by Steve Turner
All of us who deal with plants – big or small, annual or perennial, flowers or foliage – have one thing in common. Working with the soil in which they grow. It is often the most overlooked aspect of the garden for new gardeners and the most cherished aspect for seasoned gardeners, who have grown to appreciate the value of good soil. It quite simply is the difference between success and failure for many gardens and can be the difference between the pleasure of gardening and the tediousness of work.
Many of you are familiar with terms like soil structure, pH, humus, etc. But what do these terms really mean and how do they effect our efforts to improve or maintain our soil’s fertility?
Let’s start by looking at pH. Most people understand it as whether the soil is acidic (less than 7.0), alkaline (more than 7.0), or neutral (7.0), and that different plants prefer different ranges, but why? In different pH’s, nutrients move at different rates through the soil. The more acidic the soil, the faster the rate. If a plant is acid-loving and is in alkaline soil, it will not get nutrients at a fast enough rate. Fertilizer in this situation would have no effect because it is not a matter of whether the nutrients are present, it is the rate at which they are available to the plant.
Now, if you need to raise or lower the soil pH for a particular plant, you will need to understand cations and how they affect your ability to change the soil. Cations are positive-charged nutrients in the soil. The most common, in order, are calcium, magnesium, potash, sodium and hydrogen. Calcium is the most important nutrient in your soil and its proper balance to all others is vital since it helps regulate the soil’s pH. Here is why. All nutrients either have a positive or negative charge, and in nature the normal flow of electrical charge is from positive to negative. All clay and humus particles carry a negative charge on their surface which attracts desirable and/or undesirable positive-charged nutrients or minerals to them. If in proper balance to calcium, the clay and humus will keep these nutrients loosely held in a soil solution, which means the nutrients are readily available to the plants’ negative-charged root tips. The measurement of this exchange is called the Cation Exchange Capacity or CEC of the soil, and is measured on most soil tests.
CEC is important in determining how easy it will or won’t be to change your soil pH. A high CEC will make it difficult to change. In a poor soil, for example, as soon as you obtain the proper pH, you then increase the CEC by adding organic matter or humus to the soil. This will increase the amount of negative-charged particles in the soil and help “lock in place” the pH by making it more difficult to change. So, if you are preparing a new soil area, your first concern is the pH. It is critical that you make all adjustments to the pH prior to adding organic matter.
While organic matter raises the CEC, salts in the soil are the enemy to proper CEC. All commercial or synthetic fertilizers contain salt; this is how they bind the nutrients together. Every time you add one of these to your soil, you are adding salt, which in turn lowers the CEC. They lower it by using up all the negative-charged particles in the soil and, as a result, the positive-charged nutrients just leach away. This increases the need for more frequent fertilizing, which in turn adds more salts to the soil, and thus begins a vicious circle that many gardeners fall into. This is why the use of organic fertilizers is beneficial. While it is true that plants cannot tell an organic nutrient from a synthetic one, the soil can. The additives in synthetic fertilizers cause problems for the soil as a whole and a lowered CEC is just one of them.
If you have a poor CEC and lots of salts in the soil, then odds are you have compaction also, which limits root growth and the flow of water and air through the soil. This decreases the activity of beneficial soil microbes and increases the activity of pathogens in the soil, which in turn causes diseases and so the vicious circle continues.
Humus is one of the keys to breaking this cycle. Humus (or “finished compost”) is organic matter that has been broken down by soil microbes as much as possible. These microbes in turn produce ideal nutrient storage for our plants. Humus can hold 3 to 4 times more water and nutrients than clay alone. It helps store water in the soil for drought periods as well as nutrients that plants can readily absorb as they need them. This holding capacity will help reduce the leaching of nutrients out of the soil and lessen the need for additional fertilizers.
Humus also helps reduce soil erosion. For example, an acre of soil with only 1 percent humus can only hold less than 1/2 inch of water; any more rain than that will simply erode the soil. However, if the humus content is increased to 6 percent, that acre will easily hold over two inches of rain in the same soil. For those of you having drainage problems, this is something to consider.
Soil microbes are the keys to producing humus – if they are not present in sufficient numbers, then all organic matter in the soils stays in its original form or takes much longer to break down into humus. It takes many different types of microbes to accomplish this and they do so in a very systematic fashion. To get an idea of how many organisms there are in the soil, at a depth of 1 to 3 inches in good garden soil, there will be on average 9,750,000 types of bacteria, 2,080,000 types of Actinomycetes (organisms classified between bacteria and fungi), 119,000 fungi, and 25,000 algae in a single gram of soil (Microbiology: An Introduction, 4th edition). These numbers drop rapidly as you go deeper in the soil, but remain pretty high until about 18 inches in depth.
If you live in a newer subdivision, most of the beneficial microbes left with the topsoil that was probably scraped off the land before construction started, and the remaining ones were severely reduced by the soil compaction that followed during construction. This is a problem for many gardeners and up until recent times most of the research was focused on nutrients and soil structure alone. In the last 10 to 20 years, the focus has shifted to the beneficial organisms that make up a living soil.
By getting back to soil basics, we can be much more successful in our gardening pursuits by increasing our soil’s fertility. To correct problems we need to focus on the soil as a whole and stop looking for that one “miracle solution” that will supposedly give us healthier plants. The soil is a vastly complex environment. It can be, and often is, damaged in many ways that need different solutions to help correct each of the individual problems. The terms we discussed should help put you on the right track. Start by doing a soil test to determine your soil’s pH and CEC. Then adjust the pH as necessary, add humus to raise the CEC, reduce or eliminate the use of synthetic fertilizers, and fine tune from there. Good luck!
Steve Turner, Certified Arborist, is from Arboricultural Services in Fenton, MI.