In the mid- to late 1800s, investigators began their studies on determining how plants grow. One of the earliest theories proposed was known as the humus theory and it suggested that it was the earth—that is, the soil the plant was rooted in—that provided all the ingredients (food and structural material) that constituted the plant.
This theory was tested in what is known as the willow tree experiment. A willow tree sprig was planted in a carefully weighed tub of soil. Over a period of several months, the willow sprig became a small tree. Then it was harvested, dried and weighed, and the soil was reweighed.
Naturally, the willow tree had increased in weight in several degrees of magnitude, but there was essentially no loss in the weight of the soil. Thus, it was concluded that the soil was not the source for the increased weight of the willow sprig—which also partially undermined the humus theory.
However, there still exists an offshoot of the humus theory. This is the concept that the source and type of substances that roots absorb will affect the health of a plant and, in turn, the quality of produced product.
According to the theory, organically sustaining the media defines the well-being of the plant-rooting-media complex, while sustaining the media through inorganic methods leads to reduced plant growth and less-nutritious product quality. However, there exists little data based on comparative trials that supports such conclusions.
For hydroponic growing, this theory states that the inclusion of an organic substance(s) into a nutrient solution will enhance element utilization in both general absorption and plant-growth processes.
This assumes that either the organic substance(s) are root absorbable and contribute to essential biological processes, or its presence at the root interface enhances the biological activity around the root (and, in turn, the absorption process), thus resulting in greater productive plant growth.
How element absorption occurs through the roots is not a well-understood process. There are two proposed theories, one being based on a carrier concept and the other on what is called an ion-pump methodology.
Both processes require energy that is derived from root respiration, which requires the presence of oxygen (O2). In both concepts, the element absorbed by the root must be as an ion (either a cation or anion).
It is generally held by plant physiologists that an element in a combined form, or not existing as an ion, will not be root absorbed and then not translocated (it might be taken into the so-called free space within the root cellular structure, but it won’t be further translocated into the plant).
So, the question is: What role does the presence of an organic substance(s) in the solution surrounding the plant root play is the absorption process? At this time, a satisfactory answer does not exist.
Also according to the humus theory, the biological activity in the soil determines its fertility and, in turn, plant health and product quality. Today, the definition of a fertile soil has been fairly well-established; the primary parameters being physical (texture and structure), chemical (elemental content), physiochemical (cation and anion exchange capacity) and biological (organic matter content and microbial activity).
It is the combination of these parameters that determines fertility as measured by plant growth. For some growers, a healthy, fertile soil is biologically based and results in products that are different in terms of their nutritional value from those obtained from plants being grown in an inorganic environment.
It is therefore thought that the soil source of an essential element plays a significant role in determining availability and utilization of the root-absorbed element. Indeed, some believe a chemical source of an element, such as that derived from an applied inorganic fertilizer, will poison the process of availability and utilization.
That is why, for some, it is composting and compost use that play a major role in the establishment of a fertile soil, as composts, manure and crop residues provide the food needed by soil micro-organisms (whose activity establishes the criteria for a healthy soil).
It should be remembered, however, that the types and numbers of microflora in a soil are determined by the plants growing in that soil and not by the organic matter content of the soil nor that which is being added as either compost.
In a monoculture growing system, there will be few microflora species, but at high populations, while in a rotational plant growing system, there will a wide range of microflora species at moderate populations. In a monoculture system, the population of some micro-organisms might reach such a level as to become pathological, adversely affecting the following crop.
As such, rotating plants species or the use of a cover crop between growing seasons can change the existing microflora species and numbers, resulting in significantly improved plant growth and product yield. Seedling the soil with micro-organisms is only successful if there is sufficient food in the soil needed to support the organisms being added.
In conclusion, it has been suggested that both plant health and the quality of produced products are superior with organic-based growing systems; however, this cannot be confirmed until a sufficient number of studies have been conducted. Nonetheless, organic concepts based on the humus theory continue to attract much attention among growers and consumers.
More facts on organic growing systems
- In general, soil is relatively sterile as most of the micro-organisms found in a soil exist in the rhizosphere around plant roots.
- All soil microbes require a food source for their growth and survival.
- Soil microbes have the same elemental requirements as plants; therefore, their existence and activity correlates with the elemental status of a soil. Also, due to this direct competition, a high level of microbe activity can be detrimental to plant growth.
- Soil microbial activity is affected by physical factors (temperature, aeration, moisture and soil texture and structure) and the physio-chemical factors (pH, salinity, cation exchange capacity, organic matter content and nutrient element status).
- Aerobic organic material composting releases carbon dioxide into the atmosphere.
- Anaerobic organic material composting releases the gasses ammonia, carbon monoxide and ethylene into the atmosphere.
- Composting concentrates the elemental content of the material being composted.
- Compost that is the end product of microbial decay will not stimulate microbial activity when applied to a soil since all of the food substances have been exhausted.
- Bacteria and fungi can be present in the rooting medium, but at levels insufficient to be pathologic. They’ll await those conditions that will favor their growth, thereby becoming pathogenic.
- The organic matter content of a soil is determined by the physical and chemical properties of the soil, as well as temperature and cropping use, and usually remains at a fixed level that is not easily changed.
- Soils that are high in organic matter are difficult to manage, slow to warm, slow draining and require special treatment in their tillage and soil fertility management requirements.
- Adding an organic substance to a hydroponic nutrient solution invites the potential for root disease occurrence.
- Organic substances in a hydroponic nutrient solution can interfere with water and elemental root absorption.
- Organic matter decomposition, or high microbial activity in a nutrient solution or within the soil solution, can consume enough oxygen that root respiration is reduced and, in turn, there occurs a reduction in water and element absorption.