连续淋洗对山东省主要土壤磷素有效性及磷库的影响

研究采取室内模拟降雨淋洗的方法,研究了连续淋洗对山东省3大土类磷素有效性和磷库的影响。结果表明,3种土壤的磷淋失量均随着供磷水平的提高而升高。3种土壤出现磷淋失量峰值的时间有着明显差异,褐土施磷处理的磷淋量峰值分别出现在第3 d,4 d和5 d;而潮土高磷和低磷处理的磷淋洗最高值分别出现在第2 d和3 d;而棕壤不同施磷处理的磷淋洗高峰值均出现在第2 d。在相对较低的施磷水平下(P3以下),3大土类的速效磷含量基本均保持在20μg/g左右,再提高供磷水平,褐土和棕壤的土壤速效磷含量随施磷量的增加显著升高,而潮土的速效磷含量则随着施磷量的增加先升高后降低。在3大土类中,全磷含量最高的是褐土,其次是潮土,再次是棕壤。在淋洗条件下,褐土的土壤磷库相对变化不大,而潮土和棕壤的磷库则随施磷量的变化有着较大的起伏。因此,施磷水平控制在P3水平(180 kg P/hm2),3大土壤的磷淋失量较低,土壤速效磷变化不大,有利于减少土壤磷素淋失到水体的风险,也保证了土壤磷的供应能力。     

连续淋洗对山东省主要土壤磷素有效性及磷库的影响

研究采取室内模拟降雨淋洗的方法,研究了连续淋洗对山东省3大土类磷素有效性和磷库的影响。结果表明,3种土壤的磷淋失量均随着供磷水平的提高而升高。3种土壤出现磷淋失量峰值的时间有着明显差异,褐土施磷处理的磷淋量峰值分别出现在第3 d,4 d和5 d;而潮土高磷和低磷处理的磷淋洗最高值分别出现在第2 d和3 d;而棕壤不同施磷处理的磷淋洗高峰值均出现在第2 d。在相对较低的施磷水平下（P3以下）,3大土类的速效磷含量基本均保持在20μg/g左右,再提高供磷水平,褐土和棕壤的土壤速效磷含量随施磷量的增加显著升高,而潮土的速效磷含量则随着施磷量的增加先升高后降低。在3大土类中,全磷含量最高的是褐土,其次是潮土,再次是棕壤。在淋洗条件下,褐土的土壤磷库相对变化不大,而潮土和棕壤的磷库则随施磷量的变化有着较大的起伏。因此,施磷水平控制在P3水平（180 kg P/hm^2）,3大土壤的磷淋失量较低,土壤速效磷变化不大,有利于减少土壤磷素淋失到水体的风险,也保证了土壤磷的供应能力。     

