Compost, The Series, part 2 – Earthwise Agriculture

This the second part of our original series: “The Effective Use of Compost”

In Part 1 we defined compost, manure and synthetic fertilizers and briefly summarized the advantages and disadvantages of each. Now we will dive a bit deeper into why compost is generally superior to either of the other two major classes of soil fertility amendments.

The Benefits of Using Compost

1. SUPPLIES ORGANIC MATTER AND ASSOCIATED BENEFICIAL MICROBES:

Among its many other attributes compost supplies carbon-rich organic matter and associated beneficial microbes which buffer soil reactions, regulate mineral uptake in the plant, and encourage diversity in the soil foodweb. Most southern(tropical) and western(arid) soils are very low in stable organic matter, some as low as 0.25%, whereas 4-10 % is considered highly beneficial for plant growth. Synthetic fertilizers don’t supply significant soil carbon or beneficial microbes. Manure does supply soil carbon and some beneficial microbes, although the number and diversity of organisms is much reduced compared to compost.

2. HELPS REGULATE MINERAL INTERACTIONS:

There are countless mineral reactions taking place in the soil (see soil mineral wheel) at any given moment. It would be impossible for anyone to characterize, much less optimize all of these interactions. Organic matter in the soil naturally regulates and buffers these reactions and regulates soil mineral exchange in a number of ways. These include:

1. Acting as an electrically charged “sponge” to “soak up” excess nutrients, holding them loosely for absorption by the plant roots as needed.

2. Maintaining a more even distribution of free water in the rhizosphere.

3. Increasing the rate of nutrient cycling through the encouragement of a diversity of organisms in the soil.

In these ways and more compost promotes balanced uptake of minerals by the root system of the plant. Manure provides a smaller measure of these benefits, while synthetic fertilizers may actually hinder these processes (by reducing organic matter content of the soil) if used as the only fertility source over a long period of time.

3. IMPROVES SOIL STRUCTURE:

Compost supplies a wide diversity of microorganisms as well as organic matter to support their growth. The addition of compost makes “poor” (or poorly structured) soils more productive in a number of ways:

1. Sand particles are very large in size relative to clay particles – as much as 1000 times larger! Because of this, sandy soils usually have good internal drainage – a plus – but they have trouble holding and storing water and nutrients because of their low level of electrical attraction. Water and mineral nutrients are both electrically charged. One of the ways compost makes sandy soils more “loamy” is by increasing the number of electrically charged sites (both positive and negative) available for nutrient attachment and water absorption.

2. Clay soils have much less pore space, and have much more surface area, and are much more electrical than sandy soils – a thousand times more in many cases. They thus have the ability to hold water and nutrients very well, in fact their problem is often poor internal drainage and reduced oxygen in the root zone. Water saturation of the root zone drives out needed oxygen in the soil, promoting pathogenic fungi and limiting nutrient uptake. Compost promotes increased beneficial biological soil life, which in turn exudes starchy, sticky materials such as polysaccharides and glomalin. These materials then “glue” the small clay particles into clumps or aggregates, mimicking the size of the larger sand particles and increasing the particle size in clay soils by improving porosity, permeability, resistance to compaction, and aeration.

The effect of compost on soil structure can initiate a “virtuous cycle” of soil improvement. As the soil becomes better drained, more minerally balanced and better aerated, microorganisms flourish, improving structure further. This not only creates a better environment for nutrient uptake by plant roots, it also allows easier penetration of the roots into the soil. As in the other cases noted above, manure provides some of these positive effects, but in a more limited way, while synthetic fertilizers have little or no effect for the same reasons outlined in number 2 above.

4. PROVIDES ADEQUATE, BALANCED NUTRITION:

At a rate of 3-10 tons/acre (depending on compost analysis, soil condition, and crop needs) compost will provide a proportionally balanced supply of organically bound minerals that will meet the majority of the nutritional needs of a growing crop, including most micronutrients. Note: No matter what base fertilizer program is used, the use of additional foliar and soil nutrients will almost always be indicated during the season to optimize crop yield and quality. Manure and synthetic fertilizers can also supply balanced and comprehensive fertility when applied at appropriate rates. Compost and manure will only release a portion of their total nutrients in the first year of application. The leftover portion is mainly conserved for the following years of production. Synthetic fertilizers are typically applied in only the amount needed for the next crop, partly because of their tendency to leach with water movement over time.

In the next part of this series we will examine some of the ways and reasons to most effectively incorporate compost, manure, green manure, and gasp! “Synthetic” fertilizers into a sustainable farming system.