Publisher Summary This chapter reviews the cycling of nutrients within pasture soils. The major emphasis is on the central role of the grazing animal in influencing soil fertility, particularly in the dung and urine patches. In addition, the progress is reviewed that has been made in modeling the cycling of nutrients in pasture systems. Nutrients are partitioned differently between dung and urine, with K being excreted mainly in urine; P, Ca, and Mg are excreted principally in dung and N and S are excreted in both forms. Nutrients returned in excreta can be in inorganic and organic forms, depending on the particular nutrient being considered. For some nutrients (e.g., P and S) significant mineralization of ingested organic forms occurs during passage through the animal, whereas much of the N is excreted in the readily available organic urea form. From the present knowledge of the major nutrient inputs and losses for the grazed pasture system and an understanding of the pathways of nutrient flux within the system and some key measurements, simple mass balance nutrient models for various pasture systems can be constructed. Such simple models have been used to calculate site-specific maintenance fertilizer requirements of pastures based on the amount of nutrient required to replace losses in the soil (e.g., through fixation and leaching) and losses by animal transfer and in animal products.

Nutrient cycling and soil fertility in the grazed pasture ecosystem

2 – Soil Acidification, its Measurement and the Processes Involved

Effects of landscape fragmentation on ecosystem processes in the Western Australian wheatbelt

Long-term changes in some chemical properties of yellow podzolic soils under subterranean clover pastures were studied near Binda, New South Wales. The rate of decrease in pH of the surface 10 cm of soil was greatest in the early years following pasture establishment, and decreased with time. Over 50 years the decrease in pH was approximately one unit, resulting in current soil pH values of about 5.0. The increased acidity had led to increases in CaCI,-extractable manganese and exchangeable aluminium. Although the build-up of soil organic matter under a 32- year-old subterranean clover pasture was largely confined to the surface 10 cm, the decrease in pH extended to a depth in excess of 30 cm. Total soil nitrogen increased gradually throughout the 50-year period, reflecting increases in the soil organic matter. There were associated increases in cation exchange capacity and titratable acidity. Total soil phosphorus also increased with time and was correlated with the decrease in pH. This reflected the accumulation of organic and inorganic residues from phosphate fertilizers, but it seems unlikely that the acidity of the superphosphate contributed directly to the increase in soil acidity although it is essential for the establishment and maintenance of the legume, the main source of the increase in soil organic matter. It seems likely that the area of pasture soils adversely affected by acidity problems will increase in the future.

Soil acidification under clover pasture

Solution culture and greenhouse studies have both clearly demonstrated the ability of legumes to acidify their rooting medium. Furthermore, research workers comparing the pH beneath undisturbed sites versus all-legume pastures or all-grass versus all-legume (or grass-legume) pastures have observed a lower soil pH under the leguminous pastures. The processes leading to legume-induced soil acidification are reviewed and discussed.



The growth of legumes which are fixing atmospheric N2 involves the excess uptake of nutrient cations over anions from soil solution. This results in the net efflux of H3O+ ions from plant roots into the rhizosphere.



When virgin lands are sown with legumes the accumulation of soil organic matter, with a consequent increase in cation exchange capacity and exchange acidity, is an important contributing factor to the long-term decline in surface soil (0–10 cm) pH. Nonetheless, such a phenomenon does not explain the decrease in pH below 10 cm soil depth nor the lower pH below leguminous than all-grass pastures. The efflux of H3O+ ions from the legume roots may have an important effect on the soil pH under such conditions. The loss of symbiotically fixed N; from the system through leaching of NO3− - N may also contribute to soil acidification under leguminous pastures.

Soil acidification induced by leguminous crops

The total nitrogen content of the soil was significantly increased at each of four levels of superphosphate applied to a densely sown subterranean clover pasture on a coarse-textured sand at Kojonup, W.A. During the first 4 years of the experiment nitrogen build-up was similar for all four treatments, but over the 11 year term the build-up was significantly slower at the lowest level of superphosphate. The relationship between the age of the pasture and soil nitrogen was linear at the lowest level of fertilizer but curvilinear at all other levels. The relationship between total superphosphate applied and soil nitrogen was curvilinear. There were no differential effects of the fertilizer treatments on either the bulk density or the pH of the soil. At the two lower levels of superphosphate a high percentage of both phosphorus and sulphur was retained in the major plant root zone (0–10 cm) of the soil. At the two higher levels most of the phosphorus was retained but there were heavy losses of sulphur. The results of pot trials with soil taken from the experiment at the end of the 11 year period showed that the different levels of superphosphate had induced large differences in the phosphorus status of the soil but had had much smaller effects on the sulphur status. The possible effect of this loss of sulphur on the accumulation of soil organic matter is discussed.

ER Watson

Papers

The influence of subterranean clover pastures on soil fertility. III. The effect of applied phosphorus and sulphur