Critical Olsen P and CaCl2‐P levels as related to soil properties: results from micropot experiments

The usefulness of soil phosphorus (P) tests used in routine soil analyses is limited by the fact that a single measurement cannot encompass all P‐related factors potentially affecting plant performance. In this work, we performed micropot (15 mL) experiments to test the hypothesis that the predictive value of two common soil P tests (Olsen P and CaCl₂‐P) can be improved by considering properties commonly measured in laboratory analyses. Forty‐nine sets of soils ranging widely in properties were used for this purpose, each set consisting of samples with similar properties but differing in P status. Ryegrass and turnip were grown in a chamber for 30 days in two separate experiments and their yields at harvest recorded. The critical Olsen P and CaCl₂‐P levels, which were taken to be those corresponding to 95% asymptotic yield as calculated from data fitted to a Mitscherlich equation, were greater for turnip than for ryegrass, probably as a result of the difference in yield (49 and 160 mg dry matter/micropot on average for ryegrass and turnip, respectively) and hence in P requirements between the two species. Critical Olsen P spanned narrower ranges than critical CaCl₂‐P in both crops and is therefore seemingly the more robust of the two tests. Both critical P values exhibited moderate correlations with soil properties. Thus, critical Olsen P was (a) lower in soils with a medium pH – which is consistent with the fact that the bicarbonate solution method tends to overestimate plant‐available P in strongly acid and calcareous soils; (b) positively correlated with pH and carbonate content in calcareous soils; and (c) uncorrelated with soil properties in noncalcareous soils. On the other hand, critical CaCl₂‐P in some soil groups was negatively correlated with some properties increasing the P buffering capacity of soil (e.g. Fe oxide content). Taken together, our results suggest that routinely measured soil properties help to predict critical Olsen P better than critical CaCl₂‐P.     

Assessing the Feasibility of Olsen P and P Sorption Parameters in Acidic Soils

Plant-available phosphorus (P) and P adsorption capacities are important for crop growth in acidic soils. Olsen P test, which is based on extraction with bicarbonate for predicting the amount of soil P available to plants, was used in this work. Soil P-adsorption capacities were determined by Langmuir line equation. The purpose of this work was to examine the suitability of Olsen P for predicting phytoavailable P and P sorption parameters in acid soil. To this end, we (i) assessed the phytoavailable P by successively pot-cropping rice and (ii) P adsorption characteristics of soil and their relation with Olsen P. Plant-available P, estimated by Olsen P in tested soil, was correlated to labile P. Qm (phosphorus sorption maximum) was negatively correlated with K (P sorption strength). P buffering capacity of soils was P3 (the highest P rate) >P2 (the second highest P rate) >P1 (the lowest P rate) >P0 (no P adding) after 75 day’s rice growth, which indicated P replenishment capacity was different among P treatments. This also suggested that P of plant uptake may decrease soil buffering capacity, especially for soils that contained relatively lower amounts of P. Qm and K were not significantly correlated to Olsen P. Degree of P saturation and Olsen P shared the similar trend with the change of P application rates and sampling dates. We concluded P status in soil can be characterized by degree of P saturation and Olsen P in tested soil. They were able to explain P status from both agronomic and environmental aspects.

Abbreviations: Qm, P sorption maximum; K, P sorption strength; P3, highest P rate in soil; P2, second highest P rate in soil; P1, lowest P rate in soil; P0, P adding in soil.     

Development of an alternative to the Olsen bicarbonate-extraction test for determining plant-available phosphorus in basaltic soils

Abstract. Recent work has demonstrated that the Olsen test for phosphorus (P) is an unreliable predictor of plant-available P in soils derived from basalt parent material in Northern Ireland. The present study was conducted to develop a more reliable soil-P test for these soils by regressing P fractions removed from soil by various chemical extractants against herbage P indices calculated from plant tissue test data using a diagnosis and recommendation integrated system. The degree of P saturation of the soil P sorption capacity, based on ammonium oxalate extractable P, Al and Fe, provided a better prediction of P available to swards on basaltic soils than either the Olsen test or a number of other well-known soil-P test procedures. The superiority of the degree of P saturation test on basaltic soils was attributed to the fact that it simultaneously takes account of both P quantity and P buffering capacity factors in predicting P availability. The Olsen-P test, which accounts for the P quantity factor alone, was only reliable for non-basaltic soils. Re-classifying the P fertility status of basaltic soils according to the degree of P saturation test could result in considerably less P being recommended for these soils with possible consequential benefits to water quality.     

