Soil and Hydroponics

Scientifically speaking, plant growth in any rooting medium, including soil, is hydroponic, since the elements absorbed by plant roots must be in a water-based solution. The concentration and movement of the elements within this solution depend on the nature of the surrounding medium. For example, in soil, the soil solution and its elemental composition are the result of many interacting factors — an ever-changing, dynamic system of complex equilibrium chemistry (Lindsay, 1979; Tan, 1998; Peverill et al., 1999; Essington, 2004), in which the soil, soil microorganisms, and the plant root (Carson, 1974) each play unique and specific roles that alter the availability and eventual absorption by the plant root of the elements required for growth (Barber and Bouldin, 1984; Barber, 1995; Wignarajah, 1994). The complexity of the chemistry of the soil (nutrient) solution is significantly simplified when the support medium is an inert substance, such as sand, gravel, perlite, or rockwool, and becomes even simpler when the plant roots are suspended in a nutrient solution, as is the case in the standing aerated nutrient solution (see pages 123-126), nutrient film technique (NFT) (see page 127-141), and aeroponic (see pages 142-143) methods of hydroponic growing.

In soil, elemental uptake is affected by the movement of the elements within the soil solution and by the growth of plant roots; the various processes involved are discussed by Barber (1995) and Jones (1998a). The movement of elements along with soil water is called "mass flow"; it can carry elements to or away from plant roots by soil water movement. Within the soil solution itself, elements move from regions of high to low concentration by the physical process called "diffusion." Thus, as the ions of elements are absorbed by plant roots from the solution in immediate contact with the root surface, a concentration gradient is formed (a lower ion concentration exists in the soil solution next to the root, called the rhizosphere, as compared to the higher ion concentration away from the root), which provides a mechanism for resupply: ions flow (diffuse) from high to low areas of concentration. The plant also plays a role by root extension (growth) into the soil mass, bringing greater contact between root surfaces and the soil mass.

Much of the complexity of the root-soil phenomenon is reduced in hydro-ponic systems, where the plant roots are periodically bathed with a moving nutrient solution that contains the essential elements required by the plant. The flow (application) of the nutrient solution acts much like the mass flow behavior in soil systems. Therefore, the impact of diffusion and root extension on elemental availability and root uptake is reduced. It should be noted that in a soil-plant system, only a very small portion of the soil makes physical contact with plant roots, whereas in most hydroponic systems, plant roots are exposed to almost the full volume of nutrient solution. Such an extensive exposure of rooting surface to the nutrient solution has advantages, but it also poses problems that will be discussed in more detail later.

Nutrient element uptake by plants that are grown in a soilless organic medium, such as peat, pinebark, and coir, will act more like that occurring in soil where the principles of mass flow, diffusion, and root extension will significantly affect plant growth. Similarly, plants that are grown in an inorganic medium, such as vermiculite, zeolite, or expanded clay, substances that have a cation exchange capacity, will also act in a similar manner to plants grown in soil.

There are those who would consider soil growing as a system that is "out of control," while hydroponics is classed as a system "for control." This would seem at first glance to be a reasonable assessment, although not entirely true in practice. A soil system is indeed difficult to keep in control due to the complex inorganic-organic and biological nature of soil, as well as the interaction of plant roots with soil processes. Plants growing in soil are frequently competitors for the essential elements in the soil solution with other organisms (bacteria, fungi, etc.) present in the soil. These interactive processes and competition can be minimized in a hydroponic system. Therefore, the grower has the ability to regulate the composition of the nutrient solution and, in turn, control plant growth to a considerable degree. The challenge for the hydroponic grower is the control of the nutrient solution composition, a topic that will be dealt with in some detail in this book. It should also be remembered that in soil, the soil itself acts as a "buffer" that can be beneficial to plant growth, while in most hydroponic growing systems, no such buffer characteristic exists. Therefore, any error made in the composition and use of a nutrient solution can have far greater adverse impact on the plant than, say, an error made in the use of fertilizers or other amendments added to a soil. The source of the soil buffer capacity effect comes from the organic material in the soil plus the cation exchange phenomenon of both the organic and inorganic colloidal material in soil. Therefore, the use of any substance in a soilless mix that has both of these properties will also add some degree of buffer capacity to the rooting medium. An example would be the mixing of an inorganic rooting medium, such as perlite, with an organic medium, such as pinebark (see Chapter 10).

There have been those who have attempted to duplicate hydroponically what occurs in soil. The challenge is to maintain a constant level of nutrient element availability that is neither excessive or deficient. The unique charac teristic of most soils is that the concentration of elements in the soil solution is defined by equilibrium phenomena. Therefore a "fertile soil" is one in which the soil solution is kept maintained in the constant state of optimum elemental composition and content. Asher and Edwards (1978a,b), for example, have been able to duplicate the soil solution hydroponically in their study of plant nutrition on low-fertility soils. One of the procedures they used was exposing the plant roots to a rapid flow of a low concentration-ion balanced nutrient solution; the deficient, just adequate, and toxic ranges for the essential elements are given in Table 3.1. A similar effect would be obtained if a plant is grown in an infinite volume of nutrient solution in which removal of elements from the nutrient solution by plant roots does not alter the composition of the nutrient solution. Such a system could be classed as an "ideal" hydroponic growing system. The only hydroponic system in use today that would come close to this ideal is aeroponics (see pages 142-143).

Those holding the organic view of plant growth and development have considerable difficulty in accepting hydroponics as a natural system of plant production. Their contention is that unless the elements essential for plants are derived from an organic and/or natural source, plant growth and development are deficient and, therefore, unnatural. Scientific proof that such is

Table 3.1 Comparisons of Limiting Concentrations for Nine Elements in Some Nutrient Solutions Commonly Used for Experimental Purposes

Element

Deficient

Just Adequate

Toxic

Common Range in Nutrient Solutions

Concentration in Parts per Million (ppm)

Nitrogen (N)

As nitrate (NO3)

0.14 to 10

3.0 to 70

20 to 200

49 to 210

As ammonium (NH4)

0.007 to 5

0.03 to 25

0.4 to 100

0 to 154

Potassium (K)

Ammonium present

0.4 to 6

10 to 39

59 to 300

Ammonium absent

0.04 to 4

1.1 to 5

Calcium (Ca)

0.02 to 22

0.24 to 40

80 to 200

Magnesium (Mg)

0.05 to 6

0.2 to 9

24 to 60

Phosphorus (P)

0.003 to 4

0.007 to 2.6

0.03 to 4

15 to 192

Sulfur (S)

1.3

48 to 224

Concentration in

Parts per Billion

(1/1000 ppm)

Manganese (Mn)

0.55 to 71

0.55 to 2.310

16.5 to 3.850

110 to 550

Zinc (Zn)

0.65 to 3

3.25 to 16

195 to 390

0 to 146

Copper (Cu)

0.63

1.26

0 to 10

Source: Asher, C.J. and Edwards, D.G., 1978, pp. 13-28 in A.R. Ferguson, B.L. Bialaski, and J.B. Ferguson (Eds.), Proceedings 8th International Colloquium, Plant Analysis and Fertilizer Problems. Information Series No. 134. New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand.

Source: Asher, C.J. and Edwards, D.G., 1978, pp. 13-28 in A.R. Ferguson, B.L. Bialaski, and J.B. Ferguson (Eds.), Proceedings 8th International Colloquium, Plant Analysis and Fertilizer Problems. Information Series No. 134. New Zealand Department of Scientific and Industrial Research, Wellington, New Zealand.

the case is lacking, although many argue the natural point of view with considerable elegance, despite the lack of factual substantiation (Bezdicek, 1984). The possibility of growing organically using hydroponic procedures is discussed later.

Growing Soilless

Growing Soilless

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