There are times when it is necessary to purchase additional soil for the garden, especially when creating new raised beds or raising the grade of an existing or new garden bed. Purchased soil can also be incorporated into the existing soil of the bed to improve it, much like we would add organic matter.

Usually, these purchased soils are sold as “topsoil” or “garden soil.” You may also find soils available that are created especially for a particular type of plant, such as azalea soil or rose soil. When you see the words topsoil or garden soil you should be aware that these materials are not literally what the names imply. The topsoil was not striped off of a field somewhere, and garden soil was not dug out of a garden. These materials are actually soil mixes that are created by blending together a variety of materials to produce a product that plants will thrive in.

Selecting a topsoil or garden soil is very important. You need to be aware of what materials were blended together to create the mix you are purchasing. For small scale jobs, it is often easiest to purchase bagged soil mix from a local nursery. Many nurseries carry soil mixes that they have blended themselves, or that they have obtained from a local company that makes soil mixes. You will also find bagged soils packaged by national companies, but these are not necessarily better, or even as good, as locally produced soil mixes. For larger jobs, soil companies will sell you blended soil mixes by the cubic yard and deliver it (if your order is large enough), or you can go directly to the company and pick it up yourself.

Locally produced soil mixes in the New Orleans area often start with the fertile alluvial soil that is deposited by the Mississippi River. This soil is acquired from several locations, and may be called by several names depending on where it was obtained.

Some of this soil is deposited in the Mississippi River bed itself, and it is dredged or pumped out to keep the river open to the large ships that travel the river. This alluvial soil is called pumped sand or sugar sand. Soil obtained from the area between the river and the levee is called batture sand. Pumped and batture sand are also called river sand.

Alluvial soil obtained from the Bonnet Carre spillway is called spillway sand. Spillway sand is the least desirable material to use in a blended soil mix. It is deposited in the spillway when the river is high, and when the water level goes down the soil dries out and native plant species move in and grow on it. These plants, such as nutgrass, torpedo grass, alligator weed and many others, do not cause problems in the spillway, but if introduced into your garden they can be a disaster.

Generally, avoid soil mixes that use spillway sand because it may contain seeds, bulbs or bits of rhizomes of these weeds, which can then infest your garden (this also applies to using spillway sand as fill).

These sands we get from the Mississippi River are not actually sand in the way most of us think of sand, because they contain various amounts of sand, silt and clay and are generally classified as very fine sandy loams. Although they are fairly well supplied with mineral nutrients, they are low in organic matter which is not deposited along with the sand, silt and clay. This is lack of organic matter is what makes river sand light in color. These soils are also low in nitrogen, a nutrient that is very water soluble and is lost as the soil is carried down by the river.

Soil companies blend the river sand with various organic materials in a variety of proportions to create their soil mixes. It is important to know what type of organic matter is used. Organic matter – such as manure, stable sweepings, bagasse (ground up sugar cane, a byproduct of the sugar mill) and rice hulls – that has been composted is preferred. Composted finely ground pine bark is also an excellent addition, especially for increased drainage.

Less desirable is fresh ground pine bark that has not been composted and fly ash. If you use a soil mix that includes pine bark that has not been composted, you will need to carefully monitor nitrogen levels in the soil. The microorganism decaying the pine bark will tend to tie up the available nitrogen in their bodies, which deprives plants growing in the bed of the nitrogen they need. Always incorporate a nitrogen containing fertilizer into the soil mix in the bed before planting. And watch the plants carefully for stunting or overall yellowing, both indications of nitrogen deficiency. Provide additional nitrogen should you see this. Eventually (several months after installation), the bark mix in the bed will finish decaying/composting, and you won’t have to be so concerned.

I generally avoid soil mixes that contain fly ash. This material is essentially powdered charcoal, and is added to soil mixes to make them look black, a color of soil gardeners generally associate with fertility. Fly ash, however, is strictly cosmetic and adds nothing to improve the soil. It is better to purchase soil mixes that get their dark brown color from the addition of quality composted organic matter.

You should also ask the company about the pH of their topsoil or garden soil. Ideally, the pH should be about 6 to 7.

CREATING BEDS

Soil mixes are often used to fill new raised beds. Raised beds are generally built 8 to 12 inches high – but may be higher, and they are particularly useful if you are gardening in a situation where the drainage is poor. Construct the sides of the raised bed with your chosen materials such as landscape timbers, bricks, pressure treated boards or other rot resistant materials. The soil company or nursery can help you decide how much soil you need based on the dimensions of the bed.

Before adding the new soil, kill or remove any unwanted grass or weeds present. The herbicide glyphosate (Killzall, Eraser, Roundup and other brands) is appropriate to use. Till the soil in the bottom of the raised bed, add a few inches of the topsoil or garden soil and work that in, and then fill the bed with the new soil. If tilling is impractical, you can simply put the soil mix in over the existing soil after the weeds are dead.

If you add topsoil or garden soil to raise an existing bed, always try to blend it into the existing soil. This creates a transition between the new soil and the existing soil. Plants prefer this to a situation where there are distinct layers of different soils.

It is usually a good idea to dig a 3:1:2 ratio fertilizer into the bed before planting (such as 15-5-10 or anything similar – higher first number, lower second number and third number in between). This will ensure there is sufficient nitrogen in the soil for the plants, and will compensate for any nitrogen that may be tied up if the organic matter in the mix is still undergoing rapid decomposition (less of a problem if the organic matter in the mix was composted before being added). New soils often get better with age, and gardeners will talk about soil mixes “mellowing” over the first six to eight months after they are put in place. This is generally the time it takes uncomposted organic matter to break down.

Whether improving existing soils through bed preparation or using soil mixes in the garden, never overlook the importance of the soil to the health, appearance and productivity of the plants you grow.

Dan Gill
Consumer Horticulture Specialist
LSU AgCenter