Long-term effects of soil management treatments on soil physical conditions in a factorial·citrus experiment

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Summary Long-term effects of soil management and ammonium sulphate treatments on soil physical conditions in a citrus factorial experiment were examined in terms of infiltration and moisture characteristics. Permanent sod had the highest infiltration rate, followed in turn by winter- clover, winter-tick bean, and bare-surface chemical weed control treatments. Despite inevitable compaction from heavy harvesting and spraying machinery, it was found that infiltration rates for non-tilled plots receiving permanent sod or bare-surface treatments were almost identical to those reported 17 years previously when the experiment had been in progress for only 4 years. The fact that the deterioration in soil physical conditions initially observed on herbicide-treated plots was not further aggravated by long-term usage is of considerable importance when assessing possible harmful effects of herbicides on soil structure. Numerous citrus roots were found near the surface of herbicide-treated plots. These roots, which were particularly profuse in under-tree areas, contrasted markedly with the relative scarcity of roots found near the surface of plots receiving other soil management treatments. It is considered that these near-surface roots tended to compensate for any lack of organic matter normally supplied by surface vegetation and at the same time increased the tree’s efficiency in uptake of applied nutrients. Moisture characteristics, determined in the laboratory on samples taken at a depth of 0–15 cm from inter-tree areas of each plot, were not as sensitive as infiltration rates to soil management treatments. Nevertheless, moisture characteristics showed results consistent with those obtained by infiltration measurements; samples from permanent sod showed better structure stability and had higher moisture-holding capacities than samples from the bare-surface chemical weed control treatments. Increasing rates of ammonium sulphate tended to increase the infiltration rate; this effect was mainly evident on tilled plots. Moisture characteristics also showed a similar beneficial response to nitrogen applications. It is suggested that this response was due to the release of calcium sulphate, a well known soil stabilizing agent, as a result of soil acidification induced by cumulative ammonium sulphate applications. In 1947 an experiment was established at Griffith, in the Murrumbidgee Irrigation Areas of New South Wales, to assess the effects of four soil management treatments and four levels of nitrogen fertilizer application on citrus productivity. The treatments were applied to trees which had been planted in 1941. It was found (Cary, 1968) that the highest yields and the best quality fruit were obtained from adequately fertilized, non-tilled herbicide- treated plots. This paper describes the long-term effects of these treatments on soil physical conditions, as evident from infiltration rate measurements and soil water content/suction relationships. Physical measurements, including infiltration rates, had been carried out on the same trial in 1951 by Greacen and Perkman (1953). Thus it was possible to determine whether any discernible change had occurred over the period 1951 to 1968. An explanation is offered as to why the non-tillage bare-surface treatment proved best.

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