Water Retention and Nutrient Availability in Soil: Drainage and Compaction
Crop Insights written by Stephen D. Strachan, Ph.D.1 and Mark Jeschke, Ph.D.2
- Soil texture, bulk density, and organic matter content determine soil water holding capacity.
- Water’s adhesive and cohesive properties create forces to retain plant-available water within the root zone.
- Soil compaction increases bulk density, determines pore size and volume distribution, and ultimately limits water and nutrient uptake in corn plants.
- Soil compaction reduces the rate of water penetration to recharge the soil profile during a rainfall or irrigation event, reduces gaseous exchange and limits oxygen uptake by corn roots within the soil profile by reducing the macropore concentration, and limits the ability of corn roots to grow into new soil to extract water and nutrients.
- Increasing soil compaction is an unavoidable result of corn production. Ways to manage soil compaction are:
- Match implement sizes to drive on the same wheel tracks.
- Do not randomly drive across fields with heavy implements but follow established wheel tracks.
- Till soils at the proper moisture content.
- Reduce the number of tillage operations per growing season.
- Properly match equipment loads and weight distribution to tillage operations.
- Manage your operation to increase soil organic matter.
- Plant rotational or winter cover crops with root structures that tend to reduce soil compaction as these roots grow.
Water held in soil is essential for corn growth. Each rainfall and irrigation event replenishes soil water as growing corn plants deplete this water. Soil texture, bulk density, and organic matter primarily determine the soil’s water-holding capacity. The physical properties of water and soil govern a soil’s ability to retain water and the corn plant’s ability to extract this water. Soil compaction is a “hidden yield robber” (Figure 1). Management that minimizes soil compaction and increases organic matter creates the greatest opportunity for soil to retain the maximum amount of plant-available water and nutrients to support corn growth.
Figure 1. Areas of stunted yellow corn in a field likely resulting from soil compaction created during the previous harvest. Soil compaction can restrict root growth of corn as well as reduce the ability of soil to retain and supply water.
How Does Soil Retain Water?
Three physical properties of water – ionic interactions with nutrients and other ions, a strong force of cohesion, and a strong force of adhesion – create the forces that hold water within the plant root zone (Strachan and Jeschke, 2017). Cohesion is the ability of water molecules to stick to other water molecules. Adhesion is the ability of water molecules to stick to other molecules that are not water molecules. These forces interact with soil-bound cations, clay minerals, organic matter, and other solid materials that constitute soil colloids. If these forces were not present, the force of gravity would pull water molecules deeper into the soil profile where they would no longer be available for plant uptake. Soil pores retain plant-available water. Soil texture, organic matter content, and bulk density determine the distribution and size of soil pores (Hillel, 1980).
The soil mineral fraction consists of silicates and aluminum hydroxy silicates, and soil organic matter contains oxygen and nitrogen atoms essential for retaining water. Oxygen and nitrogen atoms in both soil constituents are capable of hydrogen bonding with hydrogen atoms of water molecules. In addition, the chemical structure of aluminum hydroxy silicates in the soil mineral fraction and the molecular structure of organic acids and other materials in soil organic matter create net negative charges that are dispersed among water molecules located next to these soil constituents. Negative charges associated with soil minerals and organic matter also create cation exchange sites. Cations associated with these sites produce positive charges that are dispersed among the water molecules located next to these cations. The combined forces of ionic charge dispersal and hydrogen bonding hold water molecules very tightly within the soil matrix and negate the downward force of gravity (Figure 2).
Figure 2. A combination of forces resulting from hydrogen bonding and ionic charge dispersal hold the first ring of water molecules very tightly to soil. Corn roots cannot easily extract these water molecules from soil.
Additional rings of water molecules surround the innermost ring of water molecules tightly associated with soil colloids (Figure 3).
Figure 3. Multiple rings of water molecules form around each soil colloid. As each ring of water forms farther from the soil colloid, forces pulling water molecules toward the soil colloid diminish. Rings of water farthest removed from soil colloids are more available for plant uptake.
These additional rings of water are held in place through the forces of hydrogen bonding or ionic charge dispersal. Ionic charge dispersal is a stronger force expressed over short distances because, as more water molecules disperse an ionic charge, the ionic force per interaction decreases. As rings of water molecules further from the soil colloid form, the weaker force of hydrogen bonding becomes the more dominant force. Water present in rings further from soil colloids is not as tightly associated with soil colloids and is therefore more readily available for plant uptake by corn roots. Figure 3 depicts only a few rings of water molecules. In the “real world”, there are many layers of water molecules with different levels of different forces pulling on these molecules.
