Increase in soil organic matter can affect fertility programs.
November 19, 2007 By Bruce Barker
Switching from conventional tillage programs over to direct seeding and no-till
changes soil quality and subsequent potential nutrient supplying power. One
of the most important changes comes with an increase in soil organic matter.
"Generally, over a period of years, there will be an increase in soil
organic matter, but that comes along with other improvements in soil quality
like better soil tilth," says Ross McKenzie, agronomy research scientist
with Alberta Agriculture, Food and Rural Development (AAFRD) at Lethbridge.
By eliminating tillage, crop residues are left on the soil surface which reduce
contact with the soil organisms that decompose residues. In addition, less oxygen
is added to the soil and since many soil organisms need oxygen to decompose
residue, decomposition occurs at a slower rate. As a result, crop residue stays
in its organic form longer, resulting in an increase in soil organic matter.
Another reason soil organic matter may increase is that no-till crop rotations
are generally more diversified, especially in the drier regions of the prairies.
For example, McKenzie is involved in long-term rotation studies at Bow Island,
Alberta in the Brown soil zone, where he is comparing traditional wheat-fallow
rotations with extended rotations that include more diversified crop systems.
"In those trials, we had quite a good increase in soil organic carbon,
a reflection of organic matter, in the first six years, although after 12 years
the increase has flattened out," explains McKenzie. "In Dark Brown
or Black soils, the increases may continue to rise in the longer-term."
Rick Taillieu, an agronomist with Alberta Reduced Tillage Linkages (RTL), says
low disturbance direct seeding, in addition to improving organic matter, brings
many other benefits, including improved water infiltration. When tillage is
eliminated, root and earthworm channels in the soil allow more water to infiltrate
while the straw 'duff' layer on the surface helps conserve water and improve
"Insufficient water is the major limiting factor in crop production for
many growers," says Taillieu. "Anytime you can improve water infiltration
and the soil's water holding capacity, you are creating opportunities for achieving
higher, more stable yields, which contributes to better farm sustainability."
McKenzie says that researchers have been able to identify the various components
of organic matter, adding that the light fraction carbon increases the most
with extended crop rotations and reduced tillage. The light fraction (also called
labile carbon) is the portion that is most easily mineralized. When light fraction
carbon breaks down, mineralization of organic nitrogen (N) and other nutrients
into plant available nutrients takes place.
"Light fraction organic matter cycles more easily in the soil, which can
mean the potential for greater nitrogen availability and other plant nutrients
such as phosphorus," says McKenzie.
Rigas Karamanos, manager of agronomic research at Westco Fertilizer in Calgary,
Alberta, says growers should be cautious when setting up fertility plans on
longer-term reduced tillage soils. He makes a distinction between mineralizable
(also called labile) N and mineralized N. Mineralizable N represents a pool
of soil N that contains forms that can be converted to inorganic N by micro-organisms,
and signifies what can potentially be available for mineralization. Mineralized
N is the actual amount that is converted from organic forms to inorganic forms.
"To get mineralized N, the right conditions are necessary, such as warm
soil temperature and good soil moisture," explains Karamanos. "The
potential may be there for more mineralized N, but that doesn't always occur,
especially in cooler years."
Karamanos cites research that shows, although mineralizable N in the soil is
significantly increased through direct seeding and zero tillage, actual mineralization
in the field may not be affected or even be decreased.
Other research has shown that the 'quality' of N is increased under zero-tillage.
Karamanos says University of Saskatchewan research (Ph.D. dissertation of Dr.
F. Selles) found that the amount of total N in the soil was the same between
conventional and no-till. However, Selles noted that labile N was higher in
no-till with 14 percent more available N in the zero to four inch depth, 21
percent more in the four to eight inch depth and four percent more in the eight
to 12 inch depth. "The make-up of the soil organic matter is different
and that is one of the benefits of no-till," explains Karamanos.
After converting to no-till from conventional tillage, McKenzie says growers
will likely have to alter their N fertility program slightly until soil organic
matter stabilizes. "They may have to increase their N rates in the short-term
as the N becomes tied up in organic matter," explains McKenzie. "But
you also have to remember that zero-till conserves moisture. Every tillage operation
loses one-half inch to one inch of water, so under zero-till, your yield potential
could be increased by five bushels for wheat and seven to eight bushels for
barley, for each inch of water conserved in the soil. So when developing fertility
plans, you may have to adjust your target yields as well."
What about other nutrients?
Karamanos reviewed the research literature to see what effects no-till had on
sulphur (S), phosphorus (P) and potassium (K). He found that the changes in
S crop nutrition are small and of minimum impact on the overall fertility of
Looking at P, a number of studies demonstrated that generally, available P
levels are not affected by tillage. However, both mineralization and the fraction
of P that is readily available from crop residues are higher under reduced tillage
"Increased availability of P in systems with minimal disturbance has been
linked to the greater colonization of mycorrhizae fungi, which live in association
with certain species of plants," explains Karamanos.
Conversely, a number of studies show that P has the potential to become stratified
in the top five centimetres of soil, although the zero to 15 centimetre depth
levels appear to be similar under no-till and conventional tillage. However,
since zero-till crops are generally sown shallow, because of better soil moisture,
this stratification may not be a concern.
Looking at K, Karamanos generally found that 'available' K levels are increased
under zero-tillage, although the same stratification issues exist.
Assess all factors when developing fertility plans
Taken at face value, the research would suggest that more N is required for
crop production under zero-till, at least in the transition phases. However,
Taillieu says that fertilizer efficiency must be considered when looking at
the big picture. "Low disturbance seeding generally uses banded N in the
spring, which is the most efficient method of applying fertilizer," says
Taillieu also says that including pulses in rotation is part of the bigger
story. Pulse residues have higher N content and decompose quickly, whether incorporated
or not. Adding a pulse crop in rotation may compensate for lower soil N availability
during the transitional period. Another key management tool in the transitional
period is a soil test. Basing a fertility program on soil tests will help producers
take advantage of all the benefits of low disturbance seeding.
"Low disturbance seeding will provide long-term benefits such as improvements
to soil quality and fertility. Add in the immediate benefits of fuel and labour
savings, and the move to low disturbance seeding can immediately contribute
to improved farming sustainability," says Taillieu. -30-