Changes in Soil pH and Extractable Phosphorus Following Application of Turkey Manure Incinerator Ash and Triple Superphosphate

During the incineration of turkey manure, a wide variety and concentration of nutrients are conserved in the turkey manure ash (TMA). In particular, the high concentration of citrate-soluble phosphorus (P) (43 g kg^?1) may make it a suitable P source for crop production. The ash is alkaline with a pH of 12.2. We conducted a soil incubation study using a low-P soil with a pH of 6.2 to evaluate the effects of TMA on soil pH and extractable P. Two TMA rates, based on citrate-soluble P (10.9 and 21.9 mg kg^?1), were compared with equivalent rates of triple superphosphate (TSP). In addition, a 0-P control was included. At the rates tested, TMA slightly increased soil pH, but this increase would be of minor agronomic importance. At equivalent P rates, changes in water-soluble P (WSP) concentrations with TMA and TSP were similar. Changes in iron-oxide-strip–extractable P (FeO P) and Bray 1 P concentrations were greater with TMA than with TSP. In contrast, changes in Olsen P concentrations were greater with TSP than with TMA for up to 32 days. For TMA, the ability of the tests to extract P was ranked, from highest to lowest, as Bray 1 P > FeO P > Olsen P > WSP, and for TSP they were ranked Bray 1 P ≥ Olsen P ≥ FeO P > WSP. However, the Bray 1 P and FeO P tests tended to overestimate plant-available P because they measured more citrate-soluble P than that added with TMA. We conclude that TMA can be used as a P source for crop production and that the Olsen test may provide a better estimate of plant-available P in TMA-amended soil.     

Soil Phosphorus Management Based on the Agronomic Critical Value of Olsen P

Fertilizer phosphorus (P) is generally added to agricultural soils to meet the needs of crop production. In this study, the crop yield and soil Olsen P were measured every year (5–18 years) at 16 winter wheat (Triticum aestivum L.) –maize (Zea mays L.) crop rotation sites in cinnamon soil (Luvisols in FAO system). The mean agronomic critical value of Olsen P for maize was 14.2 mg kg^?1 and for winter wheat was 14.4 mg kg^?1 when using the Liner-plateau and Mitscherlich models. The change in soil Olsen P was positively linearly correlated with the P budget (P < 0.01), and an increase of 4.70 mg kg^?1 in soil Olsen P for each 100 kg ha^?1 of P budget in the 0–20 cm soil layer. A model of P fertilizer recommendation rate that integrated values of the change in soil Olsen P in response to P budget and the agronomic critical value of Olsen P was used, in order to adjust current levels of soil Olsen P to the agronomic critical value at the experimental sites over the next 5 years, P fertilizer application rate should be in the range of 0–87.5 kg P ha^?1.     

Determining the optimum range of soil Olsen P for high P use efficiency, crop yield, and soil fertility in three typical cropland soils

Soil Olsen P level has a major influence on crop yield, efficient P utilization, and soil fertility. In this study, the optimum Olsen P range was determined from long-term (1990-2012) field experiments in three typical soil types of China under single cropping of maize or double cropping of maize and wheat. The critical soil Olsen P value for crop yield was evaluated using three different models, and the relationships among P use efficiency (PUE), Olsen P, and total P were analyzed. The agronomic critical soil Olsen P values obtained from the three models for the neutral soil of Gongzhuling and the calcareous soil of Zhengzhou were similar; however, the values from the linear-linear and linear-plateau models for both maize and wheat were substantially lower than those from the Mitscherlich model for the acidic soil of Qiyang. The PUE response change rates (linear equation slopes) under different soil Olsen P levels were small, indicating slight or no changes in the PUE as the soil Olsen P increased in all three soils. A comparison of the Olsen P levels that achieved the maximal PUE with the agronomic critical values derived from the three models indicated that the linear-plateau model exhibited the best performance. The regression equation coefficients of Olsen P response to total P decreased as follows:Zhengzhou (73 mg g^-1) > Qiyang (65 mg g^-1) > Gongzhuling (55 mg g^-1). The Olsen P level increased as the total P increased, which may result in a decrease in PUE. To achieve a relatively high crop yield, PUE, and soil fertility, the optimum Olsen P range should be 13-40, 10-40, and 29-40 mg kg^-1 at Gongzhuling, Zhengzhou, and Qiyang, respectively.     

