Here hard case to make for defect since product not unreasonably dangerous. If found to be unreasonably dangerous, look to either manufacturing or design defect. Would need evidence to show that seeds were manufactured defectively, i.e. unlike other seeds in production batch. Similar evidence needed for design defect but look to factors in Banks under risk-utility approach. Need to look at industry standard or custom or Hand formula or possibly res ipsa to determine breach.
Problem arises here with concurrent or intervening causes such as misuse of fertilizer.
This misuse might undercut causation. Most likely the economic loss here not recoverable since failure to have marketable tomatoes not connected to soil damage. Most likely warranty of fitness has been waived by language in manual but warranty of merchantability has not unless merchantability language used expressly. Contract damages available for breach of warranty of merchantability likely for sample given at dinner. Arguably the only warranty was for bruise resistant tomatoes not tender tomatoes.