Azaleas - Give Them A Happy Home
Art Frazer, Alexandria, Virginia
Reprinted with permission from Columbia Nursery's newsletter "Azaleas"
Last spring we unexpectedly opened a can of worms, as they say. Our article on earthworms, "a gardener's best friend", caused many comments, requests for more information, and a few requests to sell earthworms. Besides the repulsive thought of "selling your best friend", we imagined that we had made it abundantly clear that buying earthworms is futile unless you provide the basically attractive soil conditions - and having done that, the earthworms will come to you - free. Perhaps we didn't stress what we meant by basically attractive soil conditions.
The earthworm revels in the same kind of soil that grows good flowers and vegetables. Except the earthworm is never content to leave good enough alone; it is ever busy improving the tilth and adding to the humus. Sad to say, this is a lesson many homeowners are slow to learn. Yet it is fundamental to successful gardening. You'll never advance to Green Thumb, First Class with poor soil - it's that basic.
What is poor soil? Unfortunately it's the kind of improved soil that is prevalent in northern Virginia and southern Maryland. It tends to be worse in our burgeoning suburbs because many builders excavate and dump subsoil on top of what is eventually supposed to be our yards. But even our original "top soil" is not a great deal better. George Washington was undoubtedly a great statesman and a pretty fair general, but he didn't do too well as a farmer on his several plantations - given the kind of soil which most of us have inherited.
Poor soil has one or more of the following characteristics: 1) a low level of mineral nutrients, in which case it is infertile (worn-out un-rejuvenated farm land); 2) a low percentage of humus, the decayed vegetative material that is the elixir of soil; 3) poor percolation, the inability of rain to soak through the soil particles causing surface run-off or. internal stagnation; and 4) poor aeration, inability of air to infiltrate normal spaces between the soil particles. The first problem is easy to correct by application of fertilizers. But the other problems also can be cured, as we shall see.
The predominantly clay soil, typical of our area, is such a problem because it has three of the above characteristics - little humus, and very poor percolation and aeration. Consequently it's a wet gooey mess when wet, and hard like concrete when it's dry. It's quite understandable. The soil particles are So very fine and tightly packed that water and air cannot easily penetrate. A good illustration - and an essential precaution before planting - is to dig a small hole being certain that it is at least 12 inches deep. Fill it with water and observe how long it takes the hole to drain completely dry. If it still has standing water after two hours, the drainage (percolation) is bad. Shrubs planted in such a hole may manage to survive a year or so, but, they will not thrive. In fact they are dying a slow death from what is popularly called "root rot". To mix good soil, or organic material, with the dirt used in planting a shrub in such a hole is considerably less than half an answer to the problem. Why? Because it has not corrected the basic condition. You still have a water tight basin (dug in impervious clay) into which surface water will flow and remain. The hole becomes a water-logged reservoir shutting off the essential oxygen supplies which the roots must have. There is a vast difference between moisture (small water droplets surrounding the soil particles) and soggy wetness.
This business of adequate percolation and aeration in soil is critical. Remember soil in good tilth is only about 40% minerals (i.e. "dirt"). A small part, up to 10% or more, is organic material - decaying, or decayed, leaves, etc., which produce the humus in the soil. But the other 50% is evenly divided between moisture (water) and air. BOTH absolutely essential for healthy roots. If soil moisture habitually exceeds 25%, the tender roots suffocate from lack of air and the shrub becomes sickly and eventually dies. (In a severe case the leaves of the plant will suddenly wilt and drop, just like a plant suffering from drought.) On the other hand if the moisture level drops much below 25% (accompanied by a corresponding increase in air) the roots cannot take in sufficient moisture to sustain the plant. Remember, moisture from the soil is constantly being taken in by the roots, moved upward through the plant, and evaporated through the pores in the leaves. Professor Clement Bowers ("Rhododendrons and Azaleas", p. 79) informs us that about 50% of a plant tissue is Water. As he points out, surprisingly large quantities of water are taken up through the roots for use as a solvent for plant food, as a medium of transportation, as a cooling mechanism in warm weather, and for other purposes.
Just as clay is an exceedingly poor soil structure so, at the other extreme, is sand. The texture is so coarse that percolation is excessive, moisture cannot be retained. Soil nutrients are leached out of the soil, and it is customarily deficient in humus.
What then is good soil structure? If your memory can turn back to grandpa's day (before suburbia) you recall having heard the term "loam". Even the word sounds good, it rolls mound nicely on your tongue. In short, it connoted good farm soil. Or perhaps the soil was described as "sandy loam", or "clay loam", almost as good. "Loam" describes soil having relatively equal amounts of clay, silt (coarse clay) and sand. Consequently, it allows good percolation and aeration, but has good moisture retention qualities.
