Maize Inbreds' Response to Riecito Phosphate Rock and Triple Superphosphate on an Acidic Soil

Abstract

Tropical acidic soils require large inputs of nitrogen (N) and phosphorus (P) fertilizers to sustain crop production. Attempts to use phosphate rock (PR) as a cheaper P source have shown limited success because of low rock solubility. The objective of this study was to evaluate growth and P nutrition of aluminum (Al)‐tolerant maize inbreds fertilized with PR. Twelve Al‐tolerant inbreds from CIMMYT were planted in 2‐kg pots filled with an acidic soil very low in available P and fertilized with 0, 40, or 100 mg kg^?1 of Riecito PR or triple superphosphate (SP). Plant shoots were harvested 35 days after planting, and biomass, root length, P uptake, and soil residual P were determined. Inbreds were able to sustain growth when fertilized with PR. There was indication that various mechanisms were involved in the responses to PR fertilization. Cultivars combining high uptake and conversion efficiencies should improve maize utilization of PR.     

Effect of dicyandiamide on growth and nutrient uptake of cotton

Abstract

The nitrification inhibitor dicyandiamide (DCD) offers potential for improving efficiency of N applications to cotton grown on sandy soils of the southeastern Coastal Plain. Research has indicated that cotton is sensitive to DCD. The purpose of this greenhouse experiment was to investigate the effect of DCD on growth and nutrient uptake of DPL 90 cotton grown for 73 days in pots containing a typical Coastal Plain soil (Norfolk sandy loam, Typic Paleudult). Nitrogen (50 mg kg^‐1) as NaNO₃ or urea, and DCD (0, 2.5, 5, 10, 15 and 20 mg kg^‐1) were applied to the soil at first true leaf and plants were harvested 58 days later. Sodium nitrate increased leaf dry weight and total dry weight of plants 9.1 and 6.0%, respectively, over urea fertilized plants. Leaf area, dryweight, and stem dry weight were reduced linearly with DCD. Fertilization with urea increased concentrations of leaf P, K, and Mn and reduced the concentration of Mg in leaf tissue. Dicyandiamide increased leaf N, P, and K concentrations but reduced concentrations of Ca, Mg, and Mn. Uptake rates (μg^‐1 g^‐1 fresh root day^‐1) of Ca and Mg were increased 7.5 and 13.7%, respectively, with NaNO₃ vs. urea, while P uptake rate was 15.5% greater for urea‐fertilized plants vs. NaNO₃‐fertilized plants. Dicyandiamide reduced Ca and Mg uptake rates. Phosphorus uptake rates were increased by DCD when urea was the N source. The effects of DCD on cotton growth and nutrient uptake generally resulted from the compound itself and were not an indirect result of nitrification inhibition. Although significant reductions in plant growth did not occur unless DCD exceeded that normally applied with recommended N rates on this soil, these results suggest a need for caution when applying DCD to cotton grown on sandy soils.     

Agronomic effectiveness of phosphate rock and superphosphate for aluminum‐tolerant and non‐tolerant sorghum cultivars

Abstract

About 35% of soils in Venezuela are acid and low in available phosphorus (P). To solve this problem farmers lime and apply phosphate fertilizers to the soils, but both lime and fertilizers are expensive. A good alternative to overcome soil acidity is the use of aluminum (Al)‐tolerant cultivars. The objective of this study was to test the hypothesis, by use of a pot experiment, that sorghum cultivars tolerant to Al toxicity are able to use P from phosphate rock more efficiently than are susceptible cultivars. Three sorghum (Sorghum bicolor L. Moench) cultivars, Chaguaramas III (Ch), AI‐tolerant, Decalb D59 (D59), and Pioneer 8225 (Pi), both Al‐susceptible, were grown in the greenhouse for 20 and 35 days in two acid soils fertilized with 0 and 100 mg P kg^‐1 as triple superphosphate (SP) and Riecito phosphate rock (PR). Santa Maria soil was very low in available P (2 mg kg^‐1) and highly saturated in Al saturation (64.5%) and Pao soil was higher in available P (20 mg kg^‐1) and low in Al saturation (6.5%). Chaguaramas dry matter production, P uptake and root length was higher in Santa Maria soil as compared with Pi and D59 when grown with both SP and PR fertilization. Chaguaramas response to PR in Pao soil was not as good as in Santa Maria soil. The results of our experiment suggest that Al‐tolerant Ch is able to utilize P from PR more efficiently in soils like Santa Maria than Al‐susceptible cultivare Pi and D59.     

