EDITOR’S NOTE: Following are highlights from the University of Arkansas (U of A) Extension Service and the 2009 Georgia Soybean Production Guide. To see the complete text, please visit www.uaex.edu and www.caes.uga.edu/extension/.

The timing of irrigation is commonly referred to as irrigation scheduling, according to U of A Extension Service. Correct timing is critical to maximizing yield. Having the ability to irrigate is important, but it is essential that a grower have a commitment to apply irrigation in a timely manner. Too often growers irrigate by the appearance of the crop.

Visual stress, especially during bloom and pod set, results in yield loss. Also, once irrigation is started, the time required to finish a field will result in part of the crop suffering even greater stress. If the soil moisture can be determined, then irrigation timing decisions can be improved.

A more precise method employs tensiometers, a sealed, water-filled tube with a vacuum gauge on the upper end and a porous ceramic tip on the lower end. The tensiometer is installed in the seedbed at a depth where the majority of the roots are located. A 12-inch depth is commonly used for surface irrigation, except where a hardpan exists, and there it is placed just above this layer. Shallower settings at about eight inches deep are recommended for center pivots.

Two or three tensiometers per field are recommended to avoid a problem if one of the tensiometers quits working. Starting irrigation at a vacuum gauge reading of 50-60 centibars on silt loam and clay soils, and 40-50 centibars on sandier soils, is recommended. Tensiometers are fairly reliable and effective when checked and maintained properly. However, the time and effort required usually results in most producers not being able to use them very effectively.

Checkbook irrigation scheduling
Soil moisture accounting is used to calculate the soil-water balance in the root zone throughout the growing season. This method is sometimes called checkbook irrigation scheduling because a record is kept on the water that enters and leaves the soil like an account balance is maintained in a checkbook. Two forms of the checkbook procedure are available through county Extension offices in Arkansas – the Checkbook User’s Guide and the Irrigation Scheduling Computer Program.

The Checkbook User’s Guide is used to keep a written record of the soil moisture balance when a computer is not available. It is a three-page handout that shows how to use a water-use chart and a water-balance table to monitor the soil moisture. The water-use chart shows an estimate of how much water the crop uses each day based on the maximum temperature and the age of the crop.

Daily water use and rainfall amounts are entered into a water-balance table. Maximum temperature data can be taken from the weather, newspaper, etc., but the rainfall should be measured with a gauge at each field. Adding and subtracting these numbers in the table determines the soil moisture deficit. Table 1 shows the recommended allowable deficits that are included in the User’s Guide to help determine when to irrigate. The allowable deficits vary depending on the soil type, crop and irrigation method.

Table 1. Recommended Allowable Deficits – Soybeans
Predominant Soil	Flood, Furrow Or Border Irrigation (inches)	Pivot Irrigation (inches)
Clay	2.00	1.50
Silt Loam with pan	1.75	1.25
Silt Loam without pan	2.50	2.00
Sandy Loam	2.25	1.75
Sandy	2.00	1.50
With pan – with shallow (less than 10-inch) restrictive layer
Without pan – without shallow restrictive layer.
Source: University of Arkansas Cooperative Extension Service.

 

The Checkbook User’s Guide, water-use charts and water-balance tables are available through your county U of A Extension Service office at no cost. This method requires some record keeping, but it can be helpful in deciding when to irrigate.

Easy-to-use computer programs
If a computer is available, then the Irrigation Scheduling Computer Program can be used for the record keeping. This program operates much like the checkbook method just described except that the computer does the calculations. It also uses local daily maximum temperatures and rainfall measured at the field to determine a soil moisture deficit for the field.

The program is being successfully used by growers in Arkansas, Mississippi, Louisiana, Tennessee and Missouri. It is also being used in numerous irrigation studies and demonstrations conducted in Arkansas.

In field studies using both tensiometers and the scheduler program, it was found that they are usually within one or two days of each other on indicating when to irrigate. However, the program is much easier to use and maintain than tensiometers. The program also has the option to predict when irrigation will be needed in the next 14 days if no rainfall occurs. This offers a real benefit to managing irrigation labor and sharing irrigation water with other crops. The program is available through the county Extension office.

Georgia weighs in
To schedule irrigation for most efficient use of water and maximum production, it is essential to frequently determine the soil water conditions throughout the root zone of the crop being grown.

A number of methods for doing this have been developed and used with varying degrees of success, but the two which have proven most practical for field use are tensiometers and electrical resistance blocks. In comparison to the investment in irrigation equipment, these types of instruments are relatively inexpensive.

• Tensiometers: A tensiometer is a sealed, water-filled tube with a porous ceramic tip on the lower end and a vacuum gauge on the upper end. The tube is installed in the soil with the ceramic tip placed at the desired root zone depth and with the gauge above ground.

• Electrical Resistance Sensor: Electrical resistance meters determine soil moisture by measuring the electrical resistance between two wire grids embedded in a block of gypsum or similar material that is permanently embedded in the soil. They are best suited to heavy soils.

• Solid State Blocks: Manufacturers have recently improved their technology to allow for blocks that perform in the range of tensiometers. These type blocks work very well in coarse type soils (sandy) in the soybean belt. An advantage of solid state blocks is that they do not require refilling as does the tensiometer.

The basic location and installation of the blocks are similar to the installation of the tensiometer, except no water is required to be put into the block.

Keeping a chart
You can obtain the full benefit of using tensiometers, electrical sensors or solid state blocks by recording readings and plotting them on a chart. Readings may be plotted directly in the field.

Use different colored pencils for different depth tensiometers (or soil blocks) to make the chart easier to read. The chart lines show what has happened in the past. By projecting them ahead, you have an advance indication of what you can expect in a few days. This information is helpful in scheduling the next irrigation and in measuring the effectiveness of a previous irrigation (what depth of penetration was achieved and how soon the soil dried out).

Most manufacturers include charts with their instruments. If not, they can easily be made. Record rainfall data along with the instrument reading to aid in evaluating soil water changes.

For a complete explanation of available irrigation scheduling methods, consult the University of Georgia Cooperative Extension Service’s Bulletin 974 titled “Irrigation Scheduling Methods.”