The ability of the DGT soil phosphorus test to predict pasture response in Australian pasture soils – a preliminary assessment

Diffusive gradients in thin‐films (DGT) technology provides an alternative assessment of available phosphorus (P) for a range of crops, suggesting a preliminary examination of the performance of the new DGT‐P test, compared to existing bicarbonate extractable Olsen and Colwell P tests, for pastures is justified. This study utilized historic data from the Australian National Reactive Phosphate Rock (NRPR) study (1992–1994) that included 25 experimental sites representing a wide range of soil types and climates used for pasture production. Stored (~19 yr) soil samples were analysed for DGT‐P, Olsen P and a single point P buffering index (PBI) and re‐analysed for Colwell P. Results showed the traditional bicarbonate extractable Colwell (r² = 0.45, P < 0.001) and Olsen P (r² = 0.27, P < 0.001) methods predicted relative pasture P response more accurately, compared to the novel DGT‐P test (r² = 0.09, P = 0.03) when all 3 yr of data were examined. We hypothesize that the harsher bicarbonate extraction used for the Olsen and Colwell methods more accurately reflects the ability of perennial pasture roots to access less labile forms of P, in contrast to the DGT‐P test, which does not change the soil pH or dilute the soil and appears unable to fully account for a plants ability to solubilize P. Further studies are needed to compare the capacity of DGT‐P to measure P availability in perennial pasture systems and to better understand the soil chemical differences between pasture and cropping systems.     

Predicting Plant Phosphorus Requirements for Hawaii Soils using a Combination of Phosphorus Sorption Isotherms and Chemical Extraction Methods

Phosphorus (P) is an essential nutrient for plant growth and reproduction. One of the tasks of soil testing is to identify whether the soil P level is sufficient to meet crop requirements, and if not, to provide an estimate of the quantity of P that must be added for good growth of a given crop. Data for 12 soils (11 series) from Hawaii were used to develop correlations between critical P concentrations in soil solution derived from P sorption isotherms with P extracted with Mehlich 3, Olsen, or modified Truog solutions. Extractable P, in turn, was correlated with P fertilizer requirements. Critical P levels in soil solution reported in the literature for various crops ranged from 0.005 mg L^?1 for cassava (Manihot esculenta) to 0.30 for lettuce (Lactuca sativa) and to 1.6 for nonmycorrhizal onions (Allium cepa). The P buffer coefficient, defined as the ratio of fertilizer P added to extractable P, averaged 2.2, 4.2, and 8.6 for the modified Truog, Olsen, and Mehlich 3, respectively. Phosphorus requirements for certain crops could be estimated by the following steps: (i) obtaining (possibly one time only) soil solution P levels via P sorption isotherm for a given soil (series or family), (ii) identifying the critical soil solution P for a given crop from the literature, (iii) regressing soil solution P against extractable P, and (iv) establishing relationships between extractable P and fertilizer P.     

