Improvements to soil carbon have to be viewed within the whole system of the farm. It is undoubtedly challenging to improve soil carbon in horticulture, without bringing in large amounts of organic matter, often sourced from conventional farms. You need to look at the whole farm and see where you can build long term carbon. Features such as woodlands, hedges, shelter belts, beetle banks and field margins will be the most productive in terms of soil carbon accumulation, with more modest amounts possible within the actual growing land, primarily through the use of green manure fertility building leys and the possible addition of composted wood chips.
So you will need to consider incorporating more of these features without reducing your crop yields. To justify the inclusion of trees and other hard carbon features you have to look at the additional benefits: shelter, increased biodiversity to control pest and disease problems, enhanced soil microbial activity etc. It is important to have an idea as to what the carbon picture really is upon your farm, so do a whole farm carbon analysis. Regular soil analysis for carbon is essential and you will need to do this for many years before you get to see any real changes.