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Scouting Alfalfa Fields for Nutrient Deficiencies

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By Daniel Kaiser
Extension Nutrient Management Specialist

The dry conditions in March and April have given way to extremely wet areas in some parts of Minnesota.  Since alfalfa stands got an early start this year there were a few concerns popping up early in the southeastern part of the state on areas of fields yellowing.  While there may have been some effects due to the cool weather in April a couple of nutrient could be of concern.
Sulfur - Research over the past ten years has started to increasingly identify areas where sulfur deficiencies are becoming more common.  Much of our work in Minnesota has tied these aS def alfalfa.gifreas to 1) sandy soils where sulfur tends to leach rapidly; or 2), soils with low organic matter contents that have a limited capacity to mineralize sulfur.  Warm and moist soils are ideal to mineralize sulfur.  The dry soil conditions in March would not have been ideal and some deficiencies may have been expected.  Typical sulfur deficiencies will occur as a yellowing of the upper leaves of the plant and stands will tend to be thin.  Sandy soils with low organic matter or eroded knolls in the fields will be the most susceptible areas of fields.

Potassium - Dry soil conditions can also limit potassium uptake.  In addition, alfalfa has a higher need for potassium then some other crops and on many soils where alfalfa is grown K K def alfalfa.gifcan be limited.  Potassium is considered non-mobile in the soil but some movement is possible for soils that are sandy which have a limited capacity to hold K.  However, deficiencies can occur on many different soil types.  Deficiencies occur on lower plant leaves and show up as yellow spots towards the edge of the leaf.  Remember this when scouting to differentiate between sulfur and potassium.  Potassium is a mobile element so deficiencies will appear on lower leaves while sulfur will show up on new leaves.  Soil testing is the best way to determine rates to apply.  Rates should be applied in early spring but can be split applied early spring and after the first cutting if high rates are used or in high management situations.

Boron - Deficiencies of this nutrient are more likely to show up under dry soils since this nutrient is made available as organic matter is mineralized.  Traditionally soils in the B def alfalfa.gifeast-central and northeast region are where boron deficiencies have occurred.  Tell-tale signs of deficiencies are chlorotic and stunted plants in very sandy areas of fields low in organic matter.  A soil test can be taken to determine if boron needs to be applied.  Care should be taken with application rates since over-application can lead to poor germination of newly seeded fields or stand damage in established fields.  

The likely culprit for any deficiencies in early 2012 in the southeast is probably sulfur.  Keep an eye on stands prior to the second or third cutting to see if any symptoms remain.  Some of the deficiencies may have lessened due to mineralization after rainfall events this spring.  Plant analysis can be used to help identify the problem.  It is recommended to sample the top 6" from at least 10 plants during early bloom.  Current research in Iowa has found that plant concentrations above 0.22 to 0.25% S should be adequate for maximum yield potential.  To aid  in sampling make sure to same at least 3 areas of the field ranging from good to poor growth to be able to compare tissue results.  

If any spots still remain in the field topdressing a sulfate-sulfur fertilizer source such as gypsum, ammonium sulfate, or potassium sulfate can have an impact on yields of future cuttings.  Elemental sulfur is a poor choice if a field is deficient since it takes time to become available.  Fertilizer guidelines for alfalfa were changed over the winter to include suggested application of sulfur on field areas with 3.0% or less soil organic matter in the top 6-8".  In these cases 10-15 lbs of S per acre is suggested annually.  You can find more information on the current fertilizer suggestions at:

Fertilizing Alfalfa in Minnesota

http://www.extension.umn.edu/nutrient-management/Docs/FO-3814-B-1.pdf

Photograph Sources
J. Sawyer, Iowa State University Extension.  Photos can be found at:
http://extension.agron.iastate.edu/soybean/documents/nutrientdeficiency.pdf
http://www.agronext.iastate.edu/soilfertility/info/SulfurEmerges_Alfalfa_IPNIBetterCropsArticle.pdf


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