Shredding or grinding raw materials can produce several beneficial results, particularly when composting fibrous materials such as leaves, woody plants or corn stalks. Shredding exposes a greater surface area, which makes it more susceptible to bacterial invasion. A piece of wood, a pile of corn stalks or leaves packed together does not decompose quickly in a compost pile. Sufficient oxygen is not available in the center of such objects to permit aerobic and more rapid decomposition.

Shredding material makes it more homogenous, produces beneficial initial aeration, and provides a structure which makes material more responsive to moisture control and aeration as well as moving and handling. Shredded refuse heats more uniformly. It withstands excessive drying at the surface of the pile, is insulated against heat loss, and resists moisture penetration from rain better than does unshredded refuse. Fly control is easier when refuse is pulverized or shredded. Compost users find that shredded or ground material can be applied more readily and uniformly to the land.

The best size of particles for composting is less than 2 inches in the largest dimension, but larger particles can be composted satisfactorily. The particle size of material being composted is related to the finished product requirements and by economics. If the material is to be used on lawns or flower gardens, compost should be screened through a one-inch screen so it looks better and is easier to apply and work into the soil.

It may not be worth the added cost and labor to shred the material. Any particles that are too large can be forked or screened out and broken up when necessary. Individual farmers or gardeners are not necessarily particular about uniformity of compost structure when preparing the compost. Nor is uniformity as important for agriculture fields as for the hobby gardener.

Initial shredding of all material is not necessary in the composting operation. Often, the best practice is to limit initial shredding to only large pieces of organic materials. Some compost operators believe that permitting some larger irregular pieces to remain creates greater air spaces in the mass and hence more entrapped oxygen.

Vegetative and herbaceous matter should not be ground because it becomes soggy. The high moisture content of these materials makes it unsatisfactory for aerobic composting. The time of shredding or grinding is determined by the raw material to be composted, but it need not be a difficult operation. Regrinding can be done either after the compost is mature or ready for use, or near the end of the maturing process. Regrinding near the end of the period of active decomposition would serve as the last turn for aeration, and remaining stabilization would take place in large stockpiles.

Whether grinding or shredding should be practiced or not depends upon the nature of the raw material, the desired features of the final product, such as the appearance, size, quality, and the economic requirements of the operation. Shredding and grinding the materials will shorten the decomposition time.