Summerfallow and Soil Conservation

Introduction

Farming on the Canadian Prairies has risks that vary according to the area and farming enterprise involved. Farmers have adapted farming techniques to reduce the risk and meet challenging conditions. In drier zones farmers rely on summerfallow as part of their cropping rotation.

Summerfallowing can be defined as leaving the land without plant growth for an annual growing cycle. However, the term is often used to describe one particular type of summerfallow, black fallow.

In black fallow, a number of tillage operations are used during the fallow period to control weeds and volunteer crop growth. If the tillage operations bury most of the plant residues below the soil surface, the soil can be at risk of wind and water erosion.

The reasons to practise summerfallow include weed control, conservation of soil moisture, increased short-term nutrient availability, reduced risk of residue-borne plant diseases, and reduced risk of crop failure due to drought.

Conservation summerfallowing maintains sufficient plant residues on the soil surface to prevent soil erosion, as well as controlling weeds and increasing stored soil moisture. Tillage operations are reduced in number or intensity, or may be replaced with non-tillage alternatives.

History of Summerfallow

As the use of land for cultivated agriculture spread on the Canadian Prairies at the turn of the century, farmers encountered an environment that was much drier than what they had known in eastern Canada or Europe. Sparse and irregular precipitation and high evaporation rates during the summers reduced the amount of moisture available for crop growth. Experience and time were necessary for farmers to learn how to produce crops on the semi-arid plains.

Early recorded use
Summerfallowing was one method used to lessen the adverse effects of drought. An early example was in the late 1880s, near Indian Head, Saskatchewan. One year, when excess spring moisture prevented the seeding of some fields, wet fields were fallowed due to the short growing season remaining. In the following year of drought, fallow fields were the only fields to produce a reasonable yield. This experience resulted in a statement in 1889 by the Indian Head Experimental Farm that "fallowing was the best preparation to ensure a crop."

Summerfallowing encouraged
Summerfallowing did not become a widespread until the 1920s. One reason for this was that during the early 1900s up until 1917, relatively favorable moisture conditions were experienced and no benefit was gained from summerfallowing.

However, a drought began in 1917 that continued until 1920. An example of how the opinion of summerfallow changed over this period is found in the comments from the Moccasin Experimental Station annual reports. In 1917, the annual report stated, "Summerfallow is a waste of time and effort, and need not be practised." The 1922 annual report stated, "The highest yields obtained over the last three years have been on fallowed plots."

This drought affected other common farm practices. The severe wind erosion experienced by farmers led to the adoption of a number of soil conservation practices. These included narrower fields, alternate strips of crop and fallow, and placement of these strips at right angles to the prevailing winds. These practices were recommended by university and extension persons throughout the 1920s. Part of the fallow recommendation during this period was the technique called "dust mulch fallowing". The goal was to create a surface mulch of fine soil that would prevent the escape of moisture from continuous pores deeper in the soil.

Stubble mulch fallow
The severe wind erosion problems of the 1930s worsened when a dust mulch was created. This resulted in research trials on leaving crop residues on the soil surface. In 1936, C.S. Noble built his first wide blade cultivator, which revolutionized tillage on fallowed land. It was well-suited to stubble rather than dust mulch fallowing. By retaining more crop residues on fallow fields, stubble mulch fallow helped to reduce erosion damage.

Herbicides reduce the need for tillage
The three main reasons why tillage is used in growing crops are to prepare a seedbed, to incorporate or bury previous crop residues and to control weed growth.

Reliance on tillage for weed control was reduced when herbicides became commercially available for farm use. The use of growth regulating herbicides (i.e., 2,4-D and MCPA) started in the late 1940s and early 1950s. Farm managers then had the option to control some broad-leaved weeds with an in-crop herbicide application. In the 1960s, herbicides were developed that allowed grassy weed control (i.e., wild oats) in wheat, barley and flax. No significant replacement of tillage with herbicides under summerfallowing occurred until the release of non-selective herbicides (i.e., paraquat or Sweep and glyphosate or Roundup) in the late 1970s.

With the use of non-selective herbicides, farm managers have the option of replacing some or all tillage operations during a fallow period.

Extent of summerfallow use
During the 1930s summerfallowing became an established farming practice, occupying 30 per cent of cultivated land on the Prairies. Since then the amount summerfallowed has fluctuated between 29 and 39 per cent.

The use of summerfallow as a means of storing precipitation in the soil for the subsequent crop is most beneficial in the Brown and Dark Brown soil zones. These areas experience the largest moisture deficit during the growing season.

Research has shown that, in a fallow year, only 30 per cent or less of the precipitation is retained in the fallowed soil. However, the little moisture that is retained helps to ensure an adequate crop yield in the following year, and especially if insufficient moisture severely limits crop growth.

