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Soil Sci. Soc. Am. J. doi:10.2136/sssaj2012.0427 Received 21 Dec. 2012. *Corresponding author (Noura.Ziadi@agr.gc.ca). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Changes in Soil Phosphorus Fractions for a Long-Term Corn-Soybean Rotation with Tillage and Phosphorus Fertilization Nutrient Management & Soil & Plant Analysis

Accurate critical levels must be accompanied by predictive models on the amount of P required to increase P availability to a target value to obtain reliable P recommendations. We estimated, based on information on soil properties, the increase in soil available P after the addition of a unit of P (b coefficient) in an area of homogeneous although geographically distant soils. All 71 selected soils were noncalcareous, belonging to the Mollisol order and located in the Pampean Region (Argentina). Samples (0-20 cm) were incubated for 45 d after the addition of five doses of P. Soils were characterized for parameters related to soil P availability: pH, particle size distribution, organic C, total P, initial soil available P, and two P retention indices differing in the interaction period between the added P and the soil matrix (1 or 18 h). Obtained b values ranged from 0.27 to 0.74. Soils located at the northern area of the study region had higher b values (mean = 0.58) than southern ones (mean = 0.42). The best multiple regression model for estimating the b coefficient (R 2 = 0.70) included initial soil P, a binary variable that accounts for the location of the soil, and the P retention index with the shorter interaction period. The strong effect of the geographic position of the soil on the estimation of the b coefficient led us to hypothesize about a possible role of the mineralogic characteristics of the parent material on P dynamics. Our results indicate that single independent variable models may not be enough to predict the b coefficient in homogeneous soils.

The purpose of this 4-yr study was to evaluate tall fescue (Festuca amndinacea L.) production and changes in soil P fractions resulting from alum-treated poultry litter (ATPL) applications. Plots were established in the spring of 2000 at Orange, VA, on a Davidson loam (fine, kaolinitic, thermic Rhodic Kandiudult). Treatments included the application of ATPL, non-treated poultry litter (NPL) and triple superphosphate (TSP) at rates based on current litter management strategies in Virginia. During the 4 yr of this study, applications of ATPL resulted in forage yield and P uptake similar to that found in treatments receiving NPL. Also, no significant differences in Mehlich-1 extractable soil P between litter treatments were observed after 4 yr of application. A soil P fractionation procedure revealed significantly decreased H 2 O extractable inorganic P (Pi) concentrations in soils receiving N-based rates of ATPL compared with equivalent applications of NPL. Conversely, the organic P (Po) fraction extracted with 0.1 M NaOH was significantly elevated in treatments that had received ATPL compared with those receiving NPL or TSP regardless of P application rate. The results indicate that applications of ATPL does not adversely affect P uptake by fescue or forage production. This study also shows that ATPL can have an impact on the forms of P found in soils following continued applications; however, these differences may not always be detected through routine soil analyses such as the Mehlich-1 extraction.

Methods for environmental risk assessment of P loss potential from soils lack uniformity and are generally difficult for routine analysis. Mehlich-1 extractable P (M1-P), an approach that is widely used to assess soil P status for plant growth, was used as a soil test P (STP) estimator of the degree of P saturation (DPS) of a soil. The concept of DPS integrates the dominant properties controlling the P sorption-desorption status of soils. Soil samples from three physiographic regions of Virginia were analyzed for M1-P and a wide range of other extractable P forms and selected chemical and physical soil properties. The DPS determined by ammonium oxalate (NH 4 -Ox) extractable P (P ox ), Al (Al ox ), and Fe (Fe ox ), ranged from 2 to 155%. Mehlich-1 P, with a range of 1 to 1100 mg kg -1 was the most suitable single variable for estimating DPS. However, soil type and properties from the three physiographic regions were sufficiently different that regression models to estimate DPS based on M1-P were significantly (P < 0.001) different between regions. Addition of other chemical or physical soil properties yielded insufficient improvements to the regression models over the strong relationships (r 2 = 0.93, 0.98, and 0.75 for the Ridge & Valley, Piedmont, and Coastal Plain regions, respectively) between M1-P and DPS. Interpretations/comparisons between studies are often limited by the numerous methods that are used to calculate DPS. We recommend DPS be determined as mmol kg -1 of NH 4 -Ox extractable P, Al and Fe and calculated as 100 (P ox ) (Al ox + Fe ox ) -1 .

