Dr. Emi Kimura, Extension Agronomy\/State Extension Peanut Specialist, TAMU Dept. of Soil & Crop Sciences, Vernon, (940) 552-9941, emi.kimura@ag.tamu.edu<\/a><\/p>\n June 6, 2019<\/p>\n \u00a0<\/strong>Note:\u00a0 The following comments address field observations primarily for the Texas High Plains peanut production region but are applicable to other Texas regions as well.\u00a0 We discuss other legume crops in the document as well.<\/em><\/p>\n <\/p>\n Legumes are a valuable asset in any Texas crop rotation.\u00a0 They offer a potential contrast of susceptibility with the more likely disease and insect threats in other crops.\u00a0 Also\u2014though often incorrectly assumed\u2014legumes offer significant potential nitrogen (N) fixation to the existing crop with potential improved soil N status for the subsequent crop.\u00a0 There is one important caveat<\/strong>:\u00a0 legume and N-fixation benefits can only occur IF the roots are infected<\/u> by a crop-specific bacterial strain of either Rhizobium<\/em> or Bradyrhizobium<\/em>, which either you applied or may already be present in the soil.<\/p>\n <\/p>\n This bacterial infection of the root leads to nodulation. \u00a0The bacteria survive and thrive inside these root nodules, visibly showing a pink or red color (Fig. 1).\u00a0 That means fixation of nitrogen in the air (an N2<\/sub> molecule, which makes up about 78% of the atmosphere) is converted into a form the plants can use.\u00a0 This is the least costly form of nitrogen you can \u201capply\u201d to your legume crop.<\/p>\n <\/p>\n We will save information about effective use and application of N-fixing inoculants for a RowCrops NewsLetter in spring 2020.\u00a0 That will include discussion of choosing crop-specific inoculants, what form they are (the potential benefit declines from liquid to granular to seedbox powder forms), and ensuring accurate application and placement of the inoculant with the seed.<\/p>\n <\/p>\n Assessing Nodulation in 2019 Texas Peanuts<\/u><\/p>\n <\/p>\n We are approaching the time in mid-June after peanut planting in the Texas High Plains when peanut growers should check their taproot Bradyrhizobium<\/em> nodulation.\u00a0 Scouting 5 to 6 weeks after planting assesses early nodulation in advance of possible decisions about applying mid-season N, and if so, how much N.\u00a0 You might find it surprising we even mention applying N<\/p>\n <\/p>\n Fig. 1<\/strong>.\u00a0 Sliced Bradyrhizobium<\/em> nodules on peanut demonstrate pink and red color indicating active fixation of nitrogen from the air.<\/p>\n <\/p>\n fertilizer to a legume, but this possibility does occur.\u00a0 Nitrogen applications are almost certain if you recognize soon enough you have few or no active nodules on the roots.\u00a0 This applies to peanuts, soybean, peas and beans, guar, alfalfa, etc.<\/p>\n <\/p>\n Scout your nodules!<\/em><\/p>\n <\/p>\n Farmers, crop consultants, and supplier support staff scout field crops for insects and disease.\u00a0 We have learned we need to \u201cscout for nodules\u201d in our legume crops as well.\u00a0 Of course, this is a different situation compared to a pest.\u00a0 But if you applied an inoculant to your legume crop seed at planting and later find little to no nodulation, this is important for two reasons.\u00a0 First, of immediate concern, you are not getting the nitrogen nutrition you may have expected for your crop. \u00a0Thus, fertilizer N may be needed to obtain your yield potential.\u00a0 Second, if you have no nodules then we need to understand why (wrong inoculant, expired inoculant, improper application, inoculant introduced into hot dry soils where the bacteria did not survive long enough to infect the roots, etc.).<\/p>\n <\/p>\n Use a shovel to dig (do not pull, that can strip nodules off) several plants from different rows and field locations.\u00a0 The ideal situation is when you may also have a few rows with no inoculant applied to the seed. \u00a0Then you could truly see if and how much nodulation benefit you may have.\u00a0 For any legume crop, inoculated or not, if nodulation is deemed poor, nothing can be done to increase nodulation in the current crop. \u00a0In West Texas peanuts 20 to 25% of fields annually may be undernodulated, or worse, have only a few nodules per plant.\u00a0 Poor Bradyrhizobium<\/em> nodulation calls for supplemental N to achieve desired yield potential.\u00a0 Therefore, early scouting is recommended.