The weed seed population of arable soil: II. Influence of crop, soil and methods of cultivation upon the relative abundance of viable seeds

1. The weed seed population of the soil is greatly influenced by the type of crop grown. Soil conditions being similar, the composition of the flora under continuous wheat and barley is very much the same, but the relative abundance of the constituent species varies greatly, some being favoured by the wheat crop and others by the barley. On the whole, the spring cultivation before barley sowing tends to keep the number of buried weed seeds below those occurring in the autumn ploughed wheat soil. 2. When fallowing operations are carried out the various species in the soil population are differently affected. Most species are reduced in number, but the degree of reduction ranges over a wide percentage, while a few species may even be increased. These variations seem to depend upon the correlation between the times of the fallowing operations and the periods of maximum germination of the different species, coupled with the length of their natural dormancy. 3. If the interval between processes of cultivations are too prolonged some species are able to reach maturity and replenish the soil with so many seeds that the beneficial effect of the fallowing is entirely lost. Weed species vary considerably in their ability to recolonise the soil in this way. 4. When land is cropped the processes of cultivation affect the weed flora more variably than is the case with fallowing. On the same area some species may be drastically reduced while others may be doubled or trebled in quantity. This again depends on the correlation between the data of sowing the crop, the method of cultivation, and the habits of the weed species as regards maximum period of germination and length of natural dormancy. 5. Some weed species respond to cropping and fallowing in the same general direction, being reduced by both methods of cultivation. Other species may be generally reduced by fallowing, but behave variably under crop, being increased or decreased in different seasons. 6. From the agricultural point of view it is apparent that unless fallowing operations can be carried out with a much greater degree of thoroughness than is usual, reduction of many weeds can be effected almost as well and more economically by intensified cultivation while the land is under crop. Other species, however, which tend to increase in some seasons under crop conditions, may be more effectively dealt with by fallowing if their predominance justifies the expense, which implies loss of crop as well as the cost of numerous cultivations.