Pumping Plants Up

Try solving this riddle. What is it that surrounds the planet and can be found in every living thing, yet it’s always in short supply in the plant world? Hint—farmers use it to make crops healthy and abundant.

If you’re thinking fertilizer you’re mostly right, but the correct reply is a particular type of fertilizer. Stumped? The answer is nitrogen.

Nitrogen makes up about 80 percent of the Earth’s atmosphere. It is a common element in all living tissue and a component of proteins, genetic material and chlorophyll. However, most of the Earth’s nitrogen is in the form of atmospheric gas, which can’t be used directly by plants, or, as scientists say, is not available to plants.

That’s a bit inconvenient when you consider that nitrogen is the crop nutrient that makes plant stalks and stems grow big and strong and plant foliage dense and lush. It’s the plant world’s version of steroids. Nitrogen helps agricultural crops "bulk up."

"Of the major nutrients that are most important to crop production, nitrogen is the most limiting," said Neil Christensen, an Oregon State University soil scientist and OSU Agricultural Experiment Station researcher. "It’s limiting to plant growth in that if plants don’t get enough nitrogen during their most active growth stage (usually in the late spring or early summer) they will not grow to full potential and will not produce as great a yield of whatever the particular crop might be, such as wheat, corn, peppermint, hops or grass seed."

Illustration: Bill Lanham	A modern fertilizer plant. Photo: Index Stock

Other essential plant nutrients are potassium, phosphorus and sulfur. There are many additional elements that plants require in trace quantities. However, nitrogen in its many forms (see nitrogen sidebar, page 33) is the nutrient agricultural producers depend on for maximum yields.

That’s why nitrogen fertilizers are a key ingredient in meeting the food needs of a growing world population. Agricultural producers have achieved dramatic increases in crop yields due, in large part, to greater use of nitrogen fertilizers. Over the past 100 years this has spurred global demand and industrial production of the fertilizers.

In the United States alone, annual production of nitrogen fertilizers has grown from 600,000 tons in 1914 to 12 million tons today. Some consider the development of the Haber-Bosch process (see Fritz Haber sidebar, page 34) for manufacturing nitrogen fertilizers one of the greatest innovations of the 20th century, right up there with the automobile, the computer and space travel.

But there’s a dark cloud tucked away behind this silver lining.

Where nitrogen fertilizer is concerned, too much of a good thing can be bad—detrimental to the environment and costly to growers. How much is too much? Agricultural Experiment Station researchers and Extension Service faculty at OSU have been studying that question extensively over the past 25 years with the goal of helping Oregon agricultural producers become more efficient users of nitrogen fertilizers.

"It’s well documented that nitrogen fertilizers, especially nitrates, not taken up by crops and left in the field after harvest eventually leach (or filter) through the soil and end up in groundwater, causing contamination that is harmful to humans," said Christensen. The amount of nitrogen that may leach into groundwater depends on the drainage characteristics of the soil and the type of crop grown.

Well-drained soils let water pass through, carrying nitrates along with it. In addition, crops differ in both nitrogen needs and how efficiently they use nitrogen. Grass seed crops, for example, tend to be more efficient than many other crops in taking up nitrogen. That means at harvest less nitrogen is left in the soil and available to leach into groundwater.

Nitrate contamination of groundwater has been recognized as a growing problem in agriculture over the past 30 years, according to Christensen. "This environmental issue is one of the major reasons that efficient use of nitrogen fertilizers has become a research priority," he said.

That’s not the only reason OSU researchers are studying nitrogen efficiency.

Growers rely on nitrogen applications in the spring to boost crop yields later in the growing season. This reliance often is responsible for a tendency to apply more nitrogen than crops need, which is costly to growers.

"Nitrogen left over in the field after we harvest represents money we spent that we didn’t need to," said Larry Venell of Venell Farms near Corvallis. "The whole thing gets down to economics. There’s no point to putting more nitrogen on a crop than it can use. It costs us as well as having an environmental impact."

Venell Farms is a 10,000-acre operation producing grass seed, wheat, corn, bush beans and legume crops. "We look for cost effectiveness," Venell added. "It’s the only way we can be profitable."

