On the Viable Seeds Present in the Soil Beneath Pastures

Abstract

A KNOWLEDGE of the seeds present in the soil beneath different types of vegetation is of great value in the consideration of some of the ecological problems which underlie agricultural practice and is essential in studying the effects on plant population of interspecific competition and of soil cultivation. Such knowledge is largely still an unrealised desideratum in agriculture and ecology. A study of the seed content of pasture and arable soils has been made by Dr W. E. Brenchley (1), who found that a close association existed between the buried seed flora and the history of the soil. Soils underlying permanent pasture which had been continuously and closely grazed for many years were relatively deficient in seeds of any sort, but a marked difference was noticeable if the land had previously been under arable cultivation, evidence being obtained that the seeds of arable weeds may retain their viability in the soil for as long as 58 years. In soils under arable at the time of sampling, the seed flora was more varied and the seeds occurred in greater numbers. More recently Brenchley and Warington (2, 3) investigated the seed content of arable soils in considerable detail and again found an association between the seeds present and the treatment (in respect of manures and cultivation) which the land had received. The seeds of most species showed a marked periodicity of germination, the majority of seedlings appearing during the autumn and winter and relatively few during the late spring and summer. A definite indication was obtained that many seeds had a period of natural dormancy during which germination would not occur even under favourable conditions; a distinction was made between this phenomenon and the induced dormancy maintained in the soil by conditions unsuited to germination. In this later work no attention was given to pasture soils. These researches have shown how interesting are the results to be obtained in this field; nevertheless li.ttle work of a critical nature has been carried out on any considerable scale by other investigators in this country. This is possibly due to the laborious nature of such experiments and to the extensive glasshouse accommodation demanded; it is, however, the more to be regretted since it is obviously undesirable to generalise from results obtained with a few fields in a single district. Natural dormancy in the seeds of many agricultural weeds has, however, been demonstrated by several workers who have carried out germination tests on seeds taken directly from the plants (7, 10, 11),