Water, Soil Nutrients, and Corn Grain Yield
Crop Insights written by Stephen D. Strachan, Ph.D.¹ and Mark Jeschke, Ph.D.²
- High-producing corn hybrids are well-adapted to efficiently take up and utilize water and nutrients from soil.
- The chemical structure of water allows water to dissolve plant nutrients and carry these nutrients from the soil, through the corn plant, and into the harvested grain.
- Evaporation from plant leaf tissue and hydration of nutrient ions and organic molecules in growing points, such as developing kernels, pull water – like a chain – from the soil into and throughout the corn plant.
- As water is pulled throughout the corn plant, nutrients dissolved in this water are carried toward these same locations.
- Water uptake and plant nutrient uptake are tightly related – limited water uptake reduces total nutrient uptake.
- Corn requires a minimum of approximately 25 inches of water during the growing season to achieve maximum grain yield.
- At a population of 32,000 plants per acre, approximately 21 gallons of water is taken up by each plant where it is either transpired through the plant or is used to support growth and grain production.
- Pioneer has been focusing on improving water use efficiency in corn for over 60 years. Grain production per inch of rain has increased dramatically from the 1950s to today.
High-producing corn hybrids are well-adapted to efficiently extract nutrients from soil and incorporate these nutrients into biochemical processes for grain production (Figure 1). One of your jobs as a corn producer is to help create and support environmental conditions that maximize plant growth and grain production. There are two primary requirements for success. First, the fertility program must create a soil environment that contains adequate amounts and the proper balance of the different plant nutrients. Farmers, fertilizer dealers, and crop consultants understand this part of the grain production process well. Second, these nutrients must move into and throughout the corn plant to create grain. Corn producers may not understand the process of nutrient and water movement throughout the corn plant as well.
Figure 1. The highest yielding corn hybrids are best adapted to utilize water from soil. Water and nutrient uptake and translocation in the corn plant are highly associated – limited water uptake reduces total nutrient uptake.
The problems corn plants face as they harvest nutrients scattered throughout the soil and concentrate these nutrients in the grain are similar to the problems you face during harvest. Grain is growing on corn plants scattered throughout the corn field and you must concentrate this grain in the bin. You must first extract the grain from corn ears located throughout an entire field. You collect this grain with a combine. In an analogous manner, corn roots and organisms associated with corn roots collect nutrients scattered throughout the soil profile. For your business, you must also move this harvested grain from the field and concentrate this grain in a storage bin as you prepare this grain for market. Often a grain cart is part of your grain transportation operation. For the corn plant, the plant moves nutrients and concentrates these nutrients in the ear as the corn plant moves water. Water is the “grain cart” in the corn plant’s nutrient harvest process. In this Crop Insights article, we shall discuss how water solubilizes plant nutrients and how water moves these nutrients from the soil to the ear.
Physical Properties of Water
The chemical structure of water molecules creates six physical properties of water relevant to plant growth and grain production:
- Excellent solvent to carry ions and nutrients
- A strong force of cohesion (water molecules stick to other water molecules)
- A strong force of adhesion (water molecules stick to other molecules that are not water molecules)
- The volume of a given weight of water does not change as the pressure changes
- A very high capacity to absorb heat
- Liquid water expands as it freezes
These six properties support life in all organisms. However, for this article, we shall focus on only the first three.
Chemical Structure of Water
Water consists of two atoms of hydrogen and one atom of oxygen. Each hydrogen atom consists of one positively charged proton in the nucleus and one negatively charged electron spinning around the nucleus. The oxygen atom consists of eight protons and eight neutrons that comprise a nucleus encircled by eight electrons (Figure 2).
Figure 2. Diagram of a hydrogen atom with one electron encircling one proton on the left, and an oxygen atom with eight electrons encircling a nucleus consisting of eight protons and eight neutrons on the right.
As two hydrogen atoms combine with one oxygen atom to produce one molecule of water, the two electrons from the two hydrogen atoms and six electrons from the outermost electron shell of the oxygen atom encircle the two hydrogen nuclei and one oxygen nucleus, forming a stable molecule (Figure 3).
Figure 3. Diagram of a water molecule showing eight electrons (green) encircling the two hydrogen nuclei and the oxygen nucleus.