Disadvantages of Conventional Summerfallow

Potential for erosion
In black fallow fields, tillage operations stir the soil and bury crop and weed residues. Lack of plant residues on the surface leave the soil vulnerable to wind and water erosion. Plant residues anchor the soil, reduce wind and water speeds at the soil surface and protect the soil from raindrop impact. Raindrop impact and frequent tillage can pulverize the soil resulting in crusting and poor soil moisture infiltration. Low infiltration rates result in higher runoff and more water erosion.

Decline of soil organic matter
Tillage raises soil temperatures and increases aeration and mixing of the soil. These conditions lead to faster decomposition of both the soil's organic matter and crop residues, as compared to a soil with a growing crop. During the fallow period, certain plant nutrients that become available through decomposition are lost through leaching or gaseous emissions. Normally these nutrients would have been used by plants and retained in the plant-soil system.

A decline in the organic matter content of a soil results in less plant nutrients being stored in the soil and degrades the physical structure or tilth of the soil. Poorer soil structure results in less infiltration of precipitation into the soil resulting in increased runoff. Consequently, it becomes more difficult to prepare a suitable seedbed for crops. The ease of seedling emergence also declines because of increased resistance from crusting of the soil surface.

Groundwater recharge
During a fallow season, precipitation percolates down through the subsoil and enters the groundwater. Water-soluble crop nutrients (e.g., nitrates) are transported with the water. Excessive levels of crop nutrients can reduce the quality of the water in underground aquifers. In addition, the excess water can move salts to groundwater discharge areas. This will cause groundwater levels to rise in these discharge areas and increase the size of areas adversely affected by excess salinity. If a crop had used the water, the nutrients would have been used by the plants, and less salts would have been moved to discharge areas.

Erosion problems can be controlled by maintaining sufficient crop residues on the soil surface through the use of conservation methods. Residues do not, however, have much effect on the problems of declining soil organic matter or groundwater recharge. The best way known to conserve plant nutrients and prevent moisture from leaching through the soil into the groundwater is to have plants growing on the land.

Conservation Fallow Systems

The basic goals of conservation fallow systems are to minimize soil erosion during the fallow period, allow adequate weed control and store moisture in the soil profile for the subsequent crop.

By using residue conserving practices, adequate cover can be maintained through the fallow period until the next crop is sufficiently established to protect the soil from erosion. A minimum residue cover of 1,350 lb/acre is needed to protect most soils from serious wind or water erosion. This is roughly equivalent to the residue left after harvesting a wheat crop yielding 14 bu/acre of grain. In practice, most fields will have crop yields that comfortably exceed this level. Soil-protecting residue is reduced through natural decomposition (sunlight, oxidation, microbial activity), a process over which a farm manager has little or no control. Another way residue cover declines is through tillage, an operation that mixes the residues into the soil. Tillage operations can, however, be managed to maintain surface residues, while preventing serious erosion, controlling weed growth and preparing the seedbed for the subsequent crop.

If the residue conserved on a field can be kept standing, it will help trap snow and thereby increase spring soil moisture. Usually 45 per cent of soil moisture conserved in an 18-month fallow period is received over the first fall and winter. By trapping snow more effectively, the amount of soil moisture conserved can be increased. This increased moisture conservation can in some cases reduce the need to summerfallow altogether and allow cropping on an annual basis.

Several economically attractive options are available to farmers who wish to store soil moisture, control weeds and prevent erosion.

Reduced tillage fallow
Reduced tillage fallow is similar to black fallow, but the number of tillage operations or the amount of soil mixing is reduced.

Tillage operations do not need to be replaced with a herbicide application. However, the choice of tillage implement is very important. Table 1 lists the tillage equipment commonly available today and the relative amounts of residue left after using this equipment. Wide-blade cultivators and chisel plows are more conducive to reduced tillage than discs or plows.

Table 1. Effect of common tillage implements on surface residue conservation

Tillage Implement	% Residue Left After One Pass	% Residue Left After 4 Passes
Wide-blade cultivator	90	60 - 65
Chisel plow with low-crown shovel	85	40 - 45
Chisel plow with normal shovels	80	35 - 40
Chisel plow with normal shovels plus mounted harrows	60	10 - 15
Heavy tandem or offset disc	35 - 65	5 - 15
Moldboard plow	0 - 10	0

There are advantages and disadvantages to using each type of tillage implement. Also, the way a specific implement is used can affect how much soil mixing takes place. Proper depth, working angle and field speed are required to control weeds and maintain residue cover.

