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Tissue & Sap Test Reference.
Tissue & Sap Test Reference
Reading a tissue analysis report. Sufficiency ranges by crop, sampling protocol, sap test vs dry tissue, and what to change when readings are off.
How to take a tissue sample
- Pick the right leaf. Most reference tables specify a particular leaf — typically the "most recent fully expanded leaf" (often the 5th or 6th from the top), or for some crops the petiole at a specific stage. Crop-specific protocols matter.
- Sample 15-25 leaves. One leaf doesn't represent the field. Walk a representative pattern; collect from healthy-looking and suspect plants separately if doing diagnostic.
- Don't wash. Sticky residues on the leaf may include foliar sprays — but washing also removes nutrients you want measured. Air-rinse only if obviously contaminated.
- Refrigerate or air-dry immediately. Sap tests need fresh tissue (within 24 hours). Dry tissue tests want air-dried within 1-2 days at <100°F.
- Submit to the same lab consistently. Different labs use different extraction methods; numbers aren't directly comparable across labs.
- Track over time. A single test is a snapshot. The valuable signal is tissue trends across the season vs the same time last year.
Dry tissue vs sap test
| Dry tissue test | Sap test | |
|---|---|---|
| What it measures | Total nutrients in dry leaf | Mobile/active nutrients in fluid |
| Turnaround | 3-7 days (lab) | Same day (handheld) or 1-3 days (lab) |
| Cost per sample | $30-60 | $20-40 + handheld unit ($300-1500) |
| Best for | Strategic / season-long | Real-time feed adjustment |
| Use case | Major-element planning, deficiency confirmation | "Should I bump the feed today?" |
| Limitations | Slow; whole-leaf number can mask issues | Variable; requires careful sampling; new petiole only |
Common interpretation pitfalls
- "Sufficient" can still mean limiting. Numbers in the sufficiency range don't guarantee the element isn't the limiting factor. Look at ratios (e.g., K:Ca:Mg) and trends, not absolute values.
- Antagonisms hide in single-element readings. High K reduces Ca uptake; high Mg reduces K. A "sufficient" Ca with marginal K can produce blossom-end rot symptoms in tomatoes.
- Mobile vs immobile elements move differently. N/P/K/Mg show in OLD leaves first when deficient (they translocate). Ca/B/Fe/Mn show in NEW leaves first (they don't translocate).
- Compare to the right reference. Cannabis ranges are different from tomato. Check the crop AND the leaf being sampled.
- Time of day matters for sap. Sap N is higher in mid-morning than late afternoon. Sample at the same time across the season.
Free under CC BY 4.0. Cite as "OAT Tissue & Sap Test Reference (openagriculturetechnology.com)". Sufficiency ranges from university extension publications, Reams nutrient management literature, and crop-specific reference works.