Soil Management Handbook - Ministry of Agriculture and Lands

6. Soil Conservation
6.1 Crop Rotations
Crop rotation means growing a series of different
crops over the years on the same field. Continuous
cropping to the same crop or related crops often
results in a build-up of soil-borne plant diseases or
weeds. If this occurs, it will be necessary to grow a
different crop in the diseased or weedy field.
Continuous cropping to cultivated row crops leads to
a breakdown of soil structure, compaction, increased
erosion and a loss of organic matter. For these
reasons, it is a good practice to grow crops in a
rotation so that disease and weeds can be controlled
and soil quality can be maintained.
The most beneficial crop rotations are those which
include grasses or grass-legume mixtures. If this is
not possible, rotations which include fibrous rooted
crops should be used. These crops are most effective
in building stable soil structure, increasing soil
organic matter and improving the porosity and
structure of the subsoil (see Figure 4, page 37). A soil
which has been seeded to a hay crop for a number of
years will have better and longer lasting structure and
tilth than the same soil continuously used for annual
crops. A rotation of deep-rooted crops with
shallow-rooted crops will allow plant nutrients to be
extracted from the subsoil. Some of these nutrients
are added to the plow layer when residues from
deep-rooted crops are incorporated into the plow
layer. The best example of crop rotation in the
Southern Interior of the Province is corn/grass used
by the dairy industry. The most significant
improvement that could be made to that rotation
would be to include a winter cover crop after corn
harvest.
On steeply sloping land, crop rotations, which include
close-growing crops or perennial grass-legume hay or
silage crops, are recommended for direct protection
against soil erosion and for building up and
maintaining a stable structure which is more resistant
to erosion.
Although crop rotation is not feasible in perennial
fruit crop production, growth of cover crops and
occasional tillage to relieve traffic compaction is
recommended. Tillage will allow for the
incorporation of organic matter where traffic
compaction is a problem. Reseeding to a diverse
cover crop mix will be beneficial to the soil in
perennial forage crops. Management of the cover
crop may require inputs other than just mowing, such
as interseeding, weed control or reseeding.
6.2 Cover Crops
The current level of knowledge about cover crop use
in the Southern Interior of B.C. is limited for most
crops. However, cover crop use in vineyards is
covered quite well in the Vegetation Management
Section of the Management Guide for Grapes
1994-95. This section of the Handbook will give an
indication of the commonly used cover crop species
and their seeding rates and dates. Cover crops are
also called catch crops, green manures and living
mulches.
Catch crops are grown for the purpose of trapping
leachable nutrients in the plant. Green manures are
crops grown for the purpose of adding organic matter
to the soil. Living mulches have many uses and are
commonly seen as grass under growth in orchards or
vineyards.
Cover crops are useful for many purposes in the
region. These purposes include: control of wind and
water erosion in annual crops, suppression of weed
growth, beneficial insect habitat, nutrient capture, soil
structure improvement, increased trafficability,
production of livestock or water fowl feed and most
importantly, their influence on microclimate under
perennial crops. It should be noted that crops such as
grapes are poor competitors for water and nutrients.
Therefore, care must be taken in choosing a cover
crop species.
A cover crop may also be established between the
rows of perennial crops, to protect the soil from
erosion and to reduce the need for cultivation. The
most common use of cover crops has been for over
winter soil protection. Because rainfall intensities are
lower in the Interior, the risk of severe winter water
erosion is not great, therefore, winter cover cropping
for soil protection may be less important than it is in
the Lower Fraser Valley. However, cover crops
grown over the winter are beneficial to soil tilth. In
the South Okanagan and Similkameen Valleys, many
of the sandy soils are susceptible to wind erosion.
Soil Management Handbook – Okanagan-Similkameen Valleys 83