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SOIL BIOLOGICAL FACTORS LIMIT THE EARLY GROWTH OF DIRECT-DRILLED WHEAT

S. Simpfendorfer1, J.A. Kirkegaard2 and D.P. Heenan1

1NSW Agriculture, Wagga Wagga Agricultural Institute, Wagga Wagga, NSW 2650
2CSIRO Division of Plant Industry, Canberra, ACT 2601

Abstract

The poor early growth of direct drilled (DD) wheat was studied in intact soil cores collected from 27 farm sites throughout south-eastern NSW. Cores from each site were subjected to a factorial combination of methylbromide fumigation and cultivation. Wheat seedlings were then grown in the cores with good nutrition and in a controlled environment resembling that of a cool moist winter in southern NSW. The response to soil disturbance indicated that reduced early vigour was present in DD cores collected from 12 out of the 27 sites. The response to fumigation further demonstrated that at eight out of these 12 sites the poor early vegetative growth was primarily due to the presence of deleterious biological factors.

Key words: Reduced early growth, wheat, direct drill, fumigation, deleterious soil microorganisms.

Minimum tillage and stubble retention have been widely promoted throughout the grains industry as a key component of conservation cropping systems designed to sustain soil fertility and crop production. The benefits of such conservation cropping practices in reducing soil erosion, increasing soil organic matter, and improving soil structure have been repeatedly demonstrated (reviewed by Hamblin (4)). However, many producers are still reluctant to direct drill (DD) wheat due to concerns over crop yields. Although many factors may be contributing to the slow rate of adoption, a consistent problem across a wide range of soil types and environments has been reduced early vegetative growth of DD wheat compared to seed sown into conventionally cultivated seed-beds (1,3). Studies have found that the reduced early growth could be completely overcome by soil fumigation which indicates that soil microorganisms are responsible for reducing the growth (2, 5). However, these findings have been based on data from two long-term field trials in south-eastern NSW at Cowra (2) and Harden (5). We aimed to determine the occurrence of reduced early growth in DD wheat throughout a wider area of south-eastern NSW and investigate the soil microorganisms involved.

Materials and methods

Twenty intact soil cores (200 mm deep + 80 mm diameter) were collected from 26 different farm sites throughout south-eastern NSW and the following treatments imposed on five replicate cores: 1) no cultivation or fumigation (DD); 2) hand cultivation to 10 cm without fumigation (C); 3) no cultivation with methylbromide fumigation (Fum DD); and 4) hand cultivation plus fumigation (Fum C). Two seedlings of Janz wheat were established in each core and grown under temperature and light controlled growth chamber conditions (11oC and 12 h day) with frequent application ¼ strength Hoagland's solution. Seedling vigour was assessed around 30 days after emergence along with rhizosphere populations of total bacteria and fungi, pseudomonads, bacilli and actinomycetes.

Results

Figure 1

Hand cultivation demonstrated that reduced early vigour was present in DD soil cores collected from 12 out of the 27 farm sites (Fig. 1). Fumigation indicated that this reduced vigour was largely due to physical factors at four of these sites whilst the remaining eight sites had biological factors restricting the early vegetative growth of DD seedlings. At these eight sites, populations of total pseudomonads were elevated in the rhizosphere of DD wheat seedlings compared to those growing in cultivated soil (data not shown).

Figure 2

Fumigation by itself resulted in a dramatic increase (51%) in the growth of DD wheat seedlings averaged across the 27 farm sites. At only three farm sites (all with canola stubbles), did wheat not have reduced early vegetative growth in DD cores compared with the Fum DD treatment (Fig. 2). This indicates that deleterious soil microorganisms which restrict the early growth of DD wheat may be widespread throughout southern NSW and their effect is not totally removed by soil disturbance.

Discussion

Fumigation with methylbromide results in the sterilisation of the intact soil cores without altering the physical structure of the soil. Nutrient release does not account for the increased growth response commonly observed after fumigation (6). The significant growth response to fumigation in the DD and a number of C cores is therefore a result of the removal of soil microorganisms deleterious to the early growth of wheat seedlings. These deleterious microorganisms were found to restrict the early growth of DD wheat at 24 out of the 27 farm sites examined.

Conclusions

This study demonstrated that poor early vegetative growth in DD wheat is not a phenomenon restricted to the long-term experiments at Harden and Cowra. The response to cultivation indicated that poor early vigour due to the presence of biological constraints was evident in soil cores collected from eight of the 27 field sites. The presence of increased populations of pseudomonads in the rhizosphere of DD seedlings also appeared to be associated with the reduced early vigour at these sites. Work is continuing to identify the exact role of these bacteria and other microorganisms in the poor early vigour of DD wheat. The response to fumigation further indicates that such biological constraints to early wheat growth are very widespread throughout southern NSW.

Acknowledgments

We gratefully thank the Grains Research and Development Corporation for funding and technical assistance provided by Angus Ingram and Graeme Heath.

References

1. Chan, K.Y., Mead, J.A. and Roberts, W.P. 1987.  Aust. J. Agric. Res. 38, 791-800.

2. Chan, K.Y., Mead, J.A., Roberts, W.P. and Wong, P.T.W. 1989. Aust. J. Agric. Res. 42, 221-228.

3. Cornish, P.S. and Lymbery, J.R. 1987. Aust. J. Exp. Agric. 27, 869-880.

4. Hamblin, A.P. 1987 In: Tillage: New Diections for Australian Agriculture. (Eds P.S. Cornish and J.E. Pratley) (Inkata Press: Sydney). pp. 128-170.

5. Kirkegaard, J.A., Munns, R., James, R.A., Gardner, P.A. and Angus, J.F. 1995. Aust. J. Agric. Res. 46, 75-88.

6. Simpfendorfer, S., Kirkegaard, J.A. and Heenan, D.P. 1997. Proc. 11th Aust. Plant. Path. Soc. Conf. Perth, p 43.

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