Farmer’s questions lead to changes in tissue sampling for lettuce

by | May 10, 2011

Photo of farmer Bill Hering and regional agronomist Tim Hall

Farmer Bill Hering, left, and regional agronomist Tim hall discuss the appropriate leaf of lettuce to tissue sample. Hering's questions about his tissue test results led to changes in protocol for tissue sampling of leaf lettuce.

When farmers need nutrient management advice, they often turn to our Agronomic Services Division. Sometimes, these interactions work in reverse.

Two years ago, Bill Hering of Hering Farms in Faison called regional agronomist Tim Hall with a question about tissue test results. That call and subsequent dialogue triggered a series of field tests and, ultimately, a refined protocol for tissue sampling of leaf lettuce.

Hering, who grows a wide variety of produce, has used agronomic testing services regularly for years. In 2009, he submitted several tissue samples so he could monitor the nutrient status of leaf lettuce. The plant analysis report indicated that calcium levels were low to deficient. This news puzzled him because the crop looked healthy and he had applied sufficient calcium, in the form of gypsum, before planting. He called NCDA&CS regional agronomist Tim Hall to get a second opinion.

After visiting Hering’s fields, Hall reviewed the agronomic reports and noted that soil pH was acceptable, no lime was required, and therefore, the crop should not need calcium. Not long afterwards, another grower contacted Hall and related a similar story. Lab results were indicating a need for calcium, but in the field, the leafy greens did not seem to have a nutrient problem.

“Both of these growers are hard-working and conscientious,” Hall said. “They have years of experience with agronomic testing services. Their concerns spurred me to look more critically at what was going on in their fields.”

Hall took the issue back to the division’s home office. Brenda Cleveland, section chief in charge of plant analysis, speculated about the unexpected results.

“Usually, the most recent mature leaf is the appropriate leaf to collect when gathering a tissue sample,” Cleveland said. “With lettuce, however, leaves are in a rosette, and it is not always easy to decide which leaf is the most recently mature one. My first thought was that the sampling procedure was incorrect or inconsistent, but I knew there were other possibilities to consider. The sufficiency values themselves had to be scrutinized.”

Cleveland and Hall contacted Hering and asked if they could systematically collect tissue samples from his crop and run some follow-up tests. He agreed. They decided to sample leaves from two different locations on the plant and compare the nutrient concentrations. Each week during the spring and fall seasons of 2010, they collected samples of recent mature leaves and more mature, outer-perimeter leaves. The two sets of tissue samples showed clear differences in their nutrient concentrations.

Photo of lettuce

NCDA&CS agronomists now recommend collecting mature, undamaged, outer-perimeter leaves for tissue sampling.

The most recent mature leaves that growers had typically been advised to sample were fairly near the center of the whorl. When analyzed, these leaves consistently showed low calcium concentrations, but the outer perimeter leaves indicated the presence of adequate levels. Agronomists concluded that, for a crop like lettuce, it is critically important to define more precisely which leaf should be collected for a tissue sample.

Results of the field study helped the Agronomic Services Division provide more accurate tissue sampling guidelines for lettuce. The division now advises growers to collect the outermost, undamaged leaf. This leaf is practical because it is easy to identify and its nutrient content correlates more closely with the division’s established sufficiency ranges for calcium. The newly refined guideline should eliminate false diagnoses of low calcium levels.

“That’s one of the benefits of the Agronomic Division having a dedicated Field Services Section,” Hall said. “We are in direct contact with growers throughout the state. We hear feedback. We relay reactions and concerns back to our office and laboratory staff. We have an ongoing dialogue that helps both the division and its clients.”

Growers such as Bill Hering rely on NCDA&CS for precise nutrient management data and feel invested in the service. “Tissue analysis and soil test data help us optimize fertilization, input costs and yield. You can’t get too much of this kind of information,” Hering said. “We’re more than glad to work with the division to get things fine-tuned.”

The NCDA&CS Agronomic Services Division has regional agronomists statewide who can make on-site visits and evaluate suspected nutrient problems. Currently, they are working particularly closely with growers of lettuce and other plasticulture crops. With the help of a grant from the N.C. Tobacco Trust Fund Commission, agronomists visit and instruct plasticulture growers on how they can best use the division’s testing services to increase efficiency and productivity.