Soil Fertility

Last updated on February 23, 2014

Don't apply nitrogen to soybeans. Well nodualted soybeans will fix all the nitrogen they require. It is better to inoculate the seed with Rhizobium bacteria than to apply nitrogen fertilizer.

Nutrient Requirements and Uptake by Plants

Soybean plants (as well as the symbiotic bacteria associated with them) require all of the following nutrient elements:

  • nitrogen (N),
  • phosphorus (P),
  • potassium (K),
  • sulfur (S),
  • calcium (Ca),
  • magnesium (Mg),
  • iron (Fe),
  • boron (B),
  • manganese (Mn),
  • zinc (Zn),
  • copper (Cu), and
  • molybdenum (Mo).

Most nutrients are absorbed from the soil; however part of the N is obtained from bacterial fixation in the nodules and some S is absorbed (primarily as SO2 and H2S) from the air. Soil nutrients are absorbed into the plant roots with water and move up into the plant to the leaves and other vegetative plant parts.

The amounts of nutrients available vary with

  • soil type,
  • soil test,
  • depth of soil, and
  • tillage practices.

They are influenced by soil temperature and moisture conditions.

Roots will not grow into dry soil and moisture must be present for roots to absorb nutrients from the soil. However excess moisture in the soil limits aeration, and roots also require air (oxygen).

The amounts of nutrients taken up by the plants early in the season are relatively small because the plants are small. However the nutrient concentration in individual leaves of well nourished plants are as high during this period as individual leaves later in the season. Uptake and accumulation of some nutrients in the leaves continues throughout the season until maturity; uptake of others is completed by stage.

Redistribution of mineral nutrients from older plant parts to newer growing parts is a primary source for some nutrients. Some nutrient elements are very mobile in the plants and are readily translocated from older to newer plant parts. Redistribution of N, P, and S is a primary source of these nutrients for growth of the beans and results in severe depletion of these elements in the leaves, petioles, stems, and pods during the late seed-filling period. However, some nutrients such as calcium are very immobile in the plants and there is little redistribution of these elements from older to new plant parts. Late season redistribution of mobile materials that have accumulated in leaves and other plant parts without redistribution of Ca results in increased Ca concentration in the leaves late in the season.

Redistribution of other elements in the plant generally is intermediate between the extremes for very mobile N and immobile Ca. P and S are very similar to N. K is redistributed from the vegetative plant parts to the developing seeds, but is not redistributed from the pods. Zn and Cu are redistributed but not to the same degree as N. Mn, Mg, Fe, B, and Mo are relatively immobile but not as immobile as Ca. Very marked differences in mobility of Fe have been observed among different soybean varieties.

Fertilizer Use and Fertility Management

When the soil cannot supply the plant nutrient requirements, fertilizers and/or manure can be added to supplement the nutrient supply. Uptake of nutrients added to soils is not always an efficient process. Under good conditions, the recovery in the year of application ranges from 5 to 20 percent for phosphorus and 30 to 60 percent for potassium. However, additional nutrients are recovered in future years.

Nutrients Most Commonly Deficient:

  1. Nitrogen is fixed and made available to the plants by the bacteria in the nodules on the roots. Where soybeans have not been grown previously, inoculation is needed to supply the desired bacteria. Liming of acidic soils is usually beneficial. By making conditions favorable for N-fixation, the need for N fertilization is reduced or eliminated.
  2. The availability of phosphorus and potassium in many soils is not adequate for optimum yields so fertilizers and/or manure to supply these nutrients should be applied where needed. Depending on soil pH, lime also may be needed.
  3. Applications of some of the other nutrients are desirable on some soils where deficiencies exist. S, Fe, B, Mn, or Zn are the elements that are occasionally deficient.

Foliar fertilization micronutrients: Mo, Fe, Cu, B, Zn, Cl, Mn

Foliar fertilization may be effective, particularly if the soil is low in availability of one or more micronutrients

Effectiveness usually depends on soil pH. Soil pH has a major effect on availability of micronutrients reduced examples: available availability iron (fe) below ph 6.5 above 7.0 other micronutrients molybdenum (mo) above 7.0 below 6.0.

Iron chlorosis (deficiency)

  • widespread in prairie soils of midwest (Iowa, Illinois, Minnesota)
  • frequent in high lime (calcareous) soils, particularly in cool, wet weather and poorly drained soils
  • infected plants are light green to yellow in color
  • plants may recover later in the season, but yields will be reduced
  • best control is resistant varieties
  • foliar application of sequestrene (iron chelate) is usually helpful

Molybdenum

  • the micronutrient that is required in the smallest amounts for plant growth
  • more likely to get a response if ph is < 6 because mo is tied up (unavailable) in acid soils. It differs from the other essential micronutrients in this respect.
  • Symptoms are light green leaves
  • usually applied foliarly when a deficiency occurs
  • molybdenum is difficult to apply with granular fertilizer because it is required in such small amounts that uniform distribution is difficult.


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