Mulch, compost, fertilizers, soilless mix – do you know ... Which BAGS to buy?

By David Sprague

OSU Ext. master gardener volunteer

Walk into any garden center this time of the year and you will see bags on pallets stacked all around you.

Do you ever wonder what is in all those bags and what they’re supposed to do? How do you know which of them to buy? The choices can be quite overwhelming.

To help you know what to buy and what not to buy, let’s take a look at what’s in these bags.

For the most part, these bags contain different types of soil amendments. Soil amendments can be put in four general categories: mulch, compost, fertilizers and soilless mixtures.


Mulch is anything that covers the surface of the ground. Mulch moderates fluctuations in soil temperature and water content. It protects against erosion and suppresses weed germination and growth.

There are two categories of mulch – inorganic and organic. Inorganic mulch is semi-permanent to permanent and includes products like black plastic sheets, chopped rubber and rocks. The organic mulch is usually bark mulch or wood chips.

One advantage of using organic mulch is when it decays it becomes compost, improving soil in addition to the other benefits.


Compost is decomposed plant material that directly improves the condition of the soil.

Compost breaks up clay soils and helps sandy soils retain water. It provides nutrients for plants to use and holds those nutrients in the soil.

Compost can also be used as a mulch, although weed control is less than desired.


Fertilizers are products that provide plant nutrients.

There are many types of fertilizers, but all are either organic or synthetic.

Organic fertilizers come from natural sources. Synthetic fertilizers come in numerous formulations, such as fast- or slow-release.

Synthetic fertilizers contain one or more of the primary nutrients:nitrogen (N), phosphorus (P) and potassium (K).

Primary nutrients are the ones most needed by plants for healthy growth and flower/fruit development.

By law, fertilizer bags must show the percentage by weight of the primary nutrients they contain. This is shown as three numbers separated by dashes such as 10-6-4. In this example, the fertilizer is 10 percent nitrogen, 6 percent phosphorus and 4 percent potassium. The numbers are always shown in that order, N-P-K.

Fertilizer bags may also contain secondary nutrients and micronutrients. These are nutrients plants need in smaller amounts such as calcium, sulfur, copper and zinc. The amount of secondary and micronutrients are not required to be shown on the product.

Next to the fertilizers are the pH adjustors. The bags of lime, sulfur and other products will raise or lower the soil’s pH level (the measure of acidity). These bags are only necessary if your pH is outside the desired range of proper growth for the plants you want to grow in the area.

soilless mixtures

Soilless mixtures are used to germinate seeds and grow seedlings, as well as in container gardening.

These mixes are very light weight. They consist of some combination of peat moss, perlite and vermiculite. Because they are sterile, some of these products add small amounts of fertilizer.

So what do you need? Well, if your flowerbeds have little or no mulch, then buying mulch is a good idea.

Adding compost will help improve your soil. But have your soil tested before buying fertilizers or products that change the soil’s pH. A soil test will show exactly what, if any, of these products you require.

Once you know your soil test results you can buy what you need off the pallets, based on testing recommendations.

For more on soil amendments, visit; for details on soil testing, visit go.osu.edusoiltesting.