The oxygen atom has a much higher affinity for electrons than the two hydrogen atoms; consequently, the eight electrons encircling the water molecule tend to spend a greater amount of time near the oxygen nucleus and less time near the two hydrogen nuclei. This distribution of electron density causes the electronic charge to be more negative near the oxygen atom and more positive near the two hydrogen atoms of the water molecule. Individual water molecules therefore exist as dipoles – molecules containing regions of partial negative and partial positive charges (Figure 4).
The high electron density on the oxygen side of the water molecule provides a net negative charge to this side of the molecule.
The low electron density around each hydrogen atom provides a net positive charge around each hydrogen atom equal to one-half of the negative charge in the region near the oxygen atom.
Figure 4. The water molecule exists as a dipole – a molecule containing regions of partial negative and partial positive charges.
Water is an Excellent Solvent for Plant Nutrients
The dipole nature of the water molecule allows water molecules to arrange themselves in appropriate ways to dissolve positively charged cations and negatively charged anions. For positively charged nutrients such as potassium (K+) and calcium (Ca2+), the positive charge is dispersed through the partially negatively charged portions (oxygen-side) of water molecules (Figure 5).
Figure 5. To dissolve cations, negative portions of water molecules face toward the cation to disperse the positive charge (opposites attract).
For negatively charged nutrients such as nitrate (NO3-) and phosphate (H2PO4-), the negative charge is dispersed through the partially positive charged portions (hydrogen-side) of water molecules (Figure 6).
Figure 6. To dissolve anions, positive portions of water molecules face toward the anion to disperse the negative charge (opposites attract).
For both types of nutrients, concentric spheres of water molecules surround each ion. The innermost sphere of water consists of just a few water molecules and forces related to ionic charge dispersal for each water molecule are very strong. A second sphere of water molecules forms outside the innermost sphere. Forces related to ionic charge dispersal for each water molecule in the second sphere are less because there are more water molecules to disperse the ionic charge. Additional spheres of water molecules continue to surround the ion as ionic and other forces dictate. These innermost spheres of water molecules are highly associated with the ion. As these innermost spheres of water molecules move, so too do the nutrients.
Forces of attraction between nutrient ions and soil and water molecules determine nutrient behavior and mobility in soil (Table 1). Cations such as K+ tend to bond to negatively charged soil particles, are not abundant in the soil water phase, and tend to have low mobility. Anions such as NO3- do not readily bond to soil, are more abundant in the soil water phase, and are more mobile in soil water. Phosphorus is an exception, as it exists as an anion but has low water solubility, making it relatively immobile in soil.
Water Has a Strong Force of Cohesion
The dipolar structure of water creates an attractive force between a partially positively charged hydrogen atom of one water molecule and a partially negatively charged oxygen atom in a neighboring water molecule. This attraction, called hydrogen bonding, aligns water molecules in a manner similar to how north and south poles align magnets.
Table 1. Essential nutrients for plant growth, forms available for plant uptake, and relative mobility in soil water.
Magnets have a single pole at each end, so magnets can form only a line. Water molecules have two positive poles and one negative pole. This additional positive pole allows water molecules to link together to form a complex 3-dimensional network (Figure 7).
Figure 7. The positive and negative regions of water molecules allow water molecules to attract each other just like the positive and negative poles of magnets.
Hydrogen bonding forces associated with this 3-dimentional network allow water molecules to act like links in a chain. Water molecules can be “pulled” from one location to another just like a chain can be pulled to move materials from one location to another. Hydrogen bonding is therefore responsible for the force of cohesion – the ability of water molecules to stick to each other.
You, as a farmer, move corn grain by pulling the grain cart with a tractor. What force does the corn plant use to pull water with dissolved nutrients into and throughout the corn plant?
Forces Governing the Movement of Water in Soil and Plants
Water movement in soil and plants is not a random process. Five primary forces act on individual water molecules. These forces, in rank order from most to least powerful, are:
- Evaporation: the conversion of liquid water to water vapor
- Hydration: water tightly associated with clay mineral structure or molecular constituents in the corn plant
- Ionic attraction: charge dispersal of dissolved ions or ions bound to cation exchange sites
- Hydrogen bonding: formed from the polar character of hydrogen and oxygen atoms in the water molecule
Sunlight provides the ultimate power for the force of evaporation, the most powerful force water molecules must obey as water moves from soil and throughout the corn plant (Figure 8).
Figure 8. Water in the corn plant and in the soil continuously strive to maintain an equilibrium with water in the atmosphere, but cannot because water in the atmosphere keeps moving away from the corn plant and soil.
The movement of water from the vascular tissue to the leaf creates a water deficit in the vascular tissue. Water is pulled from corn roots to the vascular tissue to eliminate this deficit in the vascular tissue. The corn plant now has a water deficit in the corn root, so water is pulled from the surrounding soil into the root.
The second most powerful force for pulling water through the corn plant is the hydration of molecules and cellular constituents in growing points of the plant such as the developing kernels in the ear. This hydration process is dependent on the ability of water molecules to adhere to other molecules (the force of adhesion). Ionic attraction of charged ions such as potassium and the force of hydrogen bonding to organic molecules such as sugar and proteins create this force of hydration. The force of evaporation pulls water up through the vascular tissue. The pulling force created by hydration of organic materials in the developing kernels is sufficient to pull water from the vascular tissue to the ear. If the corn plant is not under heat stress and there is plenty of water in the soil, the developing kernels can siphon water from the vascular tissue to provide the water and nutrients needed to support maximum growth and yield. However, if the amount of water that can be extracted from the soil is low and the corn plant is facing a “heat stress” environment, the force of evaporation dominates. The corn plant shuttles water to support evaporative demand and less water is shuttled to support kernel and ear growth. The result of this water and nutrient deficit to the ear is reduced yield.
Water Use Efficiency and Corn Hybrid Selection
Corn requires a minimum of approximately 25 inches of water during the growing season to achieve maximum grain yield. At a population of 32,000 plants acre, approximately 21.2 gallons of water either transpire through the corn plant or are associated with biochemical and physical processes of that corn plant from planting until physiological maturity (black layer). Water carries nutrients from the soil into and throughout the corn plant and eventually deposits needed nutrients in the harvested ear. Water uptake and plant nutrient uptake are tightly related – limited water uptake reduces total nutrient uptake.
Figure 9. Average U.S. corn yield, 1980-2016 (USDA-NASS). Widespread drought in 1983, 1988, and 2012 resulted in sharp drops in corn yield.
In soils with adequate fertility, the ability of the hybrid to use water efficiently is a big factor in determining the hybrid’s yield potential across a range of environments. “Rain makes grain.” When examining long-term U.S. corn yield trends, years in which there was widespread drought immediately stand out (Figure 9).
Improved drought tolerance in corn has been one of the primary objectives of plant breeders at Pioneer over the past 60 years. Pioneer established the first dedicated drought breeding station in York, Nebraska, in 1957. Since then, Pioneer has expanded drought research around the globe. The progress that has been made in improving water-use efficiency in corn is evident in yield trends relative to growing season precipitation in rain-fed corn production. For example, in Champaign County, Illinois where all, or nearly all, corn is produced without irrigation, corn yield per inch of rainfall during the period of April through September has increased from around 4 bushels in the 1950s to around 7 bushels today (Figure 10).
Figure 10. Corn yield per inch of growing season precipitation (April-September) from 1950 to 2016 in Champaign County, Illinois. (Yield data: USDA-NASS; rainfall data: Nat. Weather Service).
Currently, Pioneer conducts field research on hybrid drought tolerance at multiple managed stress environments in North and South America, Europe, India, South Africa, and other locations around the globe with dependable levels of drought stress. Pioneer® brand Optimum® AQUAmax® hybrids, which are developed and tested to help deliver a yield advantage in water-limited environments, represent the most recent output of this ongoing research effort.
All water molecules within the soil, plant, and atmosphere strive to maintain a dynamic equilibrium in the midst of constantly changing environmental factors. Water is pulled from the soil, through the corn plant, and into the atmosphere. As this water is pulled, it carries the necessary nutrients to support plant growth. Water uptake and plant nutrient uptake are tightly related – limited water uptake reduces total nutrient uptake. In high fertility soils, the ability of the hybrid to use water efficiently is a big factor in determining the hybrid’s yield potential across a range of environments. As corn breeders develop new hybrids that require less water to achieve maximum yield potential, agronomists, fertilizer dealers, and farmers will need to develop agronomic practices for the corn plant to transport water and nutrients more efficiently in order to achieve higher grain yields for these new genetics across a range of environments.
¹Stephen D. Strachan, Ph.D., DuPont Research Scientist
²Mark Jeschke, Ph.D., Pioneer Agronomy Information Manager