Digging Deep: Soil-Plant Relationships

By Jan Allen

The other day I marveled at the number of self help books at a local bookstore.  Volumes of reference books showed me how to neatly dissect my relationships with my spouse, children, parents, food, environment, colleagues, and self, not to mention my dog.

Forget self help; we need soil help.

One of the least understood mechanisms in the soil-plant relationship is that vegetation and its cycle of growth and decay is the essential feedstock for the creation of organic matter.  Organic matter is so important in the soil matrix that it is commonly cited as the source of both nutrients and the microbial population that facilitates nutrient transfer to plants. In this relationship the plant and soil system are interrelated. Plants put 60-80% of their energy back into the soil, about half of it in the form of exudates that feed the soil food web. Microbes and fungi rely on these plant exudates as their basic food source, and the plants rely on the microbes and fungi to make enzymes (which the plants cannot make themselves) to convert nutrients into plant-usable form.  In a functioning soil system, this cycle continuously repeats, where the plant debris is allowed to fall to the soil surface, or where compost is applied to replace the plant debris.

The soil universe is much more complex than the animal world in which we live. Where an animal ecosystem may have two or three trophic levels (e.g., where one species eats another), soil can easily have seven trophic levels (e.g., bacteria, flagellates, amoebae, ciliates, nematodes, mites, and earthworms).  In a typical garden soil there are tens of thousands of different organisms within these various trophic levels. The density and vitality of the microbial population depends on the return of decayed vegetative matter to the soil to replenish organic matter that has been lost or consumed.   As the amount of organic matter decreases, so does permeability, water retention, nutrient retention, and other characteristics of properly functioning soils.

Amazingly there were people writing about this before we were breathing (437 years ago; AD 1573; Thomas Tusser; Five Hundred Points of Good Husbandry. Seventy-two years ago, a thick volume titled “Soils and Men – Yearbook of Agriculture – 1938” was written.  After reading the summary I was captivated.  This book seemed prophetic.  It talks about viewing organic matter as a fuel in the soil, hydrology and rural-urban economic balances.  It talks about the loss of organic matter and its restoration, and the nature and use of organic amendments.  Is this cool or what?

Self help or soil help, I contend that all of our relationships are rooted in the health of our soil.  Even my dog’s.  She loves to put her muzzle deep into the soil just for fun.