of experiments were done. The experiments were either started at the start of the growing season and fertiliser treatments were applied when the experimental plots were sown to barley or wheat. Alternatively, the experiments were laid out onto wheat and barley crops sown by farmers after a swathed canola crop in the previous year. These latter experiments were started after waves became apparent, about four to five weeks after the wheat or barley was sown. Material and methods The thirty-three experiments were located at sites in farmer paddocks that had grown a canola crop in the previous year, and the canola crop was swathed to harvest grain. The soil samples were collected from under burnt canola swath rows and from the adjacent areas from where the previous canola crop was cut to make the swath. The following were measured on the soil samples: th Australian Research Assembly on Brassicas Conference Proceedings

2 ph (1:5 soil: 0.01 mol CaCl 2 ), hereafter called soil ph organic carbon bicarbonate-extractable phosphorus bicarbonate-extractable 0.01 M CaCl 2 extractable aluminium sulphur extracted by 0.25 M KCl heated at 40ºC for three hours (KCl-40 extracted soil sulphur). Soil properties are listed in Table 12. The experiments were placed on four adjacent burnt swath rows and the three cut areas between the four swath rows of the previous canola crop. The width of the burnt swath rows was m. The width of the cut areas was 6 8 m. So each plot was two metres wide and from m long. The Type I experiments comprised 10 treatments randomly allocated to four replications. The 10 treatments were: Control, no treatment applied. All nutrients applied (N, P, K, S, Cu, Zn, Mn, and Mo) (hereafter called ALL). ALL with N omitted (ALL-N). ALL with P omitted (ALL-P). ALL with K omitted (ALL-K). ALL with S omitted (ALL-S) ALL with Cu omitted (ALL-Cu). ALL with Zn omitted (ALL-Zn). ALL with Mn omitted (ALL-Mn). ALL with Mo omitted (ALL-Mo). Treatments for the Type II experiments were the same, except there was no ALL-P treatment. This was because the experiment was put on a wheat or barley crop sown four to five weeks previously and the farmer had drilled phosphorus while sowing the crop so a minus phosphorus treatment was not possible. The amounts of each nutrient element applied in the Type I and II experiments were: 80 kg nitrogen/ha as Urea 20 kg sulphur/ha as gypsum (17% S) 50 kg potassium/ha (KCl, 50%K) 15 kg phosphorus/ha as triple superphosphate (20% P, negligible S) Micronutrients were applied as foliar sprays when wheat or barley was at the 6 th leaf stage. The following micronutrients were applied: 0.75 kg copper/ha as copper oxychloride (52% Cu) 0.5 kg zinc/ha Zn sulphate (22% Zn) 1.0 kg manganese/ha as Mn sulphate (25% Mn) 0.03 kg molybdenum/ha as Na molybdate (39% Mo) In the first year (1999) a nematicide (Nemacur, active ingredient 100 g/kg Fenamiphos) treatment was included in the Type I experiments only. This was achieved by either adding no nematicide or adequate nematicide (100 kg/ha) to each of the 10 treatments listed above for the Type I experiments. The following were measured in Type I and II experiments: yield of dried shoots at the boot stage (Gs59) grain yields at maturity. For each treatment plot, samples to measure shoot and grain yields were collected and kept separate from two areas. In three experiments, there were large differences in shoot and grain yields of barley for samples collected from under the swath area and in the cut areas of the previous canola crop. No nutrient element deficiencies were identified in the barley crops. For these three experiments, soil samples were taken at several locations in both the burnt swath and cut areas of the previous canola crop to measure aluminium extracted from the soil using 0.01 M CaCl 2. The soil extractable aluminium values were related to the corresponding yield of dried shoots. Table 12: Soil properties for the 33 sites measured on soil samples of the <2 mm fraction of the top 10 cm collected before the experiments began from under the burnt swath and the area cut to make the swath of the previous canola crop Burnt swath Cut area Soil property Range Median Range Median ph Colwell P (mg/kg) Colwell K (mg/kg) KCl-40 S (mg/kg) Organic carbon (%) Extractable Al (mg/kg) th Australian Research Assembly on Brassicas Conference Proceedings 35

