California Gardens - The Year Round Gardening Site

Soil Fertility with Mulch and Compost

Mulch changes the character of the garden more than almost anything else you can do. Maybe you can't put the rake away completely but the idea of a sterile clean raked or blown patch of dirt defies the biology. If the pieces are too big, shred them, or set them aside to rot awhile so they can be put back into the garden. The idea of mulching with rubber, rocks and other recycled waste really doesn't help the biology much. Since the idea is to grow plants, not to be a landfill thinking on the biology aspect is critical. The soil is supposed to be alive.

Soil health is relevant to almost all plant problems. Moisture, drainage, and soil fertility are integral to plant health, but the most important factor is the soil. Healthy plants are less likely to attract bug infestations or ward them off when they occur. I control much of the re-inoculation of fungi on roses by applying a blanket compost before the rains start. This interrupts the spores from being splashed back up onto the leaves and limits much of the occurrence of rust and black spot. Because the roses are healthy they rapidly grow new foliage and there is no long term damage due to loss of the photosynthetic power to the plant. Thus healthy plants are also less likely to be destroyed by the onset of a fungus.

If there is any one element that serves as the solution to more problems in the soil it is organic material. When the soil is too sandy and drains too well, add mulch or compost, the organic material retains moisture. When the soil is too heavy or clay-like, unevenly shaped bits of organic material break up the platelets that make the clay and allow space for water to move through the soil. Some plants prefer disturbed dry soil (mostly we call those weeds) but the majority of those with the pretty flowers prefer a loamy well-mulched soil.

As organic material breaks down it releases carbolic acid. This neutralizes the salts that are dissolved in our water the prime soil problem in the Southern half of California) and helps to gradually dissolve rocks into soil. This buffering of the soil helps counteract the effects of salt deposits due to evaporation of water and past fertilizer (those little pellets are applied as a salt) applications. In general our soils are rarely short of potassium or phosphorous. These nutrients are not particularly water soluble and thus are not washed out of the soil as rapidly as is nitrogen. As the salinity of the soil increases, the ability of the plants to withdraw these necissary elements out of the soil is compromised. And the ability of the little guys that live in the soil have a hard time making a go of it, yielding less nitrogen.

Compost provides a habitat and food for the host of organisms that feed off the soil. There are over 10,000 insects living in a cubic foot of healthy soil, as well as a host of worms, fungus, and a wealth of micro-organisms. You can think of each of these creatures as tiny roto-tillers mixing particles of organic material into the depths of the soil. Earthworms consume and excrete huge quantities of soil, changing some of the organic material into nitrogenous waste. They change the soil structure by pelletizing the particles. You can see a slime trail where the worms crawl, this slime sticks the particles together. This clumping allows water and roots to penetrate the soil more easily and provides a handy trail of fertility for the roots to follow.

By using organic mulch as the only soil conditioner the changes made to the soil from its natural state are minimized. The pH is balanced. Organisms living in the soil are encouraged by their improved habitat. Furthering this idea I apply the compost to the surface and allow the naturally occurring organisms to draw it down to their soil horizon. By turning the soil many organisms are placed too high or too low in the soil and suffer for this inconvenience. I have found that rate at which the mulch is incorporated into the soil is increased by non-tilling. When I look to nature as a model for how to incorporate my mulch I see a leaf fall on the ground. It then breaks down, (ie a host of critters go on a feeding frenzy) and the leaf becomes a part of the soil.

It doesn't require fancy devices or containers to make compost. Given enough time, moisture, and moderate temperatures organic material inevitably rots. The devices just hasten the process and visually sanitize what might otherwise look like a junk heap. With a drum composter, or regular turning and moistening of the compost, the finished product can be rotted in as little as two weeks. By allowing the natural processes to take place, cold composting, 3 - 5 months, including the rainy season are required. By increasing the availability of oxygen, nitrogen, and water decomposition rates increase. Due to a lack of time and an adequate amount of space I cold compost. If the size of the heap attains gargantuan dimensions spontaneous combustion is possible. By regulating the water availability and pile size combustion problems can be averted. The maximum size that I would like to see a pile attain is 6 foot by 6 foot windrow. If a pile starts out at these dimensions it will shrink to less than 1/3 of this volume by the time the decomposition process is complete.

The most frequent problem that I encounter is an underestimation of the amount of material that can be generated in a yard, and the amount of time and energy required to render that stuff to a texture you might be willing to mulch your garden with. It is a very positive sign that more communities are starting green-waste recycling programs. Many of us have neither the time, energy or space to do this work efficiently.

Community compost programs often provide compost containers at low cost. Almost anything that can hold the debris in place will work, including gravity. Constructions with pallets, hog wire are commonplace and cost effective. Worm bins are a great way to deal with small quantities of kitchen scraps.

I apply mulch at a rate of 4-6" thick once or twice a year on all of the open ground in a landscape. I am careful not to leave it stacked up against the trunks of woody plants as that can rot the plants that I am tending. When I notice the weeds germinating it is time to add another dose of mulch. I often mulch beds as I do my winter pruning on the plants. This allows the greatest access to the bed without trampling foliage. There is one exception to this rule for me and that is where I am trying to germinate seed directly in the ground. If the weeds can't get started then my seeds probably won't be able to either.

Again looking at nature as our model, each year large quantities of organic material are added to the soil by the natural decay of grass and leaves and the occasional fallen tree. Nitrogen is brought into the system dissolved in raindrops in amounts that are measured by the ton per acre. And is captured by leguminous plants and bacteria. As tenders of the soil it is our imperative to capture as much of this bounty as possible. Instead we are burying much of this potentially great stuff in landfills. This costs us now and will cost us more later due to lost topsoil and less efficient water use. My object in deciding how my landscape will be maintained is to determine by what method I will have the least work to do in the long run while maximizing the beauty around me. I have found that by attempting to mimic nature in as many facets of the care of the yard as I can comprehend, the health of the plants and the amount of maintenance required is optimized.

The most rapid accumulation of organic material in nature comes from a grassland. Our contrived grassland, that we call a lawn is a totally different story which in most cases has a negative impact on the carbon health of our planet (organic lawn care). I am not shy about leaving an area fallow and just knocking down the weeds periodically. Weeds are nature's answer to a soil that is short of organic material, call them a green mulch.