1 Root Ecology and Root Physiology- 2 General Survey of Root-Study Methods- 21 Historical Development- 22 Principles of Classification- 23 Selection of the Best Method- 3 Excavation Methods- 31 Introduction- 32 Outline of the Classical Method- 321 Selection of the Plant- 322 Digging the Trench- 323 Excavating the Root System- 324 Drawings and Photographs- 325 Preparation and Storage of Excavated Root Systems- 326 Review of Applications- 327 Advantages and Disadvantages- 33 Modifications of the Classical Method- 331 Excavations with Water Pressure- 332 Excavations with Air Pressure- 333 Excavations in a Horizontal Plane- 334 Sector Method- 4 Monolith Methods- 41 Introduction- 42 Simple Spade Methods- 43 The Common Monolith Method- 431 Square Monoliths- 432 Round Monoliths- 44 Box Methods- 45 Cage Methods- 46 Needleboard Methods- 461 General Survey- 462 Construction and Preparation of the Needleboards- 463 Excavating the Monoliths- 464 Washing Procedure- 465 Photographing and Sectioning- 466 Special Modifications- 467 Evaluation of the Methods- 5 Auger Methods- 51 Specific Features- 52 Sampling Techniques- 521 Techniques with Hand Augers- 5211 Sampling Procedure- 5212 Number or Replications- 5213 Special Technique for Studying Tree Roots- 522 Mechanized Techniques- 523 Core-Sampling Machines- 53 The Core-Break Method- 54 Advantages and Drawbacks- 6 Profile Wall Methods- 61 General Survey- 62 The Traditional Trench Profile Method- 621 Digging the Trench- 622 Preparing the Profile Wall- 623 Exposing the Roots- 624 Mapping and Counting Procedure- 6241 Determination of Root Number- 6242 Determination of Root Length- 63 The Foil Method- 64 Technique in a Horizontal Plane- 65 Evaluation and Applications- 7 Glass Wall Methods- 71 Introduction- 72 Glass-Faced Profile Walls- 73 Root Laboratories- 731 General Survey- 732 Features of Their Design- 733 Methods of Recording- 734 Light Sensitivity of Roots- 735 Recent Research in Root Laboratories- 74 Root Observations with Glass Tubes- 75 Evaluation of the Methods- 8 Indirect Methods- 81 Introduction- 82 Determination of Soil Water Content- 821 Gravimetric Method- 822 Neutron Method- 83 Staining Techniques- 84 Uptake of Non-Radioactive Tracers- 85 Radioactive Tracer Methods- 851 General Survey- 852 Soil Injection Technique- 853 Plant Injection Technique- 854 Root Studies with 14C- 855 Critical Evaluation- 9 Other Methods- 91 Measuring Root-Pulling Strength- 92 Measuring Root-Clump Weight- 93 Measuring Root Tensile Strength- 94 Root Measurements Using Soil Sections- 95 Root-Detecting Method- 96 Mesh Bag Method- 97 Root Replacement Method- 98 Root Investigations with Paper Chromatography- 99 Electrical Methods- 910 Determination of Growth Rings in Tree Roots- 911 Investigations of Root Hairs- 912 Determination of Root Nodules- 913 Determination of Mycorrhizae- 10 Container Methods- 101 General Features- 102 Rooting Volume and Container Size- 103 Types of Containers- 1031 Small Pots- 1032 Boxes and Tubes- 1033 Glass-Faced Containers- 1034 Flexible Tubes- 104 Rooting Media- 105 Filling the Containers- 106 Seed Technique- 107 Irrigation Problems- 108 Special Washing Procedures- 109 Modified Container Methods- 1091 Cage Method- 1092 Needleboard Method- 1093 Root Training by Plastic Tubes- 1094 Split-Root Technique- 1095 Undisturbed Soil Monoliths- 1010 Root Studies in Nutrient Solutions- 1011 Root Studies in Mist Chambers- 1012 Comparability of Results from Container Experiments with Field Data- 11 Techniques of Root Washing- 111 Dry Sieving- 112 Storing Soil-Root Samples Before Washing- 113 Chemicals for Facilitating Root Washing- 114 Washing Roots by Hand- 115 Flotation Method- 116 Root-Washing Machines- 117 Nutrient Losses from Roots During Washing- 118 Cleaning Roots from Debris- 119 Storing Roots After Washing- 12 Root Parameters and Their Measurement- 121 General Aspects- 122 Root Number- 123 Root Weight- 1231 Determination of Fresh Weight- 1232 Determination of Dry Weight- 1233 Advantages and Critical Objections- 124 Root Surface- 1241 Calculation from Other Parameters- 1242 Photoelectric Measurements- 1243 Adsorption Methods- 125 Root Volume- 1251 Calculation from Other Parameters- 1252 Displacement Technique- 126 Root Diameter- 1261 Measurements- 1262 Applications in Tree Root Studies- 127 Root Length- 1271 Direct Measurements- 1272 Intersection Methods- 1273 Root-Counting Machines- 1274 Reasons for Increasing Use of Root Length Measurements- 128 Root Tips- 1281 Technique of Counting- 1282 Root Coefficients- 129 Shoot-Root Relations- 13 Some Future Aspects for the Use of Ecological Root-Study Methods- References

Methods of studying root systems

(I) Inorganic components . . . . . . . (a) Nitrate . . . . . . . . . (b) Cation-anionuptake . . . . . . (2) Organic compounds . . . . . . . (a) Amino acids and other organic nitrogenous compounds (b) Organic acids . . . . . . . . (3) Effects on flowering . . . . . . . (4) Ammonium plus nitrate as a nitrogen source . . . ( 5 ) Metabolic changes characteristic of ammonium toxicity . VII . Summary . . . . . . . . . . VI . Physiological effects of nitrogen sources . . . .

