Soil Properties

And Nature the Old Nurse, took the child upon her knee,
Saying here is a story-book thy father has written for thee,
Come wonder with me into regions yet untrod,
And read what is still unread, in the manuscripts of God

Longfellow (5th birthday of Agassiz)

Introduction

In soil science the soil properties, particularly those of significance, are rarely the consequence of a single component. Therefore the measurement of a single soil property cannot offer insight in isolation.We can measure the pH but without knowing something about the other soil properties the measurement offers no insight whatsoever.

Similarly knowing the total nitrogen content without knowing details of the organic matter and microbial dynamics reveals nothing about the soil properties that regulate the availability of soil nitrogen. Thus if we want to know something in soil science we usually need to measure two or more soil properties and then use these measurements to infer the actual soil property we are interested in. In many respects it is an exercise in lateral thinking; one that is often imprecise revealing only tolerances and approximations of the soil properties which one must then interpret.

The following pages deal with the methods and data that needs to be gathered to start to determine the soil properties of a specific area. Whilst it is possible and desirable to acquire propriety soil maps, such maps are often at a far greater scale than is needed for routine management of plots within the map. So whilst soil maps are invaluable to the agronomist it is still a good idea to qualify those maps in relation to ones own piece of land.

As these pages are constructed they will hopefully take you down the road of understanding more about soil properties and how to measure and exploit them in a sustainable and considerate fashion. You should then be able to use these measurements to calculate and manage Calcium (lime), irrigation and nitrogen management.

The Physical, Chemical, Biological and Hydrological Soil Properties

Whilst we begin by saying that no one property should be used in isolation; for practical management it is necessary to first fracture the concept of soil properties into four distinct disciplines: the physical soil properties, the chemical soil properties, the biological soil properties and the hydrological soil properties.

the interdependency of the soil properties

These properties further extend beyond the confines of the soil environment. They are both separate, dependent upon and contribute towards the greater planetary cycles of the Geosphere, Biosphere, Atmosphere and Hydrosphere. Thus whilst each condition is as much a result of the way all the soil properties and processes interact, for practical purposes we need to consider each as if it were distinct.

The Physical Soil Properties

Within the physical body of the soil are the three physical states of solid, liquid and gas. This states are expressed as fractions of the whole volume that is filled with particles (solids) and pores (water or air) at or approaching field capacity* (the state of a soil following prolonged rain and natural drainage). The size and arrangement of the soil particles is though what determines the porosity, and through the chemically active clays and sequioxides, the availability of the principal basic nutrients (Calcium, Magnesium & Potassium) is largely controlled.

Thus the mineral state and in particular the particle size distribution is one of the most important factors in understanding and managing the soil environment. The simplest means of determining particle distribution is by a visual and hand analysis in the field with a more detailed analysis possible using grading sieves to separate the various fractions of sand, silt and clay.

The Chemical Soil Properties

Plants as primary producers require mineral elements which they remove from the soil and combine with Carbon Dioxide (CO2) (from the Atmosphere) and Water (H2O) (also taken from the soil matrix) to produce the sugars, carbohydrates and proteins, required for growth.

plant nutrition as manufacturing

Plants do this by using the energy from the Sun to combine those elements into the proteins and carbohydrates that animals, including Man, subsequently break down for their nutrition.

So whilst both processes are called nutrition, one consumes what the other has constructed. Thus if any similarity exists between them it is in name only.

Plant nutrition is concerned with the supply of those elements that a plant needs to build the proteins and carbohydrates to maintain the metabolic and physiological processes. Those elements being 22 in total.

 

The Soils Chemical properties however extend beyond 22 elements and to a greater extent encompass the entire elemental suite of planet Earth. Fortunately with respect to plant nutrition we are not concerned with the entire elemental suite of planet Earth and with many plant nutrients being passively and adequately supplied by the abiotic systems we can largely ignore the soil properties and processes that supply 10 of the elements needed for plant growth.