Laws and guidelines limiting P applications to cropland based on soil P exist in the Mid-Atlantic USA because of water quality concerns. We evaluated Mehlich 3 (M3) as an environmental soil P test using 465 soils typical to the Mid-Atlantic region and found M3-P accurately predicted water soluble P (WSP), desorbable P (Fe oxide strip P [FeO-P]), and total sorbed P (oxalate P). The M3-P saturation ratio (M3 [P/(Al+Fe)]) was linearly related to the well-established oxalate P saturation method (DPS or ) and a M3 [P/(Al+Fe)] range of 0.10 to 0.15 corresponded to reported environmental limits for DPS ox (25-40%). Rainfall simulation and column leaching studies showed M3 [P/(Al+Fe)] predicted runoff and leachate P concentrations better than M3-P. We suggest consideration of the following approach now used in Delaware for agri-environmental interpretation of M3-P and M3 [P/(Al+Fe)]: (i) Below optimum (crop response likely; M3-P ≤ 50 mg kg -1 ; M3 [P/(Al+Fe)] 100 mg kg -1 ; M3 [P/(Al+Fe)] > 0.11); (iv) Environmental (implement improved P management to reduce potential for nonpoint P pollution-in Delaware M3-P > 150 mg kg -1 ; M3 [P/(Al+Fe)] >0.15 is now used). (v) Natural Resource Conservation (no P applied even if the potential water quality impact is low to conserve P, a finite natural resource).

There is uncertainty concerning evaluation of bioavailable P in manured soils. This study assessed P availability in manured Iowa soils by measuring soil P with agronomic and environmental P tests and P uptake by corn (Zea mays L.) and soybean [Glycine max (L.) Merr.]. Soil and plant samples were collected at the V5-V6 growth stage from trials established at nine locations that received various rates of liquid swine (Sus scrofa) manure, and from farmers' fields that received other animal manures. Soil P was analyzed by the Bray-P1 (BP), Olsen (OP), and Mehlich-3 (M3P) agronomic tests, and by the Fe-oxide impregnated filter paper (FeP), anion-exchange resin membrane (RP), and water (WP) environmental tests. Soil P at a 15-cm depth ranged from deficient to 15 times optimum levels for crops. Extracted P was highest for BP, M3P, and RP, intermediate for OP and FeP, and lowest for WP. Relationships between soil P extracted by the tests were linear, trends were similar for manured and unmanured plots, and correlation coefficients were ≥0.70 (correlations were poorest for WP). There was no conclusive evidence for differences between tests in detecting manure-derived soil P at most sites. However, in some conditions BP, M3P, OP, and RP may extract proportionally more manured-derived P than FeP and WP. Only the agronomic tests were significantly correlated (0.42 for M3P, 0.44 for BP, and 0.47 for OP) with plant P uptake across sites. Agronomic soil tests would predict plant P availability for crops better than environmental P tests in soils receiving liquid swine manure.

Since the FeO-sink method to estimate plant-available P in soils was first introduced in 1983, modifications have continued to improve the procedure. Recently FeO-coated paper circles (5.5-cm diam.) were used in place of the traditional 2 by 10 cm strips. The objective of this study was to more clearly describe the preparation and use of the FeO-coated circles. We used Whatman no. 50 hardened filter paper (2.7-μm pore openings). Standard methods for FeCl 3 treatment of the papers were used. After the papers had been air dried, one paper was secured between two 10 by 10 cm fiberglass screens. By simultaneously immersing stacks of 25 of these paper-screen units in 2.7 M NH 4 OH for a 30-s immersion time, 100 papers could be uniformly coated with FeO in about 10 min. Immersion times of 30, 60, and 180 s in NH40H solution showed that papers immersed for 30 s in NH 4 OH solution extracted P as effectively as those immersed for longer times. One 5.5-cm FeO circle was as effective as three circles in extracting P from most soils. A desorption model for soil P was used that can relate soil-test P to desorption kinetics. In soils with maximum reversible P ranging from 25.1 to 106.5 mg kg -1 , desorption constants ranged from 0.141 to 0.232 h -1 with a mean of 0.191 h -1 . Low-P soils released P slower than did high-P soils. Such information may be of interest in assessing P availability in soils.

