Maine Organic Farmers and Gardeners Association

Basics of Organic Soil Fertility

by Eric Sideman, Ph.D.
Copyright 2006

In 1648, Jean-Baptiste van Helmont did a great experiment and had clear results, but he drew the wrong conclusion. Still, he was among the first on the path to understanding the role of soil in plant nutrition. He placed 200 pounds of soil in a pot and planted a willow branch. He weighed the plant at the start of the experiment (5 pounds), recorded how much water he gave it, then weighed the plant and the soil after five years. The willow weighed 169 pounds; the soil had lost only 2 ounces. Van Helmont concluded that the 164 pounds of new plant material arose from the water.

Wrong! Actually carbon dioxide from the air contributes most of the material making up plants. Not until the twentieth century did scientists learn for sure that the carbon and oxygen in plant material come from carbon dioxide, while water contributes only the hydrogen.

Of course, plant material is more than just carbon, oxygen and hydrogen, and although other elements amount to only a bit of the total weight, they are very important. How well a plant grows is most often based on the availability of these other nutrients, because carbon dioxide and water are usually plentiful. This is where soil comes into the picture. Plants get essential nutrients mostly from soil. Those needed in relatively large amounts, called macronutrients, include carbon, hydrogen and oxygen from air and water plus nitrogen, phosphorus, potassium, calcium, magnesium and sulfur from soil. Those needed in relatively small amounts, called micronutrients, are iron, manganese, boron, chlorine, zinc, copper, molybdenum and nickel. A soil that is abundant in these nutrients in the proper proportion is called fertile. (A few plants may require, accumulate or benefit from other nutrients, such as silicon, sodium and cobalt. See for an interesting discussion of a holistic way to consider plant nutrients.)

There are two basic approaches to soil fertilization. The first is to provide required nutrients to each crop in a soluble form that plants can use immediately. In other words, feed the plants directly. This approach can accurately meet a crop’s needs. During the twentieth century scientists learned how to make fertilizers from synthetic chemicals and how to provide plants with exactly what they need. The extreme of this approach is hydroponics, in which plants are grown without soil, and nutrients are supplied in water solutions in the proper proportion to grow beautiful crops.

The alternative approach to fertilization is building and maintaining stable nutrient levels in the soil using natural materials. The decomposition and natural chemical breakdown of these materials put the nutrients into forms that are available to crops. Organic farming and gardening is based on this second approach. Robert Rodale, one of the fathers of organic farming and gardening, is often quoted as saying, "Feed the soil and it will feed the crop." Organic farming is really all about taking care of the soil. Good crops can be grown using the former method of maintaining fertility with synthetic chemicals, but the great disadvantage of this approach is that it does nothing to improve the soil and the long-term productivity of our farms and gardens. In fact, the longer synthetic chemical fertilizers are used, the more dependent the soil becomes on them, because soil microbes have no organic matter for food and they disappear. I have heard such soils accurately referred to as dead.

The advantage of the organic approach is continued improvement of soil fertility, protection of natural physical and biological processes in the soil, and minimal environmental disturbance, because organic sources of nutrients are less likely to leach or run off and contaminate ground and/or surface water. Building long-term soil fertility is the kingpin of organic farming and gardening. The weakness of the organic approach is that it is not as exact a science as providing soluble nutrients, and determining precise applications of organic soil amendments is difficult, especially when microbes are limited by cold, wet or dry weather. But we put up with this challenge to protect our environment and take care of our soil.

Using natural and organic sources of nutrients not only ensures sufficient fertility to support good crop growth; their use also maintains a healthy soil with lots of biological activity to recycle and conserve nutrients and build soil structure. Good soil structure is very important, because it gives roots access to air and water while simultaneously allowing excess water to drain.

Sustainable soil fertility is based upon practices that build and conserve nutrients, maintain organic matter and promote soil microbiological activity. Organic matter plays a few key roles. It is the major source of recycled nutrients, it holds nutrients so that they don't leach, holds water, and, as it decays, releases "glues" that bind soil particles and build soil structure. An often-neglected function of organic matter is that fresh organic matter is needed to feed soil microbes. Practices that add organic matter include mulching, adding compost or manures and growing green manure crops.

