One of my New Year tasks was to collect a large quantity of two year old well-rotted horse manure from a neighbour.
This stuff is full of baby worms and the dark rich colour of black coffee. It probably has the same kick-start effect on the garden as coffee has on people. It is concentrated goodness which will be used sparingly. Because it is from the stable and well-rotted it has few weed seeds, perfect for the polytunnel borders.
Adding manures and compost to the polytunnel beds is always slightly problematic. Fresh stuff from my poultry is too hot and not sterile enough. The compost from my compost heaps can be good but rarely weed seed free. I tend to concentrate this in trenches where the weed seeds won't get light and germinate, or in the base of large containers for beans and courgettes. But concentrating compost in trenches doesn't spread the goodness and if you are cropping the polytunnel year round then you will soon exhaust the soil. Although I do use pelleted poultry fertiliser to feed the plants, I noticed that the soil was beginning to lose it's colour. The outside vegetable patch was a deep brown but in the polytunnel the soil was beginning to look more grey. The organic matter in the soil was being taken up by the plants and soil life, and wasn't being replaced fast enough.
We have cattle ourselves, but they are not housed, so collecting and rotting their dung wasn't going to be easy. You can buy (and previously I have) well rotted farmyard manure in bags from the garden centre, but it is costly if you need a good quantity. Resting one of the beds after giving it a good helping of compost from the heap wasn't an attractive option, when space is a premium, and weed seeds could come through for years to come (dock seeds can persist in the soil for up to 50 years). I have got the polytunnel to a position where the weeds are not difficult to keep under control and keeping it like that is a priority. So the offer of well-rotted horse manure from the stable was too good to refuse.
How to use? The easiest way is to add to the surface around the plants if they are big enough, like my presently growing purple sprouting broccoli and kale. The worms will take it down and as you water it will also be washed into the soil. The feeding roots of most plants are close to the surface and they will take in the goodness quite easily from a surface mulch. Another way is to dig in when preparing the ground for a crop, or add to a trench or planting hole. I will avoid adding it to the bed which will have the next crop of carrots but once they are growing strongly I will add a small amount to a watering can and water it on the soil near the plants. This suits carrots, beetroot, turnips, onions etc. Gradually the organic material in the soil will build up again and this benefits everything. The soil structure retains water and nutrients better if it has a good quantity of organic matter. The soil life has more organic material available to break down and they release the nutrients making them available to the plants. Healthy plants mean a better crop and arguably a more nutritious crop.