Besides being costly, over-fertilizing crops with nitrogen can actually reduce crop yields.

For example, said Christensen, applying too much nitrogen to winter wheat, which Willamette Valley growers often plant in rotation with grass seed crops, can make wheat plants susceptible to "lodging"— falling on the ground—before the crop is ready for harvest.

Excess nitrogen causes wheat stalks to remain green and flexible rather than drying out as they usually do in summer. This makes the wheat more apt to lodge when the field is hit by summer winds or rain. Wheat lying on the ground is much more difficult to harvest than standing wheat, with the result that yields drop dramatically where lodging occurs.

The solution to these problems seems simple—apply only as much nitrogen as the crop needs, no more, no less. However, determining the correct amount is not simple.

Oregon farmers rely on applications of nitrogen fertilizer to boost yields of high-value crops like sweet corn. This sweet corn harvest is on the Peter Kenagy farm near Albany. Photo: Bob Rost		Gene Pirelli, a faculty member in the Polk County office of the OSU Extension Service, is working with livestock producers to identify how to apply nitrogen fertilizers to pasturelands more efficiently. Photo: Bob Rost

Crops such as wheat, grass seed, sweet corn, bush beans, broccoli and peppermint have different nitrogen fertilizer requirements. In addition, as noted above, soils vary in their drainage characteristics, which can affect what happens to nitrogen fertilizers left in the field after harvest.

These factors combine to form a production system for each crop that is distinct in terms of irrigation and fertilization. Using nitrogen fertilizer efficiently with any particular crop is a balancing act—balancing the crop’s nitrogen needs with a specific rate and timing of nitrogen application that will provide maximum benefit to the crop.

Figuring out what the balance should be for many Oregon-grown crops is an ongoing research priority.

Over the past 12 years, Christensen has worked with many OSU researchers and Extension faculty members on nitrogen management and water quality studies in the Willamette Valley for crops that include hops, sweet corn, wheat and peppermint.

"One of our goals was to look at what is unique about the Willamette Valley’s climate and soils and examine how those characteristics affect nitrogen management issues," said Christensen.

The research included extensive soil and plant tissue sampling and testing to measure the amount of nitrogen in the field and in the crop.

"We’re particularly interested in nitrate in soil profiles, how it changes over the year and what you can infer about where that nitrogen is going and what kind of an environmental risk it might be," said Christensen.

"The data collected allowed calculation of more accurate nitrogen fertilizer recommendations for many crops grown in the valley," said Christensen. "The soil sampling component of the research revealed that some crops, grass seed for example, are much more efficient users of nitrogen than other crops, such as sweet corn."

During this time, Christensen also worked on adapting a nitrogen mineralization test for use in winter wheat crops in the Willamette Valley.

"This is a useful tool for helping wheat growers decide how much nitrogen fertilizer to apply to crops in spring," said Christensen. The test involves taking a soil sample from the field in January and then processing the sample via an incubation process.

Results indicate the amount of organic nitrogen in the soil that will convert (or mineralize) into a form of nitrogen that plants can use as the soil warms up in the spring. The test allows growers to anticipate the availability to the crop of organic forms of nitrogen.

Currently Christensen is studying how different classes of wheat grown in Oregon, including soft white, hard white and hard red wheat, respond to different rates of nitrogen fertilization.

"The purpose of this research is to examine how various rates of nitrogen fertilizer affect yield response and protein content," Christensen explained. "The protein content is of particular interest because some of the wheat markets require a specific level of protein in wheat kernels. Growers who don’t meet this protein target have to find other outlets for their wheat that don’t pay as well."

Oregon wheat growers would like to become consistent suppliers of high-protein wheats that command higher prices because of their superior milling and baking characteristics.

"Consistency is the key," Christensen said. "The goal of this research is a set of recommendations growers can use to make sure their management of nitrogen fertilizer applications will consistently deliver the desired protein levels."

According to Christensen, a big advantage in OSU’s efforts to help growers improve their nitrogen efficiency management is the close working relationship between Agricultural Experiment Station researchers and Extension faculty on the OSU campus in Corvallis and at county Extension offices throughout the state. John Hart agrees. He’s an OSU Extension soil scientist and cooperator with Christensen on several nitrogen management projects.

"We’ve developed a team approach that has been very effective in delivering information to growers," said Hart. "I think the word used these days to describe the arrangement is ‘seamless’." Campus-based staff have been able to work with the growers pretty effectively because of the efforts of county-based Extension faculty."

Hart and Christensen worked together on a recently completed five-year study of nitrogen efficiency in grass seed crops. Mark Mellbye, an OSU faculty member in the Linn County Extension Service office, coordinated the participation of several growers who cooperated on the project.

"The goal of the study was to fine-tune our recommendations for how much nitrogen fertilizer is needed to get maximum grass seed yields," said Mellbye. "This was a large on-farm study with 10 growers located throughout the mid-Willamette Valley area participating."

An important feature of the study was its scale, Mellbye noted.

Left to right: Bob Spinney, a crop consultant for Western Farm Service, and OSU soil scientists Neil Christensen and John Hart. They're at Venell Farms near Corvallis, which has cooperated with OSU on nitrogen fertilization efficiency research projects. Photo: Bob Rost		A device called an air-flow spreader, with 25-foot booms on the sides that ensure uniform fertilizer application, applies urea in a Willamette Valley mint field. Urea is a dry form of nitrogen fertilizer. It consists of tiny white pellets. Photo: Bob Rost

"We conducted the project on large fields—up to five-acre sections at each location rather than the small research plots we usually use for fertilizer application trials," Mellbye said. "This allowed us to get a very accurate yield response to the various rates of nitrogen fertilizer applied."

Hart is working with Gene Pirelli, an OSU faculty member based in the Polk County Extension Service office, to introduce livestock producers to a nitrogen efficiency management program called the T-Sum 200 Method.

"In the past, livestock producers had to guess at the best time to put nitrogen fertilizer on pastures," said Pirelli. "April 15 is a popular choice because growers assume that forage plants are beginning active growth about that time."

The T-Sum 200 method gives growers a much more accurate way of choosing the right time.

T-Sum 200 is a system of using averaged daily temperatures, starting Jan. 1, to calculate the time when pasture grasses will break out of winter dormancy, Pirelli explained. That break out point is when applying nitrogen fertilizers to pastures will be most effective in promoting vigorous growth.

"By using the T-Sum 200 method to time nitrogen fertilizer applications livestock producers get more value from the nitrogen they apply and grow healthier pastures that can support grazing livestock earlier in the year," said Pirelli.

At OSU’s North Willamette Research and Extension Center at Aurora, just south of Portland, horticulture researcher Del Hemphill, in cooperation with Hart and Christensen, developed a "pre-sidedress soil nitrate test" (PSNT) to help growers of sweet corn adjust nitrogen applications.

"The key component of this test is timing," said Hemphill.

Before the introduction of PSNT, growers did not have a test that would accurately predict nitrogen availability from soil during the growing season, Hemphill explained. Taking a soil sample in early spring, following heavy winter rainfall, would show very low levels of available nitrogen, reflecting leaching of nitrate by winter rains. But later in the spring when soils warm up, biological activity converts organic nitrogen into a form plants can use, he said.

"The approach with this tool is to conduct the soil test five or six weeks after planting, which is the time that growers usually apply most nitrogen fertilizer as the crop begins active growth," Hemphill said.

"By conducting the test later in the spring, growers can measure the amount of nitrogen that is already in the soil and available to the crop," he added. "Then they can adjust their spring application of nitrogen accordingly."

Tests like the pre-sidedress soil nitrate test and nitrogen mineralization soil test developed by OSU scientists are important nitrogen management tools that are catching on with growers, according to Christensen.

"The growers have been very proactive about nitrogen management in crop production," he said. "They want to be more efficient in their use of nitrogen fertilizers, just as they want to increase efficiency in all crop production practices, and they understand the environmental issue with nitrogen as well. Ultimately, all our efforts are aimed at making production agriculture sustainable in Oregon."

Larry Venell of Venell Farms puts it another way.

"If we want to keep farming, we’re going to have to get better with everything we do and use," he said. "That’s the only way we’ll be able to compete in world markets."