Asked May 15, 2019, 9:09 AM EDT

My son gives me horse manure and I add it to my garden every year. It contains about 60-70% clean sawdust and is pretty uniform and fine in texture. Usually in the fall I use a 8’ wide rotor tiller and tractor to go over at least three times to mix real good. This year we mixed the manure into the garden soil in March , planted vegetables in April and harvest thought out the year. My question is- will the vegetables be safe from bacteria and parasites . Is there a general rule on how old horse manure should be before adding to a garden. Rick Thank you.

Howard County Maryland

1 Response

Aged manure is great for improving soil quality and add valuable nutrients to the soil.

How long the manure was composted and how closely it was it managed (turned regularly and measured for pathogen killing temperatures) lessens risk. Manures are considered fully composted when a static, aerated pile reaches at least 131ºF for 3 consecutive days. This kills most plant and human diseases, and weed seeds. Most farmers with animals do not actively compost and monitor their manure to this standard. So, it’s best to treat any animal manure you can locate as un-composted. Your fall application and incorporation is recommended for home gardeners.

The rule for farmers (for fresh manure or manure not actively composted) is 120 days from application to harvest for leafy greens like lettuce, spinach, etc. which could come into contact with the soil. It is 90 days for fruiting vegetables.

Given your timing, there is some increased risk, mainly for leafy greens.

The fact that you incorporated the manure is good however, and you should wash any crops thoroughly before consuming. Come July there should be no issue.

You could consider mulching around plants to reduce the chances of contact between the soil and the crops.

Also, just as an aside, with that much (perhaps fresh) sawdust (carbon) in the mix, keep an eye on Nitrogen needs of plants.