Soils and site preparation

Most native soils in Central Oregon are a fine sand to sandy loam, and tend to be very sterile with minimal organic matter (generally less than 1 percent). This type of soil does not hold high quantities of water or nutrients without some modification. Amending the soil with organic material such as compost or aged manure (free from weed seeds, disease pathogens, and herbicide residue) will help improve water holding capacity, soil microorganism activity levels, and the overall fertility of the soil. Ideally, sandy soils should have 2 to 3 percent organic matter.

Soil testing prior to planting can be useful to determine the soil pH and nutrient levels, allowing time for adjustments. Remove weeds and sod from the planting area to avoid competition for water and nutrients. The soil pH in Central Oregon is generally between 6.5 and 7.8. Most vegetable crops prefer a more acidic soil pH of 6.2 to 6.8, so in many areas the soil may require amendments to lower the pH. Soil pH affects a plant’s ability to take up nutrients, so it is important to achieve an ideal pH.

If your soil is too alkaline, add elemental sulfur in quantities of 4 to 6 pounds per 1000 square feet in the spring. Sulfur needs time to break down and gradually lower the soil pH, so the sooner (6 months to a year ahead of time) you can apply it to the site before planting the better. (See Improving Garden Soils with Organic Matter and Acidifying the Soil.) In other parts of the country, where soil is more acidic, adding lime is a common practice to increase calcium and raise the pH; however, this is rarely needed for backyard gardening in Central Oregon.

Ideally, you want to achieve 3 to 6 percent organic matter in your site, so continually add organic matter to the planting area each season. Vegetables require more nutrients from the soil than ornamental plants, and it will be important to replenish the site as needed. It may also be important to till the soil initially to alleviate soil compaction and turn in an amendment; however, be careful not to over till which can destroy soil structure.

You can add manures to your garden area in the fall to break down over the winter. Avoid using pig, cat, and dog manure as they can harbor pathogens that affect human health. If using horse, cow, or chicken manure, allow it to age for a minimum of 6 months so that the nitrogen levels are not too high.

Before acquiring horse or livestock manure, ask the supplier what the animals were fed, the origin of the feed, and what herbicides, if any, were used to spray the pasture to avoid using manure containing herbicide residue carryover. (See Herbicide Carryover in Hay, Manure, Compost and Grass Clippings: Caution to Hay Producers, Livestock Owners, Farmers and Home Gardeners.)

Source: extension.oregonstate.edu