Soils and Soil Preparation
by Tony Avent
As Appeared in:
The American Hosta Society Journal 1993 volume 24, # 1,2
The most important factor is growing a hosta is owning a hosta...we are all well aware of the tough nature of a hosta plant. The most important factor in growing a hosta well, however is soil preparation, soil preparation, and soil preparation. Nowhere has this need been better demonstrated, than in the garden of Van and Shirley Wade of Belleville, Ohio.
Let me preface my remarks by saying that some areas of the midwest and northwest have naturally near perfect soils. These folks are most certainly skeptical of people such as myself that spend time extolling the virtues of soil preparation. Many others are satisfied with the way their hostas grow, and for them (at least until seeing the Wade garden) felt that they were doing a good job growing hostas.
Begin by remembering that the most important part of the hosta plant is what? Oh, course it's the leaves...right? This illustrates the common misconception that the most important part of something is that which we can see. In hostas as well as almost all other plants, the most important part is the root system...the underground network that provides moisture, nutrients, and anchors the plants in the ground.
We must first examine what the roots would like to be happy, because as the saying goes...happy roots are healthy roots and lead to healthy plants. Probably the most important factor in soil preparation is pH. Ph is defined by scientists as the percentage of hydrogen ions in the soil. In lay terms, pH is the acidity or alkalinity of the soil. The scale of pH in soils runs from 1 to 14, with 1 being the most acid and 14, the most alkaline.
Most garden plants, hostas included, prefer a pH near 6.0, which indicates a soil that is slightly acid. Many native soils in the south have a natural pH of 4.0, while soils in the midwest have pH's in excess of 8.0. While these numbers may seem relatively close, each number represents a ten fold increase in the acidity or alkalinity.
By example, the pH of the native soil in my garden is 4.0, which is 100 times as acid as the desired pH of 6.0. A good analogy is the use of a dilute solution of cleaning bleach that can be used with the bare hand. Imagine making the solution 100 times as strong, then sticking your hands into the solution...look ma...no skin. Begin thinking of your hands as roots of your hostas. As the solution becomes more acid (or alkaline), the root hairs, which absorb the nutrients and water are burned, rendering them unable to perform their primary function. Having the wrong pH consequently negates much of the effect of watering and fertilizing.
Due to the chemical effects of pH, as the soil acidity drops, certain essential plant elements are no longer available to the plants, while other chemical reactions render some soil elements toxic to the plants. Similar chemical reactions occur when the soil alkalinity is excessive...the reason for very few hosta growers in certain areas of the western United States.
Interestingly enough, pH is quite easy and inexpensive to counteract. Most state departments of agriculture offer soil testing for a nominal fee (free in some states). I recommend having a soil test taken ever few years. If you are uncertain about how to get a soil test in your area contact your county cooperative extension service.
I do not however recommend the home soil test kits. While some of these kits do a decent job of determining the soil acidity, they do not measure the buffering capacity of the soil. For example, two different soils of a pH 4, may take entirely different amounts of lime to raise the pH to 6.0. It is not unusual to need amounts from 10 pounds to 200 pounds to change identical pH's in different soils.
In areas of acid soil, powdered agricultural lime does the trick, while in alkaline soils, sulfur (usually used in the form of amonium sulfate) will work to drop the pH.
In using lime, it is critical that the material be mixed into the soil profile. When applied to the surface of existing landscapes, the soil neutralizing effect which takes 6 months to complete will only be effective as the lime moves through the soil at the rate of 1/2 inch per year. This poses two problems...if enough lime is added to change the soil profile all at one time, the surface pH will be too alkaline, while just below the surface, the soil remains acid. Ideally in established plantings, a small amount of lime can be applied every year until the entire soil profile has been raised.
Obviously the optimum scenario would involve having a soil test performed and add the correct amount of lime or sulfur when the beds are prepared. After this time a monitoring of the soil pH every couple of years would be adequate, with small amounts being surface applied as needed.
