The Dirt Issue 42 – Feed Your Soil, Feed Your Plants

Posted on August 30, 2019
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So far in 2019 we’ve explored three practices that play an important role in soil husbandry. We’ve discussed why so many Skagit Valley farmers rely on v-ditches to remove excess water and keep soils aerated. We’ve also shown how the planting of cover crops and adoption of alternative methods of tillage are effective techniques to maintain or enhance soil tilth. Each of these methods are individual approaches to our August topic: “Growing soil: Feed the soil, feed your plants.”
How can soil “grow”?
Ever been called out by referring to soil as dirt? If so, you probably had an earful about the enormous difference between what is essentially dead “dirt” (broken down parent rock) and dynamic, living—yes, living—soil. Good soil, fertile soil, abounds with life. In our September issue of The Dirt, we will discuss the wonder of the soil food web and it’s vital role in making soil capable of producing healthy crops. This issue, however, we are focusing on feeding the soil itself in order to grow the soil.
All fertile soils have certain basic components: weathered or worn rock of various types and in particle sizes known as sand, silt, or clay; plus air; water; and organic matter. Organic matter is the key to growing soil. According to Building Soils for Better Crops, “A typical agricultural soil has 1% to 6% organic matter. It consists of three distinctly different parts—living organisms, fresh residues, and well-decomposed residues. These three parts of soil organic matter have been described as the living, the dead, and the very dead. The living part of soil organic matter includes a wide variety of microorganisms, such as bacteria, viruses, fungi, protozoa, and algae.
The fresh residues, or “dead” organic matter, consist of recently deceased microorganisms, insects, earthworms, old plant roots, crop residues, and recently added manures. This part of soil organic matter is the active, or easily decomposed, fraction. The well-decomposed organic material in soil, the “very dead,” is called humus. Because it is so stable and complex, the average age of humus in soils is usually more than 1,000 years. Humus holds on to some essential nutrients, storing them for slow release to plants.”

Organic matter is indispensable.
Growing soil is a function of adding organic matter. Organic matter in the form of manures, compost, cover crops, cash crop residues and the like, offers different advantages depending on what is used and how it is applied. By adding more organic matter than is lost or consumed over a given period of time, soil will grow.
But the question is what type of organic matter and how much? Fresh, still green plant residues tend to decompose very quickly giving up their nutritional value in an often-volatile manner. If growing plants cannot absorb that nutrition just as quickly, much of it is lost. Brown, dried residues such as plant stalks, leaves and harvesting culls, act as storehouses and give up their nutritional value much more slowly, providing sustenance over the long haul. There is merit in each. The quick release residues give a burst of energy to a young plant giving it just what it needs to become established. The slower release residues provide the constant steady input that is necessary to reach maturity.
Manures, being rich in nitrogen, are much like the still green residues from plants. Composts, on the other hand, are already partially broken down and provide the slow, steady feeding that brings the crop to harvest.
Keep it coming. Keep it diverse.
The way it happens in nature, soil is built up by providing a continuous supply of organic matter. Successfully building up agricultural soils is much the same. By providing a rich mix of seasonably applied organic matter—selected to address the specific needs of a given field—the field is made more fertile. If other careful practices are followed to minimize soil structure degradation and prevent erosion, the resulting soil is fertile and stable. It is resilient in the face of climate threats. It is better able to absorb and release water as needed. It is more balanced in its nutrient load and it exhibits the loose, open structure that allows plants to get what they need when they need it.
Not an off-the-shelf solution.
A question for any farmer is where to source organic matter of enough diversity to add to the soil. Farmers with livestock have a somewhat easier time of it than farmers who need to acquire manure from off the farm, but any organic matter that can be generated on the farm itself is good. As it says in Building Soils for Better Crops, “What we really want in soil is a slow, steady burn of organic matter. In soil you get a steady burn by adding organic residues regularly and by not disturbing the soil too often or too greatly. Soil organic matter is the key to building and maintaining healthy soils because it has such great positive influences on essentially all soil properties.”