Soil Management: The Soil Resource
| Corn |
Soybeans | Forages
| Cereals | Dry
Edible Beans |
| Spring and Winter Canola
| Other Crops | Soil
Management |
| Soil Fertility and Nutrient
Use | Field Scouting
|
| On-Farm Stored Grain Management
| Weed Control |
| Insects and Pests of Field
Crops | Diseases of Field
Crops | Appendices |
Pub 811:
Agronomy Guide > Soil
Management > The Soil Resource
Order OMAFRA Publication
811: Agronomy Guide for Field Crops
Soil Formation
The properties of Ontario's soils are related closely to landforms that
were created by glacial ice, meltwaters, glacial lakes and wind. Glaciers
moved across all of Ontario grinding rock into fine particles, and mixed
and moved existing soil. As the glaciers retreated, they dropped soil
materials from within the ice itself. The meltwater deposited gravel and
sands as mixed layers. Flat beds of sand, silt and clay were deposited
by lakes that formed from the ponding of melt waters. The soils were further
distributed as strong winds moved across these landscapes. The soils of
today were developed on these deposits.
Examples of the landforms include shallow-over-bedrock, muck or peat,
till plain, end moraine, sand plain and clay plain. Additional information
and photo examples of the landforms can be found in Best Management
Practices: Soil Management, Order No. BMP06E. See also the Soil
Management page on the OMAFRA website at www.ontario.ca/crops.
Soil Variability
The soils of Ontario vary greatly in their make-up due to the scraping
and mixing action of glacier movement. As the glaciers melted, wind, water
and time contributed to further differences in soil development. Some
soils are very shallow to bedrock, others have more than 100 m (328 ft)
of overburden. The depth of topsoil varies as conditions for soil formation
varied. Soils vary from region to region, from farm to farm and within
fields. Observe a soil excavation to see the variation in the depth of
soil horizons (shown in Figure 8-1, Variability
in Soil Horizons). This natural variation often occurs over very short
distances.
Growers have been aware of this soil variability for years and have tried
to improve areas that were less productive.
Figure 8-1. Variaibility
in Soil Horizons
Yield monitors have allowed us to see how much that variability can affect
yields. In some fields of corn, there can be an over-6.3 t/ha (100 bu/acre)
difference between high- and low-yielding areas. Soybeans can have a 2.7
t/ha (40 bu/acre) spread and wheat a spread of up to 5.4 t/ha (80 bu/acre).
Soils can vary in:
- mineral composition, texture, fertility, pH
- soil organic matter: amount and type
- structure, aeration, drainage, density, water-holding capacity
- thickness of horizons, topographic position
- age: degree of weathering, erosion, deposition
- depth to:
- bedrock
- water table
- textural or structural changes
Intensive crop production has contributed to the variation in topsoil
depth. Soil erosion by wind, water and tillage has reduced the amount
of topsoil in parts of many fields. That topsoil, in some cases, is deposited
in other areas of the field, increasing topsoil depth there, while in
some fields, the topsoil has been lost totally from areas. Other examples
of management that can intensify variability include liming, removal of
fence rows, compost/manure piles and uneven manure application patterns.
Table 8-1. Soil Organic
Matter Pools
|
Property
|
Active Soil Organic Matter (living)
|
Moderately Stable (dead)
|
Very Stable (very dead)
|
| Activity |
- living or recently deceased
- living part includes bacteria, fungi, algae, plant roots, insects,
earthworms, etc., which help with the breakdown of residues and
manures
- supplies nutrients as well as glues for aggregate formation
- recently dead includes dead organisms, recently added manures,
old plant roots and crop residues
- supplies food for organisms
|
- recently decomposed organic material
|
- sometimes called humus
- virtually untouchable, intimate part of the soil
- holds on to some nutrients for slow release
- can lessen drainage or compaction problems
- can improve water retention in sandy soils
|
| Persistence |
- influenced by year-to-year management
|
- influenced over a farmer's lifetime
|
- existed long before the field was farmed and will exist long
after the farmer retires
|
| Percentage of soil organic matter |
10%-15% |
40%-45% |
40%-45% |
What Is Good Quality Soil?