Excluding fertilizer P, a finite quantity of soil P exists for plant uptake. To improve our understanding of sinks and sources of long-term plant-available P, soil P fractions to a depth of 30 cm were determined in soils under a continuous cropping system prior to fertilization (1975), after 10 yr of P fertilization (1985), and 6 yr after the last P fertilization (1992). Soil types for two study sites were Norfolk loamy sand (fine-loamy, siliceous, thermic Typic Kandiudult) and Davidson clay loam (clayey, kaolinitic, thermic Rhodic Paleudult). Superphosphate was applied from 1975 to 1986 at rates of 0, 10, 20, and 40 kg P ha -1 yr -1 . Average removal of P, via corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] cropping, was between 10 and 20 kg P ha -1 yr -1 from 1975 to 1992. Resin-extractable soil P increased in the Norfolk soil with annual P applications that were in excess of crop removal and decreased with annual P applications that were less than crop P removal. Resin-extractable soil P decreased to below 3 mg P kg -1 in the Davidson soil regardless of P application or removal rate. Inorganic soil P extracted with NaHCO 3 and NaOH increased with excess P additions and decreased with deficient P additions for both soils. Organic soil P in the Norfolk soil extracted with NaOH represented a P sink at the 40 kg P ha -1 yr -1 treatment in 1985, but subsequently mineralized by 1992. In both soils, inorganic P extracted in the resin, NaHCO 3 , and NaOH fractions, and organic P in the NaOH fraction of the Norfolk soil, represented the biologically dynamic P.

The effect of soil texture on P transformation and the extent and timing of crop residue decomposition affects both P availability and the fate of the P added in the residue. Our objectives were to examine P transformation in sand, sandy loam, and loam soils during surface or incorporated wheat (Thiticum aestivum L.) straw decomposition with and without N and P addition. Packed soil cores (4.8 cm diameter by 2.5 cm deep) were incubated in the dark for 10, 30, 60, and 90 d at 25°C and - 0.033 MPa initial soil water pressure. We determined CO 2 respiration, labile inorganic P (Pi), labile organic P (Po), microbial P, NaOH-extractable Pi and Po, and HCl-extractable Pi. Soil texture had a greater effect on P transformation than did residue placement. Microbial P was greatest at 10 d and was 13.0, 9.8, and 6.5 mg kg -1 for the loam, sandy loam, and sand, respectively. Labile Pi was lowest at 10 d and was 22.9, 18.0, and 11.5 mg kg -1 for the loam, sandy loam, and sand, respectively. More of the nutrient P added remained as labile Pi when the residue was surface applied. Residue placement did not affect microbial P, indicating that the effect of placement was indirect. Phosphorus transformations were closely linked to microbial activity and C dynamics.

Iron oxide coated filter paper can estimate labile P in soil by acting as a P sink. Variations in procedures and contamination from soil adhering to the sinks limit effective data interpretation, however. We evaluated two papers with pores

Testing of a simplified soil phosphorus model is described. Changes in organic P content of the surface soil due to cultivation are simulated relatively accurately for periods of up to 40 years at several locations in the Great Plains. Simulated changes in organic P contents of the subsoil are smaller than those of the topsoil. Concentrations of soil test P are accurately simulated over periods of up to 20 years with and without fertilizer P applications in several soils. Simulated responses of maize (L.) and wheat (L.) to soil test P concentrations are similar to those found in three published studies. Fertilizer P required to maintain adequate P fertility is accurately simulated for maize and wheat at several locations in the U.S.

Etude de 5 techniques et de leur valeur a prevoir la reponse en croissance des agrumes aux champignons mycorhiziens (Poncirus trifoliata × Citrus sinensis)

The effect of phosphorus fertilization of three different soils on the growth and minor element nutrition of citrus was evaluated in greenhouse studies. These soils were collected from uncultivated areas in San Diego County, Calif., where citrus had benefited from P fertilization. The soils were placed in 3-gallon crocks and treated with monocalcium phosphate in amounts equivalent to 0, 76, 360, and 900 lbs. P per acre. Budded lemons and/or sour orange seedlings were subsequently planted in these soils. After 6 months of growth the plant tops were harvested, weighed, and analyzed. In later experiments the soils were pretreated with Cu at the rate of 20 ppm. Cu, and tested for P interrelations using only sour orange seedlings as indicator plants. Severe Cu deficiencies were induced in the citrus plants by as little as 360 lbs. P per acre. Plant growth in the soils treated with high concentrations of P was greatly reduced when compared to that of the controls. Typical symptoms of Cu deficiency as well as markedly reduced leaf Cu values were associated with the heavy P treatments. Pretreating the soil with Cu at the rate of 20 ppm. Cu only partially modified the P-Cu effect. Zinc absorption also was reduced by excessive P fertilization. Iron and Mn appeared to be unaffected.

