By Ieuan Evans
Discover the world of soil organic matter.
By Ieuan Evans
Crop yields in Canada 60 or more years ago were essentially dependent on residual
soil fertility and ambient rainfall. A 30 to 40 bushel per acre crop of wheat,
60bu/ac of barley, 70bu/ac of oats and perhaps 80bu/ac of corn were considered
good yields. Initial soil organic matter generally ranged from two to 10 percent
and natural reserves of phosphate, potash and sulphur were often fair to good.
Magnesium and calcium were not generally considered, except when dealing with
acidic soils. Micronutrients such as zinc, copper, boron and molybdenum were
considered unimportant since so little was needed for crop production.
In the early part of the 20th Century the only micro-nutrient mentioned was
iodine because it was severely deficient in human consumption in many parts
of Canada. Canadian soils continued to be mined until the last 30 years or so
of the last century, when noticeable crop yield decreases were observed in many
Aside from livestock manure, ammonium sulphate and ammonium phosphate were
considered to be the answer to yield improvement, since responses were observed
for both nitrogen and phosphate. Sulphur deficiencies, particularly in canola,
did not become obvious until urea and liquid ammonia replaced ammonium phosphate
and ammonium sulphate as the primary nitrogen source. In heavy industrial areas,
air pollution generally provided adequate sulphate. Potassium deficiencies and
crop responses to potash were considered rare.
In the 1970s and '80s, growers' ambitions in the higher rainfall areas of central
and western Canada aimed at growing much elevated yield goals of 60 or more
bushels per acre of wheat, 100+ bu/ac barley, 120+ bu/ac oats and 120+ bu/ac
corn, essentially by elevating fertilizer application levels. On sandy and high
organic matter soils, which comprise 30 percent of prairie Canada and considerable
areas of Ontario, grain yields did not respond as expected to the higher nitrogen
and phosphate application despite adequate rainfall. By the early '80s, the
better yielding prairie soils had been in production for 100 years or more and
for more than 200 years in eastern Canada. Except for nitrogen and phosphorous
fertilizers, most soils had been subjected to nutrient drawdown with organic
matter levels dropping by 40 to 60 percent! Soil reserves, depending on location
and soil type, were also depleted of available phosphate, potash and micronutrients.
Consequently, in many higher rainfall areas, stepping up the nitrogen fertilizer
levels did not give the expected yield response. On sandy soils in particular,
many farmers applied livestock manure in an attempt to improve soils and boost
yields. Often after manure application, yields dropped precipitously in wheat
as a consequence of crop lodging, with shrivelled, ergot infested grain that
failed to make any grade.
Yield and quality reduction was blamed on excessive nitrogen in the manure.
However, the real culprit was the tie-up of nutrients, copper and manganese
in particular, by soil microflora feeding on the organic manure and literally
starving the crop. Unfortunately, until relatively recently, the role of soil
microflora in crop production has been completely ignored. A fertile, well manured
field in southern Manitoba will have, in the top six to 12 inches of soil, countless
billions of micro-organisms at work per acre. Translate this biomass in mid-July
into cow equivalents and you will be able to constitute about 12 to 15 cows
These micro-organisms in a well fertilized, moist, well manured soil are multiplying
and dying at very rapid rates and continually taking up and releasing nutrients
into the soil. These nutrients become available to crop roots on a daily basis
and allow for ideal growing conditions. Dry or sandy clay soils, low in organic
matter, have much lower biological activity that results in persistence of residual
herbicides, slow breakdown of elemental sulphur, poor water holding capacity
and more. In other words, it is the opposite of manured soils.
So, how do we optimize Canadian crop production in the 21st Century? Obviously,
application of high priced nitrogen with phosphate and sulphur generally works
and, presto-chango!, we have our answer – or do we? We know that a 50bu/ac
crop of barley will remove about 56 pounds of nitrogen per acre (in the grain).
Doubling this yield will take out 112 pounds of nitrogen. So, why can't we consistently
make this happen if moisture levels are adequate? Because, in many situations,
we forget to consider the balance of other macro and micronutrients, specific
nutrient availability and other soil factors such as soil pH, CEC (texture),
compaction, salinity, and especially microflora activity.
Therefore, to grow a 100bu/ac crop of barley, additional nitrogen is required
to build the factory (straw) and to compensate for soil immobilization (microbial)
and potential losses (leaching, volatilization, denitrification). Thus, a 100
bushel crop of barley removes roughly 112 pounds of nitrogen in the grain and
leaves about 39 pounds in the straw which, if left in the field, will be recycled
over the next few years for subsequent crop growth.
Nitrogen and sulphate sulphur are two of the more readily available soil applied
nutrients. If a soil is low in nitrogen, then we add fertilizer nitrogen expecting
only 40 to 70 percent to be utilized during the growing season with the remainder
either tied up by the soil microflora or lost through leaching, volatilization
or denitrification. Not so with phosphate, potash and elemental sulphur; which
may only be 10 to 30 percent available in the year of application. Soil potassium,
magnesium and calcium are key elements affected by the cation exchange capacity
(CEC), or texture, of all soils. These nutrients, along with phosphate and elemental
sulphur are not 100 percent immediately available when added to soils deficient
in any one or all of these components.