The phosphorus source determines the arbuscular mycorrhizal potential and the native mycorrhizal colonization of tall fescue and wheatgrass

The study assesses the effect of two phosphate (P) sources (soluble superphosphate (SP) and rock phosphate (RP)) on the arbuscular mycorrhizal potential (AMP), the root arbuscular mycorrhizal colonization (AMC) and the growth of tall fescue and wheatgrass of a grassland soil from Argentina. Mycorrhizal potential was assessed with soil samples collected from 2 years for tall fescue and wheatgrass swards before and after field plots were fertilized with 0 and 60 kg P ha⁻¹ as SP or RP. Mycorrhizal potential both at unfertilized and at RP fertilized plots was high (12–14 AM propagules g⁻¹), however fertilization with SP caused a decrease in AMP (0.70–0.95 AM propagules g⁻¹). A range of soil P between 4 and 46 mg P kg⁻¹ and a range of root AMC between 6% and 50% were obtained after fertilization with four rates of SP and RP (0, 15, 30, and 60 kg P ha⁻¹) in plots where tall fescue and wheatgrass were grown during 2 years. Soil P and root mass were higher in the top 10-cm depth than in the 20-cm of the soil profile, but AMC did not change with depth. Shoot dry matter (SDM) production of both grasses did not differ after fertilization with SP or RP, particularly at second year. The AMP positively correlated with the indigenous AMC, and they were not different between tall fescue or wheatgrass. Lineal-plateau relationships between soil P, relative SDM and AMC were established. Highest relative SDM was attained at 6.5 mg P kg⁻¹ in plots fertilized with RP, and at 15.2 mg P kg⁻¹ with SP. Variability in colonization was well accounted by the soil P (at 0–10 cm depth) fertilized with SP (r² = 0.48, P 0.01), but any relationship was found with RP. The AMC decreased with increasing available soil P from plots with SP until 18.3 mg kg⁻¹ (a decrease of 2.2% per mg P kg), after that AMC was stabilized at about 6.9%. Our study clearly showed that fertilization with SP or RP produced similar available soil P content and grasses SDM production. Mycorrhiza root colonization and propagules decreased after fertilization with SP, but fertilization with RP did not decrease mycorrhizal propagules nor colonization. It can be concluded that RP fertilization instead SP could allow obtaining acceptable tall fescue and wheatgrass yield enhancing mycorrhizal potential of soils and indigenous colonization of plants and thus maximizing the use of fertilizer.     

Changes in distribution of inorganic soil phosphorus forms with phosphate desorption by iron oxide‐impregnated paper strips

Abstract

The release of soil phosphorus (P) to solution has been described by extraction of soil with iron (Fe)‐oxide coated paper strips. Little information is available, however, on where this P is coming from. The effect of removal of reversibly adsorbed soil P on the distribution of inorganic P forms was investigated for 12 Italian soils. Phosphate was removed from these soils by Fe‐oxide strips after incubation with P (0 and 100 mg P kg^‐1) for 90 days. With no applied P, 3 to 17% of the total soil active P [saloid‐P, aluminum‐phosphate (Al‐P), iron‐phosphate (Fe‐P), and calcium‐phosphate (Ca‐P) was removed by the Fe‐oxide strips. The change in strip‐P following P addition (100 mg kg^‐1 soil), ranged from 12.9 to 53.5 mg P kg^‐1, with P coming almost entirely from the active P fractions. A close relationship between the changes in desorbed strip‐P after P equilibration and soil P sorption index (SI) was found for the studied soils (r²=0.96). Thus, the release of soil P for plant uptake or transport in runoff was a function of the amount of “actively”; sorbed P and an estimate of P sorption.     

