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WHEAT PRODUCTION AND SOIL CHEMICAL PROPERTIES OF ORGANIC AND CONVENTIONAL PAIRED SITES IN WESTERN AUSTRALIA

A. Deria, R.W. Bell, and G. O’Hara

Division of Environmental Science, Murdoch University, Murdoch, WA 6150

Summary. Wheat production levels and soil chemical properties on eight paired adjacent fields managed organically and conventionally in Western Australia were monitored and compared in three years (1992-1994). Sites were paired to ensure soil types, crop history before conversion of the organic paddock and, in most cases, the farm managers were the same, so that the management system was the primary object of difference. At four sites (1, 3, 5, 6), the grain yield of organic and conventional wheat was comparable, but grain yield of organic wheat was significantly depressed at the other four sites. The yield depression in the organic plots was most likely related to the lower pre-sowing Colwell-extractable phosphorus level in the soil, later sowing, and low nitrogen supply. In the organic plots, nitrate nitrogen and organic carbon had increased significantly after 2-3 years of pasture in site 3, but Colwell-P and Colwell-K were greatly reduced during this period.

INTRODUCTION

Organic farming systems have been considered as an alternative to conventional farming systems in Europe and North America, where the farmers achieved comparable financial returns. Although the yield had been depressed in the organic system by 10-15%, this was offset by decreased costs of production (1, 2). These results may not be relevant to South West Australia where soils are often highly weathered and contain low nutrient reserves (3).

The aim of this study was to compare wheat production and soil chemical properties of paired sites organically and conventionally managed in Western Australia.

MATERIALS AND METHODS

A wheat field at each of eight certified organic farms in the WA wheatbelt was selected (Table 1). Each site was paired with a nearby wheat crop on the same soil type and with similar cultivation history apart from the recent conversion of the organic sites. Organic sites were generally cropped after 2-3 years pasture whereas the conventional sites were cropped 2-3 years in the previous four. Except for sites 2 and 4 all the other paired sites were on the same farm and under the same management. At each site, four 20x20 m plots were selected for measurement of soil properties and wheat production.

RESULTS

The grain yields at sites 4, 7 and 8 were significantly higher in the conventional fields than the organic. The yield of organically grown wheat was lower by about 64%, 24% and 30%, respectively, at these sites. At site 2, oats was sown instead of wheat and the grain yield of both organic and conventional plots was low, reflecting that both soils and extractable P levels regarded as deficient for wheat (5). The higher yield obtained from the organic plots may indicate better adaptation of oats of the low P soils than wheat (1). Overall, wheat yield was depressed 15% in the organic plots compared to those in paired conventional plots (Fig 1). This yield decrease was most likely related to lower pre-sowing Colwell-P in soil, later sowing and decreased nitrogen.

Table 1. General characteristics of organic and conventional paired sites compared in this study

Sites

Farm Type

Soil Type

Crop Rotation (1987-1992)

Type of fertilisera

Colwell-P (mg/kg)

1

Organic

Red earth

PPPC

Nil

28

1

Conventional

Red earth

CCCP

DAP, Urea

21

2

Organic

Red

PPPW

Nil

3

2

Conventional

Calcareous

CCCP

DAP, Urea

3

   

Red

     
   

Calcareous

     

3

Organic

Red earth

PPWPP

Dynamic lifter

19

3

Conventional

Red earth

PWWW

DAP, Urea

19

4

Organic

Red

WPWWP

Nil

8

4

Conventional

Calcareous

WPWW

DAP, Urea

11

 

Red

       
 

Calcareous

       

5

Organic

Red earth

WPPPW

Nil

30

5

Conventional

Red earth

WPWWW

Agras No.1

50

6

Organic

YellowDuplex

PPPC

Dynamic lifter

10

6

Conventional

YellowDuplex

PPCC

DAP, Urea

19

7

Organic

Red earth

PPPW

Nil

19

7

Conventional

Red earth

PPPW

DAP, Urea

39

8

Organic

YellowDuplex

PPPC

Dynamic lifter

48

8

Conventional

YellowDuplex

PPCC

Dap, Urea

28

a) DAP: Diammonium phosphate, 17.5% N, 20% P. Dynamic lifter; 3.2% N, 2.8% P,

Urea; 46% N Agras No 1; 17.5% N

b) P: Pasture, C: crop, W: wheat.

Figure 1. Grain yield of eight paired sites organically and conventionally managed in WA wheatbelt.

ACKNOWLEDGMENTS

The authors thank Grains Research and Development Corporation for funding this study (1991-1996).

REFERENCES

1. Lockeretz, W., Shearer, G., Koli, D.H. and Klepper, R.W. 1984. In: Organic Farming: Current Technology and its Role in Sustainable Agriculture. ASA Spec. Publ. No. 46, pp. 37-49.

2. Stanhill, G. 1900. Agric. Ecosystems Environ. 30, 1-26.

3. Stace, H.C.T., Hubble, G.D., Brewer, R., Northcote, K.H., Sleeman, J.R., Mulachy, M.J. and Hallsworth, E.G. 1968. A Handbook of Australian Soils. (Rellim Tech.. Publications: Glenside, South Australia).

4. Perry, M. and Hillman, B. (Eds) 1991. The Wheat Book. A Technical Manual for Wheat Producers. WA. Dept. of Agriculture, Bulletin No. 4196.

5. Bell, W.R., Deria, A., Kanabo, I. and Rowdon, J. 1994. In: 15th World Congress of Soil Science, 10-16 July 94, Transactions, Volume 5b: Commission 1V: Poster Session.

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