Tillage, Soil Type and Weed Seed Bank Dynamics
By Clarence J. Swanton and Anil Shrestha, Department of Plant Agriculture, University of Guelph

Goals of integrated weed management (IWM) include increasing knowledge of seed bank dynamics and exploring how this information can be used by growers to improve the consistency of their weed control practices. The study of soil weed seed bank dynamics is a relatively new concept in weed science. The knowledge of seed bank dynamics can help in the management of weeds in agricultural cropping systems and in the effective implementation of IWM systems. The weed seed bank is the main source of weeds in agricultural fields.

What is a soil weed seed bank?

Soil weed seed banks are reserves of viable seeds present on the surface and in the soil. The seed bank consists of new seeds recently shed by a weed plant as well as older seeds that have persisted in the soil for several years. The seed bank is an indicator of past and present weed populations. There are enormous numbers of viable weed seeds in the soil. Although a great number of the buried seeds die within a few years, seeds of some species can remain viable for decades. It has been estimated that only 1-9% of the viable seeds produced in a given year develop into seedlings; the rest remain viable and will germinate in subsequent years depending on the depth of their burial.

How does tillage affect the weed seed bank?

Tillage affects the vertical distribution of seeds within the soil profile. Studies so far have shown that the majority of the weed seeds remain at or near the soil surface in undisturbed soils or in soils where no tillage has been practiced. Tillage with a moldboard plow buries seeds deeper within the seed bank. For example, a study in Ontario found that the top 5 cm of soil contained 90% (no-till), 61% (chisel plow), 37% (moldboard plow), and 33% (ridge-till) of the seeds. Keeping the seeds close to the surface will reduce dormancy, encourage germination and lead to a more efficient weed management strategy. Thus, adequate aboveground weed control can diminish the weed seed bank in the upper layer of soil within a few years in no-till soils. Burying seeds deep within the soil complicates future weed control efforts.

Is the effect of tillage on the vertical distribution of the weed seeds similar in all soil types?

It has been established that soil physical properties that influence freezing and thawing, rates of drying and surface cracking can affect vertical distribution in the soil. Very few studies, however, have looked at the effect of tillage systems on the vertical distribution of the seeds in different soil types. Evidence exists that tillage type has differential effects on seed depth across different soil types. We present here examples of the vertical distribution of weed seeds in a loamy sand soil and a silt loam soil in Ontario. Under a moldboard plow system, 12% of the seeds are present in the top 5 cm of the loamy sand soil as compared to 37% in the silty loam. Therefore, more seeds may be returned to the surface in sandy soils than in other soil types when they are inverted by a moldboard plow. In the sandy soil, the depth of burial of the weed seeds in the soil profile increased as the depth of tillage increased. In the silty loam soil, the distribution of the weed seeds in the moldboard system was more or less uniform in the top 15 cm of the soil profile.

How can this information be used?

The effect of tillage on the weed seed banks will vary by soil type. Understanding the processes that influence the weed seed bank will allow us to manipulate and manage weed seed banks effectively by implementing more informed weed management strategies. One strategy is to shift weed seeds from the dormant to the active part of the seed bank: keeping weed seeds on the soil surface and exposing them to harsh environmental conditions and predation can enhance the mortality of the seeds. Weeds can never be eradicated, only managed. The first step towards improving our weed control practices is to understand how tillage can influence the positioning of weed seeds in the soil.

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