​​Evaluate Soil Fertility​​

Soybeans require 13 essential elements to properly grow

Evaluate and understand soil fertility to prevent nutrient deficiency.

Soil sampling

  • Sampling annually or every other year will provide a better understanding of nutrient availability and crop needs over time.
  • Sample fields the same time every year, preferably after harvest and before the ground is frozen.
  • Core Sample Sizes should be representative of the field size.
    • For example, a 40-acre field should have 10 to 15 random surface cores and 6 to 8 subsurface cores taken.
  • Samples should include the top 8" for conventional tillage or 3" for no-till and reduced tillage.
  • Subsurface testing should occur below 8" to estimate nitrate-N in root zone.

Source: Ferguson, R.B. et. al. 2007. Guidelines for soil sampling. University of Nebraska-Lincoln. G1740.

Soil pH

  • Ideal soil pH for soybean production is around 6.5.
  • Low soil pH may result in reduced availability of calcium, magnesium and molybdenum.
  • Low soil pH increases the risk of herbicide carryover from imidazolinone herbicides.
  • High soil pH can reduce availability of iron(Fe), manganese(Mn), boron(B), copper(Cu) and zinc(Zn).
  • High soil pH increases potential carryover of sulfonylurea herbicides.

Macronutrients

  • Each bushel of soybean grain harvested per acre removes approximately the equivalent of 4.76 lbs/A of N, 0.9 lbs/A of P2O5 and 1.5 lbs/A of K2O.
  • A high amount of available potassium is essential for nodules to fix nitrogen and to help increase stress tolerance.
  • Soybean K deficiency can be identified through yellow leaf margins.
  • One option for phosphorus and potassium fertilizer is a banded application in which the fertilizer band matches the row spacing.
  • Talk to your local fertilizer dealer and/or testing lab for more information.
  • Macronutrients are mobile and deficiency symptoms first appear on the older leaves.
  • Phosphorus Deficiency — dark green or purple coloration, with necrotic spots.
  • Potassium Deficiency — chlorosis at the margins and in-between the veins.
  • Nitrogen Deficiency — chlorosis and yellowing of the leaves.

R. Hoeft and T. Peck. 2002. Soil Testing and Fertility. Illinois Agronomy Handbook 2001-2002. P 90. Univ. of Illinois.

Micronutrients

  • Maximum nutrient demand occurs during seed fill.
  • Micronutrients can have limited availability at high and low pH levels.
  • Manganese(Mn) Deficiency — Mn deficiency can lead to yellowing between green veins and when severe, the yellow becomes white and possibly necrotic. With many types of Mn fertilizer, 1 to 2 lbs/A of Mn can help minimize the impact of deficiencies.
  • Iron Deficiency Chlorosis — IDC is a common yield-limiting factor in high pH soils. IDC can be treated with a combination of management practices, including selecting appropriate soybean products, improving drainage, managing nitrogen(N) rates from previous year corn, maintaining high levels of available phosphorus(P), applying iron materials with the seed and using foliar iron sprays.

1Davis, V. and F. Fernández. 2009. Yellow soybeans in high pH soils: sign of Mn deficiency. University of Illinois. The Bulletin. Issue No. 15.

Optimize Fertility

  • One of the most overlooked areas of high-yield soybean production:
    • DO NOT TREAT SOYBEANS AS NUTRIENT SCAVENGERS.
  • One bushel of soybean removes up to:
    • 4.76 lbs N
    • 0.90 lbs P2O5
    • 1.5 lbs of K2O
  • 50-80% of the soybean plant's N uptake requirement comes from biological nitrogen fixation (BNF) the remainder must be supplied from soil mineralization or fertilizer.
  • In high-yielding conditions, soil N and BNF may not be sufficient to sustain N uptake rates during seed-filling period.
  • If soil N supply is low or BNF declines during reproductive stages, the N needed by the plant must be remobilized from vegetative tissues to the seeds and leaf senescence occurs thus impacting photosynthesis and seed yield.
  • Nitrogen fixation is an extremely water-sensitive process within soybeans.
    • Excess moisture or deficiency can greatly impact soil bacterial populations.
    • Moderate to severe moisture stress reduces nitrogen fixation even if sufficient nodulation has occurred.
    • In drought conditions or extremely wet conditions soybeans will increase the proportion of nitrogen derived from fertilizer sources.

S. Brouder, et al. 2003. Manganese Deficiencies in Indiana Soils. Purdue Univ. http://www.agry.purdue.edu/ext/pubs/AY-276-W.pdf
C. Grau, et al. 2003. Soil pH influences soybean disease potential. The Yields II Project: Research-Based Management Information. http://www.planthealth.info/yields2/grau2.pdf
S. Naeve and G. Rehm. 2006. Genotype X environment interactions with iron deficiency chlorosis-tolerant soybean genotypes. Agron . J. 98:808-814.
G. Rehm. 2006. Musings on Iron Defiency Chlorosis (IDC) http://www.extension.umn.edu/cropenews/2006/06MNCN34.htm

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