Soil Fertility

Originally written February 1, 2006 | Last updated August 21, 2014

Yield Goal v. Yield Potential

Illinois, Iowa, Minnesota, and Wisconsin : estimated productivity of field's predominant soil type - adjusted using N credits and organic matter

Numerous states: yield-goal approach - 1.2 pounds of N per bushel for corn following corn, with credits given when corn follows a legume or where manure is applied

Nebraska and Colorado: expected yield approach - based on expected yield, the amount of residual soil nitrate-nitrogen, and soil organic matter.

Indiana, Ohio, and Michigan: yield potential and previous crop

Corn yields in the U.S. increased substantially between 1950 and 1970 for several reasons

  • Increased use of fertilizer, particularly N
  • Better hybrids
  • Improved technology and management
  • Earlier planting
  • Better equipment
  • Higher plant populations, etc

Fertilizer applications should be balanced according to SOIL FERTILITY levels.

Applications are influenced by:

  • Grain Harvest versus Silage Harvest
  • SOIL pH (6.0-7.0 is desirable) pH affects nutrient availability
    1. Intensive farming gradually lowers soil pH, so soils in many parts of the Midwest are limed every 4-5 years
    2. Liming improves soil microbial activity, which
    3. Increases nutrient availability
    4. Reduces plant diseases
    5. Aids in decomposition of crop residues
    6. Reduces herbicide carryover problems
    7. Liming also adds Ca and Mg

Levels of fertilizer elements (N, P, and K) needed for optimum yields


  1. 160-190 lbs available N/a
    • Amount of N mineralized from OM in the soil:
    • (% OM) (20) = lbs N mineralized/a/yr
    • EX: (3.0% OM) (20) = 60 lbs N mineralized/a/yr
  2. 60-80 lbs P2O5/A
  3. 280-300 lbs K2O/A
Crop Nutrient Removal
Crop Quantity N P2O5 K2O   Yield N P2O5 K2O
Corn grain 100 bu 90 36 26   150 bu/A 135 54 39
Corn silage 1 ton (~65% moisture) 7 24 7   20 T/A 140 48 140
Soybeans 100 bu 355 82 100   50 bu/A 178 41 50
Wheat 100 bu 125 53 31   70 bu/A 88 37 22
Alfalfa 1 ton (dry hay) 50 11 45   5 T/A 250 55 225
adapted from "The Fertilizer Handbook", The Fertilizer Institute, 1982
derived from Hoeft et al., 2000

Soil Testing

Soil tests: Field sampling of soil (5 acres/composite sample)

Tissue tests: Measures N, P and K in cell sap

Apply MANURES: Manure is often spread at a rate of 5-10 tons/a

Commercial Fertilizers


Forms of N fertilizer

  • GAS
  • SOLID (Granular)

GAS - Anhydrous Ammonia (82+% N)


  • High analysis of N (82%)
  • Doesn't leach (NH3 form)
  • Traditionally the least expensive type


  • Difficult to apply
  • Must be knifed 6-8" into the soil
  • Soil conditions should be ideal
  • Shouldn't work the soil for 5-7 days after applying anhydrous
  • Anhydrous requires expensive equipment
  • It is stored under pressure as a LIQUID and VAPORIZES when pressure is released, so it is a GAS when it is deposited in the soil
  • Anhydrous is dangerous. Can burn skin critically and cause blindness. When applying, a person should wear goggles and gloves and cover all skin
  • Do not apply on SANDY soil

LIQUID N - Nitrogen solution (28-0-0 or 10-34-0)

Not as dangerous as anhydrous

Is incorporated, either as preplant or as starter, but not incorporated as deep as anhydrous

Is expensive, so use in Wisconsin has been somewhat localized

Useful in reduced tillage systems, as liquid starter can be placed closer to the seed than granular - important when planting early in heavy, cold soil

DRY (Granular)


  • NO3 form leaches readily
  • Easy to handle, but difficult to store because it absorbs moisture and hardens
  • Requires considerable energy to produce, is expensive, and has dropped in popularity in the 1980's