Thus, in a phosphate deficient soil, if you double the nitrogen application,
you may have to greatly increase your phosphate levels since a 100bu/ac crop
of wheat requires nearly 80 pounds of available phosphate, 64 pounds in the
grain and 16 pounds in the straw. In addition, if soil tests reveal low levels
of potash, other secondary nutrients (sulphur, calcium or magnesium) or micronutrients
(boron, copper, manganese, zinc), these levels may have to be significantly
increased to prevent crop failure, since any one deficient nutrient (macro,
secondary or micro) could act as the 'lowest stave in the nutrient barrel'.
For example, a 50bu/ac crop of wheat removes about 0.25 pounds of zinc from
the soil and a 100bu/ac crop removes about a half a pound. Zinc levels in the
soil are especially critical in corn production.
In order to optimize crop yields for given moisture regimes, we must have adequate
or surplus availability of all nutrients. These nutrients, particularly nitrogen,
must always be in balance in the soil. Excessive nitrogen in either wetter or
dryer growing seasons upsets crop growth and production.
How then do we obtain optimum soil fertility conditions for top yield production
in any given season or moisture regime? The simple answer is soil organic matter,
naturally occurring, added as livestock manure, or worked in as crop residues.
Our naturally occurring prairie soil organic matter levels took 10,000 years
to build up and only 100 to 200 years to run down. What then is so fantastic
about the organic content of soil? Our superficial and immediate answer is the
nutrients contained within, especially in added manure or crop residue. The
full and real answer is what adding organic matter to soils really does for
optimizing crop production.
The nutrients in the manure, or returned residue, are only the tip of the 'beneficial
iceberg'. These organic residues also:
- Enhance soil friability for seedbed preparation.
- Increase soil water holding capacity in sandy soils.
- Increase soil porosity in clay soils.
- Allow much easier root penetration by crop plants.
- Allow easier water percolation into sandy and clay soil types.
- Reduce erosion by binding soil matter.
- Hasten the breakdown of residual herbicides by enhanced bacterial presence
in the organic matter and directly adsorbs and inhibits herbicides.
- Reduce the traction (pull) needed to work the soil.
- Allow for better water infiltration, allowing for earlier access onto the
land in the spring.
- Reduce pH levels in alkaline soils and increase pH in acid soils.
- Modify crop root rhizosphere by allowing for increases in saprophytic fungi
and bacteria breaking down the organic matter and suppressing the growth of
parasitic crop root rotting fungi (biological control).
- Lessen weed competition by producing more robust weed suppressing crops.
So, there you have the additional 'dirty dozen' as a consequence of manuring
your soils. Obviously, the nutrient component is only the tip of the soil organic
Recently, on the grapevine, I've heard of individuals in the US growing greater
than 450 bushels of corn per acre. Did you check the 'ingredients'? It's corn
after corn; with about 10 to 20 tons of annually returned stover, along with
30 to 50 tons of applied manure, in addition to the appropriate availability
of all crop essential nutrients. Why not skip the manure? Well, this manure
had another major function up its 'hypothetical sleeve' calculated to produce
this phenomenal yield of corn. Simply put, its carbon dioxide (CO2)!
Under very high yield conditions it has been well documented for corn that the
limiting nutrient can be CO2. Under a still, sunny day
at noon time, either in Ontario or Brandon, in late July, in the middle of a
corn field, CO2 is the missing ingredient. The corn plants
have adequate water, lots of mineral nutrients, plenty of sunshine, but are
way short of CO2 to fix into sugars for growth. At this
time of year, heavily manured fields are releasing huge amounts of CO2
from the soil surface directly as a consequence of microbial activity. This
on-site CO2 can greatly enhance carbohydrate production
especially in the corn crop (benefit 13).
Greenhouse growers routinely add CO2 to their growing
crops, increasing tomato production by around 30 percent in most seasons. CO2
production by microbial action, or manure, could be significant in optimizing
higher yields in all crops. Under ideal growing conditions of light, nutrients
(hydroponics) and CO2 injections, greenhouse growers
can produce 50 to 70 kilograms of tomatoes per square meter in a growing season.
This translates into 250 to 350 tons per acre, way above the outdoor tomato
field production of less than 100 tons per acre.
So, if you aim to be a top crop producer, remember that you have to provide
your crops with all the nutrients necessary to achieve these goals. In addition,
you must reduce, eliminate or remove those factors that impede optimal production
such as residual herbicides, root diseases or poor soil structure by returning
crop residues, manure, or both to your land base. Manure and crop residues provide
soil inhabitants – fungi, bacteria, nematodes, worms and insects –
a food source which promotes a vigorous, dynamic ecosystem that results in the
rapid recycling of plant nutrients. In fact, another function of active, manured
soils is the relatively rapid breakdown of elemental sulphur (benefit 14).
Manure and crop residues are certainly not waste products but the very means
by which we can maintain and sustain dynamic cropping systems and optimize our
yield returns. Such a process does not happen over night, it can take five to
10 years of regular manure and crop residue application to turn your crop production
significantly upwards. If we are prepared to wait another 10,000 years and leave
our cropland alone, no doubt it will slowly recycle to its fertile unbroken
prairie soil status of the 19th Century. Rapidly increasing the organic content
and consequently the dynamism of your cropland will help you achieve the goal
of 'Tops in Crops'.
Ieuan Evans is a Senior Agri-Coach with Agri-Trend