Effect of phosphorus fertilization on legume green fallow and subsequent wheat
A greenhouse study was conducted to determine effects of P fertilization on green manure and N production by chickling vetch, cv. AC Greenfix, in a degraded Brown sandy loam and to assess if P applied to the legume and N fixed during vegetative growth will be re-mineralized during partial fallow to adequately supply a subsequent crop of spring wheat. The soil from a stubble field near Cadillac, SK was placed in large plastic tubs which were randomized for the seven green fallow treatments consisting of Rhizobium inoculated and uninoculated AC Greenfix, each at three P levels (0,36 and 72 kg P ha-t), plus an uninoculated high N control fertilized with 72 kg P ha-t and 100 kg N ha-t. At full bloom, the legume top growth was coarsely chopped and soil incorporated. Then the green-manured soil was managed for 12 weeks as ‘partial fallow’ with occasional watering and periodic soil sampling to monitor available nutrients. Thereafter, all green-manured tubs plus tubs with the ‘conventional fallow’ control were seeded to spring wheat, cv. AC Barrie, which was harvested 14 weeks later when all treatments had reached maturity. Despite suboptimal legume growth due to midwinter light deficiencies in the greenhouse there was still a positive response to P fertilization. On a DM per plant basis, high P fertilized inoculated and uninoculated treatments produced 35% and 30% more green manure biomass than the respective unfertilized controls. Fertilization with P tended to enhance symbiotic N2 fixation while N fertilization at 100 kg ha-t resulted in complete inhibition of root nodule formation. Upon soil incorporation, legume decomposition in the greenhouse proceeded so rapidly that much of the green manure P and N was re-mineralized within the first three weeks of fallowing. Green fallow P fertilization accelerated the maturity of subsequent spring wheat by 12 to 18 days but it had no effect on grain yield. Grain protein and P contents were generally also unaffected by preceding P fertilization. This lack of green fallow P fertilization response in the greenhouse, in terms of wheat yield and quality, points to the need for a similar study under natural field conditions and greater crop nutrient demands. Nevertheless, all green fallow treatments in the greenhouse effected significant wheat yield increases over the yield on conventional fallow. Thus, benefits of green manuring per se on wheat were confirmed by an average 54% increase in grain and 20% increase in straw production. Green fallowing also increased the efficiency of grain production (e.g. harvest index) and it improved grain quality with an average 29% increase in kernel size and 14% increase in protein content. These results imply that legume green fallowing per se was more important to wheat production in the greenhouse than the rate of legume fertilization.
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