I did some tests on the horse manure with lots of wood shavings, that was delivered to our site last year. One pot had a mixture of my soil with the manure and the other pot just had my soil. I sowed some salad mixture seeds in both pots and treated them exactly the same. The pot with the manure mixture only grew plants half the size as the soil on its own.
The reason could be depletion of nitrogen as the wood shavings decayed, and maybe the increase in acidity caused problems.
Either way, I left the manure in my heap and will not put it on the soil till next winter. Other plotters on our site who dug in the manure to their soil had very poor crops. I rest my case!! If you must add it to your soil, I would suggest that you leave it on top of the soil, and do not dig it in for at least a season.
I now only add fertiliser to my soil at the same time as I plant crops, thus ensuring that my plants get the maximum effect of the available nutrients.
I am not convinced by the argument that you will get the nitrogen used up by the bacteria during the time that they rot the wood, back again once the would chips have rotted. I suspect that some of the nitrogen may be converted into ammonia gas by the bacteria and lost into the atmosphere. And some of the nitrogen present will be leached out of the soil by the rain in the intervening period. It is 50 years since I did chemistry and I am very rusty on the subject, and I would be interested to hear to views of a chemist or biologist.
