Soil Management: The Soil Resource

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Pub 811: Agronomy Guide > Soil Management > The Soil Resource

Excerpt from Agronomy Guide for Field Crops
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.

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 8-2. Types of Soil Structure

Type		Importance
	Structureless
	Single grain: Soil breaks into individual particles (i.e., low organic matter sands).	prone to crusting, especially soils with higher clay contents have little resistance to wind and water erosion
	Massive: Soil breaks into large chunks.	compact, have very few pores, restrict root growth and water movement
	Granular
	Soil breaks into small aggregates or crumbs.	usually found in the topsoil layer ideal topsoil structure very good water-holding capacity, lots of pore space, good water movement and root growth
	Platy
	Soil particles are arranged in relatively thin horizontal plates.	often found in the top 8 cm (3 in.) of long-term no-till soils. Coulters cutting through the soil will chop up the plates to form a granular structure with time can be found in compacted soil layers
	Blocky
	Soil aggregates are cube-like or irregular in shape.	usually found in the B horizon promotes good root growth, aeration and drainage
	Columnar or prismatic
	Soil particles are arranged vertically to form prisms or pillar-like aggregates.	usually found in C horizons with higher clay content the vertical areas between the aggregates allow root growth and water movement

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.