长期施磷黑土中磷的吸附–解吸特征及其影响因素

  【目的】  比较长期不施磷与施磷黑土对外源磷的吸附–解吸特征,为黑土区磷素管理提供理论基础。  【方法】  供试黑土长期定位试验位于吉林省公主岭市,始于1990年。2018年选择其中不施肥(CK),施氮钾肥(NK),施氮磷钾肥(NPK)和氮磷钾+有机肥(NPK+M) 4个处理小区,采集0—20、20—40和40—60 cm土层的土壤样品,分析了土壤理化性质,采用恒温平衡法测定了土壤磷的吸附–解吸特征,并由此计算得到磷最大吸附量(Q_m)、吸附亲和力常数(K_Q)、最大缓冲容量(MBC)、磷吸附饱和度(DPS)、最大解吸量(D_m)和解吸率(D_r)。  【结果】  随着平衡溶液中磷浓度的增加,磷的吸附量与解吸量均呈先快速增加后逐渐趋于平衡的过程。与不施磷处理(CK和NK)土壤相比,施磷处理(NPK、NPK+M)减少了磷的吸附量,增加了磷的解吸量。与不施磷处理相比,施磷处理在0—60 cm剖面上的Q_m和MBC值分别降低了4.94%～63.46%和15.90%～75.18%,D_r值增加了8.52%～474.0%,以NPK+M处理变化量最大。施磷处理比不施磷处理在0—60 cm土壤剖面上全磷和有机质含量分别增加了34.40%～145.5%和12.77%～50.07%,游离态铁铝氧化物(Fe_d+Al_d)含量降低了5.14%～11.35%。冗余分析表明,不施磷处理土壤的有机质、Fe_d+Al_d和全磷,以及施磷处理中的Fe_d+Al_d、络合态铁铝氧化物(Fe_p+Al_p)、pH和有机质是影响磷吸附解吸特征参数的主要土壤因子,分别解释了不施磷和施磷处理全部变异的77.59%和90.62%。土壤有效磷(Olsen-P)与磷吸附饱和度(DPS)相关关系表明,所研究的黑土DPS环境界限值为8%左右,NPK+M处理中3个土层的DPS_M-P值(由Mehlich-3 浸提的磷、铁和铝计算)为7.77%～25.96%,DPS_O-P值(由Olsen-P和Q_m计算)为17.24%～24.75%,均高于此环境界限值,具有磷素流失的风险。  【结论】  长期施磷降低了黑土对外源磷的吸附量,增加了磷解吸量。长期不施磷肥,土壤对磷的吸附和解吸主要受有机质、游离态铁铝氧化物的影响。而长期施肥,特别是有机肥与化肥配合处理,土壤中游离态和络合态铁铝氧化物、有机质及pH是影响磷吸附解吸特征的主要因素。     

宁夏主要土壤的磷酸吸附特征及影响因素

本文研究了宁夏10种主要土壤对磷的等温吸附特征,结果表明,宁夏主要土壤的等温吸附曲线与Langmuir等温吸附方程吻合。最大吸附磷量(Xmax)介于99.0～666.6μg/g之间,与土壤物理性粘粒的含量呈极显著相关。根据吸附指标预测的施磷量为54-77kg/hm2,与肥效函数法确定的用量较一致。     

间作对红壤磷素吸附解吸平衡效应的影响

磷素的吸附和解吸特性对土壤磷素迁移及其环境效应具有重要影响,过量磷肥施入易造成土壤磷素固定和流失,但合理间作可促进磷素吸收利用,减少固定,研究间作和不同施磷量条件下红壤磷素吸附解吸特性的平衡效应对促进红壤磷的高效利用,兼顾环境效应具有重要意义。本研究采取2因素裂区区组试验,主因素为种植模式,分别为与玉米||大豆(IM)、单作玉米(MM);副因素为施磷水平,分别为P0 [0 kg(P_2O_5)·hm~(-2)]、P60 [60 kg(P_2O_5)·hm~(-2)]、P90 [90 kg(P_2O_5)·hm~(-2)]、P120 [120 kg(P_2O_5)·hm~(-2)] 4个施磷水平,通过田间试验,研究间作和施磷量对红壤磷素吸附解吸平衡效应的影响;应用结构方程模型(SEM)和邻接树法(ABT)定量分析间作和施磷水平对磷吸附和解吸的相对贡献,揭示间作影响红壤磷素吸附解吸的关键因子。结果表明:1) Langmuir等温吸附方程最适合红壤对磷的吸附特征拟合,土壤磷吸附量随平衡溶液磷浓度的增加呈先增加再趋于饱和的趋势,土壤磷吸附量随施磷量的增加逐渐降低。2)种植模式和施磷水平以及交互作用极显著(P0.01)影响红壤磷素的吸附量和解吸量。间作处理较单作磷素吸附量和解吸量分别增加22.9%和9.2%(P0.05);不同施磷水平下,间作磷吸附量较单作显著增加13.0%、19.4%、41.5%和23.9%(P0.05);磷解吸量在P0和P60处理间作较单作显著增加90.2%和194.4%(P0.05),而在P90和P120处理间作较单作减少52.1%和34.1%(P0.05)。3)不同种植模式与施磷水平下,土壤磷吸附量与土壤pH、有机质、树脂磷、有效磷、全磷以及磷吸附饱和度呈极显著负相关(P0.01),与游离氧化铁、游离氧化铝和磷吸持指数呈极显著正相关(P0.01),土壤磷解吸量与标准需磷量呈极显著负相关(P0.01)。红壤磷素的吸附和解吸主要受pH、有机质和游离氧化铁的影响,间作通过改变土壤的pH、有机质和游离氧化铁含量影响红壤磷吸附量和解吸量。玉米||大豆间作具有较好的土壤磷缓冲能力,低磷水平下促进磷素大量解吸供植物吸收利用,高磷水平下促进磷素吸附有效减缓磷素的损失。     