3 Results This is a summary of the major findings. In all years, the burnt swath area of the previous canola crop produced % higher wheat and barley shoot and grain yields than the cut areas of the previous canola crop. All differences in yield were significant (P <0.05). Potassium was the major deficiency identified at 23 sites, representing 70% of experiments (Table 13). Soil acidity resulted in molybdenum deficiency at 16 sites, comprising 29% of Type I experiments and 54% of Type II experiments (Table 13). Potassium was identified as the only deficiency at 8 sites, representing 24% of the experiments (Table 14). Molybdenum was the only deficiency at three sites, representing 9% of the experiments (Table 15). However, most potassium and molybdenum deficiencies occurred concurrently at eleven sites. Other elements identified as deficient at a total of eight sites, phosphorus, sulphur and copper, always occurred concurrently with potassium and molybdenum deficiencies (Table 16 to Table 19). At three sites, all sown to barley, though barley shoot and grain yields in the poor growth area were 30 60% lower than the good growth area, no nutrient element deficiencies were identified in the poor growth areas. Aluminium toxicity resulting from low soil ph was evidently responsible for the poor barley growth at these sites (Figure 3). Root lesion nematodes (Pratylenchus spp.) were found in small numbers in soil samples collected from experimental plots. Nematicide in 1999 had no effect on shoot and grain yields of wheat or barley. Table 13: The percentage decrease in grain yield of cereal on the cut area where a particular nutrient was omitted. The yield of ALL nutrients on the swath is taken as 100% for both Type I and Type II experiments. 30 experiments showed a significant decrease in yield resulting from the omission of K, Mo, S, Cu and P. Range (%) Median (%) Number of times a deficient element was identified 1 Significant responses (%) Type I Nil All ALL-P ALL-K ALL-N ALL-S ALL-Cu ALL-Mo ALL-Mn ALL-Zn Type II Control All ALL-K ALL-N ALL-S ALL-Cu ALL-Mo ALL-Mn ALL-Zn As there was often more than a single nutrient effect at some sites so the total of the experiments in this column for either Type I or Type II is not equal to the number of trials done. 2 In Type I experiments a nil nutrient treatment was included. 3 For Type II experiments, a range of various N P compound fertilisers were applied when the farmer sowed wheat or barley four to five weeks before the Type II experiments were started on the established farmer crop th Australian Research Assembly on Brassicas Conference Proceedings

4 Table 14: Shoot and grain yields of wheat from the burnt swath and cut areas of the previous canola crop at sites where K deficiency was the only deficiency identified (24% of experiments). Control All ALL-K Control All ALL-K Soil K ph Control was a basal of 14.9 kg P/ha at seeding, 3.8 kg S /ha at seeding, 14.9 kg N/ha in the compound fertiliser and the remaining urea topdressed to the soil surface to give a total of 43.7 kg N/ha applied. 2 Relative yield was calculated by dividing the yield for each treatment by the yield of the ALL treatment on the swath areas, so that, by definition, relative yield is 1.00 for the ALL treatment on the swath. 3 Measured on samples of the top 10 cm of soil collected before the start of the experiment. Details of concurrent deficiencies are: at two sites, both molybdenum and sulphur deficiencies occurred concurrently (Table 16). at two sites potassium, molybdenum and copper deficiencies occurred concurrently (Table 17). at two sites potassium, molybdenum and sulphur deficiencies occurred concurrently (Table 18). finally, at two sites potassium, molybdenum and phosphorus deficiencies occurred concurrently (Table 19). Note that molybdenum deficiency was involved at all eight of these sites, and potassium deficiency also occurred at seven of the eight sites. In both Type I and Type II experiments, omission of either manganese or zinc had no effect on shoot and grain yields. Table 15: Shoot and grain yields of wheat on the burnt swath areas and the cut areas of the previous canola crop at sites where Mo was the only deficiency identified (3 experiments). Control ALL ALL-Mo Control ALL ALL-Mo Soil K ph Control was a basal of P at 15.2 kg P/ha at seeding, 5.7 kg S /ha at seeding, 11.2 kg N/ha in the compound fertiliser and the remaining urea topdressed to the soil surface a total of 48.7 kg N/ha applied. 13 th Australian Research Assembly on Brassicas Conference Proceedings 37

