1. This article discusses the principles of comparative productivity and the net primary productivity of different types of plant community.

2. Primary production is denned as the weight of new organic matter created by photosynthesis over a period; expressed as a rate it becomes productivity. Biomass is defined as the total weight of plant present at a particular time. Crop, yield and standing crop are comparable with production, productivity and biomass respectively, but refer to the parts of the plant normally harvested or sampled.

3. Net production is that part of the gross photosynthetic production which is not respired by the plant, and hence becomes available for utilization.

4. Ways of adjusting source data to a common form are examined at length, for meaningful comparisons are impossible if this is not done. Source data are published according to a great variety of criteria such as fresh weight, dry weight, oxygen production and carbon fixation. Standing crop or yield data need correction for omitted parts of the plant. The determination of productivity from changes in biomass may involve corrections for material accumulated from earlier periods and for losses due to death or grazing. Conversions from gross production to net production are usually required when photosynthetic determinations are made.

5. Problems raised by the use of different units are discussed and selected factors for conversions to the recommended units are listed.

6. The basis adopted for comparisons is the maximum average annual net productivity of organic (ash-free) matter that can be attained over a large area. This facilitates the comparison of the productivity of different types of community by minimizing differences due to local site conditions and weather, and is the most useful measure for general ecological purposes. For some selected examples the productivity and biomass are expressed in a variety of other ways to facilitate direct comparisons with source data.

7. Methods for determining productivity are only discussed in so far as the details affect the comparability of the results.

8. The most productive temperate communities appear to be fertile reedswamps which may produce 30–45 metric tons per hectare in a year. Coniferous forests, and perennial plants under intensive cultivation, may produce 25–40 m.t./ha. Deciduous forests, uncultivated herbs and cultivated annual plants are less productive (10–25 m.t./ha.).

9. The most productive communities of all appear to be found in the tropics. Rain forests and perennial plants under intensive cultivation may produce 50–80 m.t./ha. in a year and it is probable that swamps are similar. Cultivated annual plants only attain 25–35 m.t./ha.

10. The phytoplankton of lakes and oceans are relatively poorly productive even on fertile sites, with an annual production of only 1–9 m.t./ha. Values greater than 3 m.t./ha. are only achieved in waters enriched by man's activities, or in the tropics. Submerged freshwater macrophytes are no more productive in the temperate region but may attain 13–21 m.t./ha. in warmer climates. Benthic marine plants in shallow waters may produce more; from 25–33 m.t./ha. in the temperate zone, rising to nearly 40 m.t./ha. by tropical coral reefs. Algae cultivated in sewage can produce up to 45 m.t./ha. and algae cultivated in mineral media, with carbon dioxide supplied artificially, may produce even more.

11. If it were possible to devise cultivation techniques which would enable plants to grow all the year at the rate normally attained for only short periods in their seasonal cycle, much greater annual productivities, up to 150 m.t./ha. year, might be attained. Eichhornia crassipes might be a suitable plant for such cultivation.

12. Assuming that the soil structure is good and that ample nutrients are available there appear to be three main ways of increasing yields; irrigating, using plants which maintain an active cover throughout the year, and developing techniques to obtain valuable products from plants, or parts of plants, not directly useable.