Looking into the Soil
For their growth plants need energy (sun), water (from soil and air) and nutrients (from soil and air) the actual scientific consensus acknowledges 16 to 22 essential elements for plant nutrition. The main part of the plants mass consists of C (~45%), O2 (~42%) and H (~6%). The part of mineral elements (e.g. Ca, Mg, S, N, P, K, Zn, Mn, Cu, Fe, B, Na, Mo) only account for about 7% (Bergmann 1993, Schilling 2000).
The majority of mineral nutrients are absorbed out of the soil by the roots, more precisely out of the soil solution. During the absorption the solute elements can mutually hinder, boost or behave neutral (see figure “interactive structure”).
A long known fact is, that above a certain nutrient level, the ratios of the elements to one another are much more important for crop development than their absolute concentration (see figure). We can assume that more than 90% of Western Europe soils exceeded that level by far.
Nutrient Provision of the Soils
Based on the geological source material a soil can provide all or only parts of the essential nutrients in a sufficient quantity. The supply of nutrients in the soil is determined by the entire available nutrients, the nutrient concentration of the soil solution, the subsequent delivery rate of nutrients and the diffusion rate including the mass fluxes of ions to the plants roots, which vary from soil to soil. Moreover soil innate parameters like soil texture (content of sand, silt and clay), concentration of humus, biological activity and characteristics of sorption figure prominently. Tillage, crop rotation, type of cultivation and fertilization can directly influence the microbiological activity and with that the dynamic processes in the soil (see figure). Strong fixed elements can be mobilized, highly concentrated elements in solution can be converted into steadier bonds.
Soil Survey / „Fractional Analysis“
A pedological laboratory has to meet the modern needs of agriculture by adequately depicting the above mentioned kinetic-dynamic processes. Therefore it is necessary to bring “nature into the laboratory” Methods have to be applied which are capable to simulate the specific conditions of the site extensively.
Therefore we use the „Fractional Analysis“, a method co-developed by us which is a standardized procedure since 2004 (ÖNORM S2122-1).
Bond Types and availability Stages
The „Fractional Analysis“ considers the different bond types of the elements in the soil. Depending on the bond type the elements are more or less difficult to access by plants. The figure below shows the different availability stages using the example of potassium (K), for reasons of simplification the figure is limited to mineral bond types.
K can belong to the structural elements, is extremely strong bound and consequently doesn’t play a part in actual as well as middle-term plant nutrition (F4 – fraction 4).
As the distance to the inner zone increases, the weathering is increasing Fragments / molecules / ions from the weathered edge areas can become relevant for plant nutrition within the next 10 to 15 years (F3 – fraction 3).
The surfaces of the clay minerals are charged negatively and create the „sorption complex“ with the organic matter. In nature free charge differences cannot exist, so opposite charged elements are attached. This accretion isn’t a constant (ion)-bond, the attached elements can be exchanged by other elements (e.g. root excretion) (F2 – fraction 2). The „exchangeable cations“ create one of the most important pools for nutrition of plants and microorganisms and also for aggregate stability.
As the distance to the sorption complex increases its magnetic effect decreases. Gradually a zone arises where negative and positive loaded particles occur in equilibrium. Their composition respectively the relations of the solute elements are of prime importance. Plant roots are only able to absorb solute substances (exceptions are little fragments of amino acids and the process of endocytosis).
In order to get optimum nutrition the soil solution should hold an ideal composition of substance concentrations, substance ratios and substance species. There is no surplus of substances without a simultaneous lack! A surplus of K implies a simultaneous lack of Ca, Mg, (N).
Substances like for example the ion pairs NO3/Cl, SO4/MoO4, Mg/Mn heavily interfere each other’s absorption because of their chemical analogy. For instance a high concentration of SO4 entails a decreased absorption of MoO4, subsequently the protein metabolism is heavily disturbed because Mo is the central atom for the processes of nitrogenase and nitrate reductase.
Root excretions can’t operate selective. The excreted acid exchanges substances of the sorption complex, those substances join the soil solution and wash around the plant roots.
Used and further References
Bergmann (1993). Ernährungsstörungen bei Kulturpflanzen. 3. Auflage, Gustav Fischer Verlag.
Fachbeirat für Bodenfruchtbarkeit und Bodenschutz beim Bundesministerium für Land- und Forstwirtschaft, Umwelt und Wasserwirtschaft (BMLFUW) (2003). Richtlinien für die sachgerechte Düngung im Weinbau.
Husz (1974). Standortuntersuchung als Grundlage einer agrarökologischen Produktionsplanung. Habilitationsschrift, Univ. für Bodenkultur.
Schilling (2000). Pflanzenernährung und Düngung. Verlag Eugen Ulmer.
Scheffer-Schachtschabel (2010). Lehrbuch der Bodenkunde. 16. Auflage. Spektrum Akademischer Verlag.
Schröder (1992). Bodenkunde in Stichworten. Gebrüder Borntraeger Verlagsbuchhandlung.