Compost for gardens

Asked October 2, 2020, 7:49 PM EDT

I have used manure compost for our garden but find I get too much weeds. I've used some mint compost on flowers with success and no weeds. Can you tell me how mint compost compares to manure compost for our garden? Likewise for our blackberry bush?

Benton County Oregon

1 Response

Yes, composted manures can be quite weedy, and the weed seed can come from one of at least 2 sources:

  • The digestive tract of the animal source of the manure does not completely kill off the weed seed.
  • In my experience the more likely source is how the fresh manure or the compost was stored. If left uncovered, either is subject to host weed seeds blowing in from elsewhere.

Of course, it depends on exactly which type of animal manure is referred to, but composts of animal manures tend to have nitrogen (N) and potassium (K) running 1-4% and 1-2%, respectively. Composts of plant waste origin run somewhat less, about 1-2% for N and K. In the case of phosphorus (P), manure sources are usually about 1-3% P, and composted plant wastes are much less – like 1/5 to ½ of 1%

Mint composts, specifically, run a bit higher in N and K: 3-4% N, <1% P, 3-4% K. Remember that mint is grown as a crop, and so has more care paid to the fertilizers supplied.

Generally composted plant wastes begin at a pH lower than neutral (acidic) and approaches neutral (pH 7) as the composting process approaches completion. With composts of animal origin (manures, meats, blood, etc.) the process is just the opposite – they begin at pH above neutral (alkaline) and approach neutral as the compost process approaches completion. In either case, fully composted materials would tend, if used routinely for years, have a neutralizing effect on garden soils. It keeps them near pH 7.

October is too late to apply manure to the garden or landscape. Nitrogen is very soluble in water, and would be lost by mid-spring as a result of winter rains.