Unsaturated Water Flow and Nutrient Uptake in Corn
Crop Insights written by Stephen D. Strachan, Ph.D.¹ and Mark Jeschke, Ph.D.²
Crop Insights written by Stephen D. Strachan, Ph.D.¹ and Mark Jeschke, Ph.D.²
Soil must supply an estimated 336 pounds of nitrogen, 153 pounds of phosphorus (P2O5), and 405 pounds of potassium (K20) per acre to the corn plant to support a grain yield of 300 bushels per acre (IPNI 2014). In addition to this total seasonal demand, soil must also supply these nutrients rapidly enough to properly feed the corn plant daily during the highest nutrient-demand growth stages from V6 to R1 (Figure 1) and at sustainable rates during the rest of the growing season (Strachan and Jeschke 2018). Water carries these nutrients and all other soil-supplied nutrients as the corn plant pulls water from the soil profile into the corn root. A better understanding of how water moves in the soil profile may provide added insight regarding soil fertility. This Crop Insights article discusses how plant-available water moves in soil as the soil provides water and associated nutrients to feed the plant.
Behavioral characteristics of water movement in soil change dramatically as soil water content decreases from saturated soil conditions through plant-available water conditions to plant-wilting water conditions (Hillel 1980). Immediately after a saturating rainfall or an irrigation event, very nearly all soil micropores and macropores near the soil surface fill with water and water moves via saturated flow. Rate of water infiltration into the soil profile depends on soil porosity. Soil porosity is a function of soil texture, structure, and bulk density. These three physical characteristics of soil determine the amount and size of macropores and connective channels that enable water flow.
During saturated flow, water moves as a singular mass over and through the soil profile. Water movement at this stage is much like water flowing down a stream. Farmers have spent billions of dollars shaping the land and adding waterways to allow surface water to withdraw from a field while mitigating soil erosion resulting from this moving mass of water and even more money to tile the soil to remove excess water from the plant root zone. During saturated flow, as water soaks into the soil, this water moves as a series of continuous bands deeper into the soil profile (Figure 2).
Saturated flow is responsible for removing excess water from the soil profile. This water is not available for plant uptake. Unless rainfall or irrigation is excessive, saturated flow occurs for approximately the first day after the rainfall or irrigation event until conductive forces of soil colloids to hold water in the soil profile negate the force of gravity to pull water through the soil profile. The soil is at field water holding capacity when the osmotic and matrix forces produced by soil colloids and chemicals associated with soil colloids are in balance with the force of gravity (Figure 2C). Subsequent movement of water is only via unsaturated flow until the next rainfall or irrigation event.
During unsaturated flow, water moves like water in a sponge. Micropores in the sponge retain water. An external force stronger than the force of retention in sponge micropores must be expressed for water to move from saturated micropores to different locations in the sponge. It is therefore possible to simultaneously have a portion of the sponge wet while another portion of the sponge is dry. This same phenomenon is true in soil. Chemical constituents in the corn root produce osmotic and matric forces that pull water from micropores in soil into the corn root (Figure 3).
Unsaturated flow in soil moves water very slowly. Pulling forces of corn roots are only slightly stronger than pulling forces of soil colloids when soil is at field capacity. This slightly greater strength originating from corn roots pulls water toward the roots. As water molecules are removed from soil, these water molecules move along the edges of soil colloids. Although a direct path from the soil to the root may be very short, the tortuous path that water molecules follow can be relatively long. When enough water is removed, pulling forces originating from soil colloids negate pulling forces originating from corn roots and the corn plant no longer can pull enough water into the plant to sustain growth. Water content of the soil has then reached the wilting point. When this occurs, corn plants wilt and show moisture or drought stress. The last remaining water in the soil is hygroscopic water, which is a thin layer of water held tightly to soil particles that cannot be taken up by plants. Figure 4 illustrates the different moisture levels and how water moves at each of these moisture levels for a well-granulated silt loam soil (Brady 1990).
Essentially all water movement from the point of field capacity to the wilting point is via unsaturated flow. We need to understand where the nutrients are within the soil structure to understand better how nutrients flow toward the corn root. Electrostatic charges of cation exchange sites and the physical structure of the soil influence the solubility and mobility of nutrients in soil water (Hillel 1980). Cation exchange sites on soil colloids and soil organic matter have net negative charges dispersed along surfaces of the colloids (Figure 5).