Short‐term evolution of phosphorus in an ultisol

Abstract

Evolution of residual phosphate was monitored as function of time on an Ultisol without cropping in a field trial at Sembawa Rubber Research Station, South Sumatra, Indonesia. Three treatments were imposed and either triple superphosphate at a rate of 600 kg ha^‐1 containing 21.54% phosphorus (P) or lime at the rate of 4.14 tons ha^‐1 was applied. During the 20 months of incubation, extractable phosphate as evaluated by hydrochloric acid‐ammonium fluoride (Bray‐I) declined in all treatments. The decrease in the P fertilized plot was more pronounced in relation to the control and the limed soil. Only 56% of P added to the soil was initially recovered as extractable P. This percentage was 30–40% in limed soils. No significant pH difference was recorded for all treated plots except for limed soils. An increase of 1.3 pH units was initially obtained by liming, but the pH dropped to about 0.9 unit after 20 months of incubation. In addition, total P in arable layer remained fairly constant. According to the recoveries of P added referring to 4.3 P 100 g^‐1, half‐life of a single P dose was estimated at 31 months for a P fertilized soil and almost threefold delay for combination of P fertilization and lime application, which was obtained from the following equation:     

Alkalinity exacerbates phosphorus deficiency in subtropical red soils: Insights from phosphate-solubilizing fungi

Red soils in subtropical regions are often low in available phosphorus (P), a vital plant nutrient. Phosphate-solubilizing microorganisms (PSMs) can release P from phosphate reservoir, making it accessible to plants. However, the complex interactions between PSMs and minerals in red soils are not yet fully understood. In this study, we investigated the effects of Aspergillus niger, a typical phosphate-solubilizing fungus (PSF), on phosphate dissolution in two representative red soils – an acidic soil and an alkaline soil. In the acidic red soil, the fungal abundance reached 3.01 × 10 ⁷ cfu g⁻¹ after a 28-day incubation period, with respiration of ~2000 mg C kg⁻¹. The secretion of oxalic acid promoted P release from inorganic phosphate (from ~1 to 187 mg kg⁻¹). Additionally, the contents of amorphous Fe/Al oxides decreased, which otherwise could have contributed to P sorption in the soil. In contrast, P availability declined in the alkaline red soil after the addition of A. niger, regardless of the P source (inorganic or organic phosphate). Meanwhile, the fungal respiration decreased to ~780 mg C kg⁻¹. Therefore, alkaline red soils with abundant carbonates are susceptible to P deficiency due to both the diminished function of PSMs and strong soil buffering. These findings have important implications for sustainable agriculture on alkaline red soils, as they suggest that the use of PSMs to improve P availability may be limited.     

Agronomic Efficiency of Indian Rock Phosphates in Acidic Soils Employing Radiotracer A‐Value Technique

Abstract

A greenhouse pot culture study was conducted to evaluate the agronomic efficiency of two rock phosphates from Mussoorie (MRP) and Purulia (PRP) in two acidic soils from Dapoli (Maharashtra) and Aruvanthklu (Karnataka), India, by growing maize (cv. Ganga) as the test crop and using ³²phosphorus (P) single superphosphate (³²P=SSP) as a tracer (A‐value technique). Dry‐matter yield and P uptake increased significantly with the application of P fertilizers compared to control treatment (without P) in both the soils. There was no significant difference with respect to dry‐matter yield among the P fertilizer treatments. However, P uptake by the shoots was found to be significantly higher in the PRP treatment in only Dapoli soil compared to other P fertilizer treatments. Phosphorus derived from fertilizer decreased in rock phosphate treatments compared to standard ³²P‐SSP treatment in both the soils, indicating an excess availability of P from the rock phosphates. A‐values of soil and rock phosphate indicate a relatively higher P availability from Aruvanthklu soil compared to Dapoli soil; A‐values for the rock phosphates were in the order PRP>MRP. The substitution ratio showed that the availability of P from both the rock phosphates were less than SSP in both the soils.     

Rapid microbial phosphorus immobilization dominates gross phosphorus fluxes in a grassland soil with low inorganic phosphorus availability

Gross phosphorus (P) fluxes measured in isotopic dilution studies with ³³P labeled soils include the biological processes of microbial P immobilization, remineralization of immobilized P and mineralization of non-microbial soil organic P. In this approach, isotopic dilution due to physicochemical processes is taken into account. Our objectives were to assess the effect of inorganic P availability on gross P mineralization and immobilization in soil under permanent grassland, and to relate these fluxes to soil respiration, phosphatase activity and substrate availability as assessed by an enzyme addition method. We used soils from an 18-year-old grassland fertilization experiment near Zurich, Switzerland, that were collected in two treatments which differed only in the amount of mineral P applied (0 and 17 kg P ha⁻¹ yr⁻¹ in NK and NPK, respectively). Water-extractable phosphate was low (0.1 and 0.4 mg P kg⁻¹ soil in NK and NPK, while hexanol-labile (microbial) P was high (36 and 54 mg P kg⁻¹ soil in NK and NPK). Extremely fast microbial P uptake under P-limited conditions in NK necessitated the use of a microbial inhibitor when determining isotopic dilution due to physicochemical processes. At the higher inorganic P availability in NPK, however, isotopic exchange parameters were similar in the presence and absence of a microbial inhibitor. Phosphatase activity was higher in NK than in NPK, while soil respiration, water-extractable organic P and its enzyme-labile fraction were not affected by P status. Together, the results showed that inorganic P availability primarily affected microbial P immobilization which was the main component of gross P fluxes in both treatments. Gross P mineralization rates (8.2 and 3.1 mg P kg⁻¹ d⁻¹ for NK and NPK) during the first week were higher than reported in other studies on arable and forest soils and at least equal to isotopically exchangeable P due to physicochemical processes, confirming the importance of microbial processes in grassland soils.     