Plant‐available soil phosphorus. Part I: the response of winter wheat and spring barley to Olsen P on a silty clay loam

Increasing fertilizer costs have prompted farmers to ask whether soils could be maintained at lower levels of plant‐available phosphorus (Olsen P) than currently recommended without losing yield. To help answer this question, we assessed the response to Olsen P by spring barley grown from 1986 to 1991, followed by winter wheat from 1992 to 2008, on a silty clay loam soil. Each year the curve relating grain yield to Olsen P was fitted statistically to determine the asymptotic yield and the Olsen P associated with 98% of that yield, that is, the critical level of Olsen P. The variance accounted for by the relationship ranged between 83 and 97% in all but two years, suggesting that the availability of soil P was the major soil factor affecting yield and that Olsen P was a reliable measure of plant‐available P in soil. Asymptotic annual yield of spring barley ranged from 2.34 to 7.12 t/ha and of winter wheat from 3.87 to 10.36 t/ha. In part, this range in yields was because of changes in the cultivar grown while the range of yields for any one cultivar was probably due to differences in weather, principally rainfall, between years. Critical Olsen P ranged from 7 to 18 mg/kg for both cereal crops (with one outlier at 26 mg/kg for winter wheat) most probably due to seedbed and soil structure conditions affecting root growth, and thus acquisition of available soil P, and the way these soil factors were affected by weather. Thus, a general recommendation for cereals grown on this silty clay loam, which is comparatively easy to cultivate, would be to maintain Olsen P at about 20 mg/kg in the plough layer to minimize the risk of losing yield in some years. This value, 20 mg/kg, equivalent to 20 mg/L, is the midpoint of P Index 2, the recommended P Index given in the Fertiliser Manual (RB209) (Defra 2010) for soils growing arable crops and grass in England, Wales and Northern Ireland.     

Phosphorus Management of Lucerne Grown on Calcareous Soil in Turkey

ABSTRACT

Lucerne or alfalfa (Medicago sativa L.) is grown as a forage crop on many livestock farms. In calcareous soils in eastern Turkey, lucerne production requires phosphorus (P) additions as the soils are naturally P deficient. Phosphorus sorption isotherms were used to estimate P fertilizer needs for lucerne grown for two years in a 3-cut system on a calcareous P deficient Aridisol in eastern Anatolia, Erzurum province, Turkey. Annual P applications ranged from 0–1200 kg P ha^?1. The Langmuir two-surface adsorption equation was used to derive the maximum P sorption capacity of unamended soil and to determine soil solution P, maximum buffer capacity (MBC), equilibrium buffer capacity (EBC), and P saturation at the optimum economic P rate (OEPR) for dry matter (DM) production. Soils were tested for Olson P at the onset of the study and after two years of P applications. In both years, tissue was analyzed for P content at flowering prior to first cutting. The OEPR (2-year average) was 754 kg P ha^?1 yr^?1 corresponding with a soil solution P concentration of 0.30 mg L^?1, a DM yield of 8725 kg DM ha^?1, and $528 ha^?1 annual profit. The P content of leaves at flowering increased linearly with P application beyond 100 kg P ha^?1 and was 3.2 g kg^?1 P at the OEPR. The unfertilized soil had an EBC, MBC, P saturation, and X_max of 3304 mL g^?1, 3401 mL g^?1, 6%, and 1086 mL g^?1, respectively, whereas two years of fertilization to the OEPR decreased EBC and MBC to 358 mL g^?1 and 540 mL g^?1, and increased P saturation and Olsen P to 56% and 32 mg kg^?1, respectively. These results suggest a P saturation >50% or Olsen P >30 mg kg^?1 are needed to maintain an optimum soil solution concentration of 0.30 mg L^?1 in this calcareous Aridisol. Similar studies with different soils and initial soil test P levels are needed to conclude if these critical soil test values can be applied across the region.     