Linear correlations are presented to compare the Bray and Olsen availability tests for soil phosphorus with four inorganic P fractions, clay content, and pH. The soils represent two great soil groups, several general profile textures, several parent materials, and three horizons. Although trends are indicated when general profile textures, parent materials, and horizons are considered separately, the clearest relationship between the availability tests and the four inorganic P fractions is with great soil group. When all Chernozem samples are considered together the Bray method gives higher positive correlations with the P fractions than the Olsen method. For Chestnut soils considered together the Olsen method gives higher positive correlations than the Bray method. Only one correlation among the Chernozem and Chestnut soils had a significant z value, however. Among the Chernozems when the r values of the textures, parent materials, and horizons are placed within their appropriate great soil group the Bray method has higher r values for 28 of the 31 positive correlations involved. Two of these had significantly higher r values as determined by the z test. When this procedure is followed for the Chestnuts the Olsen method has higher r values for 18 of the 29 positive correlations involved. Eleven of these correlations had significant z values. Clay content, pH, and Ca-P generally were involved in more negative than positive correlations with the Olsen and Bray methods with the Chernozem and Chestnut soils. The difference in the behavior of the Bray and Olsen reagents between the Chernozem and Chestnut soils used in this study probably is related to climate and its influence on the degree of development of the two great soil groups involved.

The inorganic soil phosphorus of the upper three horizons of 16 profiles was fractionated. The soils were selected to provide for a comparison of four kinds of parent materials and two great soil groups. The data show that, in general, the four glacial parent materials had similar proportional amounts of the inorganic P fractions. Climate, as expressed through great soil group, did affect the proportional amounts of two of these fractions, however. Among Chernozem soils the Fe-P fraction makes up a proportionally larger amount, and the Ca-P fraction a proportionally smaller amount, than they each do in Chestnut soils. Neither parent material nor climate alters the proportional amount of Al-P present in these soil series.

A survey was conducted to determine the relationship between the yield of soybeans and the nutrient level of the soil. The yield of soybeans was related to the soil phosphorus level and the soil pH. When the yields of soybeans were placed into the various soil test categories for phosphorus, it was found that the lowest average yield was in the low soil phosphorus category and the highest average yield in the high soil phosphorus range. Studies in the greenhouse also showed that the yields of soybeans were related to the soil phosphorus level. The response of soybeans to the addition of phosphorus was dependent upon the soil phosphorus level. A much better evaluation of the effect of the phosphorus level of the soil on the yield of soybeans was obtained by placing the soils into categories ranging from low to very high than was obtained by use of rank correlation.

A greenhouse study was conducted to determine the level of soil P adequate for satisfactory growth of Ocala sand pine (Pinus clausa (Chapm.) Vasey) seedlings. Various quantities of Lakeland sand with low and high P contents were blended to provide soil mixtures with a range of available P. Ocala sand pine seedlings were grown in these mixtures for 10 months. On the basis of the seedlings' dry matter production, the level of soil P adequate for satisfactory growth of Ocala sand pine is in the neighborhood of 1 ppm available [NH₄OAc (pH 4.8)-extractable] P. Seedlings growing in soils containing < 1 ppm available P showed a marked reduction in dry weight. Tissue analyses indicate that concentrations of foliar P of < 0.15% may denote a deficiency of available soil P.

Yield of both pea vines and canning peas were increased significantly by phosphorus fertilizers. Yield response was in the order: current season applied phosphorus > 1 year residual > 2 year residual. Yield of pea vines and canning peas were increased significantly for increases in water application in the order: 5 inches 12 inches. Time of sampling pea vines for chemical analysis was shown to be an important factor if it is desired to relate yields to the soluble phosphorus content of plant tissue, or soluble phosphorus content of plant tissue with irrigation regime. Relationships between sodium bicarbonate-soluble phosphorus and phosphorus in the plant and between yield of peas and phosphorus content of plant tissue are shown.