施磷量对小麦物质生产及吸磷特性的影响

在低磷土壤条件下,以中筋小麦扬麦12号和弱筋小麦扬麦9号为材料,研究了施磷量对小麦物质生产和吸磷特性的影响。结果表明,在施磷量(P2O5)0～180.kg/hm2范围内,植株对磷的吸收量、吸收速率和磷的积累量随施磷量增加而上升;以施磷量108.kg/h2处理的叶面积指数(LAI)、植株茎蘖数、茎蘖成穗率、干物质积累量、花后干物质积累量和子粒产量最高。当施磷量超过108.kg/hm2时,相关物质生产指标则呈下降趋势,说明即使在缺磷土壤上,施磷量有其适宜值。小麦一生对磷的吸收存在两个高峰,出苗至越冬始期为第一个吸收高峰,拔节至孕穗期为第二个吸收高峰。植株磷素积累量的70%～75%是在拔节后吸收,表明拔节期施磷对满足小麦第二个吸磷高峰和磷的最大积累期需磷有重要意义。     

不同施磷量下稻田土壤磷素平衡及其潜在环境风险评估

【目的】对南方赤红壤区不同施磷量下稻田土壤的磷素平衡及其潜在环境风险进行评估,为该地区合理施磷、 减轻农业面源污染提供依据。【方法】采用大田定位监测试验,3个不同年份(2011~2013年)早、 晚稻分别设置4个施磷水平(施磷范围为P2O5 0、 63~81、 126~162、 252~324 kg/hm2,分别用P0、 P1、P2、P3表示),连续3年测定早、 晚稻的稻谷和稻秆产量,分析其磷养分含量,以施磷水平与水稻地上部磷素累积量间的差值表示土壤磷素表观盈余量。同时,采集施基肥和穗肥后1、 2、 3、 5、 7和9 d的田面水,测定总磷含量,利用Split-line模型对2011~2012年每造水稻收获后小区耕层土壤Olsen-P含量和所有监测时间点的田面水总磷平均浓度进行分段回归,并对二者之间的相关关系进行分段回归拟合。【结果】 施磷量P2O5 63~81 kg/hm2的处理稻谷产量显著提高,但磷肥施用量增至2倍时,稻谷产量无明显增加,继续增至4倍时,前3造水稻稻谷的产量增加也不明显。施磷可不同程度地提高水稻地上部的磷素累积量、 土壤表观磷素盈余量和Olsen-P含量,且三者均随施磷量的增加而增加。在施肥后1~3 d内无磷处理田面水总磷浓度较高,是磷素流失的高危险期; 施磷量P2O5 63~81 kg/hm2的处理显著提高了施肥后2 d内田面水的总磷浓度,而P2O5 252~324 kg/hm2的处理在监测期间田面水总磷浓度均显著高于无磷处理。 Split-line模型模拟土壤Olsen-P与田面水总磷的关系,得出在本试验区土壤环境条件下,可能导致田面水中磷激增的土壤Olsen-P临界含量为19.0 mg/kg,对应的施磷量为P2O5 63 kg/hm2,与土壤磷素持平的施磷量一致。【结论】综合考虑水稻产量效应、 土壤磷素表观平衡和磷素环境风险,在本研究区域目前的土壤环境条件下,P2O5 63 kg/hm2为水稻产量较高、 环境风险较小的推荐施磷量。     

