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Is cow manure good for onions?

Using cow manure as a fertilizer for onions can provide many benefits. Cow manure contains nutrients that onions need to grow, like nitrogen, phosphorus, and potassium. The organic matter in cow manure also helps improve soil health. However, there are some important factors to consider when using cow manure for onions.

Nutrient content of cow manure

Cow manure contains macronutrients that are essential for onion growth:

  • Nitrogen (N) – Onions need moderate levels of nitrogen. Too much can cause excessive foliage growth. Cow manure contains 1.5-3.5% nitrogen.
  • Phosphorus (P) – Important for root growth and bulb formation. Cow manure contains 0.5-1% phosphorus.
  • Potassium (K) – Critical for plant metabolism and improving yield. Cow manure has 1-2% potassium.

Cow manure also provides other nutrients like calcium, magnesium, sulfur, and micronutrients like iron, manganese, zinc, copper, and boron.

Improving soil health

The organic matter in cow manure can improve the physical, chemical, and biological properties of soil:

  • Adds organic matter and improves soil structure and water retention.
  • Supplies beneficial microorganisms that promote soil health.
  • Increases the soil’s cation exchange capacity (ability to hold nutrients).
  • Buffers soil pH.

When to apply cow manure

Ideally, cow manure should be applied in fall before planting onions. This gives time for nutrients to become available and any weed seeds in the manure to decompose before spring planting. Avoid applying fresh manure right before planting.

Application rates

Recommended application rates for cow manure as fertilizer:

  • 5-10 tons per acre for soil building and moderate fertility needs.
  • Up to 20 tons per acre to correct severe fertility deficiencies.

Excessive amounts of manure can cause soil salt buildup and heavy metal contamination. Test soil and manure nutrients to determine proper rates.

Using composted manure

Composted cow manure may be a better choice than fresh manure for onions. Benefits include:

  • More stable organic matter and nutrients.
  • Lower risk of burning plants or causing rapid, excessive growth.
  • Fewer viable weed seeds.
  • Less odor.

Well-finished compost can be applied at planting or as a side dressing. Follow manufacturer rates if buying commercial cow manure compost.

Fresh or aged manure

Fresh cow manure can burn plants due to high ammonia levels. Aged manure (over 6-12 months) is lower in ammonia and safer to use. Ideally, incorporate fresh manure into soil well before planting onions.

Proper application

Steps for safe, effective application of cow manure for onions:

  1. Test manure and soil nutrients to determine appropriate rates.
  2. Apply aged or composted manure before planting if possible.
  3. Avoid direct contact between manure and plant roots or foliage.
  4. Till or work manure deeply into soil to prevent runoff and nitrogen loss.
  5. Supplement with other organic or mineral fertilizers as needed.

Potential concerns

Some potential issues with using cow manure for onions:

  • Weed seeds – can lead to weed problems. Composting kills most seeds.
  • Odors – fresh manure has an odor that can persist in onions.
  • Food safety – follow good agricultural practices and suggested wait times between application and harvest.
  • Salt content – excessive amounts can damage soil structure and inhibit plants.
  • Heavy metals – may accumulate if manure is applied in very high quantities for many years.

Using other organic fertilizers

While cow manure can provide excellent nutrition for onions, it may be beneficial to combine it with other organic soil amendments. Some options include:

  • Compost – improves soil texture and nutrients. Can be mixed with manure.
  • Cover crops – legumes add nitrogen. Non-legumes improve organic matter.
  • Bone meal – provides phosphorus.
  • Potassium sulfate – supplies potassium.
  • Kelp meal – adds micronutrients.
  • Rock phosphate – long-term phosphorus source.

A blended approach with manure, compost, and other amendments can provide balanced nutrition for optimal onion growth.

Using inorganic/mineral fertilizers

While cow manure supplies many key nutrients for onions, it is lower in some nutrients compared to mineral fertilizers. Combining manure with small amounts of inorganic fertilizers may help maximize onion growth and yields. Some options include:

  • Urea – supplemental nitrogen source.
  • Ammonium sulfate – nitrogen and sulfur.
  • Superphosphate – concentrated phosphorus.
  • Potassium chloride (muriate of potash) – potassium only.
  • Calcium nitrate – nitrogen and calcium.

Always follow soil test recommendations for fertilizer rates. Excess nitrogen from mineral fertilizers can reduce onion bulb size and cause issues like increased disease susceptibility.

Fertilizing onions throughout the growing season

Onions require different levels of nutrients at various growth stages. While cow manure provides a good preplant nutrient charge, additional fertility may be needed:

  • 4-6 weeks after planting – Side dress with nitrogen (30-50 lbs N per acre) to support leaf and root growth. Organic options like blood meal, feather meal, or Chilean nitrate can be used.
  • Bulbing stage – Reduce nitrogen to redirect energy into bulb formation. Follow soil test recommendations.
  • 2-3 weeks before harvesting – Potassium application via manure, compost, muriate of potash, or kelp meal helps size bulbs.

Avoid excessive nitrogen rates during bulbing as this causes delayed maturity and weaker bulb skins.

Using cow manure tea

Some gardeners make a liquid fertilizer “tea” by steeping cow manure in water. This provides a quick nutrient boost. However, precautions should be taken when using manure tea on onions:

  • Use aged/composted manure to reduce risk of burning plants.
  • Dilute tea to no more than 10% manure to water ratio.
  • Test nitrogen content and dilute accordingly.
  • Apply early in the day to allow foliage to dry before night.
  • Discontinue use during bulb formation.

Manure tea can provide soluble nutrients, but overapplication risks salt buildup and excessive nitrogen levels for onions. Use in moderation.


Cow manure can be an excellent organic fertilizer source for onions with proper application. It provides essential nutrients, improves soil health, and is readily available in many farming areas. However, care should be taken to properly compost or age fresh manure, follow soil test recommendations for application rates, and combine with other organic and mineral amendments as needed to balance nutrients throughout the onion’s growth stages. Following sustainable practices for using cow manure optimizes benefits while minimizing risks of over-fertilization, odors, weed seeds, and food safety concerns.