- Author: Rachael Freeman Long
Plant tissue testing is the only way to confirm a molybdenum deficiency. Collect samples from the top 6-inches or one-third of a plant, or from whole plant samples collected from baled hay. Plant samples with less than 0.3 ppm are considered deficient, 0.4 to 1.0 ppm marginal, 1 to 5 ppm adequate, and 5 to 10 ppm high. Concentrations over 10 ppm may be toxic to livestock. High moly concentrations in alfalfa should be offset with copper concentrations that are twice as high as molybdenum concentrations to prevent livestock toxicity.
The most common moly fertilizer is sodium molybdate (40% molybdenum), but ammonium molybdate can be used as well. Follow the label carefully and apply during winter or before re-growth has resumed after cutting. Broadcast on the soil surface only and avoid application to any plant foliage. A single application of 0.4 pounds per acre of molybdenum should last from 5 to 15 years. Thorough records of molybdenum application times and amounts along with repeated tissue testing are essential to determine when to apply or reapply this nutrient.
Do not apply excessive molybdenum (that is double or triple coverage with the sprayer at the end of the field) because the concentration of the element in alfalfa may become so high that the forage becomes toxic to livestock. For the same reason, do not apply molybdenum directly on foliage. Analyzing the top one-third of the plant for both copper and molybdenum can detect deficiencies and suboptimum ratios of these elements in forages.
Deficiency often occurs on slightly acid to very acid soils. Increasing the soil pH increases solubility and availability of soil molybdenum.
I am very interested in Cu - Mo - Co Plant analysis levels.
From Alfalfa Hay samples analyzed for essential elements by Midwest Labs, Omaha, Nebraska.
Avg Cu is 7 ppm CNO 9.6 Min CNR 7.7 a little lower than the low desired.
Mo 0.95 ppm on the low side.
Co 0.20 ppm I have no feel for desired level or range for cobalt.
K 1.97% ok Critical Nutrient Concentration 1.90%
Mg 0.24% a little low but ok
S 0.24% a little lower than 0.26% but ok and a 7.6/1 N/S
The soils are calcareous & a high shrink Vertisal Clay.
What can you tell me about the cobalt?
Every 10 to 20 years this soil needs some copper sulfate and some Moly.
I have looked at cobalt because a Mo Co analysis is not much more expensive than Mo alone. Just need a sample three times as large so they can find some cobalt.
Bill Wilson
Texas Hay Grower & CAFA member