矿山生态型和非矿山生态型粗齿冷水花富磷能力的比较

【目的】研究适宜施氮条件下磷富集植物粗齿冷水花的磷积累能力,可为有效利用该植物提取土壤中过剩的磷、减少磷的面源污染提供理论依据。【方法】以两种生态型粗齿冷水花 (Pilea sinofasciata) 为研究对象,进行土培盆栽试验,供试土壤为灰潮土,每盆 (8 L) 装土8 kg。分别施磷0、400、600、800 mg/kg,陈化8周。幼苗种植前,每盆施入140 mg/kg尿素。移栽9周后收获,测定植株生物量、磷含量,分析了土壤速效磷含量。【结果】1) 在供试施磷量范围内,两种生态型粗齿冷水花地上部、地下部生物量在施磷600 mg/kg时达最大,此时矿山生态型地上部、地下部生物量分别是其不施磷对照的2.37和3.69倍,非矿山生态型地上部、地下部生物量是其不施磷对照的4.63和7.36倍,矿山与非矿山生态型地上部生物量最大值分别可达28.6 g/株和31.9 g/株。矿山生态型生物量在不施磷和施磷400 mg/kg时显著大于非矿山生态型,在施磷600、800 mg/kg时两生态型间无明显差异。2) 随施磷量增加,两种生态型粗齿冷水花地上部磷含量和矿山生态型地下部磷含量均表现为先升高后降低,在施磷800 mg/kg时显著降低,而非矿山生态型地下部磷含量随施磷量增加而增加,在施磷800 mg/kg时达最高。各磷处理下,矿山生态型地上部磷含量均显著高于非矿山生态型,而地下部磷含量低于非矿山生态型。3) 高磷处理显著增加了两种生态型粗齿冷水花地上部、地下部磷积累量,且在施磷600 mg/kg时植株磷积累量最大,矿山生态型与非矿山生态型地上部磷积累量最高分别可达195 mg/株和182 mg/株。在不施磷和施磷400 mg/kg时,矿山生态型地上部磷积累量显著高于非矿山生态型,在施磷600、800 mg/kg时两生态型间无明显差异。4) 不同磷处理下,两种生态型磷富集系数远大于1,磷转移率大于80%;矿山生态型的磷提取率和移除量在不施磷和施磷400 mg/kg时均大于非矿山生态型,在施磷600、800 mg/kg时两生态型间无明显差异。【结论】适宜施氮条件下,粗齿冷水花在施磷600 mg/kg时表现出较强的磷富集能力。与非矿山生态型相比,矿山生态型在不施磷和施磷400 mg/kg时对磷的积累能力和富磷潜力更强;在施磷600、800 mg/kg时两生态型植株对土壤磷的富集能力无明显差异。     

玉米秸秆还田及施磷量对黑土磷吸附与解吸特性的影响

以探究松嫩平原玉米连作条件下,秸秆还田与施磷量互作对黑土磷吸附与解吸特性的影响为目的。该试验采取二因素裂区试验设计,主因素为玉米秸秆还田方式,分别为秸秆不还田(S0)、秸秆翻埋还田(S1)和秸秆焚烧还田(S2);副因素为施磷水平,分别为0(P0)、34.50(P1)、69(P2)、103.50(P3)kg/hm~2(P_2O_5)。结果表明:1)Langmuir等温吸附方程最适合拟合黑土对磷的吸附特征。2)秸秆还田与施磷量均显著影响黑土对磷的吸附与解吸特性,且两者互作效应显著。在相同秸秆还田方式下,随着施磷量的增加,土壤对磷的吸附能力均逐渐降低,而土壤中磷的解吸量和解吸率均逐渐增加,其中以S0条件下差异最大,S2条件下次之,S1条件下差异最小。在相同施磷水平下,与S0处理相比,S1和S2均能降低土壤对磷的吸附能力,增加土壤中磷的解吸量和解吸率,其中以不施磷肥(P0)处理下差异最大,而在施高磷(P3)处理下差异不显著,此外,S1与S2在各施磷水平下差异均不显著。3)不同施磷处理下的标准需磷量(standardPrequirement,SPR)为71.02～91.67 kg/hm~2,其中以S1P2处理的SPR(73.58 kg/hm~2)与P2施磷水平(69 kg/hm~2)最相近,是松嫩平原黑土区较为适宜的施磷方式。     

