Compost, Fertilizer, More Fertilizer and… What’s Mulch?

by Bill Swantner, Bexar County Master Gardener

It’s almost Spring garden time and there will be a lot of talk about compost, fertilizer, and mulch. What is the difference?

Compost is beneficial to the soil. Photo: Texas A&M AgriLife Extension

Compost is a soil amendment. Good soil is 25% air, 25% water, 45% mineral (sand, silt, clay) and 5% organic material (decomposed plants & animals). The soil in Bexar County is less than 0.5% organic. Compost adds back the badly needed organic material into the soil. Good soil also has many beneficial fungi and organisms living in it. These organisms help break down the organic fertilizers into a form that the plant can take up. Earthworms and other beneficial organisms help create tunnels and air pockets to allow for good air and water penetration in the soil. Colloquially speaking, a fertilizer feeds the plants, but compost feeds the soil.

Compost generally is either plant-based or manure-based. Cotton burr and mushroom are two examples of a plant-based compost. While cattle, horse, rabbit, and chicken manures are composted to create manure-based composts.  

Fertilizers come in both organic and synthetic forms. Photo: Tom Gentle, OSU Extension Service.

Fertilizer- synthetic. Inorganic fertilizers (synthetic) contain few minerals but are formulated to be taken up quickly by the plants. The Texas A&M AgriLife Extension Service recommends a 19-5-9 slow-release fertilizer. 19-5-9 means that the fertilizer formulation is 19% nitrogen, 5% phosphorus, 9% potassium. A slow release fertilizer is coated in sulfur or a plastic-covered urea. Why 19-5-9? Texas A&M University research shows that plants are 19% nitrogen, 5% phosphorus, 9% potassium.

More fertilizer- organic. Organic fertilizer is usually manure-based but the nutrients are in low formulations which means much more organic fertilizer may be needed to properly feed plants. For example, Medina’s “Growin Green” is a poultry-based fertilizer with a 3-2-3 ratio. It will take a lot of organic fertilizer to come close to the 19-5-9 recommendation. Organic fertilizer needs to be applied and scratched into the top couple of inches of soil 6 weeks before planting so that soil microbes can convert the organic nutrients into an inorganic form that can be absorbed by the plant.

Mulch helps prevent moisture loss and weeds. Photo: Bill Swantner, BCMG

Mulch is material placed on the soil to create a stable environment for the soil by protecting it from sun and wind damage.  Mulch helps to hold water by preventing evaporation. It also helps to prevent weeds. Organic mulch is ground up trees, leaves, shrubs, and/or grasses. Single shredded mulch is coarse and less absorbent; double shredded (recommended) is finer and more absorbent.  There are inorganic mulches—rocks and mulch made from rubber. You can also use compost as a mulch by applying a 2-inch layer on top of the soil instead of working it into the soil.

Note: Any product name in this article is used as an example only. There are many products on the market.

For more information:

Here’s the scoop on chemical and organic fertilizers. Kim Pokorny, Oregon State University Extension Service.

Understanding Fertilizers and Their Uses. Texas A&M AgriLife Extension Service.