Increased concern about environmental degradation and a move towards sustainable farming systems has lead to closer attention being paid to farm dairy effluents (FDE). Treatment of FDE in New Zealand is mainly through land application, or through oxidation ponds. Since the introduction of the Resource Management Act, 1991, regional councils require dairy farmers to be more accountable for the management of effluent from their dairy farms. Regulations have been imposed to limit the application of nitrogen (N) to land from FDE, and these limits range from 150 to 200 kg N ha -1 yr -1 . Farmers, consultants, and regional councils require information on the chemical composition, particularly N content, of effluents, so that land effluent application systems can be designed and managed within the guidelines or regulations imposed. Data gathered from previous investigations on effluents in New Zealand found an average solids content of 0.9% dry matter. Between 1977 and 1997 the mean N content of FDE doubled from approximately 200 to 400 mg N I -1 . The trend for higher N concentrations is likely to continue as dairy herd numbers increase. The most likely reason for the increase in N is that volume of washwater used per cow has proportionately decreased as herd size has increased, thus, FDE has become more concentrated with levels above 400 mg N I -1 increasingly common. Average values of phosphorus (P) and potassium (K) in FDE were 70 and 370 mg I -1 , respectively. Slurries obtained from anaerobic ponds, which require periodic desludging about every 5 years, had average nutrient concentrations of 1650, 290, and 510 mg 1 -1 for N, P, and K, respectively, representing an accumulation of minerals over the 5 years.

Summary Solution cadmium (Cd) concentrations and sorption and desorption of native and added Cd were studied in a range of New Zealand soils. The concentration of Cd in solution and the concentrations and patterns of native soil Cd desorbed and added Cd sorbed and desorbed varied greatly between the 29 soils studied. Correlation analysis revealed that pH was the most dominant soil variable affecting solution Cd concentration and sorption and desorption of native and added Cd in these soils. However, organic matter, cation exchange capacity (CEC) and total soil Cd were also found to be important. Multiple regression analysis showed that the log concentration of Cd in solution was strongly related to soil pH, organic matter and total Cd, which in combination explained 76% of the variation between soils. When data from the present study were combined into a single multiple regression with soil data from a previously published study, the equation generated could explain 81% of the variation in log Cd solution concentration. This reinforces the importance of pH, organic matter and total Cd in controlling solution Cd concentrations. Simple linear regression analysis could at best explain 53% of the total variation in Cd sorption or desorption for the soils studied. Multiple regression analysis showed that native Cd desorption was related to pH, organic matter and total Cd, which in combination explained 85% of the variation between soils. For sorption of Cd (from 2 μg Cd g–1 soil added), pH and organic matter in combination explained 75% of the variation between soils. However, for added Cd desorption (%), pH and CEC explained 77%. It is clear that the combined effects of a range of soil properties control the concentration of Cd in solution, and of sorption and desorption of Cd in soils. The fraction of potentially desorbable added Cd in soils could also be predicted from a soil’s Kd value. This could have value for assessing both the mobility of Cd in soil and its likely availability to plants.

The effects of soil pH on the desorption of native soil cadmium (Cd), and on the sorption and desorption of added Cd at low concentrations, have been examined for 6 New Zealand soils ranging from pH 4·9 to 6·2. The effect of contact time with added Cd on subsequent desorption from soil has also been studied. Cadmium desorption was determined by repeated equilibrations in 0·01 М Ca(NO3)2 solution. Cadmium sorption ranged between 38% and 96% from an initial addition of 2 µg Cd/g soil. The effect of increasing soil pH was to increase substantially the amount of Cd sorbed. Sorption isotherms were all linear, with a negative intercept on the y-axis. Sorption data also fitted a linearised Freundlich sorption equation. Cadmium desorption was also very sensitive to pH, with a dramatic reduction in the amount of native Cd desorbed from the soil as pH increased, as was observed for samples where Cd was added. The cumulative amounts of native Cd desorbed represented only a relatively small proportion (0–22%) of total soil Cd concentrations. Added Cd desorption ranged between 22% and 99% of the Cd initially sorbed on the soil at varying pH. Organic matter appeared to be the most important soil component controlling both sorption and desorption in the soils studied. In the contact period experiment, the proportion of Cd desorbed was decreased by increasing initial contact time to 70 days before desorption for all 4 soils studied. Contact time had the greatest effect on Cd desorption in soils with the highest amounts of soil oxide components. Implications of the study are that, for the soils studied, soil pH, Cd contact time, and soil organic matter content are controlling factors on Cd desorption into soil solution, and are therefore likely to play an important role in Cd phytoavailability.

