Compost vs Fertilizer

Photo by John Cappadonna

By John Cappadonna – Master Gardener

Why do some gardeners claim that compost is so much better than fertilizer? To answer that question, it is important to understand a little about the relationship plants have with soil. Plants take up almost all the nutrients they require from the soil dissolved in water. There are about 17 nutrients that plants take up from the soil. The ones we hear most about are nitrogen, phosphorus, and potassium.  They are considered macronutrients because plants use them in the largest quantities, but many others are used in smaller amounts, such as calcium and iron. All of them are still essential to plant health. They dissolve in water and are taken up through the roots and distributed throughout the plant via the plant's vascular system.

Commercial chemical fertilizers typically contain at least the three macronutrients, nitrogen, phosphorus, and potassium, and some contain other nutrients as well. Sometimes we mistakenly think of fertilizers as plant food, and that is not the case. Plants are unique organisms because they make their own food through photosynthesis. The nutrients we supply are used by plants as the building blocks to make food and reproduce cells so the plant can grow. Since the nutrients with the exception of Carbon Dioxide, which is absorbed through the leaves, are taken up by the roots dissolved in water. The nutrients are towed along with the water. As the water drains away with rain or irrigation, so too are the nutrients. This is why commercial fertilizers require multiple applications each season, especially for vegetables.

The relationship between plants and the soil is symbiotic. In nature, plants take up nutrients from the soil; they grow and die, or they shed leaves, and the organic material from the plants returns to the soil, where it is decomposed by microorganisms that live there. The microorganisms, primarily bacteria, fungi, and nematodes, consume organic material, and in turn they are consumed by larger organisms, such as protozoa, arthropods (including insects), and earthworms, and so on up the food chain. Microorganisms produce the nutrients plants need and store them in their bodies. The nutrients are not released until the microorganisms die or are consumed and expelled as waste by an organism of a higher trophic level. Because these microorganisms are present in fertile topsoil and live and die there, the nutrients they produce remain there continuously. It is nature's perfect time-release fertilizer!

In a vegetable garden, we remove the plants to harvest the vegetables, breaking nature's natural cycle. Compost replaces the natural organic material, completing nature's soil food web cycle. If you are growing native plants in your landscape, our native soil works well because native plants evolved with it and are adapted to it. This is not the case with our vegetable garden, where the plants we grow come from more fertile places all over the world. Compost not only provides food for the microorganisms that produce plant nutrients, but it also improves the soil. Compost builds soil structure by building aggregate. Aggregates are microscopic spaces in the soil that hold water and air. Good soil holds water and air like a moist sponge but allows the excess water to drain away.

How do you get your hands on some compost? You can make your own from yard waste like leaves and kitchen waste like vegetable peels and coffee grounds, as in the photo. You can also buy composted cattle manure or composted cotton burrs at your local nursery. Once your soil is fertile, it only takes a little compost to maintain the fertility.

If you have questions, call the AgriLife office in Odessa at 498-4071 or in Midland at 686-4700.   Additional information and our blog for access to past articles are available at westtexasgardening.org.  Click on “Resources.