不同农业土壤中有效磷的变化与磷平衡的关系

Maintaining soil phosphorus(P) at adequate levels for plant growth requires assessing how the long-term P balance(viz., the difference between P inputs and outputs) results in changes in soil test P. The hypothesis that routinely measured soil properties can help predict the conversion factor of P balance into Olsen P was tested at 39 sites in agricultural areas of the Mediterranean region in Spain. A set of soil samples from each site was analyzed for Olsen P, inorganic P(P extracted using 0.5 mol L~(-1) H_2SO_4), pseudototal P(P extracted using 0.5 mol L~(-1) H_2SO_4 following ignition at 550℃), and organic P(the difference between pseudototal P and inorganic P). Organic and Olsen P were uncorrelated in most of the 39 soil sets, which suggests that organic P content changed little with P inputs and outputs. The slopes of the regression lines of Olsen P against pseudototal and inorganic P, which were used as two different measures of the conversion factor, ranged widely(from 0.03 to 0.25 approximately), with their average values(about 0.10) being similar to those found in long-term experiments conducted in temperate areas. Neither conversion factor was significantly correlated with any routinely measured soil property; however, the conversion factor for inorganic P was significantly lower for calcareous soils than for noncalcareous soils. Our negative results suggest the need to isolate the influence of soil properties from that of management systems and environmental factors relating to P dynamics in future studies.     

Soil Phosphorus Tests for Flooded Rice Grown in Contrasting Soils and Cropping History

Soil phosphorus (P) tests for flooded rice (Oryza sativa L.) generally present uncertainties for estimating P availability. Bray 1, 1% citric acid, Mehlich 3, Olsen extractants (dry samples), and Bray 1 extractant after 3 days (BI3) and 7 days (BI7) of anaerobic incubations were evaluated to estimate P availability for rice in 43 Uruguayan soils. Field trials were conduced at each site (0, 13, 26, and 39 kg P applied ha^?1). Relative yield and absolute and relative yield increases were determined. Extracted P was variable for the different tests. For silty soils, P availability was better estimated by citric acid, Mehlich 3, and Bray 1, with similar soil P critical concentrations (6?8 mg P kg^?1). The BI3 and BI7 tests showed greater soil P critical concentration but poorer correlations with yield indexes. This study contributes to the scientific basis of P fertilization for flooded rice, promoting more effective fertilizer use and minimizing environmental P losses.     

Changes in soluble and equilibrium phosphate concentration in selected soils from Italy

Abstract

Soil phosphate in solution (PsolCpand equilibrium P concentration (EPCo) are important soil P parameters both for agronomical and environmental purposes. Solution P is the main source from which plant roots adsorb P, whereas EPCo give information about the amount and direction of changes between soluble and particulate P that occur during transport of sediment in stream flow. Changes in Psol and EPCo with soil extractable P (Mehlich 3‐P and Olsen P) were determined following equilibration of 13 Italian soils with five rates of fertilizer P (0 to 100 mg kg^‐1). The slope of the regression equation (Y=a+bX) for the relationships between Psol and EPCo (Y), and soil extractable P (X) decreased with increasing soil P sorption. Furthermore, at the same level of Mehlich 3 and Olsen P, higher sorbing P soils had lower values of Psol and EPCo. As a result, changes in Psol and EPCo per unit increase of soil extractable P were closely related (R² of 0.86 to 0.93) to soil P Sorption Index (SI). Equilibrium P Concentration could be estimated from soil solution P for the studied soils.     

Plant‐available soil phosphorus. Part II: the response of arable crops to Olsen P on a sandy clay loam and a silty clay loam

The increasing cost of fertilizer has prompted farmers to ask whether soils could be maintained at lower levels of plant‐available phosphorus (Olsen P) than currently recommended, without limiting yield. To help answer this question, critical levels of Olsen P have been determined for spring barley, winter wheat, potatoes and sugar beet grown on a sandy clay loam and a poorly structured heavy textured silty clay loam. On each soil, there were plots with a range of well‐established levels of Olsen P and, in one experiment, two levels of soil organic matter (SOM). For each crop and each year, the response curve relating yield to Olsen P was fitted statistically to determine the asymptotic yield and the Olsen P associated with 98% of that yield, that is, the critical Olsen P. Maximum yield of all four crops varied greatly from year to year, in part due to applied nitrogen (N) where it was tested, and in part to seasonal variation in weather, mainly rainfall. The wide range in critical Olsen P, from 8 to 36 mg/kg, between years was most probably as a result of differences in soil conditions that affected root growth and thus acquisition of available soil phosphorus (P). Generally, a larger asymptotic yield was not necessarily associated with a larger critical Olsen P. Spring barley and winter wheat given little N required more Olsen P, 20–34 mg/kg, to achieve the asymptotic yield, compared to 10–17 mg/kg where ample N was given; presumably, more roots were needed to search the soil for the smaller amounts of available N and root growth is affected by the amount of plant‐available soil P. In a field experiment on one soil type, soil with little SOM required 2–3.5 times more Olsen P to produce the same yield as that on soil with more organic matter. Soil organic matter most probably improved soil structure and hence the ability of roots to grow and search for nutrients in field conditions because when these soils were cropped with ryegrass in controlled conditions in the glasshouse, the yields of grass were independent of SOM and there was the same critical Olsen P for both soils. Overall, the data confirm that, for these soil types, the current recommendations for Olsen P for arable crops in England, Wales and Northern Ireland are appropriate.     

