If soybeans turn yellow during an early growth stage, you may have a case of soybean iron deficiency chlorosis (IDC). The distinctive yellow symptoms of soybean IDC often appear as soybean enters the first- to third-trifoliate leaf stage. Soybean IDC is characterized by distinct interveinal chlorosis (yellow leaf with green leaf veins) in the newest leaves and may result in substantial yield loss. Soybean IDC is not caused by low soil iron but instead caused by soil conditions that decrease iron uptake by soybean roots.
Soybean IDC risk and severity are primarily related to soil carbonate content (calcium carbonate equivalent, CCE) and exacerbated by soluble salts (electrical conductivity, EC). Soybean IDC can be common in soybean-growing regions of the Upper Midwest, Northern Plains, and Canadian Prairies, where soils frequently have high carbonates and/or soluble salts. Within a field, chlorosis symptoms are usually confined to soybean IDC hotspots with high carbonates and soluble salts; however, symptoms may appear across a field if high carbonates and salts are present throughout a field. Soybean IDC severity is made worse in cool, wet soils and soils with high residual nitrate. Soil pH is not a good indicator of soybean IDC risk because some high pH soils lack high carbonates and soluble salts, which are the two principal factors.
Unlike a nitrogen or sulfur deficiency, soybean IDC is not correctable with an in-season fertilizer application. Foliar application of iron fertilizers, including FeEDDHA products, may have short-term cosmetic effects, but foliar Fe applications have not consistently increased yield of soybeans affected by IDC. Chlorosis symptoms may naturally alleviate as environmental conditions improve (e.g., drier, warmer weather). North Dakota State University research has shown that chlorosis persisting into the fifth- and sixth-trifoliate leaf stage will reduce yield. For fields with soybean IDC now, you should take preventative action for following years: (1) evaluate which soybean varieties are most tolerant or susceptible to IDC and (2) identify soybean IDC hotspots on field maps.
Guidelines for managing soybean IDC:
- Plant soybeans in fields with low carbonates and soluble salts (principal soybean IDC risk factors). This may require soil sampling prior to soybeans (possibly outside of your usual soil sampling rotation) or consulting previous soil sampling records.
- Choose an IDC tolerant soybean variety. This is your most practical option to reduce soybean IDC risk in previously problematic fields or fields with moderate to high carbonates and soluble salts. Consult seed dealers, university soybean IDC ratings, and neighbor experiences when searching for IDC tolerant soybean varieties.
- Plant soybeans in wider rows. Soybean IDC tends to be less severe in wide-row spacings (plants are closer together) than narrow-row spacings or solid-seeded spacings.
- Apply chelated iron fertilizer (e.g., high quality FeEDDHA) in-furrow at planting. In-furrow FeEDDHA application may not be enough to help an IDC susceptible variety in high IDC risk soils (see points #1 and #2).
Please call AGVISE Laboratories in Northwood, ND (701-587-6010) or Benson, MN (320-843-4109) with questions. AGVISE offers routine soil testing for carbonates and soluble salts.