Instructor: Mike Goss

Land Resource Science Rm 140, Richards Bldg  
Ext. 2491  
 

INTRODUCTION 

To develop an understanding of the interrelationships amongst soil characteristics, root growth, water and nutrient absorption and the mineral nutrition of plants. 

On completion of the course the student should be able to explain the scientific basis of recommendations for soil management for crop production, and some of the observations of variations in plant growth in agricultural and ecological contexts. 

A study outline is provided for each major subject area, which indicates the concepts to be discussed and the associated reading assignments. You will also find a series of notes in the form of questions and the key information required in answer. These questions have been framed to encourage and help you to integrate information from the readings with materials you will have had in other courses. Students are expected to study the material ahead of classes. Class time is devoted to clarification of the concepts, and discussion of their practical significance. 

An understanding of the concepts is developed further through course assignments. These together with the exams are designed to develop and test your ability to integrate concepts from different subject areas and use them to explain observations on plant growth and crop nutrition, or agronomic practices. 

No text is required for the course. However, Mineral Nutrition of Higher Plants (Second Edition), by Horst Marschner (1995) will be used extensively and its purchase is highly recommended. Water Relations of Plants and Soils, by P.J. Kramer and J.S. Boyer (1995) is used as the text for concepts in soil-plant water relations. 

Students should refer to the following books for general background information. 

Biology of Plants - Raven, Evert and Eichhorn. 
Plant Physiology - Salisbury and Ross. 
Soil Science (3rd Ed.) - Hausenbuiller. 
 
  

SOIL PLANT RELATIONS OVERVIEW 

There is no provision in the course to give detailed information on general plant anatomy. Equally, a basic understanding of the physiology of soluble carbohydrate production by carbon fixation is assumed. 

First steps 

Q What are the generalised processes that determine the directions of flow of photosynthates ? 
 

Genetics	Environmental responses
Under a given set of growing conditions for a plant, the shoot:root ratio remains constant and will revert back to the same ratio if growth is perturbed.	Q What strategies do plants employ to survive adverse conditions ?
	Severe: roots tend to become modified for storage
	Drought: plants invest more in roots
	Nutrient deficiency: plants invest more in roots

Q How is the relative growth between organs maintained ? 
 

Growth regulator	Role	Sites of biosynthesis	Transport
Cytokinins	Cell division and expansion, RNA and protein synthesis, enzyme induction, apical dominance	root meristems (shoot meristems, embryo in seed)	In xylem from root to shoot
Gibberellins	Cell expansion, dormancy , induction of flowering, induction of enzyme synthesis	shoot apex, expanding leaves, fruits (roots?)	In roots to shoots in xylem, and shoots to roots in phloem. In symplast within shoots
Auxins	Cell expansion and division,  apical dominance, induction and  activation of enzymes	meristems of young tissue	shoots to roots in phloem. Cell to cell by polar transport.
Abscisic acid	Abscission of leaves and fruits  Cell extension, stomatal closure. Inhibits DNA synthesis. Enzyme activation, membrane permeability	fully differentiated tissues of roots and shoots	mobile in both xylem and phloem
Ethylene	Aerenchyma formation, fruit ripening,  tissue senescence. Enhances germination, epinasty, enhances flowering.	All tissues, but ripening fruit and senescing tissue important	Xylem transport from root to shoot.
Jasmonic acid	Senescence, storage promotion, stomatal closure, inhibits cell growth.	roots, shoots, fruits	phloem mobile

 

Q What characteristics of the soil are likely to modify the growth of roots, and possibly affect the allocation of resources between roots and shoots ? 
 

Soil water	Soil aeration	Soil structure
Gravimetric analysis  qg = mass per unit mass soil	Air space =  total porosity - volumetric water content
Volumetric analysis  qv = volume per unit volume soil  qv = qg x  rb

Water movement in the soil and plant depends on gradients of potential. 

Q What roles does water play within the plant ? 

1. Source of protons in carbon fixation: 2-3 % of water use only 

2. Nutrient and assimilate transport 

3. Control of plant temperature 

The exchange of water between plant and atmosphere is driven by the difference: 

VPD in atmosphere - VPD in the sub-stomatal cavity 

Availability of soil water 

Available water : the difference in the amount of water present after the soil drains under gravity, and that still present when the plant undergoes permanent wilting. 

Basic Soil Water Relations 
 
Moisture release characteristics                 Hydraulic conductivity varies with water content 

Soil Aeration 

Oxygen diffusion is 10⁴ times slower in water than in air. Consequently, factors that reduce air-filled porosity can affect soil aeration. 

Oxygen is important as an electron acceptor for catabolic processes. 

Q What are the consequences of poor aeration in soil ? 

1. Toxic substances are produced. 

2. Nutrient availability changes. 
     - some nutrients are less available, which leads to deficiency eg NO₃^-
     - some nutrients are more available, even to excess, which can lead to toxicity eg Mn. 

In the plant  

Unlike the case of water, membranes represent a boundary for nutrient movement. 
Nutrient uptake may be by two mechanisms:- 
 

Active	Passive
Catabolic energy  is required	Depends on electro-chemical gradients
Energy is released in respiration

In the soil  

Movement is by:- 
     1. Mass flow - significant if much is in solution eg N (NO₃^-) 
     2. Diffusion - needs a concentration gradient, important for nutrients where there is little present in the solution eg P 

Fertilizer placement depends on this knowledge. 

Roots can have a marked effect on the soil through which they grow. Soil that is directly affected is generally considered to form the rhizosphere. Soil in contact with the root is considered to form the rhizoplane soil. 
Part of the effect that roots have is due to the release of carbon compounds that provide a C-source for microbes, but may also act as selective chelation agents. 
Another part is due to roots modifying the pH within the rhizosphere depending on the release of protons or hydroxyl ions into the soil. 

Root symbionts are also active within the rhizosphere. 
     Mycorrhizas increase the surface area for nutrient absorption. 
     Nodulating bacteria (eg rhizobium) are important for N₂- fixation through symbiotic association. 

We shall also consider the function of nutrients within plants, and the physiological background this gives for fertilizer use within crop production systems. 

In addition to class discussion of the issues, three assignments are used to allow students to test their knowledge by evaluating experimental data.  A mid-term exam is used to assess progress, and there is also a final exam.

Students can also consider taking this course as a practicum, where they explore an issue in soil-plant relations, and prepare a written report on their findings.  They could also be involved in an experimental program.