\u00a0 We need to know which field, for example peanuts, are not nodulating early in the cropping season.<\/p>\n <\/p>\n Extension assessment of many peanut fields in the Haskell, Comanche, Atascosa, and Frio Counties typically have nodules all over the lateral roots suggesting a source of nodulating bacteria in the soil.\u00a0 These lateral root nodules contrast to the masses of nodules you hope to find on the taproot (Fig. 2), i.e. evidence of inoculant application.\u00a0 Nodules on the lateral roots, though often high in number (even hundreds per plant) tend to be less active and may not be fixing N for your peanut crop (not pink or red inside).<\/p>\n <\/p>\n <\/p>\n Fig. 2<\/strong>.\u00a0 A) Clusters of Bradyrhizobium<\/em> nodules on the tap root are ready evidence of effective inoculation of peanut.\u00a0 B) Comparison of root nodulation on uninoculated plants (top) vs. early (~5 weeks) cluster of nodules from an applied Bradyrhizobium<\/em> inoculant for peanut.<\/p>\n <\/p>\n Active nodules are pink to dark red inside.\u00a0 If nodules are white inside, they are not yet active\u2014check again in 7-10 days.\u00a0 Nodules no longer active are black, gray, and may be mushy. This becomes common later in the cropping season even though the crop may still be growing.\u00a0 (But applying late-season N after nodules slough off is not recommended.)\u00a0 Nodules which never turn pink or red inside are from soil Rhizobium\/Bradyrhizobium<\/em> that may not be specific for peanuts or your other legume crop.\u00a0 You need to differentiate these types of nodules, mostly on the lateral roots, versus the mass of \u2018supernodulation\u2019 on the taproot, which is ready evidence that your inoculant worked (Fig. 2).<\/p>\n <\/p>\n Is nodulation adequate?<\/em><\/p>\n <\/p>\n For West Texas, the following simple guideline<\/u> rates nodulation levels 5 to 6 weeks after planting. \u00a0We are particularly interested in any developing clusters of nodules on the taproot. \u00a0If 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\u2019t going to improve<\/u>.<\/p>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/u><\/p>\n \u00a0<\/u><\/p>\n Early season Bradyrhizobium<\/em> nodulation rating for peanuts (Texas High Plains).<\/p>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/u><\/p>\n Early-season\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Nodules<\/p>\n Nodulation Rating<\/u>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 per Plant<\/u>\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Management Consideration<\/u><\/p>\n \n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Excellent\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 >20\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Likely excellent late-season<\/p>\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 nodulation, N response not expected<\/p>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Very Good\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 16-20\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0Late-season nodulation also strong,\u00a0 mid-season N fertilizer needs minimal\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 if any<\/p>\n \n \u00a0 \u00a0 \u00a0 \u00a0 Good\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 11-15\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Will produce good crop, anticipate some<\/p>\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 possible mid-season N needs<\/p>\n \n \u00a0 \u00a0 \u00a0 \u00a0 Fair\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 6-10\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Would have liked higher nodulation,<\/p>\n \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0mid-season N program likely necessary<\/p>\n \n Poor\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 0-5\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0May be background soil Rhizobium<\/em> only, N fertility program<\/p>\n essential; Try to determine why nodulation is poor if field was<\/p>\n inoculated (review planting equipment, planting conditions,<\/p>\n how inoculant was handled (avoid heat, direct sun, etc.)<\/p>\n \u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 <\/u><\/p>\n \n In the above situation, as an example, a producer intends to apply 80 lbs. N\/Ac mid-season.