The wide-blade cultivator is more appropriately used in the drier, warmer areas of the Prairies. The V-shaped blade that be from 0.9 to 2.1 m (3 to 7 ft) wide. It is designed to undercut weed growth at an operating depth of 5 cm (2 in.). Most weeds will be killed as long as the weather is hot and dry at the time of use and remains so for a few days afterwards. If it rains within a day or two of cultivation, many weed species will reroot and regrow. Because of more frequent rains and moist soil conditions in the Black and Gray soil zones, the wide-blade cultivator is seldom used in these areas.

A chisel plow, equipped with V-shaped shovels or sweeps ranging from 0.25 to 0.45 m (10 to 18 in.), is a common tillage implement used when fallowing. When used for fallowing the shovels are operated just deep enough (5 to 10 cm (2 to 4 in.) ) to cut off weeds with a minimum of surface disturbance. Three basic sweeps are used: low-crown, regular-crown and high-crown sweeps. Low-crown sweeps cause the least soil disturbance, high-crown sweeps cause the most, and regular-crown sweep cause an intermediate amount. A farm manager who wants to increase the soil conservation aspects of a fallowing operations may choose to change the chisel plow sweeps from high-crown or regular-crown to low-crown.

The use of mounted harrows on a chisel plow for fallowing is discouraged because it can increase burying surface residue by an additional 20 per cent (Table 1).

The heavy tandem or offset discs are not commonly used when fallowing. However, when plant growth from weeds and/or green manure crops is substantial, the disc can slice and bury from half to three quarters of the plant material. Additionally, the farm manager may want to thoroughly mix in a soil incorporated herbicide for controlling certain weeds while fallowing or for weed control in the subsequent crop.

It is difficult to justify using a moldboard plow for fallowing due to the amount of residue it buries (90 to 100 per cent), compared to the other implements discussed here.

Six to 8 km/h (4 to 5 mi/h) is the recommended speed of travel while tilling with the above noted sweep and disc implements. Travel speeds greater than this significantly increase the stirring of the soil and leave less residue cover to control erosion.

Chemical or herbicide fallow
In chemical fallow, weed growth is controlled exclusively by consecutive and timely applications of herbicides. The choice of herbicides is based on the variety of weeds present. For example, a selective herbicide may be used for a specific weed species or a non-selective herbicide for a wide variety of weeds. Application rates are selected to control target plants economically.

The stubble and other plant residues from the preceding crop are left undisturbed, erect and anchored, as are the remains of the dead weeds. This practice protects the soil from erosive and moisture-robbing winds, and the forces of water erosion. The shaded soil surface is cooler, which lowers soil temperatures. The lower soil temperatures, along with less tillage induced aeration of the soil, reduces evaporation. Over the course of the summerfallow period, approximately 60 to 80 per cent of the protecting stubble will remain. Losses of stubble are due to the normal decomposition from ultra-violet radiation, chemical oxidation and microbial activity.

Table 2. Herbicides used for Fallowing

2,4-D
MCPA
Trifluralin (TREFLAN, RIVAL)
Glyphosate (ROUNDUP, WRANGLER, LAREDO, RUSTLER)
Dicamba (BANVEL, RUSTLER)
Bromoxynil (PARDNER)
Paraquat (SWEEP)
Linuron (LOROX)
Mecoprop (TARGET)
Source: Alberta Agriculture, Crop Protection with Chemicals, 1992, Agdex 606-1

Minimum tillage fallow
Minimum tillage fallow management often unites the positive aspects of tillage and weed control with chemicals. Herbicides can be strategically applied to target specific perennial, biennial and winter annual weeds, minimizing the number of tillage operations. Other tillage operations may be replaced with non-selective herbicide applications.

The focus of minimum tillage is to keep the greatest amount of the stubble from the previous crop anchored, erect and undisturbed. Standing stubble is vital for soil protection, snow trapping and evaporation reduction. Tillage operations are used when weeds are more economically and practically controlled than with herbicides. Stubble can also be further conserved by the use of minimum soil mixing tillage implements (e.g., wide-blade cultivators).

Cover crops
To protect erosion-prone fallow land, farm managers may choose to plant a cover crop. A cover crop is usually planted later in the growing season (e.g., early August) and is solely intended to achieve enough leafy top growth to provide protection against wind and water erosion.

Cover crops are typically spring cereals, which are inexpensive to seed, killed by freezing over the winter, and competitive with weeds but not competitive with the following crop.

Cover crops use some of the plant nutrients in the soil, but only for a short time. The nutrients used by cover crops are cycled back through decomposition, becoming available to subsequent crop plants. The amount of soil moisture used by the cover crop is small and comes from shallow depths, and it is usually replaced over the winter.

Green manuring
Green manure crops can be used as an alternative to summerfallowing. Green manuring is the practice of specifically growing a crop to work it into the soil using tillage. Almost any crop may be used for green manure, but legume crops (peas, clovers, lentils, etc.) are preferred because they add nitrogen fixed from the air to the soil.