Assessment of gross and net mineralization rates of soil organic phosphorus – A review

The quantification of net soil organic P mineralization rates is hampered by the potentially rapid sorption of released phosphate. Here, isotopic dilution approaches to assess gross and net organic P mineralization rates under steady-state conditions are reviewed, including different analytical and numerical solutions to assess P transformation rates based on incubation experiments with ³²P- or ³³P-labeled soils. Non-isotopic approaches are also commented on. Published isotopic dilution studies show that isotopically exchangeable P during incubation can partly or even predominantly (20–90%) result from biological and biochemical rather than physicochemical processes. The relative contribution of biological and biochemical processes tends to be lower in arable soils than under grassland and forests and is negatively related to the availability of inorganic P and positively to concentrations of soil organic carbon. Typical basal gross organic P mineralization rates range between 0.1 and 2.5 mg P kg⁻¹ d⁻¹, but rates up to 12.6 mg P kg⁻¹ d⁻¹ have been observed in grassland and forest soils. The further partitioning of gross organic P mineralization remains uncertain, but a dominance of microbial immobilization and remineralization is likely under most conditions, at least during the initial weeks of incubation. Over longer time periods, the relative importance of mineralization of non-living soil organic P increases, with the contribution of extracellular hydrolysis remaining to be elucidated. This requires other approaches than enzyme activity assays, since measurements of phosphomonoesterase activity in soil render organic P mineralization rates that are one to two orders of magnitude greater than those determined by isotopic dilution. The numerical modeling approach will enable assessment of soil P transformation rates under non-steady-state conditions, where P fluxes are likely to be greater than under steady-state conditions. Ultimately, an improved understanding of the biological and biochemical processes in soil P dynamics may help to improve P management in agroecosystems.     

Effect of one phosphorus rate placed in different soil volumes on p uptake and growth of wheat

Abstract

The volume of soil treated with P fertilizer affects P uptake by the crop. Earlier studies have shown that the stimulation of root growth in P‐fertilized soil was similar for both corn (Zea mays L.) and soybean (Glycine max L. Merr). The objective of this research was to determine the effect of fertilizer P placement on P uptake and shoot and root growth of spring wheat (Triticum vulgare L.). Wheat was grown for 34 days in Raub silt loam (Aquic Argiudolls) in a controlled climate chamber. One rate of phosphate per pot, 150 mg P per three kg of soil, was mixed with 2, 5, 10, 20, 40 and 100% of the soil in the pot. The P was equilibrated with moist soil for 5 days at 70°C followed by 21 days at 25° C before transplanting 8‐day‐old wheat plants into each 3 L pot. The P stimulation of root growth in the P‐treated soil was similar to that for corn and soybeans. The effect could be described by the equation y = x^0.7 where y is the fraction of the root system in the P‐fertilized soil where P is mixed with x fraction of the soil. The greatest P uptake and plant growth occurred when added P was mixed with 20% of the soil.     

