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 "Open Agriculture Technology Amendment Calculator (openagriculturetechnology.com)". Rates from extension publications and standard agronomy references.