With farm input costs rising and profit margins tighter than ever, peanut growers are looking for every opportunity to cut unnecessary expenses without sacrificing yield or quality. One of the most cost-effective tools available is pegging zone calcium testing. By identifying whether gypsum is truly needed, this test can help you avoid spending money on unneeded applications while still protecting your crop.
This post highlights key recommendations from the fertility section of the UGA Peanut Production Guide and explains how to make the most of your calcium management program.
Why Calcium Is Critical but Often Mismanaged
Peanuts require unusually high levels of calcium, and they absorb it differently than most crops. Once the peanut plant begins pegging, calcium must be absorbed directly by the developing pod from the surrounding soil—not through the plant.
If calcium is lacking in the top 3 to 4 inches of soil (the pegging zone), you can see serious problems at harvest:
- “Pops” (undeveloped kernels)
- Poor seed germination
- Higher aflatoxin contamination risk
- Pod rot and breakdown
The challenging part is that you won’t see signs of calcium deficiency until harvest, when it’s too late to fix it. That’s why taking action early, with the right information, is so important.
Pegging Zone Testing: A Simple Way to Avoid Wasting Money
It is time to begin pulling pegging zone samples. The best window for sampling is between 14 and 45 days after planting (DAP), just before the plants begin early bloom and pod development. Most peanut varieties begin flowering (R1 stage) around 30 DAP, and pegs typically become visible around 40 DAP. Because it takes about 10 days after flowering for pegs to appear and begin growing into the soil, calcium must already be present and available in the pegging zone by that time. Sampling now allows enough time to process test results and make gypsum application decisions before the crop reaches this critical stage of reproductive development.
Many growers base calcium decisions on a standard winter soil test, taken from 6 inches or deeper. However, that won’t tell you what’s happening where pods actually form. That’s why UGA recommends a pegging zone soil test.
How to collect a sample:
- When: Shortly after emergence, before early bloom
- Where: 3 inches deep, slightly offset from the row
- Why: To evaluate calcium levels right where pods will absorb it
To see a demonstration of proper sampling technique, check out this helpful video by former Irwin County ANR Agent Phillip Edwards and former Turner County ANR Agent Guy Hancock: https://fb.watch/A9x6U1gv7F/
How to Know If You Can Skip the Gypsum
For runner and Spanish peanuts, the UGA Peanut Production Guide recommends gypsum only if one or both of the following conditions are met:
- Soil calcium level is less than 500 lb/A
- Calcium-to-potassium (Ca:K) ratio is less than 3:1
If both thresholds are met, you can skip gypsum and save significant money.
Note: This test is valid only if analyzed using the Mehlich 1 extraction method.
Important exceptions:
- Virginia-type peanuts always need gypsum, regardless of soil test
- Seed peanuts always need gypsum to ensure seed quality
How Much Can a 500-Acre Grower Save?
Here’s a side-by-side look at the costs of applying gypsum to 500 acres with vs. without pegging zone testing.
| Scenario | Cost Item | Rate | Total Cost |
|---|---|---|---|
| Without Testing | Gypsum (1,000 lb/A) | $60/acre × 500 acres | $30,000 |
| Spreading | $10/acre × 500 acres | $5,000 | |
| Total Cost (No Testing) | $35,000 | ||
| With Testing | Pegging Zone Tests | 20 samples × $5 | $100 |
| Shipping Estimate | $20 | ||
| Gypsum (only 200 acres needed) | $60/acre × 200 acres | $12,000 | |
| Spreading (200 acres) | $10/acre × 200 acres | $2,000 | |
| Total Cost (With Testing) | $14,300 | ||
| Estimated Savings | $20,880 |
Bottom Line: By investing just $120 in pegging zone soil testing, a 500-acre peanut grower can avoid wasting over $20,000 in unnecessary gypsum applications—while still ensuring crop health on the acres that actually need it.
Also, if you limed the field earlier this year—particularly with calcitic or dolomitic lime that was left in the surface layer—you may already have adequate calcium in the pegging zone. If so, you can often skip gypsum and save on those acres too. However, the only way to know for sure is to pull a pegging zone test and confirm that calcium levels are in good shape.
The good news is that a pegging zone test through the UGA Extension Soil Lab only costs $5 per sample plus shipping. It’s a small investment that protects your budget and your yield.
What to Apply (If Needed)
If your test shows gypsum is required, here’s what to use:
- Product: Gypsum (CaSO₄), also called landplaster
- Rate: 1,000 lb/A supplies approximately 200 lb/A elemental calcium (double that for Virginia peanuts)
- Timing: Apply at early bloom
Common sources include:
- FGD gypsum (“smokestack” or synthetic): A byproduct of coal power plant scrubbers
- Mined gypsum (e.g., USG 500): Contains less moisture and spreads more easily
- Recycled wallboard: Can be used if the paper content is not a concern
All of these sources contain approximately 20 percent calcium.
Moisture Matters: Irrigation and Rescue Options
Because calcium moves with water, availability depends heavily on soil moisture:
- Too dry? Irrigate after gypsum application to help dissolve and move calcium into the soil
- Too wet? Excessive rainfall (4 to 6 inches) within three weeks may leach calcium below the pegging zone
In these situations, a rescue application of 10 gallons per acre of calcium chloride (CaCl₂) through a center pivot around 60 to 75 days after planting may help.
Do not rely on foliar calcium—it is not effective for pod development.
Key Takeaways
- Pegging zone testing is the best way to determine if gypsum is truly needed
- You can save approximately $70 per acre—or over $20,000 on a 500-acre operation
- Apply gypsum only when pegging zone calcium is below 500 lb/A or the Ca:K ratio is below 3:1
- Always apply gypsum to Virginia peanuts and seed peanuts
- Rescue treatments like CaCl₂ through irrigation can help if deficiencies arise later
- A pegging zone test only costs $5 per sample plus shipping through the UGA Extension Soil Lab
Source Materials
- Chapter 4: Soil Fertility – Glen Harris and R. Scott Tubbs
- Chapter 7: Peanut Physiology – C.K. Kvien, C.C. Holbrook, P. Ozias-Akins, C. Pilon, A.K. Culbreath, and T.B. Brenneman
Access the full guide (UGA Peanut Production Field Guide)