The size of the opening at the base of the soil pore determines the pore’s ability to retain water (Hillel, 1980). Water molecules stretch across a pore space formed between soil colloids like a chain stretches between two poles (Figure 4).
Figure 4. Water molecules behave like links in a chain.
For very small openings, the combined forces of ionic charge dispersal and hydrogen bonding hold water molecules in place. As the pore opening increases, the weaker force of hydrogen bonding becomes more dominant (Figure 5).
Figure 5. Multiple forces are pulling on all water molecules in the soil profile.
The chain of water molecules breaks when the downward force of gravity is greater than the lateral and upward forces of hydrogen bonding. When the chain breaks, water drains from the soil pore (Figure 6).
Figure 6. For soil pores with larger openings at their base, the force of gravity is stronger, and water drains from the center of the pore.
If the soil pore is small, water remains in the pore. If the pore is larger, water drains from the center of the pore. Liquid water associated with crushed ice in an excellent model to show how soil pores retain water (Figure 7).
Figure 7. Model of liquid water in crushed ice illustrating how soil pores retain water. Micropores (red ovals) are full with water while macropores (blue oval) retain water only along edges of solid surfaces.
Macropores and micropores comprise approximately 40-50% of the volume of an undisturbed, well-granulated silt loam soil (Brady, 1990). The amount of water present in this soil depends on when the last rainfall or irrigation event occurred and the water demand of the corn crop (Figure 8).
Figure 8. Volumes of water and air associated with soil pores in 100 grams of a well-granulated silt loam soil.
All soil pores contain water shortly after a substantial rainfall or irrigation event. The force of gravity pulls water molecules downward and drains water from the macropores. This continuous mass of water moves via saturated flow. As the water drains, the retentive forces exerted by soil colloids and the water molecules themselves eventually negate the force of gravity. When these forces are in balance, the soil is at field capacity for water retention. Subsequent water movement is via unsaturated flow, a very slow process for water movement. Soils at field capacity can stay at, or very near, field capacity for a long time if there is no water demand from growing plants. The balancing of these retentive forces with gravity and the very slow water movement of unsaturated flow allow soils to “recharge” with water during the winter months in preparation for the growing season.
Corn roots pull water from soil until the retentive forces exerted by soil colloids equal the pulling forces of plant roots. When these soil retentive forces become greater, they overpower corn root pulling forces and corn plants wilt. Although water is still present in the soil, soil colloids hold this water so tightly that this water is not available for plant uptake. Proper water management must therefore focus on how to maximize the time that water levels in the soil are between field capacity and wilting point levels. This includes tiling and other forms of drainage to drain saturated or nearly saturated soils more quickly and irrigation to meet evapotranspiration and corn crop demand. If the water content of the soil goes below the wilting point, the first water added to this soil must address soil demand. After the needs of the soil are satisfied, additional irrigation water addresses plant demand.
Soil Compaction Determines Pore Size and Volume Distribution and Ultimately the Amount of Soil Water and Nutrients Available to the Corn Plant
Highly productive, well-aggregated, agricultural soils tend to consist of about 50% solids and about 50% pore space with an equal distribution of macropores and micropores in this pore space (Brady, 1990). This ratio of macropores to micropores allows soil to store ample water for plant growth while allowing for gaseous exchange in the soil profile to provide oxygen to plant roots. Soil minerals have a particle density of about 2.6 g/ml, so a soil consisting of 50% pore volume will have a bulk density near 1.3 g/ml.
A noticeable reduction in the percent of macropores is apparent if soil bulk density of a silt loam soil approaches 1.6 g/ml and macropores are almost non-existent as the bulk density approaches 2.0 g/ml. Modern corn production requires heavy machinery to pass over the soil. Soil is compacted with each machinery operation. Based on published studies, soil in corn production increases bulk density (more compacted) by 19% and decreases pore volume by 15% when compared to undisturbed soil of the same soil type (Brady, 1990). The soil’s first response to compaction is to decrease the size, percentage, and distribution of macropores.
Compaction reduces the soil’s ability to supply water to the corn plant because:
- Compacted soils drain more slowly, allowing less water to penetrate the soil profile during rainfall or irrigation.
- A reduction in macropores slows the rate of gaseous exchange and water movement associated with root uptake.