Effect of Low‐Rate Commercial Humic Acid on Phosphorus Availability,Micronutrient Uptake,and Spring Wheat Yield

Abstract

A greenhouse study was conducted to determine the effects of low‐rate commercial humic acid (HA) on phosphorus (P), iron (Fe), and zinc (Zn) availability and spring wheat yields, in both a calcareous soil and a noncalcareous soil. In Phase I, soluble P concentrations were monitored at 1.9, 3.8, and 5.7 cm from a monoammonium phosphate (MAP) fertilizer band that had either been coated with one of two HA products at the equivalent of 1.7 kg HA ha^?1, a label rate, or left uncoated. Sampling occurred periodically up to 48 d after fertilizer application. In Phase II, uptake of P, Fe, and Zn and grain yield were measured in soils that had been fertilized with 7.5 or 25 kg P ha^?1, either coated with HA or left uncoated. In Phase I, only three significant differences (P=0.05) out of 66 comparisons were found in soluble P concentrations between HA and control treatments at time points ranging from 4 to 48 d after fertilization. In addition, no significant differences were found in nutrient uptake, shoot biomass, or grain yield between HA and control treatments. These greenhouse results suggest that low commercial HA rates (～1.7 kg HA ha^?1) may be insufficient to enhance spring wheat growth.     

Effect of Lime and Flooding on Phosphorus Availability and Rice Growth on Two Acidic Lowland Soils

Abstract

Loss of soil‐water saturation may impair growth of rainfed lowland rice by restricting nutrient uptake, including the uptake of added phosphorus (P). For acidic soils, reappearance of soluble aluminum (Al) following loss of soil‐water saturation may also restrict P uptake. The aim of this study was to determine whether liming, flooding, and P additions could ameliorate the effects of loss of soil‐water saturation on P uptake and growth of rice. In the first pot experiment, two acid lowland soils from Cambodia [Kandic Plinthaqult (black clay soil) and Plinthustalf (sandy soil)] were treated with P (45 mg P kg^?1 soil) either before or after flooding for 4 weeks to investigate the effect of flooding on effectiveness of P fertilizer for rice growth. After 4 weeks, soils were air dried and crushed and then wet to field capacity and upland rice was grown in them for an additional 6 weeks. Addition of P fertilizer before rather than after flooding depressed the growth of the subsequently planted upland rice. During flooding, there was an increase in both acetate‐extractable Fe and the phosphate sorption capacity of soils, and a close relationship between them (r²=0.96–0.98). When P was added before flooding, Olsen and Bray 1‐extractable P, shoot dry matter, and shoot P concentrations were depressed, indicating that flooding decreased availability of fertilizer P. A second pot experiment was conducted with three levels of lime as CaCO₃ [to establish pH (CaCl₂) in the oxidized soils at 4, 5, and 6] and four levels of P (0, 13, 26, and 52 mg P kg^?1 soil) added to the same two acid lowland rice soils under flooded and nonflooded conditions. Under continuously flooded conditions, pH increased to over 5.6 regardless of lime treatment, and there was no response of rice dry matter to liming after 6 weeks' growth, but the addition of P increased rice dry matter substantially in both soils. In nonflooded soils, when P was not applied, shoot dry matter was depressed by up to one‐half of that in plants grown under continuously flooded conditions. Under the nonflooded conditions, rice dry matter and leaf P increased with the addition of P, but less so than in flooded soils. Leaf P concentrations and shoot dry matter responded strongly to the addition of lime. The increase in shoot dry matter of rice with lime and P application in nonflooded soil was associated with a significant decline in soluble Al in the soil and an increase in plant P uptake. The current experiments show that the loss of soil‐water saturation may be associated with the inhibition of P absorption by excess soluble Al. By contrast, flooding decreased exchangeable Al to levels below the threshold for toxicity in rice. In addition, the decreased P availability with loss of soil‐water saturation may have been associated with a greater phosphate sorption capacity of the soils during flooding and after reoxidation due to occlusion of P within ferric oxyhydroxides formed.     

Phosphorus dynamics in a long‐term P fertilization trial on Luvic Phaeozem at Halle

The soil of the long‐term experiment laid out 1949 in Halle has the potential to supply much P. The P taken up by plants where no P (P0) or 15 kg ha⁻¹ yr⁻¹ (P1) was applied was much greater than the P applied as fertilizer (P1). A decrease in yield was measured only after the first 25 years on P0 soils but the P1 treatment has, so far, shown no decrease. Lactate extractions of the soil did not reflect P‐uptake suitably. The release of P from insoluble into water soluble forms was at a minimum after 30 years in P0 soils. P1 soils have now also declined to this minimum value and it remains to be seen whether yields decrease in this treatment in the future. Parallel to this trend, the P sorption increased in P0 soils. The subsoil also seems to be an important source for P supply, possibly influenced by root exudates. Further work is needed to gain a better understanding of soil P dynamics in connection with root exudates and microbes and to identify parameters which will provide more reliable means of calculating fertilizer P requirements.     

