Too Much Organic Matter

Fall 2009

Compost is great stuff. The material shown here is recycled into a vibrant soil that feeds the beautiful perennial flowers and herbs on MOFGA’s grounds. Too much compost or other organic matter, however, can increase the phosphorus concentration in soils to the point where the element may become a pollutant. So have your soil tested regularly to make sure it holds 20 to 40 pounds per acre of available phosphorus. English photo.

By Eric Sideman, Ph.D.

I started working for MOFGA more than 20 years ago as the association’s “answer man,” giving organic growers solutions to problems as well as explanations for doing things one way rather than another on their farms and in their gardens. I am amazed at how many of my answers are the same today as back then.
But a few are different. One of my early questions in the ’80s was, “Can I have too much organic matter in my soil?” I thought about it for a minute, which is a long time of silence on a phone call, and then, because I couldn’t think of any negative reasons, said, “The more, the better.”
That was the wrong answer. Now I know that too much organic matter could be a problem.

Properties of a Good Soil
Managing soil organic matter is the foundation of soil care, and soil care is the foundation of organic crop production. Crops need 18 chemical elements to grow and can be grown without any soil if these nutrients are available in the proper proportion. But that is not organic production.
Organic production really does reflect that now age-old slogan of Robert Rodale, “Feed the soil, not the crop.” The keystone of organic farming and gardening is the idea of producing healthful food in a way that does not harm the environment and that improves the soil for future production.
Managing organic matter well gets to the heart of this. Supplying the nutrients needed to grow crops from decomposing, local organic matter has much less impact on the environment than getting nutrients from bags of manufactured synthetic chemicals, especially from chemicals from afar, such as petroleum, which is the basis of most nitrogen fertilizers. And, decomposing organic matter improves the soil.
Whether you refer to it as soil health or soil quality, what farmers and gardeners are looking for in a good soil is one that supports crop growth. A good soil will supply crops with the nutrients they need and will hold water and air. I will come back to nutrients but want to consider air and water first.
Everyone knows that crops need water, and it is easy to picture organic matter benefiting soil water-holding capacity – soaking up water like a sponge. Crops need air in the soil too. Every living plant cell needs oxygen, but plants cannot move oxygen from the atmosphere down to their roots. (Of course there are a few exceptions, such as rice.) That means the soil must contain oxygen. This oxygen is stored in the air in the spaces between soil particles.
Because soil lacking spaces will lack oxygen, roots will not be able to take up oxygen. Or, if the spaces are tiny, they tend to get filled with water, which displaces air – so roots die from lack of oxygen. This is what happened to many of our plants this summer: A month or more of rain resulted in yellow, dwarfed plants.

Here’s to Your Tilth

Improving the size and stability of the spaces between soil particles is often referred to as building soil tilth. A soil with good tilth is porous and allows water to enter rather than running off the surface, and allows excess water to percolate through instead of becoming waterlogged. Larger spaces occur when soil is composed of larger particles.
You cannot increase the basic size of soil particles (sand, silt and clay), but you can get the soil particles to stick together, forming aggregates. This aggregation of soil particles is referred to as soil structure.
A porous soil is well aerated, holds some water but does not become waterlogged, and enables good root growth and exploration. Soil aggregates are held together by sticky substances produced when organic matter decomposes.
Organic matter, being the remnants of living organisms, is loaded with just the right nutrients needed for plant growth. However, most of the nutrients in organic matter are in large, complex molecules, such as proteins, fats and carbohydrates, and cannot be used by plants. Before these nutrients become soluble and can be absorbed by plant roots, the organic matter has to be decomposed by organisms living in the soil. Some are big, such as earthworms and pill bugs, and some are microscopic, such as fungi and bacteria.
These decomposers use organic matter as a food source, and their waste product includes simple minerals that can be absorbed by plants. The process of converting minerals in organic matter to simple, plant-available nutrients is called mineralization.

For example, proteins are mineralized to ammonium ions (NH4+) and then to nitrate ions (NO3-), which plants take up and use to build new proteins. Mineralization is recycling and supplying crops with an environmentally friendly source of nutrients.
A healthy and diverse population of decomposers is key to efficient recycling and abundant plant nutrition. So organic matter is key to soil health not only because it is a reservoir of plant nutrients, but also because it is the food that maintains a good population of decomposers. Feed the soil and it will feed the plant.
If organic matter is fundamental to building good soils, how can you have too much of it? I guess I just wasn’t listening years ago when my elders told me you can have too much of a good thing. The problem with organic matter is that while plant nutrients are needed for crop growth, they can also stimulate excessive growth of the wrong plants – mainly, algae.
In freshwater systems, such as ponds and lakes, excessive phosphorus will lead to excessive growth of algae. Then, when the algae die, their decomposition uses up the oxygen in the water. That lack of oxygen makes fish die.
Excess phosphorus pollution in lakes and ponds is a major problem caused by lawn keepers, gardeners and farmers. Misuse of fertilizer and livestock manure is a problem, but excess organic matter can be a problem too. Remember that organic matter in the soil is a reservoir of nutrients and that decomposition of organic matter mineralizes those nutrients. So, if that reservoir is too large, then the amount of nutrients available from mineralization at any time may also be too large.
Good management of organic matter not only includes adding enough to build a good reservoir of crop nutrients and to build good soil structure, it also includes monitoring what you are doing and not adding too much. The most serious concern is phosphorus. No farm, lawn or garden, whether organic or conventional, should become a source of phosphorus pollution.
Test your soil. If your soil test results say that your available phosphorus is 20 to 40 pounds per acre, your soil is in good shape. If it is a bit more than 40 pounds per acre, keep an eye on it and reduce soil amendments that contain phosphorus. If it is well over 40 pounds per acre, for example, 60 or 80 pounds per acre or more, then don’t add anything to your soil that contains phosphorus, including such organic matter as compost or manure. Other nutrient needs will have to be met with sources that include no phosphorus.

Eric is MOFGA’s organic crops specialist. You can contact him with questions about organic farming and gardening at 568-4142 or [email protected].

Scroll to Top
Sign up to receive our weekly newsletter of happenings at MOFGA.