The influence of periodic fallowing on the prevalence of viable weed seeds in arable soil

In view of the favourable reduction in the weed-seed population on Broadbalk wheat field (Rothamsted Experimental Station) by the 2-year fallows between 1925 and 1929 a cyclic fallowing of one section out of five per year was adopted to maintain control over the weeds. Annual samples for determining the weed seeds were taken from plots receiving farmyard manure, minerals and sulphate of ammonia together, and minerals and sulphate of ammonia in alternate years. The numbers of seedlings obtained were grouped to show the reduction in weed-seed population during fallow and the behaviour of various species in succeeding years. By this grouping the effect of the variable seasonal and cultivation factors is greatly reduced, as several years are included in each figure. Most species are reduced by fallow, rise more or less rapidly during succeeding years, and are again reduced by a later fallow, usually to a lower level than by the first. Alopecurus agrestis showed the greatest variation from year to year. Papaver rhoeas was the outstanding exception to the general rule, as, once reduced by fallow, it failed to reassert itself except in a few cases, and was further reduced by the later fallow. These two species, with Alchemilla arvensis, Veronica arvensis and Medicago lupulina, contributed about 90% of the total seeds. Alternate manuring with minerals and sulphate of ammonia greatly influenced the production of weed seeds. With most species nitrogen induced heavier seed production, as was shown by the average number per section compared with those produced with minerals. Alopecurus agrestis, Alchemilla arvensis, and usually Papaver rhoeas responded thus, Veronica arvensis was the only one of the main species to behave irregularly, whereas Medicago lupulina showed a reverse response, being more benefited by minerals than nitrogen. In the first year after fallowing the crop and weeds alike were usually able to benefit by the stored fertility and did not appear to interfere with one another except where farmyard manure was used, when the competition of the crop at first prevented the weed-seed population from rising. In the later years the crop tended to decrease, and the weeds to increase, until the balance was restored by another fallow. Field observations, as well as laboratory samples, have shown a steady improvement in the cleanness of Broadbalk field, the most noticeable reduction being in poppies. The various weed species fall into definite groups, in two of which the reduction or increase in numbers induced by fallowing is maintained, while in the third there is an irregular increase and decrease in numbers over a period of years. The general conclusion is that the fallowing of one section per year has fully justified itself, as the total weed-seed population has not only been kept in check, but has also been gradually decreased during the 15 years the system has been applied.