Reconstructing Plant Nutrition
Soil Fertility and Plant Nutrition are related but distinct concepts with the former a qualitative issue and the latter a quantitative. Whereas soil can be loosely defined as ‘the material that forms at the interface of the atmosphere and the lithosphere which is capable of supporting plant growth’ (White 1997), fertility is more the degree to which the soil fulfils this function of ‘supporting plant growth’. It is therefore a more complex and relative concept that takes into consideration aspects such as soil composition and hydrology; properties that are not directly related to plant nutrition but are still important to plant growth.
Similarly plant nutrition has a very different meaning to the nutrition of animals. Plants are primary producers. They take the basic elements and by harnessing the power of sunlight combine these elements into the carbohydrates, sugars and proteins that sustain the rest of life. Plants build the organic compounds that animals break down: It is an important distinction and whilst both processes are called nutrition it is in name only that any similarity exists.
Origin of Concept of Plant Nutrition
The science of plant nutrition began when Justus Von Liebig (1850) published his discourse on the chemical nutrition of plants. Through observation and speculation rather than precise experimentation Liebig deduced that plants were composed of seven basic elements and that these were absorbed via the plants roots as dissolved elements in the soil solution, the mineral element theory. His initial work identified nine nutrients which over the last 100 years has now expanded to include a further 12. These are arranged into two essential groups of macro-and micro-nutrients.
This classic table arranges the nutrients depending on growth requirements as determined by plant tissue content. It reveals nothing about the specific function within the plant or the environmental origin and whilst it broadly correlates with plant need it is little more than a list of elements arranged from the concentrations found in the ashed remains.
It bears little relation to the actual quantities required for complete growth (seed to senescence) or the function of the nutrient within the plant.
Furthermore it effectively denies the concept of nutrient turnover within the plants life cycle or variation in nutrient requirement and function depending on the stage of plant development (seed – vegetative – flower – fruit – senescence). So whilst it is crudely based on plant need it sheds no information whatsoever on the environmental origin, supply, reserve or fate of these nutrients within a system.
An alternative table, the metal/non-metal system, identifies a distinction in terms of chemical property; and whilst this roughly correlates with soil mechanisms it is not the determining factor.
So whilst the metal/non-metal system has some bearing on the physiochemical properties of the mineral elements in plant nutrition it is still little more than a list that fortuitously identifies a property (ionic charge) relevant to soil processes. From an applied context it fails to relate the elements to either their soil function or their function in plant nutrition.
Plant nutrition relates to the function and fate of the elements required for plant growth: a function that may vary depending on life cycle stage and or environmental conditions. Soil fertility relates in part to the ability of the soil to supply those elements. Therefore plant nutrition and soil fertility are interdependent concepts and any management of one implies a corresponding requirement to manage the other.
However both the two current systems, particularly from an applied perspective, are inadequate with both failing to relate to either the fate of an element within the plant or the soil mechanisms that maintain them and thus leading to the belief that “as a general rule there is a great discrepancy between the mineral nutrient concentrations in the soil and the mineral nutrient requirement of the plants”(Marschner 1996).
The 3-4-5 Dynamic Plant Nutrition Model
If we are to achieve a sustainable agriculture then we need to understand plant nutrition in terms of the supply of elements within the soil system as well as their metabolic function within the plant. The 3-4-5 Dynamic Plant Nutrient Model that I propose here and have been working to within this document reflects both the principal soil mechanism that maintain the 19 soil nutrients of importance, their ionic status, role in plant metabolism and degree of essentialness. Unlike the two proceeding systems The 3-4-5 Dynamic Plant Nutrient Model is geared towards application rather than classification.
Rather interestingly the concept of arranging the elements according to biochemical rather than physiochemical properties is not a new one with, at the University of Minnesota, a spiral periodic table to better understand microbial bio-catalytic reactions and bio-degradation pathways having been adopted.
The UM-BBD biochemical periodic tables are designed to provide an overview of microbial interactions with essential and non-essential chemical elements. Through two representations of the periodic table: the traditional periodic table and the spiral periodic table the UM-BBD group maintains a database on microbial reactions and pathways in which they describe the spiral table as:
“a more biologically relevant representation of the chemical elements“.
A statement I wholeheartedly agree with. The spiral periodic table is one that places hydrogen in a central position with the other primary elements of life; Oxygen, Carbon, Nitrogen, Phosphorus, and Sulphur clustered about. These first three (Oxygen, Carbon and Hydrogen) are the ‘God elements‘ of the 3-4-5 DNM and are supplied to the plant via the air (CO2 and O2) and water (H2O). It is only the last three in this group, Nitrogen, Phosphorus and Sulphur; the biotic elements in the 3-4-5DNM which are maintained within the soil matrix and of concern to us.
Similarly the four basic elements of the 3-4-5DNM are grouped together within the spiral periodic table as the major cations and the minor nutrients (with the notable exception of Boron) all within the Transition metals group XII-III.
If we compare the two graphical representations we can see the similarities between the two spiral periodic table and the 3-4-5 DNM.
It though, to this author at least remain a mystery as to why botanists, biologists and agronomists continue to work with the classical periodic table when it neither reflects the mechanisms within the system that maintain the supply or provides insight into the eventual use within the biotic.
The Biotic Nutrients (N,P,S)
The Basic Nutrients (Ca/Mg/K/Na)
The Minor Nutrients (Fe/Mn/Cu/Zn/B)
The Incidental Nutrients (Cl/Mo/Ni/Co/Se/Si/Al)
References and External Links
Marschner (1996) Mineral Nutrition of Higher Plants
plant mineral nutrition a nice pdf from Shao Jian Zheng of the Bioscience dept of the Zhejiang University (China) that draws heavily from Marschner
University of Minnesota BBD
Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011