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Selection of plant cultivars tolerant of low nutrient supply may increase productivity on low fertility soils and reduce fertilizer requirements. Considerable effort has been directed towards identifying 'nutrient efficient' species and germplasms, but the many different definitions for efficiency make the use of the term ambiguous. The concept of nutrient efficiency was evaluated using data from a study of differences in germplasm response to phosphorus (P) availability in white clover (Trifolium repens L.) and alfalfa (Medicago sativa L.) grown in a sand-alumina culture. Application of various criteria identified in the literature as measures of nutrient efficiency did not clarify differences between purportedly P efficient and inefficient germplasms. Germplasms differed in maximum shoot and total dry mass and in solution P concentration required to achieve 80% maximum yield, but not in tissue P concentration, internal P utilization, or P uptake per unit of fine root dry mass. Differences may have resulted from factors other than efficient use of available P. To reduce the confounding effects that other factors have on nutrient efficiency, we propose that equivalent yields of germplasms be demonstrated where nutrients are not limiting. Mechanisms that enable enhanced nutrient efficiency can be identified less ambiguously using this improved approach.

Abstract. Efficient and effective nutrient management decisions are critical to profitable and sustainable milk production on modern Australian dairy farms. Whole-farm nutrient balances are commonly used as nutrient management tools and also for regulatory assessment on dairy farms internationally, but are rarely used in Australia. In this study, nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) imports and exports were measured during a standardised production year on 41 contrasting Australian dairy farms, representing a broad range of geographic locations, milk production, herd and farm size, reliance on irrigation, and soil types. The quantity of nutrients imported varied markedly -with feed and fertiliser generally the most substantial imports -and were principally determined by stocking rate and type of imported feed. Milk exports were the largest source of nutrient exports. Nitrogen balance ranged from 47 to 601 kg N/ha.year. Nitrogen-use efficiency ranged from 14 to 50%, with a median value of 26%. Phosphorus balance ranged from -7 to 133 kg P/ha.year, with a median value of 28 kg P/ha. Phosphorus-use efficiencies ranged from 6 to 158%, with a median value of 35%. Potassium balances ranged from 13 to 452 kg K/ha, with a median value of 74 kg K/ha; K-use efficiency ranged from 9 to 48%, with a median value of 20%. Sulfur balances ranged from -1 to 184 kg S/ha, with a median value of 27 kg S/ha; S-use efficiency ranged from 6 to 110%, with a median value of 21%. Nitrogen, P, K and S balances were all positively correlated (P < 0.001) with stocking rate and milk production per ha. Poor relationship between P, K and S fertiliser inputs and milk production from home-grown pasture reflected the already high soil fertility levels measured on many of these farms. The results from this study demonstrate that increasing milk production per ha will be associated with greater nutrient surpluses at the farm scale, with the potential for greater environmental impacts. We suggest that simplified and standardised nutrient balance methodologies should be used on dairy farms in Australia to help identify opportunities for improvements in nutrient management decisions and to develop appropriate industry benchmarks and targets.

Soil phosphorus (P) buffering capacity (PBC) is an important soil property that influences the amount of P fertiliser available for plant uptake. However, current methods of determining PBC are time-consuming and uneconomic in most commercial soil testing programs. The current study examined simpler methods of measuring the PBC of a wide range of Australian soils. Phosphorus sorption and extractable P data from 290 soils (initial data set) were collated to define the range of PBC values of Australian agricultural soils. Independently, detailed chemical and physical analyses were undertaken on a second set of 90 agricultural soils (principal data set), which were selected to represent the range of soil properties measured on the initial data set. Relationships between PBCO&S (Ozanne and Shaw 1968) values (P sorbed between solution P concentrations of 0.25 and 0.35 mg P/L) and 11 different single-point P sorption indices and selected soil properties were examined for the principal data set. Whilst relationships between PBCO&S values and selected soil properties such as oxalate-extractable iron and aluminium, and clay content, were generally poor, strong relationships existed between all of the single-point P sorption indices and PBCO&S. Results suggest that PBCO&S values were most closely related to the P buffering indices (PBI+ColP and PBI+OlsP) when a single addition of 1000 mg P/kg was added to soil and either the Colwell or 4.59 Olsen extractable P were added to the amount of P sorbed: PBI+ColP = (Ps + Colwell P)/c0.41 PBI+OlsP = (Ps + 4.59 Olsen P)/c0.41 where Ps is the amount of P sorbed (mg P/kg) from a single addition of 1000 mg P/kg, and c is the resulting solution P concentration (mg P/L). This index provides a simple and accurate method for estimating PBC, a fundamental soil property that influences the P fertiliser requirements of different soil types. phosphorus sorption capacity, single-point phosphorus sorption index, phosphorus retention index, soil properties, Colwell phosphorus, Olsen phosphorus.

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