P. W. G. Sale

A discussion of the methods for comparing the relative effectiveness of phosphate fertilizers varying in solubility

Because various phosphate (P) fertilizers differ widely in their solubility, it is commonly observed that crop response to P fertilizers varies under the same soil and crop conditions. Furthermore, a major problem encountered in the methods for determining the relative effectiveness (RE) of water-insoluble P fertilizer (e.g., phosphate rock) with respect to water-soluble P fertilizers, e.g., single superphosphate (SSP) and triple superphosphate (TSP), is that their growth response curves are usually nonlinear and often do not share a common maximum yield. In this paper, we review and discuss the advantages and disadvantages of the three most commonly used methods for calculating the RE of phosphate rock with respect to TSP (or SSP). The three methods are vertical comparison, horizontal (substitution rate) comparison, and linear-response comparison.

Persistence and productivity of perennial ryegrass in sheep pastures in south-western Victoria: a review

Loss of perennial ryegrass (Lolium perenne L.) from the pasture within several years of sowing is a common problem in the higher rainfall (550–750 mm annual rainfall), summer-dry regions of south-eastern Australia. This pasture grass came to Australia from northern Europe, where it mostly grows from spring to autumn under mild climatic conditions. In contrast, the summers are generally much drier and hotter in this region of south-eastern Australia. This ‘mismatch’ between genotype and environment may be the fundamental reason for the poor persistence. There is hope that the recently released cultivars, Fitzroy and Avalon, selected and developed from naturalised ryegrass pastures in south-eastern Australia for improved winter growth and persistence will improve the performance of perennial ryegrass in the region. Soon-to-be released cultivars, developed from Mediterranean germplasm, may also bridge the climatic gap between where perennial ryegrass originated and where it is grown in south-eastern Australia. Other factors that influence perennial ryegrass persistence and productivity can be managed to some extent by the landholder. Nutrient status of the soil is important since perennial ryegrass performance improves relative to many other pasture species with increasing nitrogen and phosphorus supply. It appears that high soil exchangeable aluminium levels are also reducing ryegrass performance in parts of the region. The use of lime may resolve problems with high aluminium levels. Weeds that compete with perennial ryegrass become prevalent where bare patches occur in the pasture; they have the opportunity to invade pastures at the opening rains each year. Maintaining some herbage cover over summer and autumn should reduce weed establishment. Diseases of ryegrass are best managed by using resistant cultivars. Insect pests may be best managed by understanding and monitoring their biology to ensure timely application of pesticides and by manipulating herbage mass to alter feed sources and habitat. Grazing management has potential to improve perennial ryegrass performance as frequency and intensity of defoliation affect dry matter production and have been linked to ryegrass persistence, particularly under moisture deficit and high temperature stress. There is some disagreement as to the merit of rotational stocking with sheep, since the results of grazing experiments vary markedly depending on the rotational strategy used, climate, timing of the opening rains, stock class and supplementary feeding policy. We conclude that flexibility of grazing management strategies is important. These strategies should be able to be varied during the year depending on climatic conditions, herbage mass, and plant physiology and stock requirements. Two grazing strategies that show potential are a short rest from grazing the pasture at the opening rains until the pasture has gained some leaf area, in years when the opening rains are late. The second strategy is to allow ryegrass to flower late in the season, preventing new vegetative growth, and perhaps allowing for tiller buds to be preserved in a dormant state over the summer. An extension of this strategy would be to delay grazing until after the ryegrass seed heads have matured and seed has shed from the inflorescences. This has the potential to increase ryegrass density in the following growing season from seedling recruitment. A number of research opportunities have been identified from this review for improving ryegrass persistence. One area would be to investigate the potential for using grazing management to allow late development of ryegrass seed heads to preserve tiller buds in a dormant state over the summer. Another option is to investigate the potential, and subsequently develop grazing procedures, to allow seed maturation and recruitment of ryegrass seedlings after the autumn rains.

Yield and composition of soybean seed as a function of potassium supply

SummaryA reduction in K supply to soybean plants to deficiency levels during both vegetative and reproductive development resulted in reductions not only in yield, but also in oil and K concentrations in the seed and a concomittant increase in seed protein concentration. Correlations between mean fruit yield and oil, protein and K concentrations, over a wide range of K regimes, were 0.97, −0.94 and 0.98, respectively. When K supply was increased well above the level necessary to produce maximum yields,i.e. luxury consumption, there was no significant change in K concentration in the seed, indicating a high degree of control in the movement of K to the developing seed under high K regimes. When the K supply to the plant was limiting, the rate of accumulation of oil and carbohydrate fractions, but not of seed protein, declined during the latter part of podfilling. This resulted in a fall in the C/N ratio in the non-structural seed components during this part of seed development. Depriving plants of K only during seed development had no effect on seed composition or yield, whereas resupplying K to deficient plants after anthesis resulted in almost the same seed composition and yield as that which occurred with control plants. Possible mechanisms whereby K deficiency influences soybean seed composition and yield are discussed in terms of movement of carbohydrate and nitrogen to the seed. We suggest that potassium-deficient soils are likely to produce crops with low yields and low seed oil levels; the crop may respond to K fertilizers as late as anthesis.

