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Compost and grassing

What, why, when and issues to look out for

What

Compost is often the most effective method to cover soil, control erosion and establish vegetative cover. It is more effective than topsoil. Compost is the best available soil medium for root development and other biological activities. It gives significant protection to the subsoil and increases the soil’s absorption capacity, because it holds more water than clay and loess subsoil layers.

Compost is often very effective at adhering to soil particles as their particles have opposite charges. Laying compost is not a complete stabilisation measure itself – it needs to have the seeding added and successfully established. Establishing vegetation on compost will be the easiest of all soil cover methods.

Minimising erosion risk by applying a compost blanket.

Why

Photograph of slope.

Compost being applied to a highly erodible loess slope on the Port Hills (Source: RST Solutions Limited).

Compost and seeding of grasses or other vegetation, eg native grasses or wood plants, can provide short- or long-term cover on disturbed areas. Once established to 80% coverage, the vegetation protects exposed soils from raindrop impact, reduces the speed and amount of runoff and binds the soil particles together.

Rapid-growing annual grass gives short-term cover. It is mainly used when project works are still progressing but need temporary coverage (eg during winter shutdown).

Perennial grasses (lasting for several years) give permanent erosion protection after the earthworks are finished. The ideal method is to add permanent grassing progressively as each area within the project is finalised and brought to final grade.

When

When choosing temporary or permanent seeding
  • Either temporary or permanent seeding is useful for short to medium-term stockpiles, the outside of pond embankments or diversion bunds, on cut and fill slopes, access/haul road embankments and any other disturbed areas
  • Either may also be used on rough graded areas that will not be disturbed again for 12 months or more
  • Permanent seeding is used on completed sections of work. It may require a higher quality of surface preparation and a different seed type.
When laying compost is recommended
  • When the texture and/or the organic component of the exposed subsoil or parent material cannot produce adequate vegetative growth
  • When the soil material is so shallow that the rooting zone will not be deep enough to support plants or provide continuing supplies of moisture and plant nutrients
  • For all exposed loess soils on gentle to moderate slopes
  • If you need to establish high quality vegetative cover.

Issues to look out for

Compost and grass seeding have the following limitations:

  • The time taken to achieve a stabilised area of grass can vary considerably depending on factors including soil types, seed types and weather conditions. It can take from 4 to 8 weeks, or longer in difficult conditions. You need to plan your time and allow enough time.
  • It is harder to establish an expanse of grass during periods of temperature extremes or low rainfall. You may need to plan and sequence your construction so that you can carry out the composting and seeding during periods suitable for vegetation growth, or make sure that you plan for and water during dry periods. Compost retains moisture better than any other option, so will be the most effective option for establishing vegetation.
  • Seeds are less likely to wash away from compost than they are from topsoil. However, seeds on the surface can be dislodged by rainfall or very strong wind.
  • Grass seed that has not struck can be mobilised (which means, moved out of place) by intense rainfall, so you may need to re-sow with several applications to achieve effective coverage and stabilisation.
  • Compost is very effective, but alone is not a complete stabilisation method. Once the grass strike has reached 80% coverage, stabilisation has been achieved.
  • For slopes, use texturing or roughening to subsoil surfaces to help the compost adhere.
Design essentials

When preparing the compost seedbed

  • If the site has contaminated material, fully remove this from the area to be worked on (see contaminated land section for more information).
  • For moderate slopes, roughen/texture the subsoil before you apply the compost to get better adhesion than on smooth cut surfaces. This also prevents erosion and wash-off (which would re-expose the highly erodible subsoil) during heavy rain).
  • Lay the compost at least 50 mm deep, so that the surface is loose and friable (easily crumbled).

When amending the compost

  • Apply fertiliser at the rate following the advice of your fertiliser supplier.
  • For large sites or unusual soil conditions, you may need to arrange soil testing. For example, some soils need lime to improve their pH and/or trace elements for grass growth.

When applying seed and fertiliser

  • Consider the site conditions and time of year before going ahead. Establishing grass can take from 4 to 8 weeks, or longer in difficult conditions. You need to plan your time and allow enough time.
  • The best seeding windows for both temporary and permanent grassing are autumn and spring. With good irrigation, you can establish vegetation over summer.
  • If you are seeding during dry or cooler periods, use compost to protect the seed and soil and provide a better microclimate for germinating and growing the grass.
  • See Table 9 for typical seed mixes. However, seed mixes vary, so consult a seeding contractor before buying.
  • Use only fresh, certified seed with a high purity and germination percentage from reputable suppliers that are preferably local. Species selection must consider the project’s ecological context. If permanent seeding is required, consider the final landscape plans.
  • Apply seed uniformly across the site. If hydroseeding is required, see the related section for more information.
  • For areas flatter than 25%, traditional agricultural techniques such as drill seeding, broadcast seeding, or no tillage can be useful. Make sure that your method achieves a good seed-to-soil contact, to enhance seed survival and germination rates.
  • For small areas, hand-broadcasting and raking may also be used to apply seed and fertiliser.
  • Apply establishment and maintenance fertiliser at the rate outlined in Table 9.
  • If you need to irrigate, deliver a volume at least equal to the evapotranspiration rate (the rate at which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants). Continue this until natural rainfall provides the necessary soil moisture levels for plant survival.
Table 9: Typical seed and fertiliser application rates
  Typical seed mix Application rate
Temporary seeding Annual Ryegrass 100–250 kg/ha
Permanent seeding Perennial Ryegrass — 70%
Fescues/Cocksfoot — 20%
Clover/Lotus — 5%
Browntop — 5%
200–400 kg/ha
Fertiliser application N:P:K (15:10:10) 200–800 kg/ha
Maintenance fertiliser N:P:K (15:10:10) and Urea As required

Note: Always check that the seed and fertiliser application rates and mix are appropriate for your site, and discuss with your suppliers before you use them.

Construction, operation and maintenance

When using and operating compost and grassing

  • Consider the wind strength before and during applications of compost. Blowing compost around under windy conditions could cause nuisance to neighbours. Consider other application methods.
  • Prepare a good seedbed so that the grass seed can grow well. The seedbed should be loose, uniform and free of large lumps and other objectionable material. The compost surface should not be compacted or crusted.

When maintaining compost and grassing

  • Check the condition of the compost regularly, and re-grade or replace it if necessary. Keep the compost at least 50 mm deep, with the appropriate surface roughening.
  • Heavy rainfall can wash new seeding away from smoother hard surfaces, steep slopes and overland flow paths. If the grass hasn’t established itself well enough, you will need to re-seed the area or consider other stabilisation techniques.
  • Apply additional fertiliser dosing at the ratio of 15:10:10 (N:P:K) approximately 6 to 12 weeks after seeding, or as required.
  • Protect all re-vegetated areas from construction traffic and other activities such as the installation of drainage lines and utility services. If required, erect temporary barrier fencing and/or signage to restrict uncontrolled movement.

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