UREA CO(NH2)2 46% N

  • N is in the ammonia form, so it doesn't leach as readily as ammonium nitrate, but can VOLATILIZE (should incorporate or be rained in within 1-2 days)
  • Is blended with other grades of fertilizer
  • Many areas have switched from ammonium nitrate to urea
  • Urea has a higher analysis of N
  • Urea is not as dusty
  • Urea is easier to store-doesn't absorb much moisture

Dry fertilizer at Coop's

  • Sold in bag or bulk form
  • Broadcast and incorporated or banded at planting
  • Most coop's keep four grades of dry fertilizer on hand
    • UREA: 46-0-0
    • POTASH: 0-0-60
    • Triple Super Phosphate (TSP): 0-46-0
    • Diammonium Phosphate (DAP): 18-46-0
  • These are mixed in various formulations eg. 9-23-30 is made by mixing equal amounts of 18-46-0 and 0-0-60


Combinations of liquid and granular

A certain amount of solid (undissolved) material is suspended in a liquid medium


  • Suspensions can hold a higher analysis of N, P, and K than liquid fertilizer
  • (Liquid fertilizer can salt out fairly easily)
  • Almost any level of micronutrients can be added to suspensions



Fall plowdown

  • Traditionally, many CORN BELT farmers who apply large amounts of N for corn (150-200 lbs/a) apply ½ to 2/3 of it in the fall
  • N should be in the NH3 form, so anhydrous or granular forms are used
  • Best to apply when soil temp at 2-4" depth is 50 F or cooler
  • NH3 will not convert to nitrate until the soil warms above 50 F the following year
  • NITRIFICATION INHIBITORS used if soils have high N loss potential
  • SANDY SOILS: Fall application of N should be avoided

Advantages of fall application

  • Availability of equipment is usually good
  • Avoids ‘spring rush', which can delay planting if the following spring is wet
  • Anhydrous may be cheaper in the fall


  • Some N may be lost (Groundwater)
  • Although anhydrous may be cheaper, paying 6 months interest can make it more expensive

Spring pre-plant incorporated

  • As anhydrous, liquid, granular, or suspension
  • In Wisconsin, most fertilizer is applied at this time. Normally there is plenty of time between thawing and 50o soil temp's. This may not be true in areas of the corn belt that warm more rapidly in the spring.


  • Usually little response to starter fertilizer in Wisconsin, even in fertile soils
    • exception is full season hybrids planted late
    • most WI fields have had high manure rates
  • Growers who use starter usually apply 150 to 300 lbs/a
  • Placement near SEMINAL root system is necessary because plants may have difficulty absorbing nutrients from the soil early in the season
  • Seminal root system is small
  • Soil and air temp's usually are cool
  • Soil may be too moist for rapid root growth
  • Not as popular in areas of the corn belt that warm more rapidly in the spring and where farms are larger in size


Usually have a better response to starter fertilizer as the number of tillage operations is reduced

  • Especially true in heavy soils
  • May not be true in lighter soils

Response to starter fertilizer usually decreases as fertility level increases

Sidedressing or topdressing

  • Sidedressing of anhydrous is usually done 2-4 weeks after emergence
  • Sidedressing was decreasing in the Midwest until the mid-1980's. When interest rates are high and farmers use lower rates of N to be more cost efficient, sidedressing increases in popularity


N-cycle Ammonification 
and Nitrification
Nitrification Denitrification

Form of N

  • NH3 - many possibilities
  • NO3 - during or after planting
  • Soil type
    • Heavy versus light
    • Soil type and soil temp are closely associated
    • Heavier soils warm more slowly, so conversion to NO3 occurs more slowly

Amount of N applied

  • If applying low rates of N, time of application can have a substantial effect on yield response.
  • If applying high rates of N, time of application usually does not influence yield response

Soil fertility level

  • Soils that are low to medium in fertility may have a significant yield response when starter and/or sidedressing are used


Inhibitors - make timing of N application less critical

  • Nitrification inhibitors reduce N losses because nitrification (conversion of NH3 to NO3) is delayed
  • Activity of Nitrosomonas bacteria is suppressed
  • Most often used with anhydrous, but can also be used with liquid or solid forms of N
  • Most often used with fall-applied anhydrous
  • Results with nitrification inhibitors have been somewhat inconsistent
  • Usually more effective in wet years than in dry years
  • Response has been consistent when used on irrigated sandy soils

Slow release compounds

  • Supposed to improve N efficiency by reducing losses from LEACHING and DENITRIFICATION


  • Loss of oxygen and conversion to N2 gas
  • Occurs in water-logged soils; can lose 10 lbs N/a/day


Is extremely immobile in the soil

Frequently bound up and unavailable

Mineralization is slow: Only 10-20% of the P2O5 applied is used by plants in the year of application

Virtually all starter fertilizer that is applied to corn has some in it

Purpling of plants

Corn grown in wet, heavy soils that test high in P may have temporary P deficiency symptoms until soils warm enough for some mineralization to occur. Symptoms usually disappear by early June.


