Soil Amendment Calculator

Lime, sulfur, gypsum, and nutrient amendments. Move your soil pH and nutrient levels in a controlled, organic-compliant way.

pH adjustment (lime / sulfur)

Current pH

Target pH

Soil texture Sandy / sandy loam Loam / silt loam Clay / clay loam

Area unit per 1000 ft² per acre per hectare (10,000 m²)

Direction — —

Recommended amendment — OMRI-listed where indicated

Rate — per chosen area

Application timing — Lime takes months; sulfur weeks

Nutrient amendments

Common organic amendments for outdoor soil — what each provides and typical rates.

Amendment	N	P₂O₅	K₂O	Rate (per acre)	Notes
Feather meal	12-15%	—	—	500-1500 lb	Slow-release N; OMRI
Blood meal	12-13%	1-2%	—	200-500 lb	Fast-release N; OMRI
Fish meal	9-10%	5-7%	1%	500-1000 lb	Balanced; OMRI
Alfalfa meal	2-3%	1%	2-3%	800-2000 lb	Slow-release; growth hormones (triacontanol)
Soybean meal	7%	2%	2%	500-1500 lb	Slow-release; non-GMO required for organic
Cottonseed meal	6-7%	2-3%	1-2%	500-1500 lb	Slightly acidifying; pesticide-free source required
Bone meal (steamed)	3-4%	15-23%	—	200-1000 lb	Slow-release P + Ca
Rock phosphate (soft)	—	3% available	—	1000-2000 lb	Very slow-release; mined; OMRI
Sul-po-mag (langbeinite)	—	—	22%	200-500 lb	K + Mg + S; mined; OMRI
Greensand	—	—	5-7%	500-2000 lb	Slow-release K + iron + trace; mined
Kelp meal	1%	—	2-3%	200-500 lb	Trace minerals + plant hormones; OMRI
Compost (typical)	1-2%	0.5-1%	1-1.5%	2-4 tons	Slow-release; OM-building; allowed if compliant
Vermicompost	1.5-2%	1%	1%	1-2 tons	OM + microbes; OMRI; concentrated benefit
Worm castings	1-2%	1%	1%	1-2 tons	Same as vermicompost

Calcium / Magnesium amendments

Amendment	Ca	Mg	Effect on pH	Notes
Calcitic lime (CaCO₃)	38%	trace	Raises pH	Standard pH-raising lime; OMRI
Dolomitic lime (CaMg(CO₃)₂)	22%	12%	Raises pH	Adds Mg too; useful for low-Mg soils
Gypsum (CaSO₄·2H₂O)	23%	—	Neutral (does NOT raise pH)	Ca + S without pH change; mined; OMRI
Sulfate of potash magnesia (sul-po-mag)	—	11%	Neutral	K + Mg + S; primary K source for organic
Magnesium sulfate (Epsom)	—	10%	Neutral	Quick Mg + S; foliar use too
Magnesium oxide	—	~50%	Slight raise	Concentrated Mg; uncommon

How pH amendment math works

Soil pH adjustment depends on three things:

1. Direction — pH up (lime) or pH down (sulfur)
2. Magnitude — how many pH points to move
3. Buffer capacity — soil's resistance to change, driven by texture and organic matter

Lime requirement (lb/acre to raise pH by 1 point):

• Sandy soil: ~1500-2000 lb
• Loam: ~3000-5000 lb
• Clay: ~5000-7000 lb

Elemental sulfur requirement (lb/acre to lower pH by 1 point):

• Sandy soil: ~600-1000 lb
• Loam: ~1500-2500 lb
• Clay: ~3000-4500 lb

These are rough rules. A proper soil test reports a "lime index" or "buffer pH" that gives a specific recommendation tuned to your actual soil. For accurate amendments at scale, get a soil test.

Application timing matters

Amendment	Time to take effect	Best application timing
Calcitic / dolomitic lime	3-12 months	Apply in fall for next-spring readiness
Hydrated lime (Ca(OH)₂)	Days-weeks	Faster than ag lime but harsher; not OMRI
Elemental sulfur	3-9 months	Apply in fall; sulfur-oxidizing bacteria need time
Aluminum sulfate	Days-weeks	Faster but adds Al; not organic
Gypsum	Days-weeks	Surface-applied; rain works it in; doesn't change pH
Slow-release N (feather meal, alfalfa)	Weeks-months	Pre-plant or as side-dress
Quick-release N (blood meal)	Weeks	Pre-plant or growth-phase boost
Compost	Months-years (cumulative)	Annual application builds OM over time

Outdoor vs container

This calculator is geared toward outdoor / field / raised-bed soil amendments where you're moving the existing soil's chemistry. For container growing (engineered substrates like ProMix, coco, peat blends), the approach is different — you typically build the substrate to a target profile rather than amending existing soil.

For container amendments, see the Nutrient Calculator which handles fertigation-level chemistry, and the Pot Size Calculator for container sizing.

Free under CC BY 4.0. Cite as "OAT Amendment Calculator (openagriculturetechnology.com)". Rates from extension publications and standard agronomy references.