Weeds belong in a discussion of soils for several reasons. For one, modern chemical weed controls introduce problems of toxic residues, which may produce either short- or long-term injury to desirable plants as well as weeds. For another, deep-rooted weeds such as bindweed, Canada thistle and nut grass often make a soil unusable for gardening until they can be exterminated. Third, the amazing longevity of certain weed seeds in soils is a factor in gardening.
Longevity of weed seeds is a serious problem and one which often determines how a certain soil should be treated for lawn or garden purposes. It often becomes critical when a home owner, following "curbstone advice" from neighbors, insists upon buying a load of "good black dirt" to start his new lawn, bringing in someone else's accumulated weed seeds to further complicate matters.
My own most striking experience with seed persistence was with crabgrass. In laying a new water line to my home in 1955, plumbers uncovered an old carriage drive which must have been laid down either in 1868 when the house was built, or in 1900 when it was remodeled. Underneath the old bricks was rich black prairie loam. I dug this out and spread it over my vegetable plot. This was done in early spring, before any crabgrass plant could have contaminated the soil with fresh seed. Nevertheless, that spring, a heavy crop of crabgrass sprang up from this long-buried soil. Thus those seeds retained their viability for a minimum of 55 years.
There is only one 100 per cent control for long-lived buried seed- sterilization of the soil with live steam for several hours. This is
feasible only in greenhouses, where special covers are used on benches to retain heat and pressure at a lethal level. Practical control can be had in home gardens or future lawn areas by the use of one of two chemical materials.
Calcium Cyanamid: Used at a rate of 75 pounds to 1,000 square feet of surface (a heavy dose) this has killed out all weed plants and seeds for me, even on fairly heavy clay soil. Calcium cyanamid, called Garden Cyanamid, is a granular material that at first breaks down into substances poisonous to seeds but later converts into valuable nitrogen and lime. It is a grim coffin-gray in color and even looks poisonous to handle, but is perfectly safe if used as directed. The soil to be treated should be plowed or rotary tilled and leveled just before application. After 60 days you can plant seed, but disturb the soil surface as little as possible, to avoid bringing up new weed seeds. The 60-day wait is one drawback to this material. Since soil should be warm during treatment, this means you have an unplanted lawn or garden during the major part of the growing period, which some gardeners find too unpleasant a sight to face.