5 Table 16: Shoot and grain yields (t/ha) of wheat on the burnt swath and the cut areas of the previous canola crop at sites where Mo and S were the only deficiencies identified (2 experiments). Also listed are relative yields and soil properties. Control All ALL -Mo ALL -S Control All ALL -Mo ALL -S Soil K ph Al Soil S Control was a basal of P at 15 kg P/ha at seeding, 5.5 kg S /ha at seeding, 11.0 kg N/ha in the compound fertiliser and the remaining urea topdressed to the soil surface a total of 50 kg N/ha applied. Table 17: Shoot and grain yield of wheat from the burnt swath and cut areas of the previous canola crop at sites where wheat responded to K, Mo and Cu deficiencies were identified (2 experiments). Also listed are relative yields and soil properties Control ALL ALL-K ALL-Cu ALL-Mo Control ALL ALL-K ALL-Cu ALL-Mo Soil K ph Al Control was a basal of P at 16 kg P/ha at seeding, 14.4 kg N/ha in the compound fertiliser and the remaining urea topdressed to the soil surface a total of 42 kg N/ha applied th Australian Research Assembly on Brassicas Conference Proceedings

6 Table 18: Shoot and grain yield of barley and wheat measured on the burnt swath or cut area of the previous canola crop at a site where K, Mo and S were the only deficient elements identified (2 experiments). Control ALL ALL-K ALL-S ALL-Mo Control ALL ALL-K ALL-S ALL-Mo ph Al Soil K Soil S Control was a basal of P at 13.2 kg P/ha at seeding, 5.5 kg S /ha at seeding, 10.4 kg N/ha in the compound fertiliser and the remaining urea topdressed to the soil surface a total of 42.2 kg N/ha applied. Table 19: Shoot and grain yields of barley measured on burnt swaths and cut areas of the previous canola crop at a site where P, Mo and K deficiencies were identified (3 experiments). Also listed are relative yields and soil properties. Nil ALL ALL-P ALL-Mo ALL-K Nil ALL ALL-P ALL-Mo ALL-K ph K P th Australian Research Assembly on Brassicas Conference Proceedings 39

