Fertilizer Feeds Plants, Compost Feeds the Soil
We tend to think of our soil based only on its texture and mineral content. Yet a healthy soil is literally teeming with life. Billions and in fact trillions of tiny organisms can be found in a spoonful of rich garden soil. Bacteria, actinomycetes, fungi, protozoa and a host of other organisms live in healthy soil.
As we add composted leaves, grass clippings, manure, and other forms of organic matter to the soil, these organisms go to work, breaking the materials down into humus and releasing their nutrients to the growing plants. Living things eat and excrete, they exchange gases from one form to another, they secrete substances into their immediate environment that increase nutrient availability and enhance plant growth. In short, they change their environment. When you add compost or material that can become compost the soil literally comes to life. This process and the organisms involved is called the soil food web.
An important distinction needs to be made here though, compost is not fertilizer. Fertilizers supply nutrients in the form that plants can immediately utilize. Compost must be broken down by the soil food web before nutrients can be absorbed by plants. This is why compost is considered a soil conditioner.
Impact of Compost on Soil Structure
Sandy soils cannot hold water and nutrients very well. Like crushed glass, everything just runs right through a sandy soil. Compost can help provide a solution to this problem. Think of organic matter in a sandy soil like tiny sponges mixed in with the finely crushed glass. More significantly, the presence of compost increases the fungal population of the soil. Fungi attaches itself to soil particles with tiny threads, called fungal hyphen, which then serve as a net to hold sandy particles in place. The combination of stabilized sand grains, root hairs, fungal hyphen, and bits of organic matter allows the soil to hold water and nutrients for plants to use days and weeks later.
Heavy clay soils are generally compacted and dense due to the flat nature of the clay particles. The addition of compost increases the pore space in the soil, thus allowing the clay mass to become looser and more friable. Aeration is improved, internal drainage is enhanced, and roots are able to more thoroughly fill the soil and absorb its nutrients.
Compost is Especially Important in Central Texas
Plants were designed to live with their roots surrounded by the decaying materials they produced in previous seasons. Like the forest floor or the soil surface in a meadow, these materials provide the fuel for soil life that in turn feeds plants. Our modern landscape practices of removing leaves and grass clippings have interrupted the natural processes plants depend on.
Here in the Austin area, the warm night time temperatures keep the soil microorganisms actively consuming organic matter twenty-four hours a day. Without a constant source of plant material to feed the soil food web, soils become depleted. This is why purchasing or making your own compost as part of your regular gardening activities is so important to maintain healthy soil.
Benefits of Compost
Compost builds soil. It feeds microbes, earthworms and other organisms that in turn feed our gardens. Soil built with regular additions of compost gets more fertile each year. These microbes produce glues that hold the particles together into loose clumps, acids that wash over the mineral components releasing more nutrient elements, and growth promoting substances that supercharge plant roots.
After a few seasons of adding compost you may find the need to fertilize limited to occasional slight corrections and perhaps the use of a starting solution for new transplants. Nutrient imbalances in a purely mineral soil can cause some dramatic negative symptoms. In a soil high in organic content they are seldom noticed because all the macro and micronutrients from the leaves, grass clippings and other materials are now available to the growing plants.
Types of Compost
Most compost is made from yard debris, whether it’s from your own backyard or part of a larger municipal recycling program, and can be utilized for any purpose. There are several other types of compost to consider that may be readily available to you.
Leaf mold is compost made almost entirely from leaves and devoid of woody branches that are most often included with regular compost made from yard debris. Decomposition depends on fungi and can take two to three years to complete the process. Most commercial compost operations don’t want to tie up their facilities for that length of time, so this type of compost can be hard to find. You can make your own by just mulching with leaves and letting nature do the rest.
Cattle, poultry, rabbit, and horse are the most common animal manures available. They sometimes are combined with other materials like straw or wood chips depending on the source. While higher in nitrogen than other types of compost, they are also higher in phosphorus. Travis County soils are already high in phosphorous and adding more of this element may cause other nutrient deficiency problems. Be sure to test your soil at least once a year if you wish to use animal manures. If you have livestock and want to compost their manures, make sure that the material is well-rotted before using. The end product should be dry and not have a strong ammonia odor.
Worm castings are another option. Worm manure does not need to be composted and does not contain phosphorus.
Biosolids are made from wastewater sludge and shredded yard waste. Here in the Austin area it is sold as Dillo Dirt. It may contain trace amounts of heavy metals and other petrochemicals. Like livestock manures, it will be higher in nitrogen and phosphorous than other types of compost. If adding to soils already high in phosphorus, monitor with an annual soil test so that you can switch to other low-phosphorous compost in warranted.
Often a mix of manure, peat moss and other organic materials. A by-product from commercial mushroom growers. It’s usually comprised of very small materials that will break down quickly.
Composted Pine Bark or Needles
A by-product of the forest industry. Usually only partially composted. Best used as a mulch.
Additional Compost and Soil Resources
Soils and Composting for Austin Overview
Wood-and-Wire Three-Bin Turning Compost Bin
Compost Trouble Shooting
Calculate How Much Mulch, Soil, or Compost to Buy
Compostaje (composting) EHT-069S, see AgriLife Bookstore-Easy Gardening Series
How to Build a Vermicomposting Bin
Sheet Mulching — aka Lasagna Gardening
Managing Soil Health
Soil Health Assessment
Travis County Soil Survey Map