As a straightforward method of assessing on-farm biodiversity, plant species numbers were compared in a fixed sampling area for each type of habitat distinguished independent of farm size and applicable for comparing different farm strategies. Using the species–area relationship, the minimum sampling areas were determined. For ditch banks, crops and field margin strips sampling areas of 400 m2 (independent of the ditch bank width), 100 and 25 m2, respectively, were proposed. Indicator threshold values for conventional farms were defined based on the best 10% of the variation in species richness among farms. With appropriate farm management these targets could be achieved. In comparison to ditch banks, crops and sown field margin strips were relatively poor in plant species. The respective numbers of naturally occurring plant species found in crops and field margin were both significantly smaller than those found in ditch banks. In contrast, ecological management of ditch banks appeared to be a promising means of increasing species richness.