Ammonium and nitrate nutrition of plants

Plants that absorb nitrogen as NO3
− tend to raise the pH in the rhizosphere. Those absorbing nitrogen as NH4
+ or N2 lower the pH. The change in pH near the root surface may be calculated approximately from the H+ or HCO3
− efflux and radius of the root; and the pH buffering capacity, moisture content, initial pH and pCO
2 of the soil. An accurate equation, solved numerically, also takes account of root hairs, mass flow and slow acid-base reaction in the soil. The pH at the root surface will often differ from the pH a few mm away by 1–2 units.

Changes of pH across the rhizosphere induced by roots

In areas that remain unaffected by industrial pollution soil acidification is mainly caused by the release of protons (H+) during the oxidation of carbon (C), sulphur (S) and nitrogen (N) compounds in soils. In this review the processes of H+ ions release during N cycling and its effect on soil acidification are examined. The major processes leading to acidification during N cycling in soils are: (i) the imbalance of cation over anion uptake in the rhizosphere of plants either actively fixing N2 gas or taking up NH 4 + ions as the major source of N, (ii) the net nitrification of N derived from fixation or from NH 4 + and R-NH2 based fertilizers, and (iii) the removal of plant and animal products containing N derived from the process described in (i) and losses of NO3-N by leaching when the N input form is N2,NH 4 + or R-NH2. The uptake of excess cations over anions by plants results in the acidification of the rhizosphere which is a “localized” effect and can be balanced by the release of hydroxyl (OH−) ions during subsequent plant decomposition. Nitrification of fixed N2 or NH 4 + and R-NH2 based fertilizers, and loss of N from the soil either by removal of products or by leaching of NO3-N with a companion basic cation, lead to ‘permanent’ acidification.

Processes of soil acidification during nitrogen cycling with emphasis on legume based pastures

Rape (Brassica napus, var. Emerald) plants were grown in thin layers of a phosphate-deficient soil supplied with a P-free nutrient solution containing N solely as NO3- (3 5 x 10-3 M). Inter-root competition (excluding root hairs) for soil P began on the 14th day of growth, after which all soil in the thin layer was considered to be rhizosphere soil. During subsequent plant growth the pH in the rhizosphere decreased as much as 2-4 units (from pH 6&5 to 4-1) before rising slightly towards the end of the experiment (35th day). The pH decrease was associated with an increase in solution P concentration in the depleted rhizosphere soils, compared to the P concentration attained when similar amounts of P were desorbed from unplanted control soils by shaking them in 102 M Ca(N03)2 at the original soil pH of 6-1. P desorption in 10-2 M Ca(NO3)2 at pH values between 6&2 and 4-5 indicated that a decrease in the soil pH from 6-5 to 4-1 could result in at least a 10-fold increase in the P released into solution, which would be available for plant uptake. These results may explain reports in the literature that rape is efficient in absorbing P from P-deficient soils.

Plant-induced changes in the rhizosphere of rape (brassica napus var.

No yield differences in tops or roots were measured between ammonium and nitrate nitrogen sources in flow ‘United 108’ grown for 14 or 28 days in constant flow culture solution. There was a greater uptake of N from the nitrate source but approximately 25% of the total uptake remained as nitrate within the plant. Higher plant levels of phosphorus and sulphur were measured in the ammonium treatment and higher Ca and Mg in the nitrate treatment indicating a predominately cation-anion balance effect.

Nitrate and Ammonium as Sources of Nitrogen for Corn and Their Influence on the Uptake of Other Ions1

Ammonium effects on phosphorus absorption through pH changes and phosphorus precipitation at the soil-root interface.

Influence of Nitrogen Source on Phosphorus Uptake by Corn from Soils Differing in pH1

Short term phosphate uptake by 10‐day old corn plants was markedly influenced by the nitrogen status of the plants and by the presence of N0‐3 ‐N or NH4 + ‐N forms in the uptake solution. These factors should be considered in the use of short‐term phosphorus uptake as a measure of available soil phosphate.

M. H. Miller

Papers

Nitrate and Ammonium as Sources of Nitrogen for Corn and Their Influence on the Uptake of Other Ions1

Ammonium effects on phosphorus absorption through pH changes and phosphorus precipitation at the soil-root interface.

Influence of Nitrogen Source on Phosphorus Uptake by Corn from Soils Differing in pH1

Effect of nitrogen status on short‐term phosphorus uptake