Soybeans throughout the county are beginning to look better. Development has been slowed and many fields have areas (sometimes large areas) with a light green or yellowish tint to the beans. What is causing this yellowing? One possibility is:
Nutrient Deficiencies
NITROGEN DEFICIENCY
Caused many fields to turn yellow in late June and July. A dry spring delayed nodulation, then heavy rainfall saturated soils just as plants required large amount of N during the rapid growth phase. There is often a lag phase when nodulation and N fixation is insufficient to supply enough N for the plants demands early in the season.
The good news is that most fields will recover as soils dry out and biological nitrogen fixation catches up to demand.
Some farmers have asked if N fertilizer could be added to these fields to help them through this yellow phase. Although applying N fertilizer does help green up the plants there is no economic benefit. Numerous trials conducted in Ontario, Canada show the average yield response to N fertilization is less than two bushels per acre.
Reposted from Tim Barnes, OSU Extension Marion County July 16,2023
POTASSIUM DEFICIENCY
Potassium (K) deficiency symptoms are frequently seen on soybean at early development stages (starting as early as V3) but can also develop on plants throughout the growing season. Potassium deficiency symptoms include yellowing of leaf tissue starting at the leaf tip and moving down the leaf margin (outer edge of leaf) (Figure 1). Severe K deficiency can lead to pre-mature leaf fall of the older leaves.
Potassium deficiency symptoms are often more severe in years where root growth is slowed or delayed (wet or dry years) and can be observed in fields with adequate soil test K levels. A poorly developed root system can limit nutrient uptake causing what is likely a temporary nutrient deficiency. These deficiency symptoms often decrease or disappear once adequate root development and soil conditions improve.
IRON DEFICIENCY CHLOROSIS
Iron deficiency chlorosis (IDC) is most common in poorly drained areas of the field. Plants with IDC tend to be stunted and yellowing occurs between the leaf veins while veins remain green (interveinal chlorosis). Leaves may also develop brown and necrotic spots in leaf margins. Iron is an immobile nutrient, so these symptoms most often occur first on newer, upper leaves.
IDC symptoms are more common in years where root growth is slowed or delayed (wet or dry years). A poorly developed root system can limit nutrient uptake causing what is likely a temporary deficiency symptomology. These deficiency symptoms often decrease or disappear once adequate root development and soil conditions improve.