Anyone reading our newsletter eventually realizes that I am quite different (putting it mildly) in my recommendations about the use of compost.
Compost, when used as an amendment (planting mix) or as a substrate (growing medium in a container) is the #1 cause of unhealthy, unstable and dead plants.
Be aware that compost continues to decompose. Many gardeners feel that when the compost pile cools off their compost if finished. No way. The faster (heat producting) decomposition may be done, but the slow, cool decomposition will continue (under proper conditions) until there is virtually nothing left.
Compost is not evil. It is, along with duff (dead leaves, twigs) the method by which an ecosystem conserves and recycles its mineral nutrients and improves the soil. As dead stuff is decomposed by either fungus or bacteria the mineral nutrients become available to the surrounding plants. The natural plant glues (lignins) that remain are the attracted to the soil particles (sand, silt, clay) below and causes them to aggregate into larger clumps creating a desirable "friable" soil. The problem is the way we use it.
In Nature dead stuff piles up on top of the ground. It is typical for a mature ecosystem to have 5" of dead stuff on top of the ground. On the surface there is ample oxygen to decompose the material aerobically.
In Nature the organic content of the soil below the surface averages less than 1%. This consists of plant fibers that are decomposing very, very slowly and charcoal. Charcoal is "cooked" organic matter. It differs from ash in that the carbon is still present, but has been altered by high heat, in conjunction with inadequate oxygen, to an inert form. Charcoal and fibers are known as the humus content. Even though it is a very small amount, it plays a big part in how the soil stores moisture and mineral nutrients. Charcoal attracts and stores minerals better than any other material known. Soil organisms thrive in the presence of humus.
There are areas in Nature where the ground is nearly 100% organic. These are dead areas known as peat bogs. Annuals and some perennials will grow, but woody plants have a difficult time. Peat bogs have very poor oxygen penetration (poor permeability) and decomposition occurs very slowly (recall the
bog people). Roots of plants require oxygen to survive and thrive. In bogs roots can only survive close to the surface. Lack of oxygen promotes root diseases.
The University of California notes that there are a few farms that operate on soil that is up to 40% organic. These farms can only grow annual crops (tree and perennial crops don't have a chance) and they have to carefully monitor irrigation. Does this sound like your garden?
The average farm in our state has soil with an organic content of 1%. The University of California Davis considers this to be the ideal amount.
Why then are most commercial potting soils 85-90% organic? Why do planter mix instructions recommend amending your soil up to 50% with their product? There is way to much literature suggesting than all soil problems can be cured with more compost. The more the better. How can this be?!!
What you may not realize is how most of the research is done. Most projects are funded for a relatively short period of time (months not years). The composted bark and sawdust that are the bulk of most amendments and potting soils are excellent growing mediums for up to 5 months. Peat moss and coconut coir are excellent for nearly 12 months. These materials can provide a superior growing environment than soil for a certain period of time. Unfortunately, after their time is "up" the "soil" becomes toxic and only a few hardy plants can survive. This happens after the research project has concluded.
When compost is used as a soil the relative coarse size of the particles can create extreme airflow. This highly permeable environment provides enough oxygen for root health and aerobic decomposition of the compost. Unfortunately as decomposition continues the permeability declines. Eventually there is not enough oxygen for aerobic decomposition to continue and/or to keep roots healthy. When anaerobic decomposition occurs sewer gasses are created. These gasses are not only smelly, they are toxic to roots and they turn the soil black.
Believe it or not, there are still "soil experts" that will tell you to amend your soil with lots of compost to turn it
rich and
black. When soil becomes "black" after adding compost, you have essentially killed it. You have created a toxic wasteland that can only be cured by carting the soil off to a landfill and replacing it with loam.
There are rich "black" soils in Nature. These soils have an unusually high charcoal content of between 1-3%. It doesn't make much charcoal to make the soil appear black. Just think what happens when you spill the toner from your printer. (Hey I wonder if toner would make a good soil amendment?) The black soils are usually the result of one extremely fast, hot wildfire. Volcanoes are a common cause. Extreme heat burns so instantaneously that there is not enough oxygen to oxidize organic matter (primarily the wood of thick branches and trunks) to ash. Charcoal can persist for thousands of years. Scientists note that they can easily find ancient human fire pits from over 10 thousand years ago because plants still grow significantly better over them.
OK WHAT DOES ALL THIS MEAN WHEN YOU ARE DEALING WITH CLAY SOIL?
Most important, don't make clay soil worse by amending with compost.
OK, a little is ok. Farms add compost to improve mineral content. They add about 5% by volume. They till it in to make certain that it doesn't blow away. This is equivalent to 1/2" turned into 10" of soil. Don't go overboard! I prefer that you just apply it to the surface and leave it there.
The only way to change clay is to keep it covered with organic matter and allow the products of decomposition to make it friable. This does take some time.
Before we go further here's a quick soil revue.
The other 99% of soil is a mix of sand, silt and clay. These are particles that differ primarily in size and shape. Most everyone knows what sand looks like. Silt is similar but about 1/10 the size. Clay is smaller yet and is flake-like in shape. Sand and silt are permeable because their shape leaves a lot of gaps between particles. Their gaps amounts to around 30% of volume. Clay packs much tighter and gaps may only be 1 or 2% of volume. Clay holds water and mineral nutrients much better due to its much larger surface area. Good soil is called "loam" and is a blend of sand, silt and clay. Farmers prefer "sandy loam" which is roughly 60% sand, 30% silt and 10% clay. If the clay content is above 30%, all of the air gaps are filled and the soil is called a clay soil. A few percentage points can make a huge difference.
You can make clay more permeable immediately by amending it. Here are materials that can work:
Pumice This volcanic rock is 70% air. Horticultural pumice is between pea gravel size to coarse sand sized pieces. It is the most efficient material we have. If you had solid clay (nobody is that unlucky) mixing 4 parts soil with 1 part pumice will make it permeable.
Sand If you had solid clay amending 3 parts with 7 parts sand (70%) will make it permeable. This is generally not practical, however most local soils are called sandy clay and have soil that is about 40% clay. "Clay" soils occur when the clay content is above 35%. Therefore it doesn't require nearly that much sand to drop the clay content below 35% to allow air gaps. The worst clay soils may require 1 part sand to 1 part soil to improve them.
Rice Hulls If you can get rice hulls they will work. Rice hulls are 90% silicon, the same mineral that sand and pumice are made of. They can hold their shape for decades. Anyone know a local source?
You can improve any soil by adding the following:
Charcoal Make black soil the proper way. Most garden centers sell small bags of charcoal. Anyone know a bulk supplier?
Leonardite This material is often called humic acids. Leonardite is very, very, very old compost and is found associated coal deposits. This may be as old as 300 million years! This material acts much like charcoal and is found in significant amounts in products like
John & Bob's Organic Soil Optimizer and
Gro-Power.
Most local soils can also benefit from adding:
Peat Moss Peat moss holds moisture and increases the acidity. Our soils are usually slightly alkaline to moderately alkaline and many plants grow better when acidified. Fortunately peat moss decomposes very slowly and won't cause trouble if the soil is otherwise permeable.
After installing plants liberally apply compost to the surface. Uncomposted leaves work fine also. Some landscapers love to liberally apply chicken manure before covering with compost. SOIL FOR BARE ROOT TREES
OK, the real reason I'm talking about compost is because it is bare root season. Bare root trees should be planted in the native soil, not compost found in potting soils and planter mixes. The majority of bare root plants and trees perform fine in the native soil. If the native soil is bedrock or pure clay it is permissible to amend with pumice, sand, or our Laguna Hills Nursery potting soils (pumice & peat moss). For some reason some homeowners and professional gardeners can't keep themselves from planting bare root trees in compost. We get rotten trees returned every year because of this. Usually the roots are slimy and have a sewer smell.
I've talked to our bare root tree suppliers about this subject. The field representatives have told me than orchards never, ever add compost to the planting hole. It kills the trees.
If you have a raised bed, purchase loam or sandy loam to fill it. If you can't find either, use sand or decomposed granite. Do not purchase anything called "top soil". I wish real topsoil was actually available. Commercially available topsoils are actually about 50-60% sandy loam (good) blended with 40-50% compost (terrible). I've tried these and the result is a root system that is but 10% of normal.
If you are planting bare root trees in containers do not use commercial potting soils (other than our Laguna Hills Nursery soils). A few might produce good results for up to 18 months, but many will kill plants within months. In these soils plants can look good during cool spring weather, but look poorly during warm weather due to an inadequate root system.
I plant almost all our bare root trees in containers in a blend of 1 part Laguna Hills Nursery ACID MIX Potting Soil with 1 part sand. For high water use plants like apple trees I will use 2 parts ACID MIX to 1 part sand. The higher peat moss content holds more water. For low water use trees like persimmon, I will increase the sand content.
Our Laguna Hills Nursery TROPICAL POTTING SOIL has an intermediate moisture holding capacity and can be used "as is" for just about any tree.
About 150 years ago French horticulturalists grew fruit trees in massive pots with a soil blend that was about 97% sand. This would be equivalent to growing in a soil that was 9 parts sand to 1 part native soil. This will work just fine, but would be slightly heavier than pure sand. Pure sand weights about 100 pounds per cubic foot. The advantages of being heavy are less blow over from wind and virtually no possibility of theft.
By the way, pure sand will work. You'll have to irrigate and fertilize frequently but the tree will look extremely healthy.
A 15" container requires about 1 cubic foot of soil. A 24" container requires about 5 cubic feet of soil.
All plants in containers would also be aided by adding a layer of compost on the top of the soil.
If you want your gardener to help you install these trees, just have him dig the holes.