A legume green manure crop is typically planted the first week of May, grown for seven to eight weeks or until it reaches the full-bloom stage at which time it is worked down using a tandem disc. After discing, conventional fallow methods are followed for the remainder of the growing season. This allows sub-soil moisture levels to increase to traditional fallowing levels. Under severe drought conditions, legume growth may reduce the amount of stored soil moisture compared to black summerfallow, but this decrease is less than if a full-season grain crop had been grown.

The advantages of legume green manuring versus a black fallow are to:

• Increase protection from erosion due to the legume top-growth.
• Conserve and recycle plant available nutrients normally lost by leaching or gaseous emissions during conventional fallowing.
• Decrease the need for nitrogen fertilizer because of biologically fixed nitrogen from the air.
• Maintain soil organic matter at levels greater than when summerfallow is used frequently (once every two years), although this may be less than the organic matter level maintained under continuous cropping.

Soil Zone	Location			Per Cent Carbon Content
		Duration of Study (yr)	Soil Depth (cm)	Contin. Wheat	Fallow Wheat	Fallow Wheat
Brown	Swift Current	1967-82	0-7.5	2.15	1.7	1.94
Dark Brown	Lethbridge	1912-75	0-13	1.91	1.55	1.36
Dark Brown	Lethbridge	1951-84	0-15	1.78	1.58	1.36
Black	Indian Head	1958-84	0-7.5	2.43	2.25	2.21
Black	Melfort	1957-84	0-7.5	5.92	5.3	n/a
Solonetz	Vegreville	1958-80	0-15	4.24	n/a	3.79

• Four tillage operations with a chisel plow (sweeps), the first operation in mid-May then every four weeks depending on precipitation and weed growth
• Cost 4 x $5.00 = $20.00/acre
• Tillage reduction of residue = 60 per cent

• Four tillage operations with a wide-blade cultivator, at the same times as the Conventional Tillage example
• Cost 4 x $5.00 = $20.00/acre
• Tillage reduction of residue = 34 per cent

• Fall application of 2,4-D (0.3 L/acre); three tillage operations with a wide-blade cultivator, one in each summer month
• Cost 1 x $3.70 + 3 x $5.00 = $18.70/acre
• Tillage reduction of residue = 27 per cent

• Fall application of 2,4-D (0.3 L/acre); June and August applications of Rustler (1.0 L/acre)
• Cost 1 x $3.70 + 2 x $8.45 = $20.60/acre
• Tillage reduction of residue = zero per cent

• Five tillage operations with a chisel plow (sweeps), the first operation in mid-May then every four weeks depending on precipitation and weed growth
• Cost 5 x $5.00 = $25.00/acre
• Tillage reduction of residue = 67 per cent

• Four tillage operations with a chisel plow (sweeps), the first operation in mid-May with the remaining operations as needed
• Cost 4 x $5.00 = 20.00/acre
• Tillage reduction of residue = 59 per cent

• Fall application of 2,4-D (0.3 L/acre); application of Rustler (1.0 L/acre) in the first week of June; two tillage operations with a chisel plow (sweeps) as needed in July and August
• Cost 1 x $3.70 + 1 x $8.45 + 2 x $5.00 = $22.15/acre)
• Tillage reduction of residue = 36 per cent

• Fall application of 2,4-D (0.3 L/acre), June and August applications of Rustler (1.0 L/acre)
• Cost 1 x $3.70 + 2 x 8.45 = $20.60/acre)
• Tillage reduction of residue = zero per cent

• Fall tillage operation with a chisel plow (chisel points); six tillage operations with a chisel plow (sweeps) during the fallow season, starting at the end of May
• Cost 7 x $5.00 = $35.00/acre
• Tillage reduction of residue = 74 per cent

• Five tillage operations with a chisel plow (sweeps) during the fallow season
• Cost 5 x $5.00 = $25.00/acre
• Tillage reduction of residue = 67 per cent

• Fall application of 2,4-D (0.5 L/acre); application of Roundup (10.0 L/acre) in the first week of June three tillage operations with a chisel plow (sweeps) during the rest of the season as needed
• Cost 1 x $4.45 + 1 x $13.45 + 3 x $5.00 = $32.90/acre
• Tillage reduction of residue= 49 per cent

• Fall application of 2,4-D (0.5 L/acre); applications of Roundup (1.0 L/acre) in the first week of June and again in early July; application of Roundup (0.7 L/acre) + Banvel (0.3L/acre) in early August
• Cost 1 x $4.45 + 2 x $13.45 + 1 x $18.42 = $49.77/acre
• Tillage reduction of residue = zero per cent