Most important stages are when crops are in reproductive stages
Corn = before, during and for several weeks after pollination
Soybeans = during mid-seed fill
One to two inches of water from a timely, soaking rain or from irrigation may provide a greater increase in yield than any other factor.

Corn uses large volumes of water to produce grain but is still one of the more efficient crops in producing dry matter.

Water requirements for production of dry matter by different crops. The lower the number, the higher the water use efficiency of the crop.
Crop	Pounds of water used per pound of dry matter produced
Alfalfa	844
Soybean	646
Oats	583
Potato	575
Wheat	545
Sugar Beet	377
Corn	349
Sorghum	305

Evapotranspiration = water lost from the soil to the atmosphere via soil evaporation + plant transpiration

Estimated evapotranspiration of corn during various stages of growth when grown on a fine sand.
Growth stage	Evapotranspiration
	inches per day
Seedling to 4 leaf	0.06
4 leaf to 8 leaf	0.10
8 leaf to 12 leaf	0.18
12 leaf to 16 leaf	0.21
16 leaf to tasseling	0.33
Pollination	0.33
Grain filling	0.26
Maturity	0.23

The Soil as a Water Reservoir

The key to understanding water storage and movement is to consider the distribution of large and small pores in the soil.

Small pores: hold water against the forces of gravity
Large pores: tend to drain freely and provide pathways for roots and air.

Key terms:

Saturation: All pores are filled with water
Field Capacity: The condition that exists after a soil has been thoroughly wet and allowed to drain freely for 48 hours. This point defines the water content of the soil when it is holding the maximum amount of plant-available water.
Wilting Point: The water content at which plants can no longer extract water from the soil.
Plant-Available Water: The amount of water held between field capacity and the wilting point.
- Plant available water is closely related to soil texture: sands and clays < sandy loam, loam, clay loam.
- Better structure and aeration is associated with silt loams and clay loams allowing better better water supply capabilities and deeper root exploration.
- Do amendments help? No - economics

Water Movement in Soils

Infiltration: Entry of water into the soil. Suction forces
- Dependent upon the quantity and quality of soil pores at the soil surface.
- Best managed by protecting the soil surface with organic matter and by avoiding practices that destroy soil structure.
Soil Permeability: Water movement under saturated conditions. Positive pressure forces
- Dependent upon the quantity of large, interconnected pores
- Important characteristic to determine what sort of drainage system is used on poorly drained soils
- Heterogeneous soils have layers of different soil textures which affects the way water moves
- Best management is to avoid compaction and protect soil aggregates
Soil Crusting
- Develops when soils are overworked with tillage and then dry rapidly after an intense rain or irrigation
- Most common in soils with high silt content and low organic matter
- Best managed by reducing tillage or leaving residue on the soil surface
Reducing Soil Erosion and Increasing Stored Soil Water
- Select the right fields for corn and soybean when using tillage
- Manage the soil surface to increase infiltration
- Use no-till cover crops on sloping land
- Consider engineering practices

Root Growth and Soil Water

Water always moves from wetter to drier zones

Roots cannot grow in soils with low water content. They grow in moist soil, but they cannot grow through dry soil to reach moist soil.

Saturated soils inhibit root growth and soybean nodule growth

Planting date is key to having roots grow downward always staying in moist soil.

Drought

Utilizing Drought-Damaged Corn National Corn Handbook - 58

At planting, the amount of evaporation depends upon:

Amount of water in the soil profile.
Hydraulic conductivity of the soil. Altering structural conditions of the soil surface (upper 4 to 6 inches) will decrease water flow to surface and will dry upper soil profile.
Cultivation will dry the upper soil profile.
Residue will decrease evaporation from the soil surface.

After leaf area is established by the crop, evapotranspiration is dictated by:

The planting pattern, leaf area index, and canopy development of the crop.
Rooting depth depends upon depth to the water table. Under irrigated conditions, corn roots grow 1 to 3 feet deep, while under dryland drought conditions corn roots will grow 5 to 7 feet deep.
If water stress is observed on corn (leaf rolling by 10:00 a.m.), yield will be decreased
3% per stress day at V12
4% per stress day at V18
7% per stress day at R1

What management recommendations should be considered, if there is concern for drought conditions before planting?

Do's

Deeper planting to insure germination. Place seed to moisture: 1.5 to 2 inches.
Prevent water evaporation from the soil surface. The best management practice for this is residue on the soil surface. Each ton of residue (up to 3 T/A) increases available soil water by about 0.5 inches in the winter wheat cropping systems of the western U.S.
Minimize spring tillage and till at shallower depths.
Work and plant fields as quickly as possible.
Minimize chances of anhydrous ammonium injury by applying at an angle and 8 to 10 inches deep.
Early planting dates will help corn pollinate earlier during less stressful times of the growing season.
Good weed control is essential. Weeds compete with crops for resources. Dry conditions reduce effectiveness of herbicides.
Consider a different crop such as soybeans. Corn has better water use efficiency (dry matter produced per amount of water used), but produces more dry matter and therefore uses more water. Soybeans probably have the advantage and also flower over an longer period of time.

Don'ts

Don't skimp on P and K fertilizer
Don' lower population
Don't change hybrid maturity to shorter-season hybrid. We cannot predict what kind of season it will be at planting. Need to go with averages. If you are SURE that the year will be dry, then a shorter season will pollinate earlier during a potentially less stressful time during the growing season.

Drainage

Typical system

Plastic or concrete tubing 3 to 6 inches in diameter
- buried 2 to 4 feet deep
- spaced 60 to 120 feet apart
- lateral drains are tied to central collector main tiles of larger diameter
- outlets (drainage ditch) receive water from the drain system and convey it to streams or rivers
Arrangements
- Random
- Parallel
- Herringbone
- Double main

Drain tiles accept water only when saturation - water content exceeding field capacity - occurs above them. It does not soil moisture unless saturation occurs.

Methods to improving surface drainage (Technical assistance from NRCS)

land smoothing (leveling or planing)
shallow trenches
parallel tile-outlet terraces
sub-irrigation

Irrigation

Goal of irrigation is to keep plant growth and development continuing without stress.

Selecting an Irrigation System
- sprinklers: center pivot, towable, lateral move - High Pressure v. LEPA
- furrows - Upstream and Downstream ends versus Surge
- traveling guns
Evaluating Water Supply
- one acre inch of water = 27,000 gallons
- a one acre pond would be lowered 4 feet to supply one inch to 50 acres
- groundwater: Ogallala
- surface water: Western U.S.
- Salinity
Supplemental Irrigation Strategies and System Sizing
- In sub-humid and humid regions, irrigation systems are designed to supplement rather than replace rainfall.
- In the West, daily water use can be as high as 0.35 to 0.40 inches per day.
- In the Midwest, daily water use rarely exceeds 0.25 inches per day.
Irrigation Economics
- "Will irrigation pay for me?"
- Requires careful analysis of crop rotation, soils, time, management availability, land ownership, leasing arrangements, and water supply.
- Increased adoption of irrigation almost always occurs after a drought year
Adapting Other Management Practices to irrigation
- Irrigation raises the yield potential of a field and thus requires that other management decisions change accordingly
- Plant population should increase 2,000 to 3,000 plants/acre
- Weed and insect control are more important
Applying Nutrients with Irrigation (Fertigation)
- N is best suited for application through an irrigation system
Crop Protection Chemicals and Irrigation (Herbigation)
- limited use
- some herbicides and insecticides are labeled for use in irrigations systems
Irrigation Timing
- Corn usually does not need irrigation before V4
- Ways to measure or estimate stored soil water
  1. form a ball manually from soil taken from an 8" depth
  2. use tensiometers, gypsum blocks
  3. use checkbook water balance method by measuring water additions (rainfall + irrigation) and water loss (evapotranspiration) to estimate current water content
- Observe crop indicators
- Timing effects

Corn response to irrigation during different periods on a sandy soil in Illinois during 1988
Irrigation Treatment	Yield
Unirrigated	26
Full-season	150
From planting to silk emergence	40
From silk emergence until maturity	132
Source: F.W. Simmons

Irrigation

Water

Water - either not enough or too much - is often the limiting factor controlling corn and soybean yields.

Typical situation in the Corn Belt

Corn and Soybean Water Use

The Soil as a Water Reservoir

Water Movement in Soils

Root Growth and Soil Water

Drought

Drainage

Irrigation

Further Reading