If at all possible, use existing soil as the primary source to fill the beds. If your garden site is elevated and blessed with good quality soil, consider scraping off an appropriate amount of topsoil during site preparation and setting it aside. This procedure is recommended only for sites located on high ground. The depression created will be prone to flooding unless water can be channeled off the site.
The same scenario occurs when soil located in pathways between the beds is removed and used to fill the beds. A heavy rain will fill the lowered pathways, turning them into quagmires. Unless the water can be channeled away from the plot to a lower area, the problem will remain.
The ideal soil for your beds is a loam. Loam soils contain varying amounts of sand, silt and clay. Most gardeners prefer a sandy loam because of its favorable drainage characteristics and ease of tilth.
By design, raised beds are endowed with superior drainage characteristics. Consequently, they can utilize a broad spectrum of soil types and be effective. Clay loam soils, which are often unacceptable, can work quite nicely in raised beds if amended with sand and organic matter.
In situations where existing soil quality, quantity or site topography is inadequate, an alternative source of fill will be needed. Ask to examine any sample of fill you are considering purchasing. Spend a few dollars to have the soil tested for salt content and texture in addition to nutrient content.
For best results, consider amending the fill soil with additional materials. At the Noble Foundation Horticulture Center, good results have been obtained by using a mix consisting of equal parts (volume) of either fine sandy loam or silt loam and peat moss. Fine-textured soils such as clay loams can be amended with equal parts sand and peat moss. Avoid using sand exclusively or in combination with only peat moss. Sand, even with copious amounts of added organic matter, tends to excessively drain, making it prone to nutrient leaching.
If you are working with large numbers of beds or deep beds, the amount of peat moss recommended might be cost prohibitive. Don't worry. Simply use the amount you can justify. With continued applications of peat moss or other sources of organic matter such as compost, the tilth and water and nutrient holding capacity of the soil will be improved over time.
Calculate the amount of fill required by determining the total volume of your beds. Multiplying width times depth times length will give volume. As an example, the volume of a bed 3 1/3-foot wide (40 inches) times 1/2-foot deep (6 inches) times 30-feet long is equal to 50 cubic feet or approximately 2 cubic yards (27 cubic feet/cubic yard). If you constructed 10 of these beds, you would need to order 20 yards of soil or a combination of soil and sand. Do not consider volume of organic matter in your calculations, as it compresses easily and once mixed with soil, doesn't displace much volume.
Begin the soil preparation process by spading or tilling the existing soil as deeply as is feasible. Tilling a tight clay soil to a depth of 6 inches can be quite a chore, requiring several passes of the rototiller. If you have a strong back and are not opposed to hard work, consider turning the soil with a fork or spade. Turning the soil prior to tilling makes the tilling process much easier. If the site was thoroughly worked during site preparation, this process of loosening the subsoil prior to adding fill will proceed better. Don't disturb the soil when it's wet. If soil won't dislodge easily from your spade while digging, you shouldn't be working the soil. Serious damage to soil structure can occur when working with soil that is too wet, especially with clay soils.
Be sure to mix a small amount of fill with the existing soil prior to adding the remaining fill. This will help avoid problems that can arise from having two different soil layers. Plan on incorporating about 2 inches of fill into the existing soil.
Don't attempt to uniformly blend a full bed of soil, peat moss and sand with one pass of the tiller. Rather, spread an inch layer of peat moss, an inch of sand or soil, etc. Till until thoroughly mixed and repeat the process until the bed is full. The growing mix will settle over time, so don't be afraid to overfill the beds.
If you plan to use plastic mulch over the beds, you'll want to prepare enough mix to form a nice crown on the bed. A crowned bed is essential to insuring a tight fit of the plastic to the soil surface. During the mixing process, some of the mix will spill over into the pathways. Be sure and utilize this fallout to insure a crowned bed. Once you've filled a bed, you'll have a better idea of how much material to add to produce the crown you want.
Think twice about using compost as a substitute for peat moss in your growing mix. Depending on the source, compost can be loaded with soluble salts. As a rule, plant-based composts are not as 'hot' as manure-based composts. Manure is high in soluble salts which act to inhibit water uptake by plants causing wilting and even foliar burn in extreme cases.
If you use quite a bit of composted manure in your growing mix, plan on having the finished product tested. If salts are present in excess, a thorough watering to leach excess salts from the soil mix is recommended. A good soaking rain will suffice. This practice is especially important if you plan on using plastic mulch because the beds become 'leach proof' once the mulch is applied.
Soluble salts are not the only problem associated with using copious amounts of any compost. As organic matter decomposes, nitrate nitrogen (the form of nitrogen utilized by plants) becomes available to the crop. The greater the amount of compost in the growing mix, the greater the amount of nitrate generated.
Garden crops vary on the amount of nitrogen they need depending on the kind and growth stage. An excessive amount of nitrogen available to fruiting plants during early development can cause a delay in fruiting because the plants remain vegetative.
Avoid the temptation to use large amounts of compost in your raised bed growing mix. When it comes to compost, think proper use not abuse!