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                                      SOIL PHYSICS
Greg Schmick, Author
Chief Agronomist, Pioneer Drill

Soil Physics
Water movement into the soil profile is a very dynamic and complex system that is seldom at rest. The direction of its movement can be dramatically affected by the cultural and tillage practices used for crop production. An understanding of the basic components of water infiltration is essential to producers if they are going to efficiently manage their soil and water resources.

Water absorption and infiltration into the soil profile are affected by two basic factors:
1) gravity and 2) matrix potential (attraction of water to itself and soil surfaces). Gravity affects only the downward movement of the water and the matrix potential affects both downward and lateral flow. In the field, matrix potential is usually the predominate force affecting water movement. See Fig. 1.


Fig. 1.
Gravity affecting only downward movement of water and matrix forces affecting both downward and horizontal movement. Dye or "streamer effect" results when water is introduced at the soil V. Water moves in all directions as show by the dye.

In the  course of tillage, a layer or inconsistency in the soil is introduced and affects water movement in much the same way as a sand or gravel layer. (Even though there is a steep hydrolic gradient between the wetting front and dry soil, water flow can be nearly zero under these conditions.) The tilled ground has many large empty pores as does sand and gravel with few surface contacts, giving it less attraction for water (matrix potential). Before water movement can proceed the large pores or channels must fill to an appreciable volume or hydrolic conductivity. It is only at this point when the wetting front can continue its downward path. Coarse materials such as straw turned under by plowing restrict rather than aid water flow in much the same way. Each inconsistency or layer, weather it be worked ground, sand, gravel or layer straw must be near saturation before water can move down into the next stratification of soil. See fig. 2 and fig. 3.

Fig. 2. Water retention over sand layer. Saturation of a consistent soil type is necessary before the water infiltrates to the next soil type.

Fig. 3. Water retention over straw layer. This inconsistent layer can be compared to a paper towel. The towel will absorb water in all directions. However, if you tear the towel in half, only the half touching the water can absorb the water. A plowdown of straw into the soil produces an inconsistent layer or "tear" in the soil profile.


The extremely small pores or compacted soil found in hard pan from over tillage also severely restrict water movement. This compacted soil wts very quickley when first contacted by water due to its high absorbtion capacity (high matrix potential due to large surface area) yey, the rate of water movement is drastically slowed because of its compacted nature. See fig. 4.

Fig. 4.

Water retention over hard pan. Hard pan results from tillage or a calcium carbonate or caleachy layer.


Fig. 5. Contrasting Systems (No till on left and conventional tillage on right)

Fig. 6.
Snow trapped by standing stubble (added moisture and protection for winter crops)

In contrast to conventional and reduced tillage systems, no till provides a cropping system without unwanted tillage pans and layers. When soils are not worked, fissures and old root channels remain, and combined with increased earthworm activity, provide additional channels for water infiltration. Another factor that contributes to rapid infiltration is greater soil stability. This reduces the extent to which surface channels are blocked by deposition of soil particles carried downward during rain storms. Standing stubble and surface mulch slow the impact of rain drops that provide superior soil and water relationships, allowing the producer to approach the full cropping potential of this area.

The No Till Pie



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