- Compacted soils limit the ability of corn roots to grow into new soil to extract water and nutrients.
Compacted soils drain more slowly, allowing less water to penetrate the soil during rainfall or irrigation. One method to view how compaction limits water movement is to view the wet edge of water as a mass of water moves through the soil profile. Figure 9 shows the leading edge of water movement as this water passes through and around a zone of highly compacted soil placed within a zone of soil not compacted. The rate of water infiltration depends on the amount and size of the macropores. As a comparison, it is much easier to pump water through a one-inch hose that it is to pump water through the tiny orifice of a spray nozzle. As the amount of compaction increases, the percent of macropores decrease. If the more compacted zone is a uniform sheet near the soil surface, the rate of water infiltration deeper into the root zone of the soil profile is restricted during rainfall or irrigation, potentially reducing the ability to fully “recharge” the water-holding capacity of the soil. Improper tillage of soils that are worked a bit too wet can cause a sheeted zone of compaction near the soil surface.
Figure 9. Water, as it drains through the soil profile, is limited by a zone of highly compacted soil (outlined by the red box). Water drains through less compacted soil more quickly and eventually begins to move below the zone of high compaction (blue arrows).
A reduction in macropores slows the rate of gaseous exchange and water movement associated with root uptake. There are two critical problems if the pore volume is predominantly micropores (Hillel, 1980). First, roots require oxygen for proper growth. If all soil pores are filled with water, there is no opportunity for gaseous exchange in the soil profile, so there is limited opportunity for roots to extract critical amounts of oxygen from the soil atmosphere. Second, plant-available water moves primarily via unsaturated flow. Corn roots penetrate about 1% of the total soil volume as the corn plant grows. As corn roots grow through the soil profile, they extract plant-available water within the soil zone immediately surrounding the roots. The soil responds by allowing water further from the roots to be pulled toward the corn roots via unsaturated flow. During unsaturated flow, water movement is very slow and becomes even slower as the pore size decreases. Corn plants growing in the same soil type are therefore more likely to show and respond to greater water stress in the more compacted soil.
Compacted soils limit the ability of corn roots to grow into new soil to extract water and nutrients. One way to illustrate the effect of compaction on corn root growth is with the following greenhouse study. Seeds of corn were planted into soil compacted to bulk densities of 1.17, 1.25, and 1.38 g/ml in soil columns. Corn plants were harvested at the V5 growth stage. Shoot growth and leaf stature differed little among corn plants growing in these three soil treatments. However, root growth decreased dramatically as soil compaction increased (Figure 10). In compacted soils, limited root growth limits the opportunity for water and nutrient uptake.
Figure 10. Root growth of corn plants (V5 growth stage) growing in soil compacted to different bulk densities before corn seeds are planted. Roots were washed and dry weights were recorded for each soil treatment. Root dry weights were 2.47 g, 1.77 g, and 1.43 g for the 1.17 g/ml, 1.25 g/ml, and 1.38 g/ml soil bulk density, respectively.
Soil Compaction is an Unavoidable Result of Corn Production
Each pass of an implement in corn production compacts the soil. Soil compaction cannot be eliminated, so it must be managed. Ways to manage soil compaction are:
- Whenever practical, match implement sizes so that various implements follow the same wheel tracks.
- Do not randomly drive across fields with heavy equipment such as full grain carts; follow already established wheel tracks when possible.
- Till soils and conduct field operations when moisture conditions are correct for tillage operations; wetter soils are more prone to compaction.
- Reduce the number of tillage operations per growing season. Tillage reduces compaction in the tilled zone but often increases soil compaction just below the zone of tillage.
- Properly match equipment weights and load distributions with tillage operations.
- Manage your operation to increase soil organic matter content.
- Plant rotational crops or winter cover crops with root structures that tend to reduce soil compaction as these roots proliferate throughout the soil.
Brady, N. C. 1990. The nature and properties of soils, 10th ed. pp. 91-152. MacMillan Publishing Co., New York.
Hillel, D. 1980. Fundamentals of soil physics. pp. 21-54 and 123-165. Academic Press, Inc., New York.
Strachan, S. D. and M. Jeschke. 2017. Water, soil nutrients, and corn grain yield. Pioneer Crop Insights, Vol. 27. No. 7.
1 Stephen D. Strachan, Ph.D., DuPont Research Scientist
2 Mark Jeschke, Ph.D., Pioneer Agronomy Information Manager