The report, the first to look at how supermarkets' CSR policies impact UK growers, compared performance by market share on UK sourcing, supplier relationships and sustainability.
It claimed: "Stronger, more trusting supply chain relationships, which give farmers longer-term security, stimulate investment by farmers and by banks," while also driving efficiency in the supply chain.
NFU director of corporate affairs Tom Hind said: "We've seen some great examples of retailers really pushing UK sourcing, others that are working closely with farmers and growers in terms of environmental concerns and even some retailers that are investing in research and development."
But he added: "All of the retailers we looked at could do more to establish clear baselines and targets that can be measured on policies such as sourcing.
"All could do more to highlight the role of Red Tractor Assurance schemes. And all could do more to strengthen relationships with farmers across all sectors."
Leading supermarkets NFU comments
Aldi "Opportunity to highlight proportion of fruit and vegetables sourced from UK."
Asda "Proportion of imported food high in some products."
The Cooperative "Lack of clarity on current proportions for fruit and vegetables. Examples of engagement with farmers focus on developing countries."
Iceland "No explicit mention of British sourcing commitments. No details on relationships or expectations of suppliers."
Lidl "Sources some fresh produce from UK but should confirm proportions."
Marks & Spencer "Extension to UK growing seasons an example of collaborative effort and creation of market opportunities for UK suppliers."
Morrisons "Strong seasonal supporter of produce but requires clarity on proportions."
Sainsbury's "Commitments to doubling British (sourcing) by 2020 (signifies) potentially a large positive impact."
Tesco "Could do more to flag importance of UK sourcing and relationships with farmers."
Waitrose "Red Tractor usage could be improved. Only British suppliers required to be LEAF Mark-certified."