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The tropical environment generally allows 2 crops/year to be grown. Controlled traffic has been suggested as a means of improving soil conditions, which may also lead to increased crop yield. A field trial at Millaroo Research Station, North Queensland, on a cracking clay (Entic Chromustert) studied the effect of controlled traffic (in conjunction with direct drilling and tillage) and conventional ridging on soil properties and crop yield. Maize (Zea mays L. cv. Hybrid 50) was grown as the winter crop and soybean [Glycine max (L.) Merr. cv. Canapolis] as the summer crop. With few exceptions, there was no significant difference between any pair of treatments in sowing line water content, bulk density, aggregate size distribution, seedling emergence, mean time of seedling emergence, and final yield. Differences that did occur between crop cycles were due to climatic variation. Cone index measurements indicated no lateral spread of compaction from the traffic lanes in the controlled traffic system to the soil in the plant growth area. Under the ridged area, however, it appeared that a plough pan began to develop just below the depth of tillage. Although no marked benefit in soil properties or plant yield resulted from controlled traffic, it was possible to grow 2 crops/year for the duration of the experiment. In one season, only the controlled traffic treatments could be planted, due to unsuitable conditions for seedbed preparation. Double cropping under conventional cultivation systems is unreliable, due to the limited opportunity for seedbed preparation at the beginning of the wet season and the large number (up to 8) of operations required to prepare a seedbed. Controlled traffic, restricting soil compaction to the traffic lanes, is a system that helps to maintain a zone more favourable for plant growth, as indicated by the cone index measurements.

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