Establishing sorption isotherm to meet phosphorus requirement for tomato seedling growth 1

An Indiana silt loam soil was equilibrated with various amounts of Ca(H₂PO₄)₂ H₂O and a 0.01 M CaCl₂ solution to construct its phosphorus sorption isotherms. Using the isotherms, the P buffering capacity of the soil was calculated and amounts of fertilizer P necessary to give several levels of P in the soil solution, for experiments conducted over a 2 year period, were determined. Twenty‐four day old tomato seedlings were grown and measured for leaf area, root length, dry weights and P concentrations in leaf, petiole, stem and root.

Phosphorus concentration in soil solution increased slowly with the first increment of P added to the soil. Subsequent P additions increased the P concentrations in solution exponentially. The maximum P absorption by the soil was 324 μg P/g soil and the constant related to P binding energy was 1.37. In addition, the soil buffering capacity decreased with an increase in the amount of P in the soil solution.

Plant shoot dry weight increased linearly with P increase in the concentration range 0.65 to 6.5 μM P in soil solution. However, beyond this level the response was low. The leaf area rate of increase in the 0.65 to 6.5 μM P solution concentration range was 75 times that in the 6.5 to 84 μM P. The root length: shoot dry weight ratio decreased with increasing P supply in the soil solution. P uptake by the plants increased with increased P concentration in soil solution. At soil solution concentrations above 6.5 μMP the rate of P uptake in the shoot was 20 times less than the rate for concentrations below 6.5 μM P. Of the P taken up by tomato seedlings about 65% was in the leaf, 13% in the stem, 13% in the petiole and 9% in the root.     

Phosphorus sorption,supply potential and availability in soils with contrasting parent material and soil chemical properties

Soil phosphorus (P) management requires a more targeted and soil‐specific approach than is currently applied for agronomic recommendations and environmental evaluation. Phosphorus buffering capacities control the supply of P in the soil solution and were measured across Irish soils with contrasting parent material and chemical properties. Langmuir sorption buffer capacities (MBCs) and binding energies (b) were strongly correlated with soil pH and extractable aluminium (Al). A broken‐line regression fitted to the relationship between MBC and Al derived a change‐point value for Al above which MBC increased linearly. Soils above the change point were predominantly acidic to neutral with non‐calcareous parent material, with larger buffering capacities and binding energies than calcareous soils. Ratios of Mehlich3‐Al and P (Al:P) were used to relate buffering capacity to supply potential in non‐calcareous soils. Large ratios of Al:P were associated with poor P availability, characteristic of strongly P‐fixing soils. Threshold values of iron‐oxide paper strip P (FeO‐P) and Morgan's P revealed Al:P ratios where soils began to supply P in available form. The change‐point for Morgan's P fell within the current target index for P availability; however, the confidence interval was more compatible with previous agronomic P indices used in Ireland. Relationships between Morgan's P and measures of extractable P, M3‐P and Olsen P, deviated in calcareous soils at large soil P contents, indicative of P precipitation processes dominating in these soils. Identifying differences in soil P buffering capacity at the laboratory scale would improve agronomic and environmental assessment at field and catchment scales.     