\u00a0 But if early signs suggest that nodulation is very good, this producer would have good reason to substantially reduce target N application, perhaps by 50% or more.<\/p>\n <\/p>\n Scout your nodules\u2014end of season follow-up<\/em><\/p>\n <\/p>\n A second nodulation check for peak nodulation for Texas High Plains peanuts, later in the season about mid-August to early September, will confirm early observations. \u00a0(For other peanuts in other regions, or other legumes make this assessment four to six weeks before physiological maturity.) \u00a0Did nodulation improve? \u00a0For peanuts, I would hope for over 50 nodules per plant.\u00a0 Also, this late scouting allows you to flag fields that nodulated poorly. \u00a0If nodulation remains low, consider why nodulation might not have occurred and what you can do to enhance nodulation in next year\u2019s crop (see additional discussion below).<\/p>\n <\/p>\n When you conduct late-season nodulation sampling do you see obvious signs where inoculant was misapplied?\u00a0 This could include primarily individual yellow rows (Fig. 3.), but also spotty green\/yellow color throughout the field or light pea-green field color suggesting N deficiency.<\/p>\n <\/p>\n Fig. 3<\/strong>.\u00a0 An inoculant application malfunction on row seven of an eight-row planter led to little or no inoculant application.\u00a0 Yellow peanuts show N deficiency.\u00a0 August nodule count on this Hockley Co., Texas runner peanut field was 9 nodules per plant in yellow rows and 73 nodules per plant on adjacent green rows.<\/p>\n <\/p>\n Assessing Nodulation in Other Texas Legumes<\/u><\/p>\n <\/p>\n Nodulation assessment in other legume crops can be conducted in the same fashion as for peanuts.\u00a0 Dig (again, don\u2019t pull) several plants from different rows and at least three areas around the field (opposite ends is good).\u00a0 Like in peanuts, look for clusters of nodules, especially on the taproot if you applied an inoculant.\u00a0 In contrast to peanuts in West Texas we do not have a scale developed for other crops.\u00a0 We and colleagues have seen enough soybeans to know that at least 10 to 20 nodules per plant should be a minimum, and 50+ is very good.<\/p>\n <\/p>\n What if I did not apply a rhizobial inoculant to my legume crop?<\/em><\/p>\n <\/p>\n We still recommend you scout.\u00a0 You hope you still receive nodulation and N fixation from native Rhizobia<\/em> or especially from remaining background Rhizobia<\/em> if the same legume crop was planted previously. \u00a0Many farmers in the U.S. Midwest never inoculate soybeans due to good carryover nodulation (two-year rotation with corn).\u00a0 We all wish we could farm those rich soils!<\/p>\n <\/p>\n But Texas is different.\u00a0 Soil conditions\u2014often hotter, drier, lower organic matter\u2014are less conducive to preserving desirable rhizobial bacteria. \u00a0You cannot assume legume roots will be infected and nodules developed.\u00a0 If you have good active nodulation (many nodules, red\/pink inside), great!\u00a0 But if nodulation is poor then you know you are not getting benefit.\u00a0 Thus, fertilizer nitrogen may be needed in lieu of fixation of \u2018free\u2019 nitrogen.\u00a0 And, you can ask yourself if the evidence suggests in subsequent years you should consider applying crop-specific rhizobial inoculant to your legumes if you have not been doing so.<\/p>\n <\/p>\n Factors that Contribute to Poor Nodulation<\/u><\/p>\n <\/p>\n Successful nodulation due to inoculation is expected but is never assured.\u00a0 Sometimes something goes wrong.\u00a0 A mass of Rhizobium<\/em> or Bradyrhizobium<\/em> nodules on the taproot is highly desirable, in contrast to large numbers of individual nodules spread randomly over the lateral roots (likely from soil rhizobial populations, which may be less effective at fixing N).<\/p>\n <\/p>\n Here are major<\/strong> causes of minimal to no taproot nodulation despite inoculant application.\u00a0 These are ranked roughly in order of the potential for failure to achieve good nodulation.<\/p>\n <\/p>\n <\/p>\n Lesser<\/strong> factors which might explain poor legume crop nodulation include:<\/p>\n <\/p>\n <\/p>\n <\/p>\n For further Texas peanut production resources visit the developing website at http:\/\/varietytesting.tamu.edu\/peanut<\/a> or contact Dr. Kimura.<\/p>\n <\/p>\n![\"\"](\"https:\/\/agrilife.org\/texasrowcrops\/files\/2019\/06\/ct-1-300x199.jpg\")
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