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VIII. Functions of the Clay Minerals

VIII. Functions of the Clay Minerals

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nels for the movement of air and water, and they prevent movements

of soil materials by wind and water erosion.

The most effective soil aggregates are those that resist destruction by

mechanical means and by physicochemical reactions. I n order for soil

aggregates to be st.able, they must meet the following conditions:

1. They must be held together by effective binding agents.

2. They must resist swelling and dispersion on contact with water.

3. They must resist coalition with neighboring aggregates when wet.

Russell (1934, 1935) has suggested a plausible mechanism by which

aggregates are bound together by clays to fulfill the first of the above

conditions. According to Russell, the exchangeable cations, since they are

positive, attract the negative ends of water molecules postulated in the

Bernal-Fowler (1933) theory of hydrogen bonding (see Section VII-1).

This leaves the positive spots on the water molecules pointed away from

the exchangeable ions. These positive spots are attracted by the negatively charged neighboring clay mineral crystals. When the water films

become thinner by drainage or evaporation, the sheetlike crystals of the

clay minerals are pulled closer and closer together, with many of their

ffat faces oriented parallel to each other. The interlacing of chains of

overlapping clay mineral crystals forms an effective matrix for holding

clayey masses together. This theory agrees with the work of Sideri

(1936) and Henin (1937, 19381, who have found evidence of preferred

orientation in the clay mineral crystals of soil structural aggregates.

The observations of Gorkova (1939) and Peterson (1944, 1946a, 1947)

give further support to these theories concerning the binding of soil

aggregates by clay minerals. They found that montmorillonite, with its

greater surface and negative charge, was more effective than kaolinite

as a binding agent for structural aggregates. The work of Du tt (1947,

1948a) suggests the possibility of soluble silicates serving as binding

agents for aggregates. Organic residues appear to be responsible for

the second and third conditions necessary for the formation of stable

aggregates in soils. Russell (1935), Myers (1937), McCalla (1945),

Martin (1946), and Kroth and Page (1946) have suggested that polar

organic compounds serve as important binding agents in soil aggregates.

According to the hydrogen bonding theory of Bernal and Fowler (1933),

these polar substances could take the place of water molecules as linkages

between exchangeable cations and clay surfaces in the mechanism proposed by Russell (1935). Gieseking (1939) found that Wyoming bentonite and gelatin, two extremely lyophylic substaiices, reacted to f o m

a clay gelatin complex which was lyophobic. The dried aggregates from

this complex were extraordinarily difficult to crush and did not swell



when they were again placed in contact with water. A complex of this

type is probably formed by attract,ions between many positive amino

spots on the gelatin molecule and the many negative spots on the montmorillonite crystals. In this way linkages with much bonding energy

could be formed which could be propagated on and on throughout t.he

system. The author has unpublished data on a number of aggregates

from various soil types which were treated with water solutions of

several different organic cations. After these aggregates were dried, they

were found to be very difficult to crush, even after placing them again

in contact with water for long periods of time. It appears, therefore,

that certain organic molecules react with the clay minerals to give complexes which resist dispersion.

The activity of microorganisms in soils is conducive to the formation

of stable aggregates. Myers and McCalla (1941) and Peele and Beale

(1941) noted that the maximum effect of microorganisms on soil structure

stability lagged behind the maximum microbial activity. They interpreted this to mean that excreted and/or secreted metabolic products of

the organisms were responsible for the beneficial effect on stabilization

of soil aggregates. These observations have been confirmed by McHenry

and Russell (1944), Martin (1945, 1946), and Kroth and Page (1946).

Kroth and Page emphasize the point that these products form physicochemical complexes with the clay minerals.

McCalla (1945) has observed that gums, waxes, and fats produced

by microorganisms are instrumental in stabilizing soil aggregates. Martin (1945) found that polysaccharides, arising from bacterial growth are

important stabilizing agents for soil aggregates. The casts of earthworms

were reported by Dutt (1948b) to be very stable.

Certain plants appear to be responsible for the production of organic

substances which are sorbed on the clay mineral surfaces of soil aggregates and thereby render these aggregates resistant to the attack of

water. Numerous investigations have shown that any cropping system

which adds organic matter to soils, especially systems involving sod

crops with their fibrous root systems, are responsible for highly stable

soil aggregates. Among these investigations have been those of Elson

(1940), Woodruff (1940), Johnston e t al. (1942), Gelzer (1943), Wilson

and Browning (1945), Feng and Browning (1946) , Olmstead (1946) , and

Shauffer (1946). Norman (1946) has suggested that the enormous activity of rhizosphere bacteria around grass roots may be responsible for

this effect of the sod crops.

There has been much discussion concerning the importance of binding

agents in soil aggregate stabilizations. The clay minerals and organic

materials serve well as binding agents. More emphasis needs to be placed



on mechanisms whereby neighboring aggregates are held apart. The

most important aspect of the mechanism of soil aggregate stabilization

is the stabilization of the clay minerals on the surface of the aggregates

so that they will not exert an attraction or binding force between neighboring aggregates. The first, step toward aggregation must be the development of weakened cleavage zones in the clayey mass of the soil

material. After this has been accomplished by the growth of plant

roots, by animal activity, by the formation of ice crystals, by shrinkage

from dehydration, or by other mechanical means, it. would seem t h a t the

negative clay minerals in the faces along these cleavage zones might

become inactivated by the sorption of positively charged hydrous oxides

and organic substances. This should prevent the dispersion of the clay

mineral crystals in the zones of incipient cleavage and the subsequent

coalition of neighboring aggregates after removal of the original mechanical condition responsible for the first cleavage. Sideri (1936, 1938)

suggests that humus coats the surfaces of soil aggregates. Kroth and

Page (1946), however, found the organic materials in soil aggregates to

be quite uniformly distributed throughout the aggregate. They compared the nitrogen contents of the shells of a number of aggregates with

the nitrogen contents of their centers and found in every case a slightly

higher percentage of nitrogen in the shells. The significance of this small

difference is questionable, but their results seem to indicate that the clay

minerals in the surface of soil aggregates are stabilized by sorbed organic

cations. More conclusive evidence is needed t o establish the certainty

of this mechanism of surface stabilization of soil aggregates.

