THE INFLUENCE OF PULSE CROPS ON SOIL PHOSPHORUS AVAILABILITY TO SUBSEQUENT WHEAT CROPS IN THE BROWN SOIL ZONE OF SASKATCHEWAN
MacDonald, Catharine Ann
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A study was done to determine if part of the rotation benefit that is often observed in cereals grown on pulse vs. cereal stubble is due to differences in soil phosphorus (P) availability. The influence of pulse crops on soil P availability and P uptake by subsequent wheat (Triticumaestivum L.) crops was assessed in a series of laboratory experiments and field studies conducted in the Brown soil zone of Saskatchewan. Pulse residues generally had higher P concentrations than wheat residue when grown under similar soil fertility and environmental conditions. Higher P contents corresponded to lower carbon (C)to P ratios (C:P),which lowered the potential for P immobilization during residue decomposition. However, in a growth chamber experiment, pulse and cereal residues all had low enough P contents to cause soil P immobilization. Lentil (Lens culinaris Medic.)residue (C:P=285)caused the least immobilization while pea (Pisumsativum L.)residue (C:P=592) and wheat residue (C:P=1335)caused similar degrees of immobilization. Total P contributions from chickpea (Cicerarietinum L.),pea and wheat residues to soil during growth of a subsequent crop were estimated at 0.92, 1.24, and 1.03 kg P ha", respectively, at residue coverage rates of 5000 kg ha". This represents a minor contribution to the short-term P supply to a following crop. Estimated nutrient losses from pulse residues weathering in the field between crop harvest and seeding of a subsequent crop the following spring were also minimal and are not expected to have significant effects on P uptake by the following crop. Field comparisons indicated no significant increases in soil P availability related to the previous crop stubble. However, P uptake by the crop was significantly greater following pulse crops as compared to wheat and fallow. Some factor other than soil P supply, such as a better crop rooting system with a larger surface area to absorb P, may be responsible for greater P uptake by wheat following pulse crops. Studies of root infection by arbuscular mycorrhizal fungi (AMF)were inconclusive, but if enhanced following growth of a pulse crop, AMF could contribute to enhanced root surface area and therefore nutrient uptake.