7 6.0 (a) (b) 8 Extractable Al (mg/kg) Dry weight of shoot (t/ha) Soil ph Soil Al Figure 3: The relationships between (a) amount of Al extracted from the soil and soil ph, both measured using 0.01 M CaCl 2 ; and (b) yield of dried barley shoots and the amount of Al extracted from the soil by 0.01 M CaCl 2. Data are for a Type I experiment that indicated no nutrient element deficiency affected yields. Discussion Potassium deficiency was identified at most sites. However, potassium deficiency, due to removal of potassium in product, became apparent in sandy duplex soils of the Great Southern region of Western Australia and large grain yield responses of wheat to applied potassium were obtained on these soils in that region (Wong et al. 2000). We have shown that potassium deficiency has now extended to sandy soils in other cropping regions of Western Australia. The redistribution of potassium in the canola biomass cut to make the swaths concentrated potassium taken up by canola plants into swaths that occupied about 20% of the cropping area, so depleting potassium from the approximate 80% of the crop area cut to make the swaths. When the swaths were burnt after harvest, bicarbonate soil test potassium increased under the swath. Soil acidification was the other major problem identified in our study. Soil ph was of a ph unit higher under the previous burnt canola swath than in the area of the previous canola crop cut to make the swath. In addition, aluminium extracted from the soil was consistently lower under the burnt swath than in the adjacent cut areas. We attribute the increase in soil ph below the burnt swath to the burning of the plant material, producing an alkaline ash that was an effective source of lime that ameliorated soil ph and aluminium toxicity relative to the adjacent areas cut to make the swath. When first cleared, most sandy soils in Western Australia had soil ph values of (McArthur 1991). However, as these soils have subsequently acidified, soil ph has decreased below 4.5 and molybdenum deficiency has been induced. Therefore, in our study, when soil ph in the cut area of the previous canola crop was , molybdenum deficiency was identified in the following wheat or barley crop, mostly in association with potassium deficiency. The best solution for the induced molybdenum deficiency is to lime the soil. At three acidified sites, aluminium toxicity was the only problem, there being no concurrent potassium and molybdenum problem as found at most acidified sites in our study. This may have been because aluminium toxicity was so severe at these sites root growth and development was so poor little fertiliser potassium and molybdenum were taken up by the plants so the treatments were ineffective. This problem can be diagnosed by measuring soil ph and extractable aluminium in the same procedure (1:5 soil:0.01 M CaCl 2 ), and applying lime to ameliorate aluminium toxicity. In addition to potassium and molybdenum deficiencies, either sulphur, or copper, or phosphorus was also deficient at some sites. Sulphur deficiency may be due to more widespread use in the region of low sulphur containing fertilisers, such as triple superphosphate and mono and di-ammonium phosphates. Copper deficiency has been induced in the region by increasing use of fertiliser-nitrogen for cropping. Phosphorus deficiency was a major deficiency for newly cleared soils in the region. However, annual applications of fertiliser phosphorus are common, and fertiliser phosphorus has a good residual value, so phosphorus deficiency is uncommon, unless inadequate phosphorus has been applied. Soil testing for phosphorus provides a reliable indication of likely deficiency th Australian Research Assembly on Brassicas Conference Proceedings

8 The distribution of root lesion nematode (Pratylenchus spp.) has been examined in soils and on roots of plants grown on a range of soil types and environments in Western Australia (Riley and Kelly 2002). Canola is a known host for P. neglectus and numbers of root lesion nematode are known to increase in soil after canola crops (Sharma et al. 2001). However, only small numbers of root lesion nematode were found in soil samples collected from our experiments. Conclusions We have shown that potassium deficiency and soil acidity (molybdenum deficiency and aluminium toxicity) are major problems for growing wheat and barley crops on sandy acidified soils in the cropping areas of Western Australia. For these soils, farmers need to: monitor soil ph values and apply lime when soil ph falls to 5.0, so preventing molybdenum deficiency and aluminium toxicity from reducing grain yields. regularly soil test for potassium, sulphur and phosphorus apply fertiliser potassium when Colwell soil test potassium is <50 mg K/kg of soil apply fertiliser sulphur when KCl-40 soil test sulphur is <10 mg S/kg soil apply fertiliser phosphorus when Colwell soil test phosphorus is <30 mg P/kg soil. Acknowledgments Funds were provided by GRDC and the cereal program of the Department of Agriculture of Western Australia. We thank chemists of the Chemistry Centre for soil analysis. Technical assistance was provided by F O Donnell, T Hilder and R Lunt. References McArthur WM (1991) Reference soils of south-western Australia. (Australian Society of Soil Science, WA Branch Inc.: Perth WA) Riley IT, Kelly SJ (2002) Endoparasitic nematodes in cropping soils of western Australia. Australian Journal of Experimental Agriculture 42, Sharma S, Wright D, Loughman R (2001) Root lesion nematodes in Western Australia: Hidden constraints to profitable crop production. Farmnote 04/2001 (Department of Agriculture: South Perth, W A). Wong MTF, Edwards NK, Barrow NJ (2000) Accessibility of subsoil potassium to wheat grown on duplex soils in the south-west of Western Australia. Australian Journal of Soil Research 38, th Australian Research Assembly on Brassicas Conference Proceedings 41