Soils cycle from saturated to very dry water conditions. When soils are relatively dry, cations in soil water associate very closely with net negative charges on colloidal surfaces to form an electrostatic double layer (Figure 5A). As soil moisture content increases, cations diffuse farther into soil water as they evenly spread ion concentrations throughout the soil water phase. Anions are repelled by the net negative charge of colloidal surfaces but are attracted to positively charged cations near these colloidal surfaces. Nutrients in soil tend toward a dynamic equilibrium between nutrients dispersed in water available for plant uptake and water associated very closely with colloidal surfaces. Cations, anions, and their salts have limited solubility in water. As the water content in the soil decreases, cations and anions may precipitate out of the soil water phase to form complex hydrated salts. Nutrients contained in these hydrated salts dissolve back into the plant-available soil water solution as the soil water content increases and as corn roots remove nutrients already dissolved in this plant-available water.
Nutrient mobility is related to water solubility of the nutrient and nutrient charge. Cations are highly associated with net negative charges of cation exchange sites and are more difficult to remove from these colloidal surfaces. In addition, if the water solubility of the cation is very low, very few cations will be in the plant-available water phase. Cations tend to be immobile in soil because it requires a lot of water to move a substantial amount of a cation (Table 1).
Although anions tend to be more mobile in soil, their mobility may also be somewhat limited (Table 1). Anions may be associated with cations that are bound to cation exchange sites. The restricted mobility of these cations and the pulling electrostatic force of these cations on anions may retard movement of these anions. In addition, if the anion has very low solubility in water or tends to form a complex with a cation, it will take a lot of water to move a substantial amount of this anion.
Table 1. Essential nutrients for plant growth, forms available for plant uptake, and relative mobility in soil water.
The slow mobility of water during unsaturated flow also influences nutrient mobility and root uptake by the corn plant (Figure 6). As the corn root pulls water from the pore, nutrients closest to the corn root are extracted from the soil. This method of nutrient uptake is called mass flow because nutrients move with the mass of water that enters the root.
Another process for nutrient uptake is called diffusion. For diffusion, nutrients are present at higher concentrations in water very near surfaces of soil colloids or as hydrated salt complexes that have precipitated on colloidal surfaces. Some of these nutrients diffuse into water farther from the colloidal surface. If this water is near a plant root, the plant root then extracts this nutrient from the water. Diffusion is active over short distances only – no more than about ¼ of an inch. Diffusion is the major route for phosphorus uptake into corn roots. For both processes – mass flow and diffusion – to work, water must be present and in contact with the soil surface and the corn root.
As the corn root extracts water from the micropore, water deeper in the pore drains from the center of the pore and moves closer to pore colloidal surfaces. Nutrients associated with this deeper water also move toward these colloidal surfaces and are less available for plant uptake.
A third mechanism of nutrient uptake, called root interception, does not directly depend on soil water but rather involves direct contact between growing roots and soil colloids leading to the absorption of nutrients. Root interception is an important means of uptake for certain nutrients but contributes less to overall nutrient uptake than the other two mechanisms (Table 2).
Table 2. Mechanisms of plant uptake for soil nutrients (Barber, 1984).
Nutrients strive to maintain a dynamic equilibrium between plant-unavailable, plant-available, and water-soluble forms in the soil profile (Figure 7). Typically, the conversion of nutrients from a form not available to plants to a form available to plants is a slow process. This is partly why soils must be fertilized with nutrients in a plant-available form to maximize corn grain yield. The conversion of nutrients from a plant-available to a water-soluble form is a much more rapid process. Once the nutrient is soluble in water, it is readily available for root uptake. The key to obtaining maximum grain yield is to have sufficient amounts of water-soluble and plant-available nutrients in supply to meet corn root demand throughout the entire growing season. This is particularly important during the vegetative and early reproductive growth stages (Figure 8).
Newly-formed, rapidly growing corn roots are responsible for nearly all nutrient uptake from soil. Maximum corn yields therefore require that newly-formed corn roots, plant-available nutrients, and ample water capable of unsaturated flow are present in the same slice of soil at the same time throughout each day of the growing season.
¹Stephen D. Strachan, Ph.D., DuPont Technical Fellow
²Mark Jeschke, Ph.D., Pioneer Agronomy Manager