Use of phosphate rocks in the tropics

The use of finely ground phosphate rocks (PRs) as directly applied P fertilizers in tropical farming systems is a cheaper alternative to acidulated, water-soluble P products. However, the effectiveness of PRs in tropical environments depends on the extent to which the required P uptake rate of the crop plant can be maintained by the rate of PR dissolution in that soil. That extent that this outcome is achieved depends on the properties of the PR, the soil, climate, plant factors, and on management practice. Environmental conditions in the surface layers of highly weathered soils in the humid tropics are generally conducive of the attainment of satisfactory rates of PR dissolution, especially as the reactivity of the PR increases. In soils with very high P sorption capacities, however, the agronomic effectiveness of PRs is reduced as the acquisition of dissolved P by plant roots is restricted by competition from P sorption processes in the soil.In determining the required reactivity of PRs for use in tropical regions one must consider the rate of P demand by the crop or pasture and the suitability of the soil environment for PR dissolution. The use of some water-soluble P in combination with the PR might enable PRs of low reactivity to also be used, where alone they would be relatively ineffective. The ability of PRs to provide Ca, in addition to P, needs further study because subsoil Ca deficiency is becoming more widely recognized as a production constraint in highly weathered tropical soils. The future use of PRs in tropical agriculture is expected to expand for plantation crops and pastures and especially for landlocked countries with local deposits of PR. Increased use of PRs will also occur where more reactive PRs can effectively be used to increase the yield of annual food crop.

Patterns of mineral nutrient accumulation in soybean seed.

Abstract The time course of accumulation of both macronutrients and micronutrients in field-grown soybean seed in presented. No two nutrients followed the same pattern of movement into the grain. At the onset of leaf senescence, about 80% of the final mineral nutrient content had been translocated to the seed. The lack of synchronous movement between nutrients, dry matter, oil and protein indicated that nutrient movement was not directly coupled to assimilate movement. Nutrient concentrations on a dry weight basis generally declined during early to midpodfilling owing to growth dilution, and then increased during late podfilling. In contrast, nutrient concentrations on a fresh weight basis rose until the beginning of leaf senescence.

A survey of farmers' attitudes, management strategies and use of weather and seasonal climate forecasts for coping with climate variability in the perennial pasture zone of south-east Australia

A survey of 62 producers in the perennial pasture zone of south-eastern Australia was undertaken to gain an understanding of farmer attitudes toward climate variability, the use of weather and seasonal climate forecasts on farms and how climatic variability affects farm management. The 3 localities surveyed were Hamilton and surrounding districts in south-western Victoria, Lucindale and Naracoorte districts of south-eastern South Australia, and Campbell Town, Ross and Bothwell districts of North Central and upper Derwent Valley regions of Tasmania. Farmers in all districts considered winter rainfall to be the most reliable in terms of consistency, while autumn rainfall was the least reliable but had the greatest impact on production. Perceptions of seasonal rainfall variability and its impact were influenced by stocking rates; farmers with more heavily stocked properties considered rainfall in the growing season to be less reliable than did farmers with lower stocking rates and that autumn and winter rainfall had a greater impact on production. All farmers had strategies to manage their grazing enterprises in response to the prevailing season’s climate conditions, but not all available strategies were used. All participants fed supplements in poorer seasons while Tasmanian farmers tended to reduce stock numbers more in poorer seasons than did Victorian farmers. All the farmers used short-term weather forecasts to help make decisions about farm management, with 100% of farmers in all 3 states using radio and television forecasts and sheep graziers’ warnings. However, farmers felt that many other forecasts were unreliable and they were often were unwilling to incorporate them into decision making. Less than 50% of farmers had read or heard about the 3-month seasonal climate outlook and they were not willing to base management decisions on these outlooks. The uptake of information technology and the use of the Internet amongst farmers in the perennial pasture zone have increased rapidly, with an average of 76% of farmers using a computer and 30% connected to the Internet. Computers were mainly used for financial and farm management, while the Internet was mainly used for farm information. The education level attained by the farmer was the main factor that influenced the uptake and use of information technology.

Soil phosphorus buffering measures should not be adjusted for current phosphorus fertility