Many of the garden centers in the acid soil belt have begun to push pelletized lime. Pelletized lime (which I call yuppie lime) is lime that can be spread without turning you or your BMW a gritty shade of light grey. The lime particles have been glued together with a water soluble binder. While pelletized lime can be surfaced applied, a good rain is necessary to dissolve the pellets so that the lime can begin to take effect. Remember that the smaller the lime particles are ground, the faster the lime will work to change the pH.
Many folks make the mistake of using pelletized lime to the rototill into the beds. Since lime works on the "reach out and touch someone" principle, the lime cannot reach out to neutralize acid particles since it is too busy holding hands with thousands of other lime particles. Pelletized lime should always be dissolved on the surface prior to incorporation. I don't mean to degrade pelletized lime, as it is has two advantages...it is much easier to spread, and it pumps much more money back into the economy of the folks who produce and sell the product.
We often hear the advice to plant your hostas in an area that is moist, but well drained...hmmm. Have you ever bothered to think about this advice. Okay, let's begin with moist. Moist is the term that describes a soil condition which is somewhere between wet and dry. That was simple enough, so how about "well drained". On the surface this seems to be a contradiction, for if the area is well drained it should be dry...right?
Wrong, well drained simply means that the excess water drains from the soil, leaving the soil with adequate air space. While we are all familiar with the need for water in the soil, very few gardeners realize the need for air in the soil. Without air in the soil, there is not root growth or development.
If root growth is not taking place, then growth of the hosta also ceases or occurs at a very minimum level. With a lack of root growth, the roots are not able to absorb water or nutrients, even though both may be available in the soil. If excess water is available in a poorly drained soil, the plant could drown and suffer from drought at the same time.
In the world of ideals, which rarely exist outside the classroom, a good soil should be composed of 50% solids (soil particles), and 50% pore space. The soil pore space should be equally divided into 25% air space, and 25% water space. In a typical clay soil on a typical building lot, the total pore space is usually between 25 and 30 percent. Due to compaction and a lack or organic matter, this soil would probably be lacking in both air and water pore spaces. What this means when translated into English, is that the less pore space in the soil, the less the plants will grow.
Most parts of the United States are blessed with clay type soils. While these soils are cursed in both wet weather and dry, the soils are quite valuable for holding moisture and nutrients. Instead of cursing the soils, we need to spend time learning to modify and work with these soils to create an ideal environment for our hostas to grow.
One of the easiest ways to convert these less than desirable soils to productive growing environments is through the incorporation of organic matter. Organic matter is basically something that used to be alive. For most gardeners, this could mean a variety of materials from peat moss to compost to manure.
The act of simply digging into these compacted soils does wonders for adding aeration. The incorporation of organic materials insures that the compaction does not re-occur. The English practice of double digging has long been realized as an important factor in producing some of the lovely and lush English gardens. Due to the intensive labor intensive investment required, it is not often used in this country. In double digging, the top 12 inches of soil are removed and piled to the side. The 12 inches of soil below this layer are dug and mixed with large quantities of organic matter. Then the first layer is reapplied, after also being mixed with organic matter.
Probably the most used and overrated organic amendment is peat moss. On top of being outrageously expensive, peat moss is not very satisfactory at adding aeration to clay soils. Have you ever tried to add peat moss to a soil, then find it again a few days later? Where did it go? Notice that when you open a bag of peat moss it is compacted tightly, reminiscent of the vacuum packed bags of coffee. Is this really going to do us any good in eliminating compaction? Am I saying that peat moss is worse than nothing...no! I am only saying that there are much much better materials available. Save the peat moss for sandy soils which need such ingredients to hold more moisture.
One of my favorite soil amendments is composted leaves. Composted leaves are usually quite abundant in most areas of the country, and are usually free. In many large cities, the towns have large stockpiles that can be accessed free by residents. Other desirable organic materials includes animal manures, compost from your own compost pile, municipal composts, and a variety of other lumber byproducts.
