Irrigation is one of the most important factors in achieving good crop yields.

Plants thrive when they have sufficient water. But when they don’t, they become stressed, putting yields and profits at risk.

In Louisiana, the growing season is typically marked by high temperatures and droughts that prompt most farmers to irrigate. This is especially true in the case of soybeans. The summer reaches its hot, dry peak at a time when most Louisiana soybeans are approaching the growth stages when pods are filled and water is most needed.

Although irrigation is critical to a successful crop, farmers must be careful in how they irrigate. Applying too much or too little water, irrigating at the wrong time and using inefficient methods all can cut the benefits irrigation offers. These factors also pose concerns about the availability and quality of water.

A team of scientists at the LSU AgCenter Red River Research Station in Bossier City have undertaken several projects to identify ways farmers can improve their irrigation practices.

Optimizing soil moisture using technology

For irrigation to be effective, water must infiltrate the soil so plant roots can use it. Accomplishing this can be a challenge. Soil type affects how well the soil holds water, and water will run off of fields if it is applied to already-saturated soil.

AgCenter irrigation engineer Stacia Davis is wrapping up a three-year study of how soil moisture sensors and the data they collect can help farmers decide when irrigation is needed for their soybean crop. She has been working with farmers across the state to evaluate two types of sensors.

Davis’ data show one type, which measures saturation in terms of pressure, works best in clay soils. Another, which indicates the volume of the water present, is better for sandy soils.

Sensors, which take some practice to use properly, have not yet been widely adopted in Louisiana. Davis said they are worth considering because the data they record offer valuable insight into when crops are stressed and need water.

“If you stress any plant, other stressors have more leverage,” Davis said. “If you already have a disease issue or a pest issue or a weed issue, and then you stress the plant from water, then those other issues are going to be more prominent.”

Sensors also can help farmers monitor soil infiltration, she said. For example, a farmer may need to irrigate even if it rained recently because very light or very heavy rain likely did not percolate through the soil. Conversely, a farmer may be able to delay irrigation and save money if the sensor shows the soil is already full of water.

Davis cautions farmers that sensors are only one part of the equation. Computerized hole-selection software, which uses information about specific fields to determine the best hole size and spacing for poly pipe, is another way to improve irrigation efficiency. The software helps ensure water flows down fields evenly and prevents a pipe from bursting because of too much pressure in one spot.

Irrigation affects nutrient uptake

Water makes it easier for plants to take up nutrients, which may mean fertilizer is more efficiently used in irrigated fields, said AgCenter agronomist Syam Dodla. He is in the second year of a three-year project on how AgCenter-recommended rates for nitrogen, phosphorus and potassium should be adjusted in soybeans.

Current recommendations are based on older research that largely does not consider irrigation, which was less common 20 or 30 years ago in Louisiana, or modern high-yielding varieties, Dodla said.

“Especially at the crucial stages of the crop, like flowering to pod-filling stages, if we can provide irrigation, irrespective of nutrient application, the plants can still utilize the existing soil nutrients very well and get the benefits,” he said.

Dodla said irrigated crops make better use of nutrients, including those found naturally in the soil. In the current study, nutrient uptake was 22 percent higher with irrigation, he said.

Yet some previous research suggests fertilizer rates actually should be about 20 percent higher in irrigated fields.

That raises the possibility that some farmers are applying too much fertilizer. Most problematic is excessive potassium, which can interfere with plants’ ability to absorb other nutrients and ultimately decrease yields, Dodla said.

In another project, Dodla is trying to identify the ideal ratio of poultry litter and inorganic nitrogen fertilizers for maximizing corn yield. He is finalizing recommendations on how much of each should be applied in irrigated and non-irrigated fields.

Poultry litter — a good source of nutrients including nitrogen, phosphorous and potassium — is readily available in northwest Louisiana. It is usually applied to pastures but is increasingly being used on cropland, Dodla said.

Water availability, quality are concerns

Much of the irrigation in Louisiana is done with groundwater pumped from wells. The alternative is using surface water, which helps slow the depletion of underground aquifers. But not all farms are located near waterways suitable for irrigation.

In northwest Louisiana, irrigation is important because droughty conditions in the summer tend to be more intense than elsewhere in the state, said AgCenter water quality specialist Changyoon Jeong. Until recently, most farmers’ only option for irrigation was groundwater, further drawing down the already-strained Sparta Aquifer.

Farmers in the Gilliam area of Caddo Parish have been able to irrigate using water from Red Bayou since the 2014 completion of a Natural Resources Conservation Service project that pumps water from the Red River into the bayou, which is dry every summer. The drawback of using this water is quality. During crop season, Red Bayou has high levels of nitrogen, phosphorous and salt, Jeong said.

“In clay soil types, nitrogen and phosphorus accumulate at the top of field,” he said. “In sandy soils it does not accumulate, and it actually moves down to the edge of field. Eventually you’re going to lose a lot of nutrients.”

For the past three years, Jeong has monitored Red Bayou water quality in an effort to find out how farmers who rely on it for irrigation can mitigate possible negative effects. Fertilizer application rates may need to be adjusted, and the use of soil amendments could be helpful. He also is trying to determine how much nutrient content is lost in runoff from individual fields.

Jeong said research on nutrient management issues with surface water is important because of the continuing decline of groundwater sources.

“If we keep using the surface water for irrigation, it’s much better for the groundwater,” he said.

Olivia McClure