The Substrate As A Source Of Nutrients

Only a few of the cultivated aquarium plants constitute genuine aquatic plants in which nutrient absorption occurs both through the root system as well as through the entire surface area. The majority of aquarium plants are marsh plants which, in contrast to genuine aquatic plants, will in most cases develop a strong root system and extract most of their nutrients required for growth from the substrate. In doing so, not all plants behave the same; the absorbed amount of nutrients and its composition are, in fact, specific to the individual species and highly dependent on the substrate. A chemical analysis of the plant's dry matter, conducted in a laboratory, can show the content and distribution of individual nutritional elements within the plant. The analysis will provide information about the occurrence of plants in specific substrates (indicator plants) and also serve to determine fertilization requirements. If plants are cultivated in the laboratory with nutrient solutions, the composition of which are known, and subsequently analyzed with the aid of the dry matter analysis to determine their content of individual nutrient elements, conclusions can be drawn about the nutrient requirements of plants which, in turn, will enable focused fertilization in cases of deficiency symptoms.

The structure of the soil is essentially determined by the size of the grains. This, in turn, defines the pore volume which plays an important role in the aeration and water circulation within the soil. Coarse-grained soils will let air, water and roots penetrate it more easily than fine-grained soils. Sand, clay, loam (mixture of sand and clay), lime and

Iron precipitation on Blyxa aubertii in a biotope in Sri Lanka.

Fine Grained Water System

Iron precipitation on Blyxa aubertii in a biotope in Sri Lanka.

humus soils, among others, can be distinguished. Pure clay soils are rich in nutrients and extremely fine-grained, thus permitting hardly any air or water exchange.

Loam soils (clay and sand parts at 20-50% each) with a high humus portion are the most suitable for plant growth. A large pore volume guarantees good aeration which in turn is also supported by living organisms existing within the soil, for example, dog periwinkles in aquariums.

Laterite-containing soils (red soils) and pure laterite soils are typical for tropical areas. They are identifiable by their red coloring and hardening, to which the term laterite (later [lat.] = brick) refers. They are extremely poor in nutrient content and in most cases rich in iron and/or aluminum. Nevertheless, they usually contain sufficient nutrients for aquatic plants because, due to the rapid mineralization of the dead plant matter as well as the generally low conductivity of the water, the released nutrients can immediately be reabsorbed by the plants.

The acid or alkaline reaction of the soil (pH-value) has a big influence on the water's constitution and the nutrient supply of the plants. Most soils within the tropics have a pH-value within the acid to neutral range; lime soils with alkaline reaction are found only in areas with low precipitation (around 1000 mm). Different plants have varying demands on the pH-value of both soil and water. Most tropical and subtropical aquatic plants grow in acid to neutral soils. There are, however, species, e.g., Vallisneria as well as several Potamogeton species, which prefer both a calcium-rich base and water with a pH-value within the alkaline range. Typical aquatic living environments with high pH-values are, for example, the great African lakes, Lake Malawi and Lake Tanganyika. Several of the lime-preferring species, however, display a large tolerance range so that their cultivation in the aquarium remains satisfactory even in a weak acid or neutral envi ronment. Lajie-avoiding species, in contrast, which prosper in very acid conditions in their natural habitats, react in a less adaptive manner. It was impossible, for example, to keep several Cryptocoryne species originating from Borneo in an aquarium without an appropriate acid environment.

Even though the substrate is of great importance as a source of nutrients, it has so far been largely neglected as an ecological factor within aquaristics. This is especially evident in the fact that, on the one hand, soil analyses of natural locations have hardly been conducted, resulting in only a very limited knowledge of it. On the other hand, daily aquaristic practice utilizes liquid fertilizers as a useful aid in the event of nutrient deficiency. They are added to the water, whereas special soil fertilizers are almost entirely absent from the retail market. By adding liquid fertilizers, however, an unwanted and difficult to control algae development is aided. The use of soil fertilizer, on the other hand, enables a focused fertilization of only those plants that display inhibited or bad growth.

For this reason the manufacturers of fertilizer products are called upon to supplement their liquid preparations with a soil fertilizer, for instance, in the form of small capsules, which, similar to preparations developed for the fertilization of potted plants, enables a targeted and effective fertilization of individual plants.

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