Soils vary in their ability to supply K depending on area of origin and rainfall

Prairie soils - tend to be high in K

Forest soils - tend to be low in K

Normally applied as KCL (= muriate of potash or potash) and K2SO4

Nutrient Management

  • legume crediting

  • manure crediting

  • fertilizer selection

  • organic nutrient amendments

  • application timing

  • application method

  • equipment maintenance and calibration

Soil and Water Management and Conservation

  • irrigation scheduling

  • irrigation systems

  • irrigation maintenance and calibration

  • pesticide selection

  • riparian zones/buffer strips "bio-curtains"

  • soil pH

  • soil quality

  • tillage systems

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


  • 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.

Further Reading

To purchase hard copies of these publications, go to The UWEX Learning Store

Management of Wisconsin Soils UWEX Bulletin A3588

Wisconsin's Pre-Sidedress Soil Nitrate Test UWEX Bulletin A3630

Corn Fertilization UWEX Bulletin A3340

Optimum Soil Test Levels for Wisconsin UWEX Bulletin A3030

Sampling Soils for Testing UWEX Bulletin A2100

Aglime--Key to Increased Yield and Profits UWEX Bulletin A2240

Credit Legume Nitrogen and Reap the Profits  UWEX Bulletin A3591

Credit Nitrogen on Corn and Reap the Profits  UWEX Bulletin A3589

Credit What You Spread and Reap the Profits  UWEX Bulletin A3580

Guidelines for Applying Manure to Crops and Pastures  UWEX Bulletin A3392

Choosing Between Liming Materials UWEX Bulletin A3671

Nitrogen Management on Sandy Soils UWEX Bulletin A3634

Soil Calcium to Magnesium Ratios--Should You be Concerned? UWEX Bulletin A2986

Soil and Applied Boron UWEX Bulletin A2522

Soil and Applied Calcium UWEX Bulletin A2523

Soil and Applied Chlorine UWEX Bulletin A3556

Soil and Applied Copper UWEX Bulletin A2527

Soil and Applied Iron UWEX Bulletin A3554

Soil and Applied Magnesium UWEX Bulletin A2524

Soil and Applied Manganese UWEX Bulletin A2526

Soil and Applied Molybdenum UWEX Bulletin A3555

Soil and Applied Nitrogen UWEX Bulletin A2519

Soil and Applied Phosphorus UWEX Bulletin A2520

Soil and Applied Potassium UWEX Bulletin A2521

Soil and Applied Sulfur UWEX Bulletin A2525

Soil and Applied Zinc UWEX Bulletin A2528

Using Industrial Wood Ash as a Soil Amendment UWEX Bulletin A3635

When and How to Apply Aglime UWEX Bulletin A2458

 Estimating residue using the line-transect method UWEX Bulletin A3533

Wisconsin Integrated Crop Management Manual: Intro | Alfalfa | Corn | Soybean | Wheat | Nutrients | Weeds

Know How Much You Haul (Manure)

Management Options for Farms with High Soil Test P

Nitrogen Guidelines for Corn In Wisconsin

Nutrient Management Fast Facts

Nutrient Management Practices for Corn Production  UWEX Bulletin A3557

Phosphorus Cycle - Poster

Soil Testing Basics

Using Legumes as a Nitrogen Source UWEX Bulletin A3517

What is a Farm Nutrient Management Plan?

When and Where to Apply Manure

Wisconsin's Preplant Soil Nitrate Test  UWEX Bulletin A3512

Wisconsin's Pre-sidedress Soil Nitrate Test (PSNT)

Non-Conventional Soil Ammendments (NCR-103)

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