石灰性土壤上两种磷效率小麦根际特征差异

采用三室根箱实验,选用黑垆土和潮土两种石灰性土壤,对两种磷水平的磷低效型小麦京411和磷高效型小麦小偃54,进行植株生物量及吸磷量、根际土壤pH值、磷酸酶含量、水溶性磷含量的测定,旨在研究两种磷效率小麦在不同石灰性土壤上的根际特征差异.结果表明,不施磷条件下,两种土壤上,小偃54的根部生物量分别为0.85和4.62 g,均显著高于京411的0.68和3.65 g;小偃54根际土壤的pH值分别比京411低0.07,0.11个单位;在施磷条件下,小偃54的根际土壤水溶性磷分别低于京411 837,1588 μg/kg,达到显著性差异.根际磷酸酶在不同土壤上存在明显的差异,黑垆土在不施磷条件下小偃54的根际土壤磷酸酶含量显著高于京411,在潮土上呈现相反的趋势.试验结果表明,两种磷效率,小麦根际特征在不同土壤上有一定的相似性,同时存在明显差异.     

粪肥对不同磷水平土壤磷流失潜力的影响

田间试验结果表明,因施肥方式不同,粪肥施用对不同磷水平土壤磷流失潜力的影响也不相同。在施用的粪肥与表土混合的情况下,粪肥对土壤磷流失的增加量随土壤磷水平的提高而升高。但当粪肥表施不与土壤混合时,情况有所不同。在粪肥表施初期,无论是高磷土壤还是低磷土壤,地表径流中磷浓度都达到较高水平,施肥引起的土壤磷流失的增加量以低磷土壤更为明显。但粪肥表施较长时间后,粪肥对土壤磷流失的增加量也随土壤磷水平的提高而升高。总的来说,控制地表径流磷流失的效果是粪肥与土混施优于表施,流域内粪肥处置应优先施于土壤磷水平较低的土壤中。     

评价土壤磷素植物有效性的物理化学指标

室内分析结合温室盆栽试验研究了土壤磷素植物有效性和无机磷酸盐的解吸特性之间的关系。供试材料为浙江省分布较广的四种代表性土壤,并以吸磷能力较强的黑麦为指示植物。结果表明,在四种供试土壤上黑麦吸收磷与解吸磷之间的相关性都比与两种常规化学方法浸提磷之间的相关性更为显著。不仅如此,黑麦吸收磷与解吸磷间在数量上较化学浸提磷要接近的多,并且在各供磷强度下以及不同土壤上都符合较好。黑麦吸收磷或解吸磷(Q)随土壤供磷强度(I)变化的Q—I关系能够很好地符合Langmuir方程;而Olsen-P和Brayl-P对供磷强度的关系与该方程的符合性在酸性土壤上要差得多。可见,植物吸收磷与磷酸盐解吸有内在联系。结果还表明,由拟合Langmuir方程计算得的最大解吸缓冲容量(MBCD)能够很好解释不同性质土壤间磷肥利用率的差异,它较之最大吸附缓冲容量(MBCA)能够更好指示土壤中磷的植物有效性。但平衡解吸缓冲容量反映土壤供磷能力更为灵敏,可望成为好的土壤磷诊断指标。     

Use of adsorption and buffer capacity in soil testing for phosphorus

Abstract

The methods used to predict phosphorus (P) fertilizer requirements use extractable P levels. The inclusion of soil P buffer capacity with extractable P levels to improve fertilizer P prediction accuracy should be further investigated for Mediterranean soils. The objectives of this study, were to characterize the P adsorption properties of selected soils and to compare different soil P buffering capacity indices to determine their contribution to predict P fertilizer requirement. Twenty‐one soils were collected and P adsorption isotherms determined. On 11 soils wheat was grown in the greenhouse and P requirement determined. The Langmuir adsorption isotherm described the P adsorption of all soils. The prediction of crop P requirement was improved by the inclusion of a soil P buffer capacity index in the prediction equation. The index proposed by Salmon is easy to determine under routine soil analyses and does not require special analytical equipment. However, clay content can be used to predict the Salmon buffer index for similar soils.     