The lumber industry used to have a disposal problem for many of its byproducts, but that was only until they met the gardening industry. Age old piles of sawdust quickly disappeared, when gardeners realized what a wonderful organic amendment was sitting untouched. While old sawdust is becoming much scarcer, a new product which has taken the south by storm...composted ground pine bark. This product, which differs greatly from the shredded mulch makes a wonderful soil amendment. Since it is slow to decompose, but low in nitrogen robbing cellulose, it is wonderful for adding aeration to clay soils.
There are new organic products entering the marketplace almost daily, from Zoo Doo to Mushroom Droppings to Kricket Krap. The key with any organic amendments is to have an adequate and economical supply. The organic amendment is also much more beneficial if it is high in beneficial microorganisms. I would encourage each of you to experiment with many of these different products and draw your own conclusions. For years, some gardeners have extolled the virtues of bone meal, blood meal, cottonseed meal, and the list goes on. What is important to remember is that the only thing that two gardeners ever agree on, is what the third gardener does wrong.
While organic products help greatly in holding moisture, and somewhat in increasing drainage, something more is needed in poorly drained soils. My preference is a material called pea gravel or #78 washed stone. This small washed gravel is spread over the newly tilled bed to a depth of 1/2 inch. The material is then tilled or spaded into the bed to help create the ultimate in permanent aeration. There are a number of new materials on the market to serve similar purposes including chicken grit, isolite (very expensive), and my favorite Stalite. Stalite (marketed under the name "Perma-till" is an expanded (popped like popcorn) slate material which looks like pea gravel but each particle is filled with air space.
Many gardeners have tried to use sand to accomplish the same results of adding drainage, but often wound up with disastrous results. In a clay based soil, sand and clay particles tend to interlock, resulting in a soil with much worse drainage than before. Here in North Carolina, the combination of clay and sand has made us the national leader in brick production. Only when a sand/clay mix becomes more than 70% sand, does drainage improve. The only exception is very organic soils. In these soils, a coarse washed sand will benefit the mix.
When blending organic amendments into the soil, it is important that the necessary nutrients be added at the same time. As you know, there are 16 essential elements for plant growth. Of these, the three that are typically added through fertilizers are nitrogen, phosphorous, and potassium. On a bag of fertilizer, these are the three numbers across the front of each bag. In each bed, I add at least 100 pounds of slow release fertilizer (we use a 21-7-21) for each 1000 square feet of bed area to be prepared. While many of the micronutrients are critical for plant growth, I recommend that these only be added after a soil test. These micronutrients can be extremely toxic if added in too great a quantity.
Nitrogen is probably the most visible element in plant growth. Slow and pale green growth is usually a sure indication that a plant is suffering from nitrogen deficiency. Any time large quantities of incompletely decomposed organic material is added to a planting site, additional nitrogen is beneficial. The bacteria which work to decompose organic matter, use nitrogen as a fuel. If the nitrogen is not adequate, the bacteria will fight the plants for the rights to the nitrogen.
Phosphorous is deficient in many areas around the country. Unlike nitrogen which is very mobile through the soil, phosphorous is not. Unless phosphorous is incorporated into the soil, it will take years of surface applications to get adequate amounts of phosphorous into the root zone. Most folks get enough phosphorous with their choice of a complete fertilizer. If however your soils are deficient in phosphorous, you may wish to supplement with a superphosphate (0-16-0) or triple superphosphate (0-46-0).
Many gardeners, especially the "organic" types, prefer the use of rock phosphate. This raw form of phosphorous is very slowly available in the soil. If rock phosphate is finely ground, and used in combination with a highly organic soil, it becomes a very valuable and long lasting phosphorous source.
Potassium is the final element in the puzzle. Potassium is also available in most commercial fertilizers. Like phosphorous, it moves slowly through the soil. Unless a soil test reveals that your soil is particularly low in potassium, the amounts provided in a standard fertilizer will be sufficient. In sandy soils, supplemental amounts of potassium are often needed. These can be applied in the form of potassium sulfate, potassium chloride, or potassium magnesium sulfate...the type depends on the native soils of the area. Consult your local extension service for specific advice. Let's see some BIG hostas next year!