Soil quality is the measure of a soil's health and its ability to resist
erosion, compaction and other stresses, while maintaining economic productivity.
Many factors are assessed to determine a soil's health. A healthy soil
will:
- have good soil structure, resist crusting and have minimal compaction
- have an abundance of earthworms
- have a fresh, earthy odour
- readily decompose residue
- have good drainage, water movement and water-holding capacity
- encourage seedling emergence and root growth
- produce uniform crop growth and colour
- have nutrient levels, pH and organic matter in the optimal range
- have suffered little wind, water or tillage erosion, and will be resistant
to it
Most of the characteristics of a healthy soil have a direct or indirect
link to soil organic matter.
Organic Matter
Soil organic matter is a very small component of the soil but it plays
a crucial role. Building and maintaining good levels of soil organic matter
results in:
- higher-yielding, healthier crops
- crops more tolerant to doughty conditions and other stresses, such
as insects and diseases
- a reduced need for commercial fertilizer and lime
Soil organic matter exists in three pools in the soil. See Table
8-1, Soil Organic Matter Pools.
Importance of Organic Matter (OM)
Crucial to many aspects of the soil and crop development and with no
single role taking precedence, organic matter:
- directly or indirectly influences the availability of nutrients obtained
from the soil, playing an important role in many nutrient cycles
- improves the cation exchange capacity (CEC) of a soil by helping the
soil hold on to positively charged ions such as calcium, potassium and
magnesium making them available to the crop. This is most important
in loams and sands, where there is little clay to provide the negative
charge to hold the cations.
- forms complex organic acids during the breakdown of organic materials,
which also hold on to nutrients, and helps keep iron, zinc and manganese
in the chelated or available form
- can buffer soil pH, slowing rapid changes in pH
- darkens soil colour, helping warm the soil faster in the spring
- helps store carbon - there is four times as much C in the soil as
in plants
- stores nitrogen - almost all the nitrogen in soils exists as part
of the organic matter. Bacteria and fungi convert the organic format
to nitrate and ammonium, which can be used by plants.
- maintains the water cycle by keeping the soil open and porous, so
more water can soak into the soil. Infiltration, rather than runoff,
replenishes soil moisture during dry conditions and contributes to recharge
of groundwater. This in turn increases the amount of water that plants
can access from the soil, by increasing both storage capacity and rooting
volume, and improves drainage of excess water through the soil.
- aerates the soil, creating greater porosity so air can enter the soil
more easily
There is also some evidence that organic matter may help prevent phosphorus
from being converted to forms that are unavailable to plants.
Soil Life
A healthy soil is full of life. The organisms living in the soil play
an important role in the health of the soil and of plants. Soil life includes
bacteria, fungi, algae, protozoa, nematodes, earthworms, insects (ants,
beetles, millipedes, etc.), larger animals (moles, rabbits, snakes, etc.)
and plant roots.
Soil organisms play an important role:
- in the breakdown of organic residues and their incorporation into
the soil. As organic materials are decomposed, nutrients become available
to plants, humus is produced and soil aggregates are formed.
- creating channels for water infiltration and better aeration
- in moving surface residues deeper into the soil
- in nitrogen fixation
- in fighting plant pests, such as weeds, insects, nematodes and diseases
- stimulating root growth with substances produced by microorganisms
Soil Structure
The term soil tilth is used to describe a soil that is in a favourable
condition for crop growth. Soil with good tilth is porous and allows water
to enter easily, instead of running off the surface, which makes it more
available to plants and results in less erosion. A porous soil allows
roots to exchange oxygen and eliminate carbon dioxide more easily, which
aids root growth.
Soils and soil structure are formed through the actions of freeze-thaw
cycles, wet-dry cycles, root growth, tillage, and soil animals and microorganisms.
The active organic matter in a soil, the decomposing residues and the
soil life play a significant role in soil structure development and maintenance.
