Organic Soil Amendment for High pH, High Mg, Low K Raised Beds

Interpreting a Challenging Soil Test

Raised beds show: pH 7.8, very high Mg (1226 lbs/acre), very high Zn (11.6), high P (113), low K (161), low salts. Extension recommended synthetic ammonium nitrate + muriate of potash – not organic.

Organic Correction Strategy

1. Lower pH (Alkaline Soil)

• Elemental sulfur (soil sulfur) – 1–2 lbs/100 sq ft, incorporate, retest in 3 months.
• Peat moss – acidic, improves texture; 2–3" layer worked in.
• Non-cedar sawdust – acidic, slow breakdown; compost first if possible.
• Gypsum – adds Ca without raising pH; helps displace Mg.

2. Raise Potassium (Low K)

• Greensand (glauconite) – 5–10 lbs/100 sq ft; slow-release K + trace minerals.
• Kelp meal – 1–2 lbs/100 sq ft; K + micronutrients + growth hormones.
• Wood ash – only if pH < 7; yours is 7.8 – avoid.
• Composted banana peels – minor K source.

3. Address Excess Magnesium

• High Mg tightens soil (clay dispersion), reduces K uptake.
• Gypsum (calcium sulfate) helps: Ca displaces Mg on exchange sites; Mg leaches out.
• Apply 10–20 lbs/100 sq ft gypsum, water in.

4. High Zinc & Phosphorus

• Stop adding manure/compost high in P/Zn (e.g., poultry manure).
• Grow P-accumulating crops (buckwheat, mustard) and remove biomass.
• Time and crop removal will slowly draw down excess.

General Organic Soil Building

• Annual compost (1–2") from diverse plant materials.
• Cover crops (winter rye, hairy vetch) – build OM, fix N, scavenge nutrients.
• Mulch with straw/leaves – feeds soil life.
• Avant Gardening Soil Building & Composting Pages – full guides.

Bottom Line

Sulfur + gypsum + greensand/kelp = organic fix for high pH, high Mg, low K. Retest in 6 months. Build soil life with diverse compost and cover crops.