An excerpt from Bill McKibben’s new book, A Grower’s Guide for Balancing Media-Based Soils, published by Acres U.S.A.
For nearly fifty years, I have been consulting on crops grown in all types of natural soils. This myriad of crops—from commercial corn, beans, potatoes, and tomatoes to pecans, coffee, dahlias, and roses—have mostly been grown in native soils in various climates, using a variety of cultural practices. Seldom did someone who came to me have a total cropping failure. Generally, they had a problem with overall production—whether it was the result of compaction on the soil physics side or nutrient deficiencies resulting in poor production and increased insect and disease pressure. Sometimes seed germination was poor, or plants started slowly and couldn’t handle various stressful situations.
After getting a soil test, I used various methods to improve the soil nutrient balance, based on the Albrecht philosophy on high cation exchange capacity (CEC) soils or the Strategic Level of Available Nutrients (SLAN) approach on low CEC soils. Cation exchange capacity, also known as total exchange capacity (TEC), is a measure of the soil’s capacity to hold and exchange positively charged plant nutrients like calcium, magnesium, and potassium. A high CEC soil has a cation exchange capacity greater than 10 milliequivalents of negative charge per 100 grams of oven-dried soil (meq/100 g), and a low CEC soil has a CEC less than 10 meq/100 g. Soils with a CEC between 7–12 meq/100g were best served by using a combination of both balancing strategies.
I measured my success rate by the number of returning clients. I worked with many of my local clients for over thirty years. The goal was to achieve new highs in yield and quality and minimize the lows in stressful years. Initially, this was accomplished with just a standard soil test from Logan Labs in Ohio, but as yields got better, more information was needed. That was why I began including paste analysis and tissue analysis in the soil test package.
Over the past ten years, primarily due to the cannabis industry, I have worked with a large number of growers using growing media, which is also known as “growing substrate,” “potting mix,” or “soilless media.” Many growers like to refer to their mix as a “living soil,” but virtually all crops grown in either a natural soil or a growing medium, with maybe the exception of hydroponics, are grown in a substrate containing living organisms. Living organisms are extremely important in crop production and contribute what I like to call the X-factor to yield and—most importantly—quality. Just compare a field-grown tomato and a hothouse tomato, and you’ll know what I mean.
What exactly am I referring to when I talk about growing media? There are two types of growing media. The first is a soilless mix of various organic components, like peat and an assortment of composts. There is virtually no natural soil in the mix. The second is a natural soil that has been modified by the addition of various organic materials,xxi such as peat and composts of various products such as leaves, bark, manure, etc. These materials have been added to such a level that they significantly change the bulk density. On the Logan Labs report, the density of growing media is referred to as media density. When enough organic material has been added to a soil to reduce the media density from 1.0 g/cm3, which is within the range for a normal soil, to a level of less than 0.75 g/cm3, I consider it to be a growing medium. The standard test for soil organic matter will usually be over 12-15 percent for this type of soil. The non-soil mixes will almost always be over 20 percent organic matter.
Whatever you choose to call your growing media is fine, but in this book I will be talking about soil chemistry because I am a soil chemist, not a microbiologist. My goal is to create a soil chemistry environment that is very conducive to microbial life. Microbial activity plays many significant roles in increasing nutrient availability. For example, microbes are essential for increasing phosphorus availability and breaking down elemental sulfur to lower pH.
I mentioned earlier that farmers growing in natural soils rarely have a total crop failure—barring a drought, flood, or some act of God—but it seemed as though growers using growing media with controlled watering, light, and temperature were having a higher percentage of crop failures. After looking at a number of test results, I quickly realized that growers were trying to treat their growing media like a natural soil—and that has proved to be WRONG.
A quick example, which will be discussed in detail later, is the matter of bulk density. After looking at a number of soil test results with extremely high levels of nutrients and consequently high salts, it occurred to me that people were not taking into account the bulk density of their mix and were overfertilizing. After discussing this issue with Susan Shaner, director of Logan Labs, we decided to include media density on all standard soil tests properly noted on the worksheet as growing media.
In this book I discuss some of the dos and don’ts of making your own mix. There are an unlimited number of ways to put together a growing medium, but hopefully I can point out some of the pitfalls of putting a mix together and increase your chance of success. I show a number of test results from ingredients that I have tested, as well as a number of final products on the market, many of which have different problems. There is no guarantee that buying a mix from a big box store or greenhouse will be the safest and easiest path to a successful grow. I show you the best test methods for identifying good or bad mixes. I give you my suggestions for the best nutrient levels needed to produce a successful crop in growing media. I admit that my desired levels are more conservative than many of the growing media producers in the marketplace. I also discuss the individual soil nutrients and their role in crop production. Lastly, I discuss irrigation water—whether it be reverse osmosis, well, or municipal water—and its effect on nutrient movement and buildup over the growing cycle.
Bill McKibben is an Ohio-based agronomist who has specialized in soil fertility balancing for more than 40 years. He is the author of The Art of Balancing Soil Nutrients, A Growers Guide for Balancing Soils, and the children’s book The Mystery of the Dying Giant Pumpkins, all published by Acres U.S.A.