Soil phosphorus (P) sorption is an important and relatively stable soil property which dictates the equilibrium between sorbed and solution P. Soil P sorption measures are commonly adjusted for the effect of current P fertility on the amount of P a soil sorbs. In the case of highly fertilised agricultural soils, however, this adjustment is likely to be inappropriate as it may mask changes in a soils capacity to sorb P, which could affect future P fertiliser applications. A study was undertaken to compare adjusted or unadjusted methods of measuring P sorption using 9 pasture soils sampled from southern Victoria which had previously received P fertiliser and lime. The P sorption assessment methods included: P sorption isotherms, P-buffering capacity (PBC) measures (slope between equilibrium P concentration of 0.25 and 0.35mgP/L), and single-point P-buffering indices (PBI), with methods either adjusted or unadjusted for current P fertility. A single application of 280kgP/ha, 6 months before sampling, resulted in a general negative displacement of unadjusted P sorption isotherm curves, indicating reduced P sorption on 8 of the 9 soils. Adding the Colwell extractable P concentration to the amount of P sorbed before calculating the slope (PBC+ColP), tended to negate this fertiliser effect and, in 2 of the 9 soils, resulted in a significant increase in PBC+ColPvalues. Increasing rates of P fertiliser application (up to 280kgP/ha)resulted ina consistent trend todecreasingPBI values (unadjusted for ColwellP), whichwas significantat 4of the 9 sites after 6 months. However, only minimal changes in PBI values were determined when PBI was adjusted for current P fertility (PBI+ColP). Phosphorus sorption properties appeared reasonably stable over time, although 2 soils, both Ferrosols, indicated significant linear increases in PBI values when these sites remained unfertilised for 30 months. Lime significantly increased both PBI and PBI+ColP values at all sites 6 months after application, but the effect generally diminished after 30 months, suggesting PBI measurements should not be taken immediately after liming. These results demonstrate that unadjusted measures of P sorption are more likely to accurately reflect changes in soil P sorption capacity following P fertiliser applications and suggest that the unadjusted PBI be used in commercial soil testing rather that the currently adjusted PBI+ColP.

Differential responses to K deficiency among soybean cultivars

The seed yield per unit of potassium applied differed for five soybean cultivars which were grown to maturity under different K regimes in a glasshouse. Whereas Dodds was the most responsive cultivar to moderate increases in K supply, the cultivar Bragg was the most efficient in its ability to produce seed with low levels of available K; Lee and Forest were the least efficient cultivars while Bossier and Dodds were of intermediate efficiency. The basis for the efficiency of cv. Bragg was that the growth of its tops, as indicated by mature stem weights and its roots, were less affected by reduced K supply than those of other cultivars. This enabled it to produce more pods under K-deficient regimes, resulting in a greater seed yield per plant. The percentage reduction in oil/protein ratios in the seed of the five cultivars under moderate K deficiency correlated closely with reductions in seed yield. However, changes in this ratio were poorly related to the K percentages in the seed. All cultivars experienced an impairment of plant senescence under K deficiency as evidenced by a reduction in leaf abcission and a delay in pod maturity. The existence of genetic diversity in K-use efficiency means that breeding programmes could utilize K-efficient germplasm in developing new cultivars for soils not naturally high in potassium.

The impact of elevated carbon dioxide on the phosphorus nutrition of plants: a review

BACKGROUND Increasing attention is being focused on the influence of rapid increases in atmospheric CO2 concentration on nutrient cycling in ecosystems. An understanding of how elevated CO2 affects plant utilization and acquisition of phosphorus (P) will be critical for P management to maintain ecosystem sustainability in P-deficient regions. SCOPE This review focuses on the impact of elevated CO2 on plant P demand, utilization in plants and P acquisition from soil. Several knowledge gaps on elevated CO2-P associations are highlighted. CONCLUSIONS Significant increases in P demand by plants are likely to happen under elevated CO2 due to the stimulation of photosynthesis, and subsequent growth responses. Elevated CO2 alters P acquisition through changes in root morphology and increases in rooting depth. Moreover, the quantity and composition of root exudates are likely to change under elevated CO2, due to the changes in carbon fluxes along the glycolytic pathway and the tricarboxylic acid cycle. As a consequence, these root exudates may lead to P mobilization by the chelation of P from sparingly soluble P complexes, by the alteration of the biochemical environment and by changes to microbial activity in the rhizosphere. Future research on chemical, molecular, microbiological and physiological aspects is needed to improve understanding of how elevated CO2 might affect the use and acquisition of P by plants.

Organic amendments initiate the formation and stabilisation of macroaggregates in a high clay sodic soil

Subsoil constraints present a substantial problem for crop production in many agricultural regions. In particular, soils in temperate grain production areas of Australia are often poorly structured due to high content of sodic clay. An alternative to the standard practice of addition of gypsum is to incorporate organic amendments deep into the subsoil. An incubation experiment was performed for 174 days using several organic amendments. These consisted of wheat shoots, lucerne pellets, canola and chickpea stubbles, chicken manure, peat, and sawdust. Gypsum, an inorganic amendment commonly applied to sodic soil, was included for comparison. The change over 174 days in soil structural properties was measured using wet-sieving. Formation of slaking-resistant macroaggregates >2 mm was most rapid with ‘green plant material’, wheat and lucerne, while the ‘stubbles’ were markedly slower in obtaining the equivalent level of aggregation. However, the largest growth in aggregates after day 56 was shown by the ‘stubble’ and chicken manure amended soils. The gypsum amendment was not capable of forming large, slaking-resistant aggregates >2 mm; this may be attributed to the inability of gypsum to stimulate soil biological processes. Peat and sawdust failed to initiate slaking-resistant macroaggregates. The study demonstrated that a variety of organic amendments have the ability to improve the physical fertility of sodic subsoil, and in the case of the green plant materials within 1 week of incubation.