Many different organic amendments can be added to any soil. These include manure, sawdust, barkdust, leaves, grass clippings, weeds, straw and peat moss. Partially decayed organic materials are often referred to as humus. Compost is usually a mixture of two or more materials which have been partially decayed. Organic materials such as grass clippings, leaves, weeds and table garbage can be accumulated in a compost pile or pit and later added to the soil after partial decay.
Undecayed organic matter can also be incorporated directly into the soil. However, if it is low in nitrogen, micro-organisms which are breaking it down will obtain nitrogen from the surrounding soil and “rob” it from plants growing in the area. Materials such as sawdust, barkdust, straw and leaves are low in nitrogen. They should be added several weeks or months before planting to allow for decay or they can be composted for a similar time period. Adding nitrogen from chemical fertilizer or manure will hasten decomposition. If incorporated shortly before planting, relatively large amounts of nitrogen must be added to provide for both the plants’ and micro-organisms’ needs.
For optimum benefit organic matter must be thoroughly mixed with soil. This usually means double spading or rototilling. Pockets of organic matter tend to remain drier or wetter than surrounding soil because water movement is restricted. Because of differences in water and nutrient content, plant growth is irregular.
Many different organic materials are available for home gardeners. Up to 1/3 amendment can be mixed with the top 8 inches of soil. Organic amendments will improve the growth of most plants.
Many types of manure are available depending upon your location. Processed manure which has been pasteurized to kill weed seeds and disease organisms can often be purchased in bags. Manure can often be obtained free or at low cost from local feedlots, stables, or barnyards if you are willing to load and transport it yourself. Horse, dairy, steer, chicken, rabbit, sheep, and goat manures are all suitable organic amendments. They are frequently available mixed with straw, sawdust, shavings or other bedding materials.
Manure is usually best if it has aged for several months. resh manure can burn if applied directly to growing plants or incorporated just before planting. Fresh manure can be composted or mixed into the soil several weeks before planting to avoid burning injury.
All manure which has been stored outside can contain weed seeds. In fact this is the most important precaution when using locally obtained manure. Check the area where the manure has been stored to see if weeds are present nearby. If noxious weeds such as bindweed or thistle are present, do not use it.
Manure also improves fertility or nutrient content of the soil as well as the physical condition. However manure is much lower in nutrients than commercial fertilizers, so a larger amount must be used if no other fertilizer is applied.
Bark Dust and Chips
Bark is one of the best and cheapest organic amendments in the Pacific Northwest. Bark is a by-product of lumber milling. It can be purchased in bags and in bulk by the cubic yard. Fine grades are best for amending. Bark breaks down more slowly than sawdust and does not rob nitrogen as badly as sawdust. Some nitrogen should be added if incorporated immediately before planting.
Sawdust, Chips and Shavings
Sawdust is also one of the cheaper amendments. It can often be obtained free or at low cost from local saw mills, if you can load and transport it yourself. Because sawdust is very low in nitrogen and breaks down rapidly, it can tie up nitrogen in the soil. If it has been aged or composted for several months, nitrogen problems are minimized. If fresh sawdust is used as an amendment, fairly large quantities of nitrogen fertilizer should be added when it is composted or incorporated into the soil. Few problems occur if incorporated several weeks before planting.
Both bark and sawdust occasionally contain pitch or resins which can be toxic to certain sensitive plants. If used in moderate amounts (not more than 25% of soil volume) and incorporated several weeks before planting, no problems are likely to occur.
Peat is an excellent, high quality soil amendment which can be purchased in large bales or bags. It is usually higher priced than other local amendments. Brown sphagnum peat is the best quality. Local black peats are sometimes available in bulk at lower cost. Some black peat deposits are very high in minerals or salts which can be toxic to plants if used in large quantities. Sphagnum peat can be incorporated into the soil immediately before planting.
Grass Clippings and Leaves
Many gardeners can save enough grass clippings and leaves to completely supply the organic amendment needs of the landscape. One of the best organic amendments for flowers and vegetables is lawn clippings. I generally apply about one inch of lawn clippings around flowers and vegetables starting in late April. As clippings dry and settle, I add another inch at about 3 to 4 week intervals. Clippings greatly reduce weed seed germination. By fall clippings are partially decayed. They can be tilled, spaded or hoed into the soil in the fall or early the next spring. Leaves can also be spaded or tilled into flower and vegetable beds in the fall. Both leaves and grass clippings can be used as a protective mulch for perennial flowers.
Another approach is to pile grass clippings, fallen leaves, weeds, and table garbage in an out-of-the-way corner. Add manure or nitrogen fertilizer to each layer to provide for the microorganisms and hasten the breakdown. The pile should be turned over or mixed as frequently as practical (weekly during warm weather) to introduce air which reduces odors and speeds up breakdown. If the compost becomes dry, sprinkle with water. Even if you have not been diligent in turning and sprinkling, the partially decayed compost can still be added to flower and vegetable beds in the fall. It is best to spade or till compost into the soil in the fall.