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Liquid polyphosphate fertilizers have shown significant yield increases compared with granular orthophosphate fertilizer on highly P-fixing soils in Australian cropping trials. Polyphosphate fertilizers contain mixed P species (predominantly pyrophosphate and orthophosphate) and are therefore chemically different from most other inorganic P fertilizers, which contain only orthophosphate. To determine the mechanisms causing the yield responses to polyphosphate fertilizer, a method for investigating the isotopic exchangeability (lability) of P added as pyrophosphate was developed and tested. This method enabled the quantification of the hydrolysis reaction of pyrophosphate to orthophosphate in soil solution. As the rate of pyrophosphate applied increased, the proportion of pyrophosphate added that was isotopically exchangeable increased, and the percentage of pyrophosphate added that hydrolyzed to orthophosphate decreased. The method was further developed to test the hypothesis that pyrophosphate addition to soil may cause the mobilization of native orthophosphate reserves. The results suggested that there was no mobilization of native P after 7 d of incubation in two calcareous soils. Abbreviations: OP, orthophosphate; PP, pyrophosphate.

Liquid polyphosphate fertilisers contain both orthophosphate and pyrophosphate, and have shown significant yield increases compared to granular orthophosphate fertiliser on highly phosphorus (P) fixing soils. The P sorption chemistry of the dominant P species in a polyphosphate fertiliser (pyrophosphate and orthophosphate) was compared on a range of Australian soils, and the effect of these P species on equilibrium solution chemistry was also examined. Phosphorus supplied as pyrophosphate had a stronger sorption affinity than orthophosphate in all soils. The greater efficiency of pyrophosphate-based fertiliser on some soil types is therefore not due to reduced sorption of this P supplied as pyrophosphate compared to P supplied as orthophosphate fertiliser. In general, the addition of pyrophosphate to soil resulted in a larger decrease in calcium concentration in solution compared to orthophosphate. In contrast, there was an increase in iron concentration in solution with pyrophosphate addition, indicating sequestration reactions associated with the dissolution of dissolved organic carbon into soil solution. The pyrophosphate ion generally mobilised more Fe into solution than orthophosphate in several soils, likely through complexation reactions leading to Fe desorption/dissolution from the soil solid phase. These findings highlight the differences in soil chemical reactions that occur with the addition of polyphosphate fertilisers, which may contribute to their yield advantage in some soil types. Additional keywords: phosphorus, partitioning, polyphosphate, ion chromatography, calcarosol., Introduction Polyphosphate fertilisers have recently gained attention in Australian agricultural research due to significant yield increases with the application of liquid polyphosphate compared to granular orthophosphate (OP) fertilisers on highly [...]

External agricultural inputs such as mineral fertilisers, organic amendments, microbial inoculants, and pesticides are applied with the ultimate goal of maximising productivity and economic returns, while side effects on soil organisms are often neglected. We have summarised the current understanding of how agricultural inputs affect the amounts, activity, and diversity of soil organisms. Mineral fertilisers have limited direct effects, but their application can enhance soil biological activity via increases in system productivity, crop residue return, and soil organic matter. Another important indirect effect especially of N fertilisation is soil acidification, with considerable negative effects on soil organisms. Organic amendments such as manure, compost, biosolids, and humic substances provide a direct source of C for soil organisms as well as an indirect C source via increased plant growth and plant residue returns. Non-target effects of microbial inoculants appear to be small and transient. Among the pesticides, few significant effects of herbicides on soil organisms have been documented, whereas negative effects of insecticides and fungicides are more common. Copper fungicides are among the most toxic and most persistent fungicides, and their application warrants strict regulation. Quality control of organic waste products such as municipal composts and biosolids is likewise mandatory to avoid accumulation of elements that are toxic to soil organisms. Additional keywords: fertiliser, compost, manure, biosolids, pesticide, soil biology., Introduction Agricultural inputs External inputs to agricultural production systems include mineral fertilisers such as urea, ammonium nitrate, sulfates, and phosphates; organic fertilisers such as animal manures, composts, and biosolids; various [...]

The effects of various management practices on soil phosphorus (P) dynamics were investigated in a field experiment in New South Wales, Australia, during 24 years of different crop rotation, stubble management, and tillage treatments. Topsoil samples collected at the beginning of the trial and after 6, 12, 18, and 24 years were analysed for resin-extractable P, inorganic and organic P, and total P. According to the calculated P input-output budget, 9-14 of the 20 kg P/ha added as superphosphate annually remained in the system, depending on the treatment. The measured increase in total P in 0-0.20 m did not differ between treatments, showing an accumulation rate of only 9 ± 2 kg P/ha.year. These results suggest a loss of 4 ± 2 kg P/ha.year, presumably into lower soil layers. Resin-extractable P at 0-0.10 m increased by 1.7 kg P/ha.year, irrespective of the treatment. The increase in total P after 24 years was almost completely accounted for by the increase in total extractable inorganic P. Changes in organic P paralleled changes in organic carbon, with a significant loss in treatments with stubble burning (wheat-lupin rotation and continuous wheat), and a significant accumulation in a wheat-subterranean clover rotation with stubble retention and direct drilling. We conclude that on the time scale of this experiment, the dynamics of carbon and organic P are closely linked. Additional keywords: P fertilisation; soil organic matter; organic phosphorus; nutrient budget; Lupinus angustifolius; Trifolium subterraneum., Introduction In southern Australia, the traditional system of alternating wheat with legume-based pastures has been increasingly replaced by continuous cropping following the decline of wool prices during the 1970s. This [...]

Isotopic labeling techniques have the potential to elucidate soil P dynamics and the fate of P sources added to the soil, but they have rarely been applied to highly weathered tropical soils. We collected soils from two crop rotations [continuous maize (COM; Zea mays L.) and maize-crotalaria (MCF; Crotalaria grahamiana Wight & Arn.) fallow rotation] in a field experiment in Kenya and incubated them for 9 wk after addition of a plant residue or inorganic phosphorus ([P.sub.i], both labeled with [sup.33]P and added at 6 mg P [kg.sup.-1] soil, or after carrier-free labeling of isotopically exchangeable soil phosphorus (soil IEP). The amount of P and recovery of [sup.33]P were determined in resin-extractable [P.sub.i] ([P.sub.resin]), microbial P ([P.sub.hex]), and in a 0.1 M NaOH extract of samples from which [P.sub.resin] and [P.sub.hex] had been removed. The [P.sub.resin] increased after addition of [P.sub.i], while [P.sub.hex] increased after plant residue amendment, involving considerable microbial uptake of soil P. The recovery of [sup.33]P in [P.sub.resin] followed the order added [P.sub.i] > soil IEP > plant residue, and decreased steadily from 7 to 22% after 1 d to 3 to 5% after 9 wk. The recovery of [sup.33]P in [P.sub.hex] remained constant throughout the incubation, being greater after plant residue amendment (15%) than in the other two treatments (4-7%). An additional 66 to 76% of [33.sup]P was recovered in the NaOH extract, as much as 27% of which was in organic phosphorus ([P.sub.o]) after plant residue amendment and 2 to 8% in the other two treatments. Similar to P dynamics after plant residue amendment, the comparison of the two rotations indicated a shift toward [P.sub.hex] and [P.sub.o] with increasing microbial activity due to previous fallow biomass incorporation.