First Step

The first step in approaching natural soil husbandry is the same as that for raising crops with synthetic chemicals: Get the soil tested. [At the Common Ground fair, stop by the Agriculture Demo Tent for a University of Maine Soil Test Kit from the University's table or from the MOFGA Technical Services table.] Results of the soil test provide the information you need to start building fertile soil.

Building fertile soil is accomplished by adding slow-release, finely ground, mineral-rich, natural rock powders and compost, and by using cover crops in rotation with cash or garden crops. The cover crops conserve the added nutrients and feed microbes. The pH, a measure of the acidity of the soil, is adjusted by adding ground limestone. Nutrients are most available to vegetable crops when the pH is between 6.0 and 7.0. Phosphorus is optimum when your soil test shows 20 to 40 pounds available per acre.

Measurements of the nutrients calcium, magnesium and potassium are given first as pounds per acre available for plant uptake. However, these numbers are not as important to the actual availability as are the numbers for phosphorus. The complicating factor has to do with the varying capacity of soils to hold and release calcium, magnesium and potassium. This holding capacity is called the cation exchange capacity (CEC).

Potassium, magnesium, calcium and hydrogen are cations (positively charged ions). Since opposites attract, these are held in the soil at negatively charged sites on soil particles and on particles of organic matter (hence the importance of organic matter in holding nutrients). The more negatively charged sites in the soil (i.e., the higher the CEC), the more cations the soil can hold. A soil with a high CEC (15) has the potential to be very fertile, i.e., to have a great reservoir of nutrient cations. A soil with a low CEC (<7) cannot hold a large reservoir.

The potential for high fertility measured as CEC is based upon soil texture, pH and organic matter content. Clay soils and soils with high organic matter content tend to have high CEC and potentially can hold high levels of cation nutrients. Sandy soils have low CEC and tend to lose cation nutrients to leaching. CEC also increases as the pH of a soil increases.

A soil with a high CEC may still have low cation nutrient fertility. Such a situation is possible if the CEC sites are filled with non-nutrient cations, such as aluminum or hydrogen. Conversely, a soil with a medium CEC and only moderate pounds per acre of nutrient cations may be fine if the sites are filled with nutrient cations in the proper relative proportions.

Percent Saturation (printed after CEC on the soil test result form) is better than pounds per acre for evaluating the balance of cation nutrients in soil, because it shows the relative level at which various nutrients occupy cation exchange sites, and consequently their availability to plants.
Desirable ranges of % saturation are:
Potassium (K) 3.5 – 5%

Magnesium (Mg) 10 – 25% (should be twice K and at least 10% of Ca)

Calcium (Ca) 60 – 80%

Acidity (represents non-nutrient cations, Al and H) < 10%

Nutrient balance is very important, because a very high concentration of one cation in the soil can adversely affect the uptake of other cations. For example, excess calcium (Ca) can induce a magnesium (Mg) deficiency and can reduce phosphorus availability. Excess potassium (K) can also suppress magnesium (Mg) uptake.

For a list of natural sources of nutrients, stop by the MOFGA Technical Services Table in the Agricultural Demo Tent at the Fair, or call the MOFGA office (568-4142).

Nitrogen, although not addressed by the soil test, is commonly the nutrient that is deficient in soils and often limits plant growth. The soil test does not measure soil nitrogen, because concentrations of this nutrient vary so much from day to day based on weather and microbial activity. Plants absorb nitrogen, a basic component of proteins, mostly as nitrate ions (NO3-). Nitrogen makes up about 80% of the atmosphere, but very little nitrogen is naturally in the nitrate form at any one time. Nitrate arises from microbial feeding on organic matter, i.e., decomposition. Of course, if you are feeding your crops with synthetic chemicals, you can use such fertilizers as ammonium nitrate or calcium nitrate, but organic growers use natural materials that are high in proteins, such as soybean meal, alfalfa meal, fish meal or livestock manure, and depend on microbes living in the soil.

For a deeper discussion on supplying nitrogen to crops using organic methods, stop by the MOFGA Technical Services Table in the Agricultural Demo Tent at the Fair or call the MOFGA office at 568-4142.

About the author: Eric Sideman is MOFGA’s organic crops specialist. You can contact him with your farming or gardening questions at 568-4142 or at