An invcstigation of the yie1c1 and plant response. to phosphorus ,on Black Chernozemlc s.ils occurring in cateniry sequellcc indicatcd that a..large proportion of rhe total r.arian& $as artributable to fields and tU thc _lnteractloll L"tiu""n ficlds and;.ih -ilL.r.ia""t from the rcsults that no simple relation-.hip "*irtr tr"."..""tr i6" pitd"crir-ity3f .tl'rc soils in thc catcnas sampled and in"it nioipn"logicat c[aiactcristics.'TIris variability.is thoughr to be due in orif ,n .Arns.i i"'r.l.i".ti.rii.fr.,ort as alTected_ by _the shape. size and ispect of thidrainage arcas samplcd and by scasottal and yearll-llucruatlons ;,i-","r""r"i.ei."i frF-ir. and als,, duc irr liarr r() tlle effect of thesc changes on the comp;titive ability of rveeds. Plalrr resp,,nse to plrosphorus \as relarcd. to lh_e topograplric,position of the soil. The l,,*"rr t.,pog'.rphii: p,,titio'r tClel cd-Calcarc-
In studies conducted from 1955 to 1961 on several soils in Manitoba, incorporation of straw into the soil did not generally induce a nitrogen deficiency in the succeeding cereal crop. When soil samples with straw were incubated in the laboratory, and when straw was applied in fallow fields, nitrate release was reduced markedly. Presumably low temperatures and lack of moisture in the fall and spring reduced the rate of straw decomposition. In this case, the level of available nitrogen would not be reduced appreciably by saprophytic microorganisms. In the area cereal crops are seeded before soil temperatures are high enough for rapid straw decomposition in the spring and the crop competes successfully with the microorganisms for nitrogen.
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