The Mehlich-III method used for the determination of available P in 82 Quebec soils was compared to five other chemical methods (Bray-II, Bray-I, Mehlich-II, Mehlich-I and Olsen) and to two anion exchange resin techniques (F -and HCO -3 ) In general, very good correlations were found between Mehlich-III, Bray-II and Bray-I methods on acid soils (r=0.95** and 0.98**). The Mehlich-III-P content is somewhat the same as that determined by Bray-I and corresponds approximately to 80% of Bray-II-P. The Mehlich--III extractant was found to be more reliable than Bray-II and Bray-I on very acid spodosols having high P-sorption capacity. The Bray methods, with high NH 4 F concentration, extracted strongly fixed Al-P in these soils and may overestimate the available P. In some acid soils containing apatite and in calcareous soils, the Bray-II and Mehlich-I (double acid) easily dissolved these Ca-P compounds and gave exaggerated high P values. The Mehlich-III and the other methods were less vigourous for these compounds and were

Some suggest that young labile soil organic matter accumulates preferentially in water‐stable macroaggregates (>250 µm) where it acts as a transient binding agent. We determined the proportions of recently deposited C (<15 yr) in water‐stable aggregate size fractions using 13C natural abundance. The experiment was performed on a silty loam (fine‐loamy, mixed, frigid Aeric Haplaquept) on plots that were under either continuous corn (C4) or a mixed C4‐C3 rotation for 15 yr and that had previously been under a permanent meadow (C3). Field‐sieved (6 mm) air‐dry soil samples (0‐10 cm) were either dry sieved or wet sieved with slaking to obtain a range of aggregate sizes. Total organic C contents and natural 13C abundances were determined in each aggregate size fraction, which allowed calculation of the proportions of aggregate C derived from either C3 or C4 (recently deposited C). The proportion of water‐stable macroaggregates in the meadow soil and their C3‐derived C contents decreased with corn cropping. The net loss of C3‐derived C was greater for larger macroaggregates (>2 mm) than for smaller ones. Assuming first‐order kinetics, the estimated half‐life of the C3‐derived C of stable aggregates >2 mm was 13 yr, which corresponds to that reported for macro organic matter in similar systems. Analysis of water‐stable macroaggregates under corn showed that they were enriched in recently deposited C relative to microaggregates and to the whole soil, which partly compensated for their loss in C3‐C. On average, 20% of the C in water‐stable aggregates >1 mm was derived from corn whereas this value was down to 9% in the whole soil and 1% in the microaggregates. The results of this study provide further quantitative evidence that slaking‐resistant macroaggregates are enriched in, and probably stabilized by, recently deposited organic matter.

(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...

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Information regarding the relationships between soil properties and the economic optimum N rate for crop yield is needed to ensure profitable use of N fertilizer. This study was conducted in 2007 and 2008 at 62 field sites in Québec (eastern Canada) to assess corn (Zea mays L.) response to N fertilizer, to calculate the economically optimum nitrogen rate (EONR) and corn yield (EOY), and to relate these two parameters with soil and crop‐based parameters. Yield response to N fertilizer rates (0–250 kg N ha−1) at each site was fitted to a linear, quadratic, or quadratic‐plus‐plateau model. The EONR and yield (EOY) were related to 12 soil and crop‐based parameters, and corn heat units (CHU). The quadratic‐plus‐plateau model best described the yield‐fertilizer relationship at 43 of the 62 field sites. The values of EOY varied from 7.4 to 13.3 Mg ha−1 in 2007 and from 5.2 to 11.2 Mg ha−1 in 2008, while EONR was between 73 and 235 kg N ha−1 in 2007 and from 48 to 200 kg N ha−1 in 2008. Correlation and principal component analysis showed that dissolved nitrogen (DNc) and dissolved carbon (DOCc) extracted with cold water and pre‐sidedress nitrate analyzed using nitrate test strips (PSNTts) were significantly and negatively related to EONR. In both years, PSNTts was consistently related to EONR, and CHU with EOY. While it remains challenging to predict EONR due to site‐specific variability and fluctuations in growing conditions, the PSNTts test shows promise in predicting the EONR for corn production in Québec.