ABSTRACT

Canola has had a very erratic history on the heavy grey self mulching clay soils (Northcote Ug5.2) in the upper central west of NSW (Dubbo, Narromine, Collie, Gilgandra). These soils are alkaline, have medium phosphorus levels and carry rosewood, myall, belah and some box timber.

Canola growth is often very unthrifty resulting in poor yields whilst wheat growing adjacent performs well.

Nutritional trials were conducted over four (4) seasons to examine the effect of P,S and Zn on canola yield and oil content. These investigations found that canola yield was extremely responsive to phosphorus applied with the seed (up to 30 kg P/ha). Very little yield or oil response occurred to sulphur. There was no response to zinc.

However phosphorus nutrition did not explain all the erratic performance. Some evidence suggests that other factors that may be affecting canola on these soils include, Blue oat mite, root lesion nematode (Pratylenchus neglectus) and sulphonylurea herbicide residues.

Introduction

Canola has had a history of very variable yield performance on the heavy self mulching grey clay loam soils in the upper central west of NSW (West of Newell highway). Some paddocks on this soil type have grown excellent canola, whilst on others canola growth has been very unthrifty, resulting in poor yields, even in good seasons. Canola generally grows very well on the red clay loam soils of central NSW.

Between 1993-98 a program was conducted to investigate whether nutrition or some other cause was responsible for this variable performance. A series of nutrition experiments investigated phosphorus, sulphur and zinc response on these soils in an area bounded by Dubbo, Narromine, Collie and Gilgandra.

Methods

These grey self mulching clay loam soils (Northcote Ug 5.2) are alkaline in the surface with pH (CaCl₂) ranging from 6.5 - 7.8. They have low-medium phosphorus levels and are timbered by rosewood, myall, belah and some narrow leaf box. They are generally capable of growing good wheat and lucerne/medic pastures.

Four rates of phosphorus (0,10,20 and 30 kg P/ha) were applied with the seed at sowing and combined with either two rates of sulphate sulphur (nil and 30 kg S/ha) and/or two rates of zinc (nil and 5 kg Zn/ha). The phosphorus source was Trifos (20%P) and the zinc source was Blueminzinc.

Plot size was 20m x 2m and treatments were replicated three or four times. A basal application of nitrogen was applied to each site depending on paddock history (range 20 - 60 kg N/ha) as urea. Mites were controlled with a residual insecticide immediately after sowing. The trials were sown with either Rainbow or Oscar varieties and harvested for grain yield and oil content.

Results

Figures 1-5 show phosphorus, sulphur and/or zinc response data from Dubbo (1993) and Collie (1995, 1997 and 1998) experiments.

Discussion

Canola yields have responded strongly to applied phosphorus in most trials up to 20 kg P/ha. This is possibly due to canola not using the VAM fungus to help extract phosphorus from the soil. Higher phosphorus rates tended to give higher oil percentage except in dry years (1997). Yield response to sulphur was not evident, apart from a small increase at higher yield levels and high P inputs in one year (1993). Higher levels of sulphur appear to be present in the subsoil on these soils and crops are probably accessing this pool. There was no response to applied zinc on these soils.

While there have been large responses to applied phosphorus, phosphorus nutrition does not explain all the erratic performance of canola on these soils.

There are a number of other factors which have emerged which could be affecting canola crops

There is now evidence that one of the root lesion nematodes (Pratylenchus neglectus) may be linked to poor performing crops on these soils. Canola is reported as being moderately resistant to Pratylenchus thornei but susceptible to P. neglectus. This nematode attacks the root system and can severely retard canola crop growth. Both nematodes have been found in moderate to high numbers on several of these heavy soil sites, where canola has performed poorly.

Testing paddocks for the presence of root lesion nematode particularly P. neglectus would appear to be an essential requirement before growing canola on these soils. On testing to date well performed canola paddocks have low RLN numbers.

Other factors such as Blue oat mite, sulphonylurea herbicide residues (canola needs a 22 month plantback), and subsoil moisture reserves (minimum 0.8 - 1m depth wet soil) can all have a major influence on canola production on these alkaline soils, however they are not considered the over-riding limiting factor.

Conclusion

Canola was found to be very responsive to phosphorus on the alkaline self mulching clay loam soils of upper central west NSW. Minimum phosphorus rates of 15-20 kg P/ha sown with the seed, should be used. Sulphur was less important than on the red soils, but some response was obtained at high P inputs and high yield levels. No response was obtained to zinc.

There is strong evidence that root lesion nematode particularly P neglectus could be affecting canola performance on these soils.

Acknowledgments

Trials were conducted on the property’s of Messrs R & B Barden, “Pine Lodge”, Collie; G & M Wiggins, “Yugilbar”, Collie and H & M Godwin, “Alton Park”, Dubbo, and their co-operation and assistance is greatly appreciated.