Soil structure is developed from soil particles held together with clay,
humus and the glues released from living and decomposing organisms. Good
soil aggregation or structure can only be maintained with a continuous
supply of organic materials, roots of living plants and soil organisms.
Table 8-2, Types of Soil Structure, describes
soil structure typically seen in soils.
Soil Compaction
Compaction is defined as increased bulk density and reduction in soil
pore space. This occurs when the soil particles are forced closer together
by the impact of equipment, animals and raindrops. The use of heavier
tractors, combines and implements, particularly with earlier spring tillage,
can cause problems under any tillage system.
A soil that is in poor tilth has structure that has deteriorated and aggregates
that are not stable. This can be seen as increased compaction, decreased
aeration and a reduction in water storage. Heavy equipment travelling
across the soil can cause significant soil compaction. This can have a
negative impact on the soil biology. The reduction in the number of medium
to large pores reduces the volume of soil available for air, water and
populations of organisms that require large spaces in which to live. There
are three types of soil compaction that can be found in the soil: surface
crusting, tillage layer compaction and subsoil compaction. See Table
8-3, Types of Soil Compaction, for a description of each.
Soils having high organic matter contents, good internal drainage and
good structure are less susceptible to compaction. For further information,
see the OMAFRA website at www.ontario.ca/crops.
Threat to the Resource
The biggest threats to the soil resource are the loss of soil organic
matter, the lack of a thriving and diverse population of organisms, and
soil compaction.
As agriculture has become more mechanized and many rotations have become
shorter and more intense, the quality of many soils in Ontario has declined
over the last 4 to 5 decades. The soil is vulnerable to degradation. Soils
that are degraded are usually the result of soil erosion and a decline
in organic matter levels. Soils in this state often end up on a downward
spiral. Further loss of topsoil due to erosion reduces the nutrient content
of the soil. The lost soil carries nutrients with it so the topsoil layer
becomes less fertile. Tillage of the soil begins to incorporate less fertile
soil from below. As organic matter levels decline, the soil becomes less
resistant to erosion. The soil also becomes less resistant to soil compaction.
As compaction increases, porosity is reduced and there is less air and
less water movement through the soil. More runoff causes more erosion.
As organic matter levels decline, the soil has less water-holding capacity.
The cycle continues in a downward spiral of soil degradation.
Soil Erosion
Soil erosion is a serious threat to the productivity of the soil. Soil
erosion is a naturally occurring process, but farming practices have accelerated
the rates of erosion. Erosion in agricultural fields involves the detachment
and movement of soil particles within and outside the field. It results
in:
- decreased crop yields
- increased cost of production
- degraded topsoil
- increased runoff and reduced water storage
Table 8-3. Types of Soil Compaction
|
Cause
|
Impact
|
| Surface crusting |
- Unprotected soil is dispersed by the action of raindrops and
pushed into a thin, dense surface layer.
|
- Soil is prone to high rates of runoff and water erosion.
- The surface becomes sealed, reducing water infiltration.
- When dry, a hard crust forms, which can delay or prevent seedling
emergence.
|
| Tillage layer compaction |
- Soil is eroded.
- Soil has low levels of organic matter.
- Heavy field equipment traffic results in soil compaction.
|
- The layer is very dense, reducing water infiltration and the
porosity of the soil.
- Root growth may be restricted.
|
| Subsoil compaction |
- Pressure from a disc or plow on the soil below the tillage layer
compacts the subsoil.
- Pressure from heavy field equipment with poor weight distribution
results in compaction.
|
- Water flow is restricted through the compacted layer.
- There will be little or no root growth through the compacted
layer.
|
The three types of soil erosion are water, wind and tillage. Water erosion
is the detachment and movement of unprotected soil particles by water.
Wind erosion is the detachment and movement of soil particles by air currents
or wind. Tillage erosion occurs when tillage equipment lifts soil and
moves it forward, gravity pulls it downhill. See the booklets Best
Management Practices: Soil Management, Order No. BMP06E, and Best
Management Practices: Field Crop Production, Order No. BMP02E,
for more information.
