Irrigation is vital to quality of nutrients

Julie Shortridge said it is important to understand what is happening in the soil, how much water it can hold before it starts to drain out, and the wilting point where the little water in the soil “is held so tightly that plants can’t pull it out.” (Photo by Nancy L. Smith)

PAINTER, Va. — “Water and nutrients are very, very closely related,” said Dr. Julie Shortridge, Extension specialist and assistant professor of biological systems engineering at Virginia Tech, to a collection of Eastern Shore farmers at the recent Corn Specialist Day at the Eastern Shore Agricultural Research and Education Center.
“If you are thinking about getting the biggest yields you can, you need to be thinking about those two things together.”
“Make sure that you’re not just avoiding dry conditions, but also excess soil moisture,” she added. “That’s really important in making sure that those nutrients stay in the root zone and are available to the plant when it needs it.”
“When there’s too little moisture, the plant is not pulling as much water out of the ground. By that same measure, it’s not pulling as much nitrogen with it.”
When there is too much soil moisture, nitrogen can be lost to percolation below the root zone or runoff, even on relatively flat fields, she said.
“The big thing is keeping water and nutrients in balance. The water conditions in your soil really impact how much nitrogen the plants are able to take up.”
Timing of nutrient application and water availability also is important. She stressed, “Know your nutrients. Know if you are using a product that may be impacted by soil moisture conditions at the time of application. If you have irrigation, you can manage that.”
She reported on research on using precision irrigation to improve both yield and nutrient use efficiency. She found corn yields were highest with precision irrigation.
Excessive soil moisture had negative results.
Precision irrigation allows growers to use less fuel and fertilizer, she said.
Shortridge said it is important to understand what is happening in the soil, how much water it can hold before it starts to drain out, and the wilting point where the little water in the soil “is held so tightly that plants can’t pull it out.”
The amount of water held in the soil depends on soil texture. “Richer, loamier soils can hold more water; that gives you more room to play with when you are trying to manage the amount of water in your soil. Sandy soils tend to have a lot less water holding capacity.”
She said 24 inches of sandy soil can hold about two inches of water.
A richer loam can hold 4 1/2 or 5 inches.
“That will tell you how often you have to water as well as how much you can put on at one time,” she said.
She outlined two ways to schedule irrigation.
One is the calendar or “checkbook” method in which “you keep track of the water that goes in and out of your soil.”
This method does not require purchasing any equipment,” she explained. “There are websites that do the math and assist in decision making.”
Shortridge noted that the checkbook method does not account for how water is used on a field, it requires some, it fails to account for the fact that crops use water in different ways and it is complicated for those growing multiple crops or who have multiple soil types.
A more accurate irrigation scheduling method uses soil moisture sensors.
Sensors are “like x-ray vision showing you what is happening in your soil.” Sensors can cost from $50 to $300 each. “The most expensive piece of any of these systems is almost always the data logger.”
Shortridge cautioned, “The data you get from [sensors] will only be as good as the installation.”
She recommended that growers “try to monitor at least a couple depths at a point” to provide the best data.
“The biggest challenge is understanding the data, understanding what to do with all these numbers that this new software system will send to you.”
A sensor will only report the amount of water in the soil.
“It’s not going to tell you whether to irrigate or not,” she said.
Growers must also determine how much to water.
“Typically, you don’t want to irrigate all the way back up to field capacity because if you get rain in the next few days, [you get] those excess soil moisture conditions where you can lose nutrients,” she said. “So irrigate up to 70-80 percent of the total amount that you could.
“Leave a little space.”