Phosphorus Buffering Capacity Indices as Related to Soil Properties and Plant Uptake

ABSTRACT

Evaluation of the plant-available phosphorus (P) in calcareous soils is commonly performed by removing a portion of solid phase P using chemical extractants. Critical soil test values, however, may be affected by variation in sorption and buffering behavior of different soils. The objective of this study was to evaluate the importance of buffering capacity indices to predict P uptake by wheat (Triticum aestivum). Eleven surface soil samples were assayed for a number of P intensity (CaCl₂-P) and quantity (Olsen-P, Colwell-P, and Resin-P) factors. Some phosphorus buffering indices were obtained from P sorption equations. A single-point index of buffering was also determined experimentally. In a greenhouse experiment, wheat was grown for 35 and 70 days on the same soils and P uptake was determined. Nonlinear and linear equations described the P sorption data (P < 0.001). Buffering indices derived from these equations were highly correlated with single-point index of capacity. Clay content was the most important soil property affecting the buffering capacity factor. The phosphorus intensity index (CaCl₂-P) was weakly related to P uptake (P < 0.05). Among the quantity factors only Resin-P was significantly correlated with P uptake. Buffering indices showed significant but inverse relationships with P uptake only at 70 days harvest (r = ?0.69 to ?0.71; P < 0.05). Combination of intensity or quantity factors with buffering capacity indices, such as intensity/capacity or quantity/capacity indicators, improved considerably the ability to account for variations in P uptake by wheat.     

Change Point in Phosphorus Release from Variously Managed Soils with Contrasting Properties

Abstract

Investigating the relation between concentration or release of phosphorus (P) into soil solution (CaCl₂‐P, determined by 0.01 M CaCl₂ extraction of soils) and soil test phosphorus (Olsen P, or 0.5 N NaHCO₃‐extractable soil phosphorus) for 10 widely ranging and variously managed soils from central Italy, a change point was evident where the slopes of two linear relationships meet. In other words, it was possible to distinguish two sections of the plots of CaCl₂‐P against Olsen P, for which increases of CaCl₂‐P per unit of soil test P increase were significantly (p<0.05) greater above than below these change points. Values of change point ranged from 14.8 to 253.1 mg kg^?1 Olsen P and were very closely correlated (p<0.001) to phosphorus sorption capacity of soils. Similar change points were also previously observed when Olsen P (and also Mehlich 3 P) of surface soils was related to the P concentration of surface runoff and subsurface drainage. Because insufficient data are available relating P in surface soils and amount of P loss by overland, subsurface, or drainage flow, using the CaCl₂ extraction of soil can be convenient to determine a change point in soil test P, which may be used in support of agricultural and environmental P management.     

Phosphorus Availability and Transformation as Affected by Repeated Phosphorus Additions in an Ultisol

Phosphorus (P) solubility and transformation in soils determine its availability to plants and loss potential to the environment, and soil P dynamics is impacted by fertilization and soil properties. A Ultisol sample was interacted with 20 mg L^?1 P solution from one to ten times. The P-reacted soils were then analyzed for water-soluble P (0.01 M calcium chloride (CaCl₂)–extractable P); plant-available P (Olsen P); ammonium chloride P, aluminum P, iron P (NH₄Cl-P, Al-P, Fe-P, respectively); and occluded P (Oc-P). The degree of P saturation (DPS) was calculated from ammonium oxalate–extractable Al, Fe, and P. The amount of P sorbed by the soil was highly correlated with the frequency of P addition with high percentage of P adsorbed initially and gradually decreased as the P addition continued. The relative abundance of the five P fractions in the P-reacted soil was in the order of Fe-P (36.5 percent) > Al-P (35.6 percent) > Oc-P (22.8 percent) > Ca-P (2.7 percent) > NH₄Cl-P (2.3 percent). Both Olsen P and CaCl₂-P were significantly increased by the repeated P addition process and highly correlated in an exponential function. The DPS was increased above the so-called critical point of 25 percent after the first P saturation process and kept increasing as the P addition continued. The P availability and adsorption in the soil were controlled by soil free and amorphous Al and Fe. The results suggest that repeated P application will build soil P to an excessive level, and consequently result in poor P-use efficiency and high P-loss potential to surface and groundwater.