No abstract

The amounts of non-exchangeable or fixed NH4+ in six soils from southern New South Wales were determined by using three methods. Using a potassium hydroxide pretreatment 2-13 % of the total nitrogen in the surface soils was fixed and between 16 and 33 % in the subsoils. Different methods showed up to a threefold difference in the amount on some soils and non-significant differences in others. The interpretation of the part played by this fraction of soil nitrogen in the chemical and biological equilibrium of soil must be qualified according to the extraction method used. The use of soil samples having a larger particle size (< 2 mm) than previously used (<0.147 mm) was found to be adequate with the soils studied. As the present definition of non-exchangeable or fixed NH4+ is misleading, it is proposed that this fraction of soil nitrogen be called intercalary ammonium (NH4+). Intercalary NH4+ is defined as that nitrogen recovered from within clay minerals by hydrofluoric-hydrochloric acid treatment, subsequent to a potassium hydroxide or potassium hypobromite/potassium hydroxide pretreatment and 0 . 5N potassium chloride leaching to remove exchangeable NH4+ and organic nitrogen from the outside of the clay particles.

In a field study on four sites, soil acidity, root rot (Phytophthora clandestina), and soil phosphorus were identified as causes of 'subterranean clover decline'. Liming increased herbage and seed production at four sites, with a tendency for lime to increase herbage yields in autumn (22%) and winter (15%) but not in spring. The presence of ryegrass with clover increased total herbage yields, and reduced clover seed production, but there was no interaction with liming. Losses caused by root rot associated with P. clandestina were quantified for the first time in New South Wales. Root rot reduced survival of seedlings as well as herbage production in autumn and/or winter at three of the four sites. In the presence of the disease, lime did not improve root health or seedling survival. On two sites with high aluminium saturation of exchangeable cations (> 17%) and high phosphorus sorption index values, subterranean clover growth responded to high levels of P fertilizer. On one site, where lime increased the soil pH to above 5.5, the P sorption index was temporarily increased, and this was associated with a temporary adverse effect on herbage yields. Some possible mechanisms underlying the seasonality of lime responses are proposed and the practical implications of our findings are discussed.

Rhizobium trifolii and root nodulation were compared in acid and limed soil to investigate mechanisms by which current soil acidity on the slopes of southern New South Wales may reduce the growth of Trifolium subterraneum. Increasing soil pH increased both the rate at which Rhizobium trifolii colonized soil and the frequency of nodules/g root (NF). Thus, numbers of R. trifolii were greater in carbonate-treated soil throughout autumn and winter compared to unlimed soil. In limed soil R. trifolii were measured at 104-105/g soil as early as seedling germination, 40-200 x more than in unlimed soil. Despite this, by spring, the numbers of rhizobia in soil without lime reached large and even similar levels as occurred in limed soil. Subsequently, with the drying of soil in summer, the numbers of R. trifolii declined markedly in both untreated and limed soil. Maximum NF could not be achieved with addition of Ca without increase in pH. From the field observations and other research, a hypothetical model is proposed to explain how differences in the rates of change in rhizobial numbers due to acidity may determine nodulation (NF). Thus, it is suggested that, when there is a repetitive decline in numbers of R. trifolii to low levels in summer, their subsequent rate of increase in the following autumn influences nodule abundance. The production of DM in an acidic soil appeared to be limited by symbiotic N2 fixation since added N fertilizer removed lime response. The limitation to N2 fixation did not appear to be due to lower occupancy of nodules by Rhizobium strains with less potential for N2 fixation than those in limed soil. It is possible that the effect of acidity on NF influences DM, though sampling limitations made it difficult to generalize in this regard.

Six surface and subsoils known to contain native intercalary ammonium were examined to assess the importance of ammonium fixation when nitrogen fertilizers were applied, the availability of native and recently fixed intercalary ammonium to plants, and the effect of potassium on fixation. Only one soil, a grey soil of heavy texture (Ug 5.4), fixed significant amounts of added ammonium sulphate, the level of intercalary ammonium being increased by 55 % in the surface and 100% in the subsoil. The native level of intercalary ammonium was reduced by 8 and 17% by the growth of plants in the red-brown earth (Dr 2.23) and the grey soil of heavy texture, respectively, but was unchanged in the other soils. The addition of small amounts of potassium prior to the ammonium did not affect fixation; however, additions of 500 ppm reduced the fixation of a 200 ppm solution of ammonium by 80 %. Intercalary ammonium is not considered to be important in the nitrogen economy of five of the six major farming soils of the slopes and plains area of southern New South Wales. The sixth, the grey soil of heavy texture, is an alluvium associated with the Murrumbidgee River and its tributaries, and as such is used extensively for irrigated agriculture. Because this soil can fix large amounts of ammonium, the use of ammonium or ammonium forming fertilizers is not recommended.