An evaluation of Colwell‐P as a measure of plant‐available phosphorus in soils of volcanic and non‐volcanic origins in the highlands of Papua New Guinea

Various soil test methods including Olsen, Colwell, Bray and Truog have been used to assess the levels of plant‐available P (PAP) in soils situated in the highlands of Papua New Guinea (PNG). Up until now, though, there has been no guarantee that these tests provide valid assessments of PAP in these somewhat atypical organic matter‐rich tropical soils. Furthermore, the critical soil‐P concentrations associated with the tests have been based on studies conducted elsewhere in sub‐tropical and temperate latitudes and as such may or may not be valid for soils or cropping situations in PNG. Soil (Colwell)‐P and leaf‐P data collected during a recent survey of sweet potato gardens in the highlands of PNG were therefore used to determine if useful relationships existed between these variables for different groups of soils, and if they do, to use these relationships to evaluate critical soil Colwell‐P concentrations corresponding to a known critical concentration of P in sweet potato index leaf tissue. Separate, highly significant linear relationships were obtained between leaf‐P and Colwell‐P for soils of volcanic and non‐volcanic origins. Based on these relationships, the critical Colwell‐P concentration for volcanic soils was found to be four times greater than that for non‐volcanic soils, presumably because much of the P extracted from the former soils with alkaline sodium bicarbonate had been chemically ‘fixed’ via sorption and precipitation reactions with sesquioxides and rendered unavailable to plants at ambient soil pH. These critical Colwell‐P concentrations if adopted as benchmark values for the soil groups in question should ensure that the results of future soil fertility surveys involving Colwell‐P assessments are correctly interpreted.     

ASI法测定土壤有效磷、有效钾和铵态氮与我国常规分析方法的相关性

用ASI法测定了土壤有效磷、有效钾和铵态氮，并与我国常规分析方法进行了相关性研究。结果表明：（1）ASI—P与Olsen—P呈极显著正相关，其相关系数在水作条件下比旱作条件下高，在酸性条件下比中、碱性条件下高，随着土壤pH值升高，两者的相关系数迅速降低；（2）ASI—K与NH4OAc—K呈极显著正相关，其相关系数在旱作条件下比水作条件下高，在碱性条件下比中、酸性条件下高，且对pH值和水分条件的变化均较敏感；（3）ASI—N比碱解N低得多，且在水作条件的中、酸性pH范围相关性较好。     

Variations in concentrations of N and P forms in leachates from dried soils rewetted at different rates

The rate at which dried soils are rewetted can affect the quantities and forms of nutrients in leachates. Both dried and moist replicated (n?=?3) samples of two contrasting grassland soil types (clayey vs brown earth) were irrigated during laboratory experiments with identical total amounts of water, but at different rates, ranging from 0 h, increasing by 30-min increments up to 4 h, and additionally a 24-h rewetting rate. Total P concentrations in leachates from dried samples of both soils generally decreased as rewetting rate increased, ranging from 2,923?±?589 μg P L^?1 (0.5 h rewetting rate) to 731?±?46.0 μg P L^?1 (24 h, clayey soil) and 1,588?±?45.1 μg P L^?1 (0.5 h) to 439?±?25.5 μg P L^?1 (24 h brown earth). Similar patterns in concentrations occurred for molybdate reactive P (MRP), although concentrations were generally an order of magnitude lower, indicating that the majority of the leached P was probably organic. The moist brown earth leached relatively high concentrations of MRP (maximum 232?±?10.6 μg P L^?1, 0.5 h), unlike the moist clayey soil (maximum 20.4?±?10.0 μg P L^?1, 0 h). The total oxidised N concentrations in leachates were less affected by rewetting rate, although longer rewetting rates resulted in decreased concentrations in leachates from the dried samples of both soils. The difference in responses to rewetting rates of the two soils is probably due to differences in the fate of the microbial biomass and adsorption properties in the soils. Results show that soil moisture could be an important factor in regulating nutrient losses and availability, especially under changing patterns of rainfall predicted by future climate change scenarios.     

