Soil Carbon Amendment FAQ
Citation: USDA Natural Resources Conservation Service (2026). Planning CPS 336 – Soil Carbon Amendment: Frequently Asked Questions. National edition, April 2026. https://nrcs-prod.azureedge.us/sites/default/files/2026-04/soil-carbon-amendment-ac-336-faq.pdf
What it is: NRCS’s question-and-answer guide for planners using the Soil Carbon Amendment practice (CPS 336): choosing biochar and compost, reading spec sheets, setting rates, and contracting payment scenarios. It’s the most-cited document in the vault’s farm notes.
Key figures
| Figure | Value | Where | Check |
|---|---|---|---|
| Starting application rate | 4–10 cubic yards per acre in general | FAQ PDF, rates | ✓ full text |
| FY2024 payment scenarios, per acre | 100% biochar 4 yd³; 80/20 biochar/compost 3.2 yd³ + 1.1 t; 60/40 2.4 yd³ + 2.2 t; 40/60 1.6 yd³ + 3.3 t; 20/80 0.8 yd³ + 4.4 t; compost off-site 3 t | FAQ PDF, Table 1, p. 13 | ✓ full text |
| Weight of biochar | Dry weight of 1 yd³ is roughly 135–540 lb | FAQ PDF | ✓ full text |
| Persistence vs compost | Roughly 60–80% of biochar carbon remains after 100 years, vs 2–14% of compost carbon (the FAQ’s summary of research) | FAQ PDF, biochar vs compost | ✓ full text |
| Soil-carbon equivalence | Soil carbon gain from 10 yd³ of biochar equaled that from 40 yd³ of compost (research the FAQ cites) | FAQ PDF, biochar vs compost | ✓ full text |
| Yield risk from biochar alone | Can reduce crop yields by binding and immobilizing nutrients | FAQ PDF, biochar vs compost | ✓ full text |
| Liming example | 1 t/acre of biochar at 9% CaCO₃ equivalency cuts a 1-ton lime need by 9%, to 0.91 t; use caution on soils above pH 7 | FAQ PDF, pH | ✓ full text |
| Salts | Biochar can intensify salinization, especially at high rates; look for spec-sheet EC under 8; manure-based biochar is higher in salts | FAQ PDF, salinity and spec sheets | ✓ full text |
| Spec-sheet targets | Organic carbon above 85% ideal; low ash; low EC; low liming equivalency unless liming is wanted | FAQ PDF, spec sheets | ✓ full text |
| Poultry-manure biochar example | Extremely high in salts and nutrients; may suit very acidic, low-nutrient, high-rainfall soils; not neutral-to-basic soils with high P or salts | FAQ PDF, spec-sheet examples | ✓ full text |
| Certification | An IBI certificate alone isn’t enough; planners still need the spec sheet. A spec sheet ≤2 years old is probably fine if feedstock and temperature haven’t changed | FAQ PDF, selecting biochar | ✓ full text |
| Excluded materials | Biochar or compost from biosolids or municipal waste (PFAS concerns); humic and fulvic substances aren’t carbon amendments but can serve as inoculants; residues that would normally stay on the land can’t be used as feedstock | FAQ PDF | ✓ full text |
| Manure as inoculant | Contract the 100% biochar scenario; the producer mixes in manure at their own expense under a nutrient management plan | FAQ PDF, contracting | ✓ full text |
| Where it can be used | Pasture, yes; native rangeland only when facilitating Range Planting (550) or Critical Area Planting (342) | FAQ PDF, contracting | ✓ full text |
| Contract terms | Practice lifespan 1 year; can be contracted up to 5 times over a contract | FAQ PDF, contracting | ✓ full text |
| Timing | Any time the ground isn’t frozen; most commonly early spring, or after harvest with a cover crop | FAQ PDF, application | ✓ full text |
Use with care
- The persistence and soil-carbon figures are NRCS’s summary of research, not NRCS measurements. For permanence numbers use models soil GHG from biochar or estimates soil C stock change from biochar.
- Payment scenarios are FY2024 values and can change each fiscal year.
- A text search of the FAQ found no mention of carbon credits. Claims that the FAQ calls CPS 336 “not a carbon credit” need a different source.
Related: defines NRCS practice 336 · NRCS soil carbon amendment in US ag · Application Rates and Methods · Key Sources