Soils, Fertility and Nutrients
Extent of Solonetzic soils in Saskatchewan (from Stonehouse, 1991)
Why Deep Till Soils
Deep tillage is not a generally recommended farm practice in Saskatchewan.
Deep tillage should never be carried out on saline soils because it increases evaporation and could bring salts from the subsoil to the surface. Neither should soils prone to erosion be deep tilled, since deep tilling can increase the likelihood of erosion.
On most soils deep tillage is of little benefit and may result in excessive loss of soil moisture.
Deep tillage can improve crop production on what are often referred to as hardpan soils. Hardpan soils have a dense layer which has few, if any, large pores such as cracks and old root channels. Large soil pores are important for good water infiltration and root growth. When there are no (or only a few) large soil pores the infiltration of rainfall and snow melt is poor. This results in water ponding at the soil surface and/or more water runoff, and thus, poor moisture conservation. Furthermore, the volume of soil that the plant roots can exploit for moisture and nutrients is restricted by the hardpan layer. Moisture and nutrient stresses of crops commonly occur in hardpan soils, particularly in dry years.
There are two types of hardpan soils where deep tillage is considered beneficial, compacted soils and Solonetzic soils.
Compacted Soils occur when the stress (weight) on the soil from farm equipment exceeds the ability of the soil to support that stress. The soil is "squeezed" into a smaller volume (i.e. compacted) at the expense of the larger soil pores. Soil compaction is greatest when the soil moisture content is high because water acts as a lubricant allowing soil particles to slide past each other as the soil is compressed. For this reason soil compaction is a greater problem in wetter environments such as Europe, the southeastern USA, and to a lesser extent the northern part of the Saskatchewan grain belt.
There are three types of soil compaction. Shallow compaction is limited to the upper 10 cm (4 inches) of soil. Shallow soil compaction or firming of the seed bed often will be beneficial in dry years. Shallow soil compaction is of short duration since the soil will loosen as it undergoes wetting and drying, freezing and thawing, or normal tillage.
A second type of compaction is plow pan or tillage pan formation, in which a dense layer forms just below the depth of normal tillage. This type of compaction has been observed where one-way discs are used extensively.
The third type of compaction is deep compaction in the wheel tracks of heavy farm equipment under wet soil conditions. Deep compaction extends well below the depth of normal tillage. Unlike surface compaction, deep compaction depends on the axle load and therefore is similar for single and dual-tired equipment.
Tillage pans and deep compaction in wheel tracks are related and are a concern because they can persist for many years.
Solonetzic or burnout soils have a very dense soil layer which is high in clay and sodium, that is referred to as the Bnt horizon or solonetzic layer. The solonetzic layer can start as shallow as 10 cm (4 inches) and may extend as deep as 75 cm (30 inches). The solonetzic layer is very hard when dry and restricts water movement when wet. Topsoil quality may also be poor, resulting in crusting problems and increased susceptibility to soil erosion. The soil below the solonetzic layer (Csk horizon) is often high in calcium and magnesium salts.
There are 1.8 million ha (4.5 million acres) of Solonetzic soils in Saskatchewan, occurring primarily in the Brown and Dark Brown soil zones (Figure 1). In some areas the soils are dominated by Solonetzic soils, which usually indicates that the soils were developed from sodium-rich parent materials. In other areas, Solonetzic soils are found in close association with "better" quality prairie soils (i.e. Chernozemic soils) and in these cases the Solonetzic soils usually occupy the lower slope positions.
Management of Solonetzic soils requires that special attention be paid to timing of tillage and seeding operations with respect to moisture conditions. When the soil is too wet tillage implements are unable to work. Conversely, when the soil is too dry tillage implements have difficulty penetrating the soil and cause hard lumps when they do.
Most hardpan soils in Saskatchewan are Solonetzic soils rather than compacted soils. The mechanical soil-loosening techniques discussed in this bulletin (deep plowing and deep ripping) apply mainly to Solonetzic soils. However, deep ripping can also benefit compacted soils.
Identifying a Hardpan Soil
There are a number of symptoms that indicate hardpan soils:
In some cases these symptoms may indicate other soil and/or plant problems (such as low soil fertility levels), and thus it must first be determined if the soil is actually a hardpan soil.
A hardpan soil may be identified by on-site field inspections. One method involves pushing a narrow steel rod into the soil to a depth of 60 cm (2 feet). Another method involves digging a small hole (the width of a shovel extending down to 60 cm (2 feet) and pushing a knife blade into the side of the pit at various depths. A hardpan layer will make it difficult to push the rod or knife blade into the soil. The rod or blade will then stick when removed. Soil samples sent to a soil testing laboratory for chemical analysis will confirm whether or not the soil is Solonetzic.
What is Deep Tillage?
Deep-tillage implements used for soil disturbance of hardpan soils:
Deep rippers (Figure 2) are subsoilers with long shanks that are usually curved for lower draft requirement. They loosen the subsoil, but result in little soil mixing. Under most conditions deep rippers will break out a slot of soil that is slightly wider than the tool point. The loosened soil resembles an inverted triangle with its base at the surface and its point at the tool point.
Paraplows (Figure 2) are subsoilers that loosen the hardpan layer by lifting up the soil and lowering it again in a wave-like motion, resulting in very little subsoil mixing.