Factors Affecting Mehlich III Soil Test Methodology for Extractable P

Agronomic and environmental testing laboratories in Texas and elsewhere have adopted Mehlich III (M3) as their official soil test phosphorus (P) methodology. However, M3-P data could be skewed due to non-homogenous soil samples or failure to follow standard protocol which could influence recommendations or restrictions. Twelve agricultural soils with a wide range of properties were collected from across Texas. Exhaustive efforts via multiple methods were made to prepare homogeneous representative soil samples. The standard M3 protocol selected was a 2 g weighed soil sample placed in a 148 ml disposable plastic cup, using a 1:10 soil:M3 solution ratio, shaken on a 200 rpm orbital shaker with a 2.5 cm throw for exactly 5 min, and filtered through Whatman No. 2 filter paper. The standard protocol was compared with nine different protocol variations with variables including soil weighing versus scooping, scooping repeatability of different technicians, soil sample weight, shaking type, speed and time, different filter papers, and varying soil:extractant ratios. Extent of soil pulverization on M3-P results was also evaluated. Tests were performed in four replications for all protocols to assess effects on M3-extractable soil P. Percent recovery of soil during grinding had no effect on M3-extractable P. Little difference in M3-extractable P was observed between scooping and weighing of 2 g soil samples. Shaker type had no effect on extractable P in soils with low clay contents, however, increasing shaking speed and using an orbital shaker resulted in higher extractable P, especially in clayey soils. Both Whatman No. 1 and 2 filter papers were found suitable for M3-P analyses. Different soil:extractant ratios resulted in a highly significant influence on the amount of M3-P extracted. However, when ratios were maintained between 1:9 and 1:11, few differences in extractable P were observed. Using sample weights below 3 g did not significantly alter precision or accuracy of results. However, technician variation in scooping of 2 or 5 g soil samples resulted in significant differences in M3-P.     

Assessing the effectiveness of manure export plus intensive silage cropping for lowering the Olsen‐P status of P‐enriched grassland

A substantial proportion of farmed grassland soils in Northern Ireland (NI) are overly enriched with P and pose a risk to water quality. To address this problem, manure could be exported rather than recycled to P‐enriched land and the latter intensively cropped with grass silage to deplete soil P. To assess the efficacy of such a strategy, a P‐ and K‐enriched grassland site was intensively cropped over a 6‐yr period with fertilizer N alone supplied to support silage growth. By year 6, soil P had declined from index 5 to index 3, and it was estimated that two more years of this management may bring it into the target index 2 range. Soil K, however, declined rapidly from index 4 to index 1 in just 4 yr, with the result that grass production became limited by K deficiency. It was concluded that nonrecycling of manure to P‐enriched grassland under silage management is probably the most effective strategy for lowering soil P status, but care must be taken to prevent K deficiency occurring.     

Simple phosphorus saturation index to estimate risk of dissolved P in runoff from arable soils

Abstract. There is increasing evidence that phosphorus has been accumulating in the surface horizons of agricultural soils to the extent that some soils represent a potential diffuse source of pollution to surface waters. The relationships between equilibrium phosphorus concentration at zero sorption (EPC ₀) of soil and a number of soil physicochemical variables were investigated in the surface layers of arable and grassland agricultural soils sampled from the Thame catchment, England. Soil EPC₀ could be predicted from an equation including soil test (Olsen) P, soil phosphate sorption index (PSI) and organic matter content (OM) (R²=0.88; P <0.001) across a range of soil types and land use. The simple index Olsen P/PSI was found to be a good predictor of EPC₀ (R²=0.77; P <0.001) and readily desorbable (0.02 m KCl extractable) P (R²=0.73; P <0.001) across a range of soil types under arable having soil organic matter contents of <10%.