. 2001. The importance of early season phosphorus nutrition. Can. J. Plant Sci. 81: 211-224. A review of studies conducted in a range of plant species indicated the importance of an adequate supply of P during early crop growth and outlined plant adaptations for accessing early season P. Potential implications of the requirement for early season P in the development of management practices to optimize P supply for crop production were also discussed. Phosphorus plays a critical role in energy reactions in the plant. Deficits can influence essentially all energy requiring processes in plant metabolism. Phosphorus stress early in the growing season can restrict crop growth, which can carry through to reduce final crop yield. Deficiencies during early growth generally have a greater negative influence on crop productivity than P restrictions imposed later in growth. Plants respond to P deficiencies by adaptations that increase the likelihood of producing some viable seed. The adaptations increase the ability of the plant to access and accumulate P and include modification of rhizosphere pH, diversion of resources to root production, increased root proliferation in high-P regions, and formation of associations with vesicular arbuscular mycorrhizae. Plants differ in strategies adopted and in efficiency of P absorption.Effective nutrient management for optimal crop production must ensure that P is supplied to the crop in adequate amounts early in the growing season. It is important that we develop methods to accurately predict the early season P supply from the soil, in order to avoid either over-fertilization or crop deficiencies. Practices to provide adequate P early in plant growth include placement of P fertilizer in or near the seed-row and maintenance of adequate concentrations of plant-available P in the soil through a long-term nutrient management strategy. Other possible management practices could include enhancement of seed concentrations of P, manipulation of tillage system and crop sequence, improved activity of mycorrhizae and other microbiological agents such as Penicillium bilaii to increase phytoavailability of soil P, or genetic selection of crops with an enhanced ability for early season uptake of P from both soil and fertilizer sources. As plants differ in relative abilities to access P from the soil and fertilizer applications, nutrient management must be tailored to the specific crop, in order to optimize P supply and crop productivity. By restricting fertilizer applications to situations where P supply is limiting to crop production and by use of effective P management practices, we can optimize the economic of fertilizer use while avoiding negative impacts on environmental quality. Une analyse des recherches effectuées sur diverses espèces végétales révèle l'importance d'un apport suffisant de phosphore au début de la période végétative et montre comment les plantes s'adaptent pour trouver le phosphore (P) dont elles ont besoin. Les auteurs ont aussi examiné les conséquences éventuelles de cette...

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The productivity of potato (Solanum tuberosum L.) systems may be strongly influenced by rotation. A field study consisting of six rotations (potato with canola (PC), wheat (PW), canola-wheat (PCW), oat-wheat (POW), wheat-canola-wheat (PWCW), and canola underseeded to alfalfa-alfalfa-alfalfa (PCAA)) arranged as an RCBD was established on a clay loam in Manitoba, Canada in 1998 and monitored for twelve years to determine effects on potato yield and quality. Higher yields for canola-containing rotations were evident in three years (P<0.10), but not beyond 2005. From 2007 onward, 2-year rotations produced a markedly lower yield than other rotations. Yields of 3-and 4-year rotations were similar, although PCW and PCAA were most frequently among the higher-yielding rotations. Overall, PC had a lower specific gravity than other rotations. Results suggest 3-and 4-year rotations, particularly PCW and PCAA, helped maintain productivity. Two-year rotations of PC and PW were not sustainable due to declining yields.Resumen La productividad de los sistemas de papa (Solanum tuberosum L.) pueden estar fuertemente influenciados por la rotación. Se estableció un estudio de campo consistente en seis rotaciones; papa con canola (PC), con trigo (PW), con canola-trigo (PCW), con avena trigo (POW), con trigo, canola, trigo (PWCW), y con canola mezclada con alfalfa-alfalfa-alfalfa (PCAA), arreglado en un diseño experimental de bloques completos al azar, en un suelo franco-arcilloso en Manitoba, Canadá, en 1998, y monitoreado por doce años, para determinar los efectos en rendimiento y calidad de la papa. Se hicieron evidentes más altos rendimientos en las rotaciones que incluían a la canola en tres años (P<0.10), pero no más allá de 2005. A partir de 2007, las rotaciones de dos años produjeron un rendimiento más bajo bien marcado que en otras rotaciones. Los rendimientos de rotaciones de 3 y 4 años fueron similares, aunque PCW y PCAA estuvieron entre las más frecuentes de las rotaciones de mayores rendimientos. En general, PC tuvieron gravedad específica más baja que otras rotaciones. Los resultados sugieren que rotaciones de 3 a 4 años, particularmente PCW y PCAA, ayudaron a mantener la productividad. Rotaciones de dos años de PC y PW no fueron sostenibles debido a la disminución en rendimientos.

The rapid increase in anhydrous ammonia use in Western Canada has created concerns about possible detrimental soil effects, particularly solubilization of organic matter. Anhydrous ammonia was injected at rates of 0.206, 0.416 and 0.834 g NHr-N into undisturbed soils taken in 20-cm-diameter cylinders. Both field capacity (FC) and permanent wilting point (PWP) moisture contents were used. Samples were taken at 0-2.5,2.5-5.0, 5.0-7.5 and 7.5-10.0 cm from the injection point, I day after injection. For the 0.416 g NHr-N rate samples were also obtained 35 days after injection. Soluble organic carbon (SOC) was measured in a l:15 soil:water extract.The SOC was closely related to the NH.*-N concentration (r, : 0.79 to 0.99; P <0.001). The SOC was increased by NH, at all injection rates for samples within 2.5 cm of injection point, at the highest rate for the2.5-to 5.0-cm zone but there were few effects in the 5.0-to 7.5-cm zone and none in the 7.5-to 10.0-cm zone. The solubilized C was 0.3-4.6Vo,0.0-3.0Vo and 0.0-0.67a of the total C in the 0-to 2.5-cm,2.5-to 5.O-cm and 5.0-to 7.5-cm zones, respectively. The 0-to 2.5-cm,2.5-to 5.0-cm and 5.0-to 7.5-cm zones represent4.3, 13.l and,2l.8Vo, Smiley and Papendick (1968). Two cyl-

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