The weed seed population of arable soil: III. The re-establishment of weed species after reduction by fallowing

1. Delay in cultivation after harvest prevents the reduction of various weeds by fallowing. Some species, already developed, continue to ripen seeds, and others have time to germinate and reach the seeding stage before they are cut down by cultivation. The numbers of extra seeds thus produced may be greater than those destroyed by fallowing, so that the reducing effect of the operation is entirely nullified. 2. When land is recropped after fallowing the first wheat crop tends to be abnormally heavy, thus introducing increased competition with the weeds. Some species fail to reassert themselves, but others are able to withstand the competition of the wheat and can replenish their stores of seed in the soil by the time the first crop is harvested. A few species vary in their response, either remaining at a low level or re-establishing themselves quickly after fallowing. 3. Species which soon begin to reassert themselves tend to increase rapidly in number and after three years under crop they may be much more plentiful than they were before fallowing began, Alopecurus agrestis and Stellaria media being notable examples. Other species tend to remain at a low level, and the varying rate of re-establishment results in a definite change in the balance of the weed flora. Though Papaver rhoeas was only reduced to about half its original number by fallowing, it has failed to increase to any great extent, so that it is no longer the dominant feature that it was before fallowing was begun. 4. Prolonged fallowing, for four years, reduces the store of buried weed seeds more drastically, but does not eliminate them all. The ultimate re-establishment of species follows the same lines as after a shorter period of fallow, though it takes longer for any species to regain its original numbers of viable seeds in the soil. 5. The period of natural dormancy of most species on Broadwalk proved to range from four to nine years, but four species may prove to be able to lie dormant for more than ten years. 6. Bartsia odontites showed very strongly marked periodicity of germination, as every seedling appeared between February and June, the majority appearing early in the year. 7. Relatively few abnormal seedlings have been observed in over 600,000 which germinated. A few albinos occurred in Alopecurus agrestis and Papaver rhoeas, and also a certain number of tricotyledonous seedlings belonging to seven species, chiefly Papaver spp., Alchemilla arvensis and Veronica hederaefolia.