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Potato (Solanum tuberosum L.) is a high P demanding crop contributing to P saturation of acid coarse‐textured soils. The P status is commonly assessed agronomically by soil testing of P (STP) and environmentally by soil P saturation or P solubility indexes. The aim of this work was to develop a P fertilizer recommendation model integrating P environmental risk and crop response probability, using P and Al extracted by the Mehlich‐III soil testing method (mg P or Al L−1 of soil) and expressed as (P/Al)M‐III percentage. The environmentally critical (P/Al)M‐III percentage was 15%, corresponding to the critical degree of phosphate saturation (DPS) value of 25% (oxalate extraction procedure) proposed in the Netherlands. Using the Cate‐Nelson procedure, the critical (P/Al)M‐III percentage across 78 field experiments was 8.2% as determined by inductively coupled‐argon plasma (ICP). Probability of potato response to P was close to 100% even above the critical STP values as shown by power tests on relative yields. The P recommendation model was computed from conditional expectations of 50 and 80% within each fertility group below 15% as (P/Al)M‐III. Above 15%, the model recommended 21 kg P ha−1, the amount of P exported by a tuber harvest of approximately 42 Mg ha−1. Using conversion equations with 114 soil samples for STP methods, fertility groups built for Quebec acid coarse‐textured soils were found to be comparable with those currently used in the Netherlands and Lower Saxony (Germany). The (P/Al)M‐III percentage provided a reliable and unifying criterion for making environmentally acceptable and agronomically efficient P recommendations for sustainable potato production.

(Mehlich 1984) is a multinutrient agri-environmental routine soil-testing procedure used in many jurisdictions in North America, but one that is affected by soil texture. The P W determined by the Sissingh (1971) method is an index of surface water contamination and desorbed P that is not influenced by soil texture and that can be used to define specific M-III critical environmental indices by soil texture group. Our objective was to define critical environmental indices by relating (P/Al) M-III to P W . We analyzed 275 soil samples from surface, and 175 from subsurface layers, varying in genesis, texture, and pH. The relationship between P W and (P/Al) M-III was influenced by soil properties, particularly soil texture and genesis. Fine-textured (> 300 g clay kg -1 ) and gleyed soils tended to release more P W at a given (P/Al) M-III compared with coarse-textured (≤ 300 g clay kg -1 ) and podzolized soils. Using a critical value of 9.7 mg P W L -1 derived from the literature, critical environmental (P/Al) M-III ratios were found to be 0.131 for coarse-, and 0.076 for fine-textured soils. Subsurface P W increased significantly with (P/Al) M-III above 0.131 in the plough layer of coarse-textured soils, but was independent of (P/Al) M-III in fine-textured soils, indicating contrasting mechanisms of P accumulation in subsurface layers (matrix vs. preferential flow). After accounting for soil texture, (P/Al) M-III appeared to be a useful index of P accumulation in Quebec mineral soils. est largement utilisée en Amérique du Nord, mais est sensible à la texture du sol. Le P W (Sissingh 1971), un indice de la désorption du P et de contamination des eaux de surface, n'est pas affecté par la texture et permettrait donc de définir des valeurs critiques environnementales M-III spécifiques à la texture. Notre objectif était de définir ces valeurs critiques (P/Al) M-III en lien avec le P W . Nous avons analysé 275 échantillons de sol de surface et 175 dans la couche sous-jacente. La relation P W -(P/Al) M-III dépendait des propriétés du sol comme la texture et la genèse. Pour un même rapport (P/Al) M-III , les sols de texture fine (> 300 g d'argile kg -1 ) et gléifiés tendaient à libérer plus de P W que les sols à texture grossière (≤ 300 g d'argile kg -1 ) et podzolisés. Utilisant une valeur critique de 9,7 mg P W L -1 tirée de la littérature, les rapports (P/Al) M-III critiques étaient de 0,131 pour les sols de texture grossière et 0,076 pour ceux de texture fine. La valeur P W dans les couches sous-jacentes augmentait significativement dans les sols de surface de texture grossière avec (P/Al) M-III > 0,131, mais était indépendante de (P/Al) M-III dans les sols de texture fine, 7113 This paper is part of a Ph.D. thesis. 4 To whom correspondence should be addressed (e-mail: leon-etienne.parent@sga.ulaval.ca).Abbreviations: ␣ m , maximum saturation factor for total sorption; ␤ m , maximum saturation factor for reversible adsorption; b, Langmuir phosphorus adsorption maximum; DPS, degree of phosphate saturation; ICP, induct...

L. 2006. Agri-environmental models using Mehlich-III soil phosphorus saturation index for corn in Quebec. Can. J. Soil Sci. 86: 897-910. Soil phosphorus (P), which is potentially a risk for environmental contamination, is currently interpreted using soil P saturation in North America. Our objective was to assess the ratio of P to aluminum (Al) in the Mehlich-III (M-III) soil test to build P requirement models for corn and soybean. We analyzed 129 corn and 19 soybean P fertilizer trials. For corn, the (P/Al) M-III ratio improved soil fertility classification compared with P M-III alone. The critical P M-III value as determined by the Cate-Nelson procedure was found to be 31.5 mg P M-III kg -1 , close to published values. The critical (P/Al) M-III ratios of 0.025 for > 300 g clay kg -1 soils and 0.040 for ≤ 300 g clay kg -1 soils differed significantly between the two soil groups. For (P/Al) M-III ratios above 0.214, there was no positive response to added P for all soils regardless of texture. Using published critical environmental (P/Al) M-III ratios of 0.076 for > 300 g clay kg -1 soils and 0.131 for ≤ 300 g clay kg -1 soils as benchmarks values, agri-environmental P requirement models were built using conditional expectations of 50 to 80% of computed optimum P values within a soil class. A validation study supported the low critical (P/Al) M-III ratios and the 50% conditional expectation model except for a high carbon soil which was outside the application range of the models. However, banded P decreased corn yield at four validation sites although the model predicted positive response to P. Soybean did not respond to P except at extremely low fertility levels ((P/Al) M-III ≤ 0.02) and behaved as a P-mining crop even in low-P soils. Corn-soybean rotations can reduce soil P to low (P/Al) M-III ratios with minimal agronomic risk. For personal use only.

Legacy phosphorus (P), corn (Zea mays L.) hybrids, weather conditions, and regulation on land-applied manure affect site-specific mineral P requirements. Mineral P fertilizers are still banded at corn seeding as insurance policy even where manure P is applied in compliance with regulation in Eastern Canada. Our objective was to measure the effects of site-specific factors on the response of corn hybrids of the "new" era (1991+) to mineral and manure P fertilization under regulatory constraints. A dataset of 370 corn P fertilizer trials conducted between 1998 and 2004 in Quebec, Canada, comprised trials using mineral P alone (229 trials) or in combination with manure (141 trials). Linear mixed statistical models tested the effect of P fertilization on seedling biomass, grain yield, grain bulk density, and grain moisture content as affected by weather and soil indices. Early plant growth proved to be a poor indicator of final crop performance. Where no manure was applied, grain yield and quality were affected primarily by soil pH, texture, and genesis. There were responses to P additions only where the saturation index (P/Al) M3 was less than 5%. Mineral P fertilizers showed no significant effect on crop performance where manure was applied, but manured soil generally showed (P/Al) M3 percentages exceeding 5%. Banding mineral P at corn seeding where manure was applied at rates complying with the regulation and the (P/Al) M3 percentage exceeded 5% appeared to be a waste of resource under the soil and climate conditions in Eastern Canada.Abbreviations: CHU, corn heat units; DAP, diammonium phosphate; Fv, filling value.