For maximum soil shattering, deep tillage operations should be carried out when the soils are dry.
The cost of deep plowing is expensive. The cost of deep ripping and of paraplowing is about half or less than the cost of deep plowing. Generally, the greater the severity of the hardpan, the greater is the cost of deep plowing or deep ripping and, usually, the greater the benefits in terms of increased crop production.
Deep Tillage and Crop Production on Solonetzic Soils
Producers should determine how much of their field is Solonetzic before deep tilling. Generally, deep tillage of entire fields is not recommended unless at least one-third of the field has a solonetzic layer. A useful method of determining if deep tillage will be beneficial is to deep till several long strips across the field and look for improvements in crop growth.
Deep plowing of Solonetzic soils physically loosens and chemically improves the solonetzic layer. Calcium and magnesium from the soil below the solonetzic layer displaces the sodium in the solonetzic layer when these soil layers are mixed. Therefore, it is important to determine that the soil below the solonetzic layer does, in fact, contain high levels of calcium and magnesium, and that the deep plow can reach deep enough to mix these soil layers. Soil samples from below the solonetzic layer should be sent to a soil testing laboratory to confirm the presence of high levels of calcium and magnesium.
Deep plowing provides long term soil improvement that can result in yield increases of between 135 to 403 kg/ha (two and six bushels per acre) of wheat per year (Figure 3).
Average wheat yields following deep plowing of a Solonetzic soils north of Radville, Saskatchewan (from Ballantyne, 1983).
The beneficial effects of deep plowing will last for more than 10 years, possibly 20 years or more.
Crop growth and yield can be severely reduced the year following deep plowing, as numerous tillage operations are required to smooth down the soil surface and these operations dry the soil.
Deep ripping of Solonetzic soils loosens the solonetzic layer, but results in only a limited amount of soil mixing. Sodium in the solonetzic layer is not displaced and in time the layer will reform. On the other hand, deep ripping is considerably less expensive, making it feasible to be periodically repeated.
The effect of deep ripping on crop production depends on the severity of the hardpan layer, the extent of soil loosening that is achieved with the ripper, and the amount of precipitation. Results from Saskatchewan and Alberta show that deep ripping increases crop production for up to five years, and in some cases longer. The second year usually shows the greatest benefit from deep ripping. Increases in crop production are usually smaller in subsequent years as the effect on soil loosening diminishes with time. In some cases, poor crop stands have been observed the year following deep ripping, due to poor seedbed conditions. In regions receiving higher amounts of precipitation, the effect of deep ripping on increased crop production will probably extend beyond five years.
Deep tillage of Solonetzic soils using the paraplow is considered too gentle, resulting in a soil loosening effect that lasts for less than two years.
In the drier soil zones, deep plowing is superior to deep ripping in terms of increased crop production. The main reason is that there is insufficient water for leaching of sodium salts, so the solonetzic layer reforms quicker following deep ripping. Furthermore, crops are usually not grown every year, making it more difficult to recover the cost of deep ripping.
Combined results from Alberta and Saskatchewan indicate wheat yield increases of approximately 269 to 673 kg/ha (four to ten bushels per acre) per year from deep plowing and 202 to 404 kg/ha (three to six bushels per acre) per year from deep ripping. The yield increases vary from year to year, showing large yield increases in dry years and small yield increases in wet years.
Solonetzic Soil in the Brown and Dark Brown Soil Zones
Solonetzic Soil in the Black and Gray Soil Zones
Deep ripping of Solonetzic soils is more successful when precipitation is more plentiful, as is the case in the Black and Gray soil zones, because:
Deep plowing is not normally recommended in the Black and Gray soil zones because deep ripping works adequately and costs much less.
The Economics of Deep Ripping in the Black and Gray Soil Zones
Recovery of the cost of deep ripping soils in the Black and Gray soil zones by revenues from increased crop production in subsequent years.
The cost of deep ripping is offset by the revenues generated by increased crop production in subsequent years. A study on the economic returns to deep ripping of Solonetzic soils in the Black and Gray soil zones of Saskatchewan suggests that the additional revenues generated by the first two crops are sufficient to pay for the cost of ripping (Figure 4). This study used average crop prices received during the 1980s and an annual discount rate of 5% for interest and inflation. The impact of subsequent crop revenues becomes smaller as the yield increases diminish and the discount accumulates.
The study also clearly shows that deep ripping non-Solonetzic soils in the Black and Gray soil zones is economically impractical, as the costs are much greater than the benefits.
Soil compaction on the prairies is more common in the Gray soil zone than other soil zones, In Saskatchewan, the Gray soil zone occurs along the northern edge of the grain belt. Reports from Alberta, where there is a much larger Gray soil zone, indicate that loosening of these soils can be beneficial if they have been compacted. Usually deep ripping or paraplowing is carried out to depths of up to 45 cm (18 inches) and has resulted in increased crop production on these soils.In other soil zones soil compaction may be a problem following wet years.
Insufficient field testing has been done to result in general recommendations on the feasibility of deep ripping and paraplowing of non-Solonetzic soils.
Revised by the Saskatchewan Soil and Crop Management Subcouncil.