Back: Table of Contents | Methods Of Applying Biosolids
SEEDING/PLANTING
Immediate Seeding
There are several options for establishing a vegetative cover on a biosolids amended site. The simplest process involves adding seeds directly to the amendment immediately prior to application. This approach was successfully used at the Palmerton, PA NPL site. Thirty lbs/ac of a grass vetch mixture was hand scattered on the amendment before loading into application vehicles. This is a very efficient and cost effective approach. It is appropriate to use this type of seeding technique with a relatively low cost seed mixture. As only a small portion of the added seed is close enough to the surface after application and incorporation, germination rates of 10-20% are not uncommon. Seeding with an annual rye would be an example of a low cost option that is appropriate for this type of approach.
Delayed Seeding
This approach will not be effective in cases where a highly reactive lime (slaked or burnt lime or a high CCE residual with pH>9) is added to a high N biosolids. These mixtures will release sufficient ammonia to kill any seeds added directly to the mix immediately prior to application. For these situations, seeds can be hand thrown on the surface of the amended soil any time after sufficient ammonia has volatilized (generally a waiting period of three days is sufficient). The surface of a biosolids amended site will be sufficiently irregular that seeds spread on the surface will fall into cracks and crevices and be able to germinate.
It should be noted that use of conventional equipment to spread seed three days after application of high rates of high moisture materials may not be effective. In these cases, the simpler the approach, the more effective and efficient. Hand seeding, using a whirly bird seeder is one approach. Using snowshoes or driving on a vehicle with floatation tires will allow early access to these sites. Biosolids amended soils are sufficiently moist and sticky or adhesive that it is not necessary to hydroseed or to use any tackifiers or mulches. The one exception is when biosolids are being applied in areas where it may be very hot during the growing season. The dark color of the material will increase surface temperature and may kill seedlings. In this case, use of light colored mulch is recommended. On the other hand, the dark color of the biosolids can effectively extend the growing season in cooler areas.
Appropriate Seed Mixtures
In many areas, there is increasing concern with reestablishing native plants on previously disturbed sites. This goal has to be combined with the more immediate goal of establishing a vegetative cover. When high rates of biosolids potentially in combination with other materials have been added to a soil, there are several approaches that may be followed to achieve a healthy stand of native species. Ongoing research to fine-tune these approaches may give different answers over time, but certain approaches seem reasonable.
Initial seeding with high rates (>20 lb/ac) of an annual cereal is generally a very effective approach. Annual cereals such as wheat or rye are inexpensive. There are also varietals that are salt tolerant (high rates of biosolids will increase the electrical conductivity of the soil for a finite period). These materials will germinate quickly and can provide a cover while the amendment stabilizes. Use of lower rates of native species seeds the following season will permit a succession to naturally occurring vegetation. If the area that has been amended is relatively small and is bordered by healthy vegetation, it is also possible to let this vegetation naturally colonize the amended area. If there is no potential for erosion and it is acceptable to leave the amended surface bare for several months, the bordering vegetation will invade the amended areas over time.
A relatively new approach involves cutting mature hay from neighboring fields and using the hay as mulch for the amended areas. The hay should be cut so that viable seeds are included in the hay. These seeds will germinate and the hay will decompose. This is a relatively inexpensive way to establish a native cover.