Quantitative models to predict the effects of soil compaction on wheat yields are being developed for the northern sandplains of Western Australia. An understanding of the relationships between soil water content (W), bulk density (p), compactibility and soil penetration resistance (P) is required. Thirteen subsoils from W.A. sandplain soils were tested for compactibility. As the amounts of very coarse sand or clay in the soil increased, the maximum density (�max.) achieved with a standard compactive effort also increased, while the critical soil water content (Wcrit,.) for maximum compactibility declined. The effects of p and W on P were investigated for five of the soils. The value of P was only slightly affected as W was reduced to less than 70% of the field capacity water content. As the soils were dried further, P increased exponentially. At all water contents, an increase in p was found to markedly increase P. Particle size distribution could be used to predict �max. and Wcrit., but could not be related to the effects of changes in p and W on P. The implications for the measurement and effects of soil compaction in the field are discussed.

The relationships between soil penetration resistance and the growth and yield of wheat were examined for a range of tillage and compaction experiments conducted on earthy sands near Geraldton, W.A. Overall, a single index of penetration resistance explained around 50% of the growth and yield variation, across sites and seasons. Equations using this index showed good potential for predicting the impact of various tillage and traffic practices on wheat yield.

The effects of the density of lupin (Lupinus angustifolius L. cv. Illyarrie) taproots on a following wheat crop (Tritium aestivum L. cv. Gutha) were investigated on a compacted, earthy sand soil near Geraldton, Western Australia. In 1985, plots were sown to lupins at densities ranging from 35 to 220 plants m-2. Because peak lupin biomass varied by less than 25%, and 100 kg N ha-1 of mineral fertiliser was supplied to the wheat, the effects of variation in residual N from the lupins were considered to be minimal. In 1986, all plots were split for shallow or deep tillage and wheat sown. There were no effects of tillage or lupin density on stored soil water at sowing. Growth and yield of wheat on the shallow tilled plots increased linearly with lupin plant density in the previous year, to equal or better wheat yields on the deep tilled plots, which were unaffected by lupin density. At normal farm lupin plant densities, the biological plough effect was estimated to improve wheat yields by 100 kg ha-1, substantially less than the benefits from nitrogen fixation and breaks in disease cycles. Nevertheless, it still suggests that lupin stand densities should be maintained or increased in crop rotations on compacted sands.

The effects of soil compaction and deep ripping on the growth and yield of crop (wheat, barley, oats, triticale, narrow leaf lupins and field peas) and pasture species (barrel medic and subterranean clover) were investigated for deep, sandy soils near Geraldton, Western Australia. In 1984 (an average rainfall season), growth and yields of all species were substantially reduced by soil compaction. Lupins were not included in the experiment. Barley, wheat and pea yields were reduced by around 45%; oat and triticale yields by 30%; and spring biomass of both pasture species was reduced by about 30%. The differences in response between the species were not significant (P>0.05). In 1985 (a very dry year), amelioration of soil compaction by deep ripping increased the dry matter at flowering of all the species, including lupins, by about 30%. Severe water stress in the cereals after flowering prevented the conversion of this dry matter advantage into grain yield differences. Because of late flowering and slow maturation, the yield of the oats was significantly (P<0.05) lower on the deep-ripped areas, compared with the yield on the compacted soil. Deep ripping increased the yield of peas by 60% and lupins by 20%, probably because flowering and seed filling were completed before the onset of severe water stress. For deep, sandy soils in mediterranean environments, maximum economic benefit would normally accrue from deep ripping prior to the cereal phase, given that current agronomic principles generally preclude the growing of peas on these soils. Selection of cereal species mainly depends on relative yields and prices; however, the results suggest that use of cultivars with rapid early growth, early flowering and quick maturity would maximise the response to compaction amelioration and minimise the risk of poor grain filling.

The phytotoxicity of the pre-emergence herbicides metolachlor, pendimethalin, and propachlor to heading lettuce was investigated in 5 experiments conducted in southern Queensland during 1990-91. Metolachlor was applied before transplanting lettuce seedlings in 2 experiments. On a krasnozem soil at Toowoomba, spraying >1.75 kg a.i./ha reduced lettuce growth and the number of marketable lettuce heads. When applied on a black earth at Gatton, 1.44 kg a.i./ha stunted the lettuce and reduced the number of marketable heads by 30% compared with hand-weeded treatments. On both soils, 2.2-2.9 kg a.i./ha of metolachlor is recommended for broadleaf weed control in other crops. The risks of phytotoxicity from such rates preclude its use in transplanted lettuce in this environment. Lettuce were severely stunted from application of 1 kg a.i./ha of pendimethalin after transplanting. Spraying at the same rate before transplanting initially reduced lettuce width in 1 of 4 experiments, but the lettuce recovered. Lettuce yields from areas treated with 1 kg a.i./ha before transplanting were unaffected. The number and size of harvested heads tended to decline where >1.3 kg a.i./ha was applied. Higher rates (up to 1.5 kg a.i./ha) may be acceptable on krasnozem soils. Use of pendimethalin at 1-1.3 kg a.i./ha would control or suppress many important broadleaf weeds in lettuce, as well as a range of grasses. Propachlor was less phytotoxic when applied immediately after transplanting the lettuce seedlings than when sprayed before transplanting. Yields were unaffected from application of 2-2.5 kg a.i./ha after transplanting on black earth and sandy soils. When sprayed before transplanting, >2 kg a.i./ha caused significant yield reductions in several experiments, although lettuce grown on a krasnozem soil appeared more tolerant. Use of 2 kg a.i./ha of propachlor would give a suppressive effect against several broadleaf weeds, but another herbicide would also be required to achieve effective weed control. A spraying strategy involving pendimethalin and propachlor herbicides in transplanted lettuce has the potential to reduce weed control costs by up to 80%.