The accumulation of cadmium (Cd), a biotoxic heavy metal, in the food chain is undesirable. A national survey of soils and plants and random testing of kidneys from grazing animals slaughtered for export was conducted to assess Cd accumulation in New Zealand (NZ) pastoral agriculture. Average total Cd content of pastoral soils (0-7.5 cm) was 0.44 (Xg Cd/g compared to 0.20 |xg Cd/g for "native" (non-agricultural) soils. Total soil Cd was highly correlated to total soil P. An increase in total soil P is a reflection of fertiliser history thus phosphatic fertiliser use is implicated in Cd accumulation in pastoral soils. The elevated pastoral soil Cd levels were not clearly reflected in grass or legume species but were displayed in weed species viz, 0.28 ug Cd/g c.f. 0.14 µg Cd/g for pastoral and native sites respectively. Over the period 1988-91, 22-28% of sheep and 14-20% of cattle kidneys sampled exceeded the NZ maximum residue level of 1 µg Cd/g. Kidney Cd content was highly age-related. Cadmium was also present in feral deer and feral sheep kidneys showing that Cd occurs naturally in the environment.

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A database was constructed comprising records from 2255 pasture phosphorus (P), potassium (K) and sulphur (S) field trials, of which 1799 included one or several rates of P. Subsets of this data were selected based on predetermined criteria to define the relationships between relative pasture production and available soil P (0-75 mm, Olsen P in µg P cm -3 soil)-the P production functions-for the major soil groups in New Zealand. These relationships, and their 95% confidence intervals, were defined using Bayesian statistics. For those soil groups for which there was sufficient data, the production functions were well defined and gave reasonably precise estimates of the relative pasture yield for a given Olsen P. the relative pasture production is most likely (P < 0.05) to be in the range 88-94% at Olsen P 25 and 98-100% at Olsen P 50. The shape of the production functions was similar for all soil groups-the relative pasture production increased with increasing Olsen P up to an asymptote-except the pumice soils and the podzols, which showed irregularities. The production function for the podzols was also flatter. There was good agreement between the empirically derived production functions and those generated from a dynamic P model. The Olsen P level required to achieve 97% maximum production was estimated for all soil groups. These ranged from 10 to 45 depending on soil group. The critical Olsen P levels were related to the soil anion storage capacity (ASC, a laboratory measure of P buffer capacity) and to soil volume weight (g cm -3 of sieved and dried soil), although not strongly. The field measured P buffer capacity (ΔP F )-the amount of soluble fertiliser P (kg P ha -1 ) required above maintenance to increase the Olsen P (0-75 mm) level by 1 unit-was estimated for selected trials. There was reasonable agreement between these estimates and those derived from the P model (ΔP M ), and these results indicated that ΔP decreases with increasing Olsen P. The results imply that factors other than those related to soil chemical properties affect the relationship between soil P and pasture production. The factors which determine the relationship between pasture production and soil P are defined and discussed. These were assigned to two categories: those factors which affect the ability of the soil to supply P for plant uptake and those that affect the ability of the plant to acquire soil P. It is concluded that further progress towards improving our ability to predict pasture responses to fertiliser P will depend on quantifying the latter effects. Based on these results and the development of a dynamic P model, an econometric P model was developed for New Zealand pastures which enables consultants to quantify the likely agronomic, financial and investment effects of any given fertiliser strategy on a given farm or block within a farm. This was not previously possible but is essential for the sustainable use of P fertilisers in pastoral farming.

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The effect of soil pH on plant cadmium (Cd) concentrations was investigated in a glasshouse study, in conjunction with an evaluation of eight soil extractants as predictors of Cd concentrations in different plant species. Results showed that in general, increasing soil pH from 5.5 to 7.0 significantly decreased Cd concentrations in clover, lettuce, carrot, and ryegrass, and to a lesser extent in wheat, although the magnitude of the reduction varied between plant species and soil types. Soil extractants which were sensitive to soil pH e.g., 0.05M Ca(NO 3 ) 2 , IMNH4CI, and O.O5MCaCl 2 or extract moderate amounts of Cd e.g., IA/NH4OAC and 0.04M EDTA were found to be the most effective in predicting plant Cd concentrations. Cd solubility as predicted using a semi-empirical equation which contained terms forpH, organic matter, and total Cd concentration was also found to be successful in estimating plant Cd concentrations for a number of plant species. It appears that pH may be a powerful tool in the management of plant Cd concentrations, however its true potential needs to be evaluated in a field situation.