Too Much Horse Manure & Grape Compost: Stunted Vegetables & Where to Test Soil

Heavy application of fresh horse manure and grape compost last winter left San Jose vegetables stunted and yellow. Likely causes: nitrogen burn, salt toxicity, or nematodes from uncomposted manure.

Why Fresh Manure Damages Crops

• High soluble salts burn roots.
• Ammonia from anaerobic decomposition toxic to plants.
• Pathogens and nematodes survive without proper composting (150°F).
• Weed seeds galore.

Immediate Steps

1. Soil test NOW — diagnose nutrient imbalance, salinity, nematodes.
2. Recommended CA labs:
   • Timberleaf Soil Testing (Murrieta) — small farms
   • Control Laboratories (Watsonville) — ag-focused
   • Soil and Plant Lab (Santa Clara) — includes bioassays
   • Wallace Labs (El Segundo) — compost experts
   • Soil Foodweb (Oregon) — premium microbial analysis
3. If nematodes confirmed: identify species, solarize, rotate, use resistant varieties.

Prevention

Only use fully composted, pasteurized manure. Age 6+ months. Test soil annually.