Effect of various microorganisms on phosphorus uptake from insoluble Ca‐phosphates by cucumber plants

Rhizospheric microorganisms can increase P availability to plants. The objective of this work was to elucidate the effects of two plant growth promoting rhizobacteria and biocontrol agents (Bacillus subtilis QST713 and B. Amyloliquefaciens FZB24), a biocontrol agent (the fungus Trichoderma asperellum T34), and Aspergillus niger CBS513.88 on P uptake from insoluble Ca‐phosphates by plants. An experiment involving microbial cultivation in liquid media and three involving cultivation of cucumber plants in a siliceous growing medium fertilized with 40 mg P kg^?1 as phosphate rock (PR), a calcareous medium supplied with the same fertilizer, and one fertilized with KH₂PO₄ or PR at 200 mg P kg^?1 were conducted. In spite of the observed PR solubilization in liquid culture, not all the microorganisms improved P uptake by plants from this P source. The effect of each microorganism also differed depending on the plant‐growing medium, revealing that its P‐solubilizing activity was affected by pH and P concentration in the medium. Overall, best results were obtained with Bacillus subtilis QST713 which increased P uptake from the siliceous growing medium and the calcareous medium fertilized with 200 mg P kg^?1. Improved P nutrition of plants was the result not only of increased P solubilization, but also of enhanced ability of plants to absorb P. The other microorganisms studied provided less promising results despite the P mobilizing strategies they exhibited in the liquid culture (acidification and organic anion exudation). Therefore, these cannot be the only mechanisms contributing to P uptake by plants. Our results support the ability of B. subtilis QST713 to enhance the use of PR as a P source for calcareous soils or to improve uptake of residual P in the form of sparingly soluble Ca‐phosphates.     

Iron chlorosis in macadamia as affected by phosphate‐iron interactions 1

Iron (Fe) chlorosis induced by heavy phosphate (P) fertilizations is a serious problem for macadamia (Macadamia integrifolia) in Hawaii. To address this problem, a study was conducted to quantify the effects of P‐Fe interaction on macadamia leaf composition and chlorosis. The soil used was a limed Oxisol (Tropeptic Eutrustox, Wahiawa Series), pH 5.5. Phosphate was added as treble superphosphate at 0, 150 and 500 mg P/kg. The 150 mg P/kg rate was designed to yield approximately 0.04 mg P/L in the soil solution, a level considered adequate for macadamia growth. The 500 mg P/kg rate was intended to produce approximately 0.2 mg P/L, a level required by many horticultural crops but considered excessive for macadamia. Iron was added as Fe‐DTPA at 0, 5 and 10 mg Fe/kg soil, and factorially imposed on the P treatments. Color Index, a numerical rating based on hue, value and chroma from a Munsell Color Chart for Plant Tissues, was correlated with leaf chlorophyll concentration and used as an indicator of chlorosis.

Phosphate concentrations in leaves increased with increasing P application rates as expected, but decreased remarkably with increasing Fe rates (at a constant P rate). Plant Fe unexpectedly remained unchanged with increasing Fe rates but decreased with increasing P rates. The results suggest that (1) soil‐solution Fe was not a limiting factor to macadamia growth as is often incorrectly assumed for high P‐fertilized soils, (2) Fe uptake was restricted not because soil‐solution Fe was low but because plant P was excessively high, and (3) Fe translocation from roots to leaves may have been hampered by high P in the plants. Consequently, Fe chlorosis was intensified primarily by P fertilization (actually, by high plant P concentrations) and secondarily by P‐Fe interactions. Chlorosis, as measured by Color Index, can be diagnosed by a leaf Fe/P ratio < 0.06, and predicted by a soil‐solution ³√Fe/P ratio < 15.     

The role of fertilization on phosphorus stratification in no‐till soils

Abstract

No‐tillage induces the stratification of soil nutrients because of the return of crop residues to soil surface, fertilization and the lack of soil mixing. In this research we have attempted to develop a phosphorus (P) balance on soybean, to study the relative importance of the causes of P stratification. An experiment was performed on a Typic Hapludoll located in mid Buenos Aires province, Argentina. The treatments were fertilized and unfertilized. Soybean biomass and P concentration in grains, stubble and roots were determined. In both treatments the P stratification was produced by the enrichment of the surface layer and the impoverishment of the deeper layers. In the non‐fertilized plots the soil lost P (7.5 kg P ha¹) meanwhile in the fertilized plots (20 kg P ha^‐1added) the soil gained P (6.6 kg P ha^‐1). The accumulation of plant residues alone is enough to redistribute P in soils, but fertilization was the main factor in P stratification.     

Influence of low-molecular-weight organic acids on the solubilization of phosphates

A range of low-molecular-weight organic acids were identified in rhizosphere soil, leaf litter, and poultry manure compost. Laboratory and greenhouse experiments were carried out to examine the effects of seven low-molecular-weight organic acids on phosphate adsorption by soils, and the solubilization and plant uptake of P from soil pre-incubated with monocalcium phosphate and North Carolina phosphate rock. Acetic, formic, lactic (monocarboxylic), malic, tartaric, oxalic (dicarboxylic), and citric (tricarboxylic) acids were used in the study. The addition of organic acids decreased the adsorption of P by soils in the order tricarboxylic acid>dicarboxylic acid>monocarboxylic acid. The decreases in P adsorption with organic acid addition increased with an increase in the stability constant of the organic acid for Al (logK _Al). Organic acids extracted greater amounts of P from soils meubated with both monocalcium phosphate and phosphate rock than water did. Although more phosphate was extracted by the organic acids from monocalcium phosphate — than from phosphate rock — treated soils in absolute terms, when the results were expressed as a percentage of dissolved phosphate there was little difference between the two fertilizers. The amount of P extracted by the organic acids from both fertilizers increased with an increase in logK _Al values. The addition of oxalic and citric acids increased the dry matter yield of ryegrass and the uptake of P in soils treated with both fertilizers. The agronomic effectiveness of both fertilizers increased in the presence of organic acids and the increase was greater with the phosphate rock than with the monocalcium phosphate. The results indicated that organic acids increase the availability of P in soils mainly through both decreased adsorption of P and increased solubilization of P compounds.     

The response of finely textured and organic soils to lime and phosphorus application: Results from an incubation experiment

A soil's responses to phosphorus (P) input differs based on its chemical composition. Soil acidity and the presence of metallic cations dictate a soil's chemical composition. Currently, soil P application recommendations are universal and do not account for differing soil composition. A targeted soil-specific approach is required to optimize P efficiency and availability. A pot incubation experiment was established to explore the effects of contrasting lime and P application rates across a range of soils (25), characterized by fine particle size and high levels of soil organic matter. Three contrasting rates of P were applied (0, 50, and 150 kg P ha⁻¹) both with and without ground lime (CaCO₃) at 5 tonne ha⁻¹ over a 140-day incubation period. The addition of lime buffered the soil, increasing nutrient availability and reducing P fixation. The 50 kg P ha⁻¹ treatment rate was required to achieve sufficient plant available P in both mineral soil textural classes. Current legislative recommendations however do not allow the application of such rates, which has an impact on agronomic performance. Loam soils experienced a greater increase in M3 soil P in comparison to clay and organic soils. Organic soils posed a major threat to water quality due to poor P retention. A re-evaluation of P recommendations is required to account for soil variability as current P allowances are insufficient on these particular soils.     

Phosphorus mineralization in subarctic agricultural and forest soils

Summary Potential P and C mineralization rates were determined in a 12-week laboratory incubation study on subarctic forest and agricultural soil samples with and without N fertilizer added. There was no significant difference in net inorganic P produced between N fertilized and unfertilized soils. The forest soil surface horizons had the highest net inorganic P mineralized, 32 mg P kg^-1 soil for the Oie and 17 mg P kg^-1 soil for the Oa. In the cropped soils net inorganic P immobilization started after 4 weeks and lasted through 12 weeks of incubation. Cumulative CO₂–C evolution rates differed significantly among soils, and between fertilizer treatments, with the N-fertilized soils evolving lower rates of CO₂–C than the unfertilized soils. Soils from the surface horizons in the forest evolved the highest rates of CO₂–C (127.6 and 89.4 mg g^-1 soil for the Oie and Oa horizons, respectively) followed by the cleared uncropped soil (42.8 mg g^-1 soil C), and the cropped soils (25.4 and 29.0 mg g^-1 soil C). In vitro soil respiration rates, or potential soil organic matter decomposition rates, decreased with increasing time after clearing and in accord with the degree of disturbance. Only soils with high potential C mineralization rates and high organic P to total P ratios, mineralized P by the end of the study. Mineralizable P appeared to be associated with readily mineralizable organic C.