But loam is not absolutely the last word in good soil, it only describes soil structure. The soil can have excellent structure and still be deficient in nutrients, or humus.
A rich or fertile soil is one that contains an abundance of organic (vegetative) material. It is, all things considered, the most active, and most important, portion of the soil. Your friendly helpers, the earthworms and the micro-organisms (soil bacteria and fungi) go to work on the vegetative material transforming it into humus. In the process, organic acids are released which act upon the mineral elements to release soil nutrients. So we have a chain reaction. The more organic material in the soil the greater the earthworm population and the greater the microorganism activity, as we discovered last spring. Speaking of microorganisms, I am told that in a singe ounce of fertile garden soil the number of microorganisms is greater than the entire human population on earth!
But the abundance of vegetative material in the soil, whether it be leaves, sawdust, peat moss, or kitchen garbage, with the resultant increase in humus, and released nutrients is only one of the benefits. The accompanying earthworm activities create a network of tunnels and galleries which facilitate the percolation of water and air, and unproved drainage. Simultaneously, as explained last spring, the earthworm castings and the produced humus tends to granulate the soil particles - that is the tightly packed clay is combined into larger particles - hence looser soil with more spaces for moisture and air.
Perhaps most important is the ability of humus to retain capillary moisture, the moisture that is retained by soil particles and slowly taken up by the plant roots. It has been ascertained that ordinary top soil may absorb moisture equivalent to perhaps 60 per cent of its weight. In contrast, "woods soil" normally high in humus can absorb moisture from 300-500 per cent of its weight.
Making a Silk Parse From a Sow's Ear
Long before the above fundamentals were understood, farmers had learned to plow cover crops and vegetable refuge into their farm land. The humus so produced enriched the soil far more than the chemical constituents of the plowed under material, and brought on the benefits we have just discovered.
How can we benefit from this experience, and how can we turn bad soil into good? Before we plunge ahead, let's pause a moment for some soul searching. I am not, by nature, attracted by punishing, hard work, in truth I'm as lazy as the next man. Nor, sometime appearances aside, do I possess a messianic dedication to detail. Finally, I do not want to be identified with the "organic cult", who have so often carried a few sound principles to fanatic and ludicrous extremes. I do admit, however, to being old-fashioned, certainly when it comes to gardening. Generally, we stick with time-proven techniques, not defiantly, but because they work best for us. Confessions over, I realize that wishing won't get the job done. Nor will shortcuts. I recall one gardener who spread two inches of sand all over the top of the landscape beds in hopes that by some mysterious action it would "get down in there and mix with the clay". As Buckminster Fuller once said, "I'm convinced you can't put out if you don't put in".
There are ways to overcome your soil problem. Ideally you can combine all three.
1. You can make raised beds, 6 to 10 inches above the surrounding soil, to assure decent drainage. This entails buying "top soil", and perhaps digging out and removing part of the present soil. Unfortunately top soil today is not the good rich loam of grandpa's day. More often it's dirt scrapped off the site of a future shopping center, and possibly no better than the soil you have. You're better off improving what you already have.
2. It's decidedly preferable to mix a generous load of sand with your clay. First spade or rototill the ground to a depth of 8 to 10 inches. Then, spread at least 4 inches of sand. Finally, thoroughly mix in the sand by spade, or rototiller (easier!). But remember although you have greatly improved the soil structure, you have not increased the humus.
3. The best approach, in our judgment, is to mix copious quantities of organic material into your soil. Ideally, you can mix in both organic material, and sand, and sand by the truckload is inexpensive. But we have found that organic material alone, in addition to providing humus, provides greatly improved porosity and aeration.
Our longtime favorite is sawdust, but leaves of sphagnum peat moss (not Michigan peat) are very good. But don't skimp. Upward to 50% sawdust is not too much. A good rule of thumb is 6 inches of sawdust plowed in with a rototiller to a depth of 8 to 10 inches. The important consideration here, as with sand, is the thorough mixing - most adequately accomplished with a rototiller. The rototiller blends the material into the soil just like your kitchen blender. But it is essential that you plow (rototill) the soil first, before spreading the sawdust and plowing a second time.
Do not insist on "old half-rotted sawdust". This myth has been disproved. In fact, we prefer fresh sawdust because it is decomposed more slowly. On the other hand microorganism activity in decomposing any vegetative material whether sawdust, leaves, or peat moss, utilizes some nitrogen which needs to be replaced. So for each wheelbarrow of sawdust, or its equivalent, we spread and plow in about one pound of acid nitrogen fertilizer. We also add superphosphate.
Rototillers, wonderful labor savers, can be rented for such projects. Rental centers usually (not always) have better models than hardware stores. Get the heaviest model you can find. It does a better job of breaking up hard clay, and is easier on your muscles.