Now is the time for peanut growers on the Texas South Plains to scout their crops for taproot Bradyrhizobium nodulation, says Dr. Emi Kimura, Extension Agronomy/State Extension peanut specialist, Texas A&M University Department of Soil & Crop Sciences, Vernon, in the June 13-issue of Texas Row Crops newsletter.

“Scouting five to six weeks after planting assesses early nodulation in advance of possible decisions about applying mid-season nitrogen,” says Kimura. "If nitrogen is needed, this early assessment will help determine how much."

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When it comes to legumes, growers may not think about applying a nitrogen fertilizer, but if a grower recognizes soon enough he or she has few or no active nodules on the roots, nitrogen applications are almost certain, says Dr. Calvin Trostle, Extension Agronomy, TAMU Department of Soil & Crop Sciences, Lubbock. This applies to legumes such as peanuts, soybeans, peas and beans, guar and alfalfa.

While legumes are a valuable asset in any Texas crop rotation, Trostle says they also offer significant potential nitrogen fixation to the existing crop with the possibility of improving the soil nitrogen status for the subsequent crop.

But there is a caveat. “Legume and nitrogen-fixation benefits can only occur if the roots are infected by a crop-specific bacterial strain of either Rhizobium or Bradyrhizobium, which is either applied or may already be present in the soil,” he says.

And root bacterial infection leads to nodulation. “The bacteria survive and thrive inside these root nodules, visibly showing a pink or red color. That means fixation of nitrogen in the air is converted into a form the plants can use. This is the least costly form of nitrogen you can apply to your legume crop,” he says.

ASSESSMENT

Application of an inoculant to legume seed at planting with little to no nodulation later on in the plant growth raises concerns, says Trostle. “First, you are not getting the nitrogen nutrition you may have expected for your crop, and therefore, fertilizer nitrogen may be needed to obtain your yield potential. Second, if you have no nodules, then we need to understand why. Was it the wrong inoculant, expired inoculant, improper application or was the inoculant introduced into hot, dry soils where the bacteria did not survive long enough to infect the roots?”

To assess the taproot, rather than pull up the peanut plant and risk stripping off the nodules, Trostle recommends using a shovel but also selecting plants from different rows and field locations.

“The ideal situation is when you also have a few rows with no inoculant applied to the seed, then you can truly see, if and how much, nodulation benefit you may have. For any legume crop, inoculated or not, if nodulation is deemed poor, nothing can be done to increase nodulation in the current crop,” says Trostle.

On the Texas South Plains, 20 to 25 percent of fields may be undernodulated, or worse, have only a few nodules per plant, says Trostle.  “Poor Bradyrhizobium nodulation calls for supplemental nitrogen to achieve desired yield potential, which is why early scouting is recommended. We need to know which fields are not nodulating early in the cropping season.”

While the peanut plants have lateral roots with nodules, suggesting a source of nodulation bacteria in the soil, Trostle says they should contrast to the masses of nodules found on the taproot, “evidence of inoculant application.” Lateral root nodules, though often high in number (hundreds per plant) tend to be less active and may not be fixing nitrogen for the peanut crop, he adds.

To determine whether a nodule is active, Trostle says the inside of it should be pink to dark red. “If nodules are white inside, they are not yet active and should be checked again in seven to 10 days.”

Nodules that are black, gray, and possibly mushy are no longer active. “This becomes common later in the cropping season even though the crop may still be growing. Applying late-season nitrogen after nodules slough off is not recommended,” says Trostle. 

Nodules that never turn pink or red inside are from soil Rhizobium/Bradyrhizobium that may not be specific for peanuts or other legume crops, he says. “You need to differentiate these types of nodules, mostly on the lateral roots, versus the mass of ‘super nodulation’ on the taproot, which is ready evidence that your inoculant worked.”

GUIDELINES

The rate chart below provides simple guidelines for early season Bradyrhizobium nodulation for peanuts on the Texas High Plains. “We are particularly interested in any developing clusters of nodules on the taproot,” says Trostle. “If early nodulation is good, you can expect it to continue to increase toward peak nodulation (about mid-August), but if early nodulation is poor, it probably isn’t going to improve.”

LATE-SEASON SAMPLING

About mid-August to early September, Kimura recommends growers check for peak nodulation, which will confirm earlier observations. For peanuts or other legumes grown in other regions, this assessment should be done four to six weeks before physiological maturity, she says. 

When assessing, Kimura says growers should consider the following:

POOR NODULATION

While successful nodulation is expected after a grower inoculates, it is never assured, says Trostle. "Sometimes something goes wrong. A mass of Rhizobium or Bradyrhizobium nodules on the taproot is highly desirable, in contrast to large numbers of individual nodules spread randomly over the lateral roots."

Trostle lists the following major causes of minimal to no taproot nodulation, despite inoculant application:

For more information about lesser factors which may explain poor legume crop nodulation or for more information about assessing nodulation in other Texas legumes, click here. 

For questions or more information, contact Kimura or Trostle.