Critical Growth Stages for Major Crops1
	Critical	Symptoms of	Other
Crop	Period	Water Stress	Considerations
Alfalfa	Early spring and immediately after cuttings	Darkening color, then wilting	Adequate water is needed between cuttings
Corn	Tasseling, silk stage until grain is fully formed	Curling of leaves by mid-morning, darkening color	Needs adequate water from germination to dent stage for maximum production
Sorghum	Boot, bloom and dough stages	Curling of leaves by mid-morning, darkening color	Yields are reduced if water is short at bloom during seed development
Sugar beets	Post-thinning	Leaves wilting during heat of the day; abnormal dark green color	Most sensitive to moisture shortages in early growing stages but peak moisture use comes later in the season when they have complete ground cover.
Soybeans	Bloom and fruit set	Leaf wilting	Any stress from R4-R6 late pod development/ early seed fill causes more yield reduction than at any other time.
Small grain	Boot and bloom stages	Dull green/bluish color, rolled up leaves; firing of lower leaves	Small grain crop injury from drought stress can appear similar to herbicide injury symptoms
Potatoes	Tuber formation to harvest	Wilting during heat of the day	Water stress during critical period may cause cracking of tubers
Sunflower	Preplant, bud and bloom	Leaf wilting	Most sensitive to moisture stress during
	and bloom		moisture stress during flowering; least sensitive during vegetative period (emergence to early bud)
1 NRCS Colorado Irrigation Guide, NDSU, Colorado State University

See the NDSU web site www.ag.ndsu.nodak.edu/drought/drought.htm for more information on coping with drought and dry conditions, covering various crop and livestock issues, with good links to other drought-related web sites as well.

Water Infiltration Rates

The volume of water that can enter an area of surface soil is called the infiltration rate. In recent years, we have unfortunately had a demonstration of its limits in some parts of the region, and the result was rainfall much higher than the soil’s ability to absorb it.

The amount of rainfall that can enter the soil is sometimes greater initially than later on during the rain. If the soil is well aggregated, as in a no-till field, water enters more quickly until larger pores are saturated. If the soil contains more clay and is cracked, filling the cracks with water initially results in initially high infiltration until the clay becomes saturated, swells and closes the cracks.

Once the pores are filled and cracks are closed, the texture of the soil becomes the most limiting factor in water infiltration. The force of the rain, the presence of shallow limiting layers, such as compacted tire tracks or tillage pans also restrict the downward flow of water. But in the absence of these factors, texture is the key to water infiltration. The following chart contains general numbers from “An Introduction to Soil Physics,” by D. Hillel, 1982. The values are useful to demonstrate the magnitude of water infiltration. Individual soils will vary around these values.

Heavy rain in the northern valley last year illustrated how soil type can affect the infiltration rate.  Near St. Thomas, which received 2 inches of rain over about 8 hours on a fine sandy loam, hardly any runoff occured. But in areas with 10 inches in 8 hours in clay soils, perhaps one inch entered the soil through cracks and normal infiltration, but the other 9 inches ponded or ran off the field.

Because of the size of the area affected, the drainage-ways that we call “rivers” but to many of us are more aptly called creeks or brooks, cannot handle the kind of water that 8-14 inches of rain over a several county area can provide. So the water backs up and is held on the field until downstream levels recede. In a clay soil, 24 hours of ponding may result in only ½ inch of water infiltration. Several days are needed to absorb 3-4 inches of ponded water.

—Dave Franzen, NDSU extension soil specialist

Textural class Steady infiltration rate inches/hour Sands >0.8 Sandy loams 0.4-0.8 Loams, fine sandy loams 0.2-0.4 Clay loams, silty clay loams and clays 0.04-0.2 Sodic clay soils <0.04