The problem of soil compaction in forestry differs from that in agriculture because of differences in the nature of the crop, in particular the weight and size of the plant members and the length of time that they persist. The roots compact the soil as they increase in size, but they also transmit the weight of the tree and forces generated by the wind onto the soil. There are important differences in management practices; in forestry modern harvesting machines apply heavy loads and, for reasons of cost, tend to be kept in operation throughout the year. As a consequence the structure of the soil suffers some damage, often manifested as compaction. Compaction arising from such sources may reduce the growth of the current trees or trees subsequently planted on the site. But it is difficult to predict the extent of such reduction, if any, because of the complex of interactions involved. Important factors concerned, namely, the soil water regime and the organic matter content, are emphasized. A conceptual model is proposed as a predictive tool. The mechanics of soil compaction, the effects of compaction on the physical properties of the soil, and techniques for the prevention and amelioration of compaction of forest soils, are discussed.

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Laboratory tillage of soils causes an increase in the oxygen uptake of soil microorganisms. The effect is closely related to the extent of aggregate disruption caused by the tillage treatment and is attributed largely to exposure of organic matter that was previously inaccessible. In some soils drying and rewetting after tillage causes a further increase in microbial activity, but this is not accounted for by disaggregation. As drying and rewetting commonly follow tillage in the field the resultant higher microbial activity could contribute to the faster mineralization of organic matter in arable land.

The mechanical strength of sandy soils under radiata pine plantations was measured with a penetrometer. Resistance to penetration was largely independent of water content in the range sampled, and was directly related to the bulk density of the soil. Soil strength at constant bulk density increased with depth owing to increase in overburden pressure and to a decrease in soil organic matter. Soil under native scrub was consistently less compact than that from adjacent radiata pine plantations on the same soil type. Soil from pasture was usually more compact in the surface 20 cm of soil than that from pine plantations, but was less compact at depth. Soil from second rotation plantations was more compact than soil on some first rotation sites, but on other sites no differences could be established. Radiata pine roots preferentially penetrated areas of lower soil strength. Root penetration was severely restricted above a critical penetration resistance of about 3000 kPa. Saturated soils were highly compacted even by light loads in a laboratory consolidometer compared to unsaturated soil. In the unsaturated condition compaction was greatest under heavy loads on soils at about 1% organic matter. Causes of the observed compaction in the field are discussed and remedial measures are suggested. Soil compaction reduced porosity but had little effect on water storage capacity. Increased organic matter at constant bulk density also reduced porosity, but greatly increased water storage capacity and unsaturated hydraulic conductivity. The importance of organic matter in maintaining a favourable structure in sandy soils and its relation to maintenance of site productivity is discussed.

Fine metal probes are being used to simulate the penetration of plant roots into soil. Existing soil mechanics theories on the bearing capacity of piles were used to estimate the point resistance of these probes but were found to be unsatisfactory. It is considered that this was due to the fact that sufficient emphasis is not given to soil compression by these theories. A new theory is proposed that is based on the model of an expanding sphere in a plastic-elastic medium at the point of the probe. Compression of the soil to accommodate the volume of the probe is assumed to occur in two main zones, a zone of compression with plastic failure surrounding the probe and, outside this, a zone of elastic compression. In estimating the point resistance to the penetrometer, the mean contact pressure on the conical surface of the point has been assumed to be equal to the pressure required to form a cavity large enough to take the probe. For three soils ranging from a loam (12% clay) to a clay soil (48% clay) at three bulk densities and two water contents the calculated point resistances were on the average within 10% of the values determined by experiment.