Amendments and Green Manures

p lants and soil organisms like a varied diet. Adding organic matter in _ different forms — mulches, topdressings, soil amendments, and green manures — ensures that both get a balanced supply of micronutrients. It can also make life easier for the gardener, as different methods of adding organic matter suit individual budgets, schedules, and energy levels.

Some methods are better suited to specific situations. Green manures are great when starting new gardens, and in a crop rotation in vegetable or annual beds. Maintaining a 2-inch-thick layer of coarse compost mulch keeps an ornamental shrub border, flower bed, or foundation planting supplied with organic matter. If slugs or snails keep you from using mulch, substitute a short-term, well-decomposed topdressing. When you renovate a Perennial bed or divide the plants in a large area, incorporate_sml amencl-ments to give plants a head start, supplementing each year with topdre^ «8 or mulch. Whichever method you choose, use overall garde, heaUh and Performance, plus periodic earthworm counts, as indicators of adequ organic matter. , . „m, wav 0f

Striving for high yields in the vegetable gardems one great w ^¡ng organic matter. The same soil conditions that p^ thel^ ^ "J also promote a large and healthy ^^^ a„d a J* amounts of organic matter, slightly have

  • of all essential nutrients. Plants that produce g > ^
  • ive root systems, which are left in the nx,e
  • r fruit, but produce lots of leaves and stems, i n ^ J* residues to turn under at the end of the season, or
  • Post pile. _______

In This Chapter

Types of Mulches

Using Organic Mulches

Using Fabric, Paper, and Plastic Mulches

Using Soil Amendments

Using Topdressings

Green Manures

Legumes and Grasses

Growing Green Manures and Cover Crops

Green Manures for Special Purposes

Combination Cover Crop and Mulch


Mulching is one of the easiest ways to add organic matter: Spread the material on top of the soil and let earthworms do the tilling for you. Mulching (or its close cousin, topdressing) is often the only way to supply organic matter to trees, shrubs, berry bushes, and perennials. If these are shallow-rooted, they'll resent any attempt to dig in amendments. Its much safer (and simpler) to spread organic materials on top of their roots. As a bonus, mulching greatly reduces watering and weeding chores. In addition to adding humus, mulches improve the soil in many ways, from reducing erosion to encouraging soil organisms.

Organic Mulches

Not all mulches provide organic matter and nutrients — only organic ones do. Organic mulches can be anything you'd put in the compost heap. For mulching, though — unlike composting — the longer something-takes to break down, the better. Wood chips and bark nuggets that are too hard for the compost heap are perfect for mulch. They don't need to be replenished as often as a mulch of grass clippings.

A few warnings are in order before you mulch everything in sight. In wet soils and very humid climates, organic mulches can promote some diseases and increase chances of rot. If spread too thickly, they can interfere with the soil's air circulation. Fresh, unweathered carbon-rich mulches such as sawdust can temporarily tie up enough soil nitrogen to interfere with plant growth. This problem is easily solved by letting them weather until their color fades, or by spreading nitrogen-rich material underneath them. Often, these same materials are flammable and should be used with care in fire-prone areas.

Inorganic Mulches

Inorganic mulches provide some benefits, although they don't supply organic matter. It's a good idea to spread 1 to 2 inches of compost or aged manure over the soil before using inorganic mulches. Once they're in place, it's hard to replenish soil organic matter. Here are a few inorganic mulches, and reasons why you might want to consider using them.

  • Black paper and black plastic. These are excellent for warming soil in spring to lengthen the growing season in cold climates; also good tor weed control
  • Gravel. A decorative mulch, gravel is good around plants that need excellent drainage or are prone to rotting (most alpine, desert, and rock-garden plants); it also keeps soil cool in hot climates, prevents wind erosion, and is effective where slugs or snails are a major problem.


Why Use Mulch?

  • Keeps soil moist.
  • Smothers most weeds.

Keeps soil soft so that any w< are easy to pull and more rail soaks in.

  • Shades soil, reducing heat stn for plants and soil organise®
  • Keeps soil warmer on cold fall nights.
  • Prevents sudden swings in soil temperature, increasing winter survival of plants.
  • Helps prevent plants from heaving out of soil in winter.
  • Reduces wind and water erosk
  • Prevents puddling or crusting from hard rains.
  • Reduces or prevents soilborne diseases by keeping rain from splashing soil onto plants.
  • Keeps fruits and vegetables clean.
  • Minimizes stress for new transplants.

If the mulch is organic, it &

  • Adds organic matter and hun'
  • Feeds and encourages earthworms and other soil orgai
  • Conditions soil.
  • Slowly adds nutrients to
  • Increases retention and ability of nutrients.
  • Recycles lawn, garden, trimmings.
  • Landscape fabric. These are plastic materials with fine holes that let air and moisture through but not weeds. They're good for weed sud pression around shrubs if covered with a decorative mulch and particu tefly useful for preventing weeds in gravel or stone paths and patios.


Mulches that disappear very quickly are more properly called top-dressings. The purpose of a topdressing is to get organic matter and nutrients into the soil as quickly as possible. Mulches need to stick around a little longer to fulfill their main purpose, which is providing a protective blanket for the soil. Providing humus and nutrients is really a secondary benefit of mulches, especially since they often take a long time to break down and release either.

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