The Influence of Pulse Crops on Soil Phosphorus Availability to Subsequent Wheat Crops in the Brown Soil Zone of Saskatchewan
MacLeod, C.A.. 1999. M.Sc. Thesis. Dept. Soil Science, University of Saskatchewan, Saskatoon, SK
Abstract
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 (Triticum aestivum 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 (Pisum sativum L.) residue (C:P =
592) and wheat residue
(C:P =
1335) caused similar degrees of immobilization.
Total P contributions from chickpea (Cicer arietinum 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.