This leaves 12 mineral elements of importance which can be arranged into 3 groups based on the soil properties and mechanism that supply them. This three groups similarly reflect the function of the nutrient in the plant and are the basis of the 3-4-5 Dynamic Nutrient Model below.

plant nutrition and soil fertility

So whilst a plant requires up to 22 elements for optimum growth we need concern ourselves with only 12 arranged into three functional groups that reflect both the mechanisms that maintain these nutrients and the primary use within the plant.These three groups are the biotic, the basic and the minor nutrients of the 3-4-5 nutrient model.

The Biological Soil Properties

Although identified early on (Dockuchaiev) that that soil organisms played a crucial role in the formation of soil properties; and Jenny (1940) subsequently distinguished the plant, animal and microbial contributions: it was not until relatively recently that the microbiological component was recognised as the most significant biological component in soil formation and maintenance of soil properties.

The microbial component is not only the largest but composed of the bacteria, fungi and soil animals (protozoa, annelids, mites, etc) it is also the most diverse and numerous. The collectively contribution of the soil microbiology to soil properties as well as it’s contribution to the maintenance of the Earth itself outweighs both the mass and the contribution of the remainder of worlds biological component by a long margin.

This microbial biomass dominates the whole soil system and whilst invisible to the naked eye it is the single largest living mass in the soil. It is responsible for regulating and recycling the soils organic matter and releasing elements (principally N, S & P) through mineralisation (converting organically bound elements into soluble mineral elements). Elements that are subsequently immobilized when taken up by another organism and once that organism dies microbial action will again liberate the mineral element.

The majority of this cycling though occurs exclusively within the microbial biomass with plant nutrition secondary and subordinate to the process of microbial nutrition.

mineralisation and immobilisation by the soil microbiology

Correctly managed, the carbon cycle and the related microbial activity within the soil can be manipulated to produce mineralisation patterns which better suit agricultural practices. In practice the more the soil is managed for the microbial community the better and more efficiently it is managed for the plant.

Furthermore a well managed microbial environment is one that is rich in soil organic carbon, carbon that would otherwise be atmospheric and contributing to global warming. Thus correctly managed the soil microbial community can be harnessed for both better plant nutrition and for the sequestration of atmospheric CO2.

The Hydrological Soil Properties

The Hydrological Soil Properties are the fourth component that connect and make the other three, The biological, chemical and physical properties one. They are similarly and paradoxically the most intimate; being engaged at a microscopic level, whilst at the same time the most detached of all the properties. For whilst the other three are dependent on the hydrology to function and interact; it is not dependent on them but part of a greater planet wide hydrological cycle. Thus the hydrology not only connects the other three and allows the system to function but connects that system with the whole biosphere.

The 3-4-5 Nutrient Model

The 3-4-5 nutrient model arranges the soils chemical properties according to the primary mechanisms that regulate their supply. In principal two mechanism dominate; the carbon cycle, which controls the supply of the elements of Nitrogen, Phosphorus and Sulphur; and the Clay Mineralogy which through the Cation Exchange regulates the supply of the Basic elements Calcium. Magnesium, Potassium and Sodium. These two mechanisms actually control and regulate the supply of nearly all the plant nutrients but are dominated and ‘regulated’ by just seven.

The Biotic (N, P & S) used by plants to build proteins and maintained in the soil by the soils organic matter reservoir (carbon cycle). Subsequently released into solution and made accessible to plants by bacterial action.

The Basic (Ca, Mg, K & Na) used by plants for metabolic purposes and maintained by the clay mineralogy. No natural mechanism for replacing loses; therefore the maintenance of these elements is an essential requirement in sustainable agriculture.

The Minor Nutrients (Fe, Mn, Cu, Zn, B) used by the plant for regulatory purposes these elements are required in very small amounts. Although largely controlled by the above 2 mechanisms they are relatively insignificant to those mechanisms and so are not considered as aspects of them. Deficiencies (when known) can be corrected by additions.

Sampling Strategies

Before any measurements are taken it it important that a suitable sampling strategy has first been devised. Not withstanding that there is considerable scope for such, the strategy chosen should conform to certain basic criteria: It should be both appropriate and feasible.

To this ends there are various strategies and statistical methods that can be adopted or combined, however for agricultural and cultivated land in general the simplest strategies are generally the best.

Free Cultural Works (CC-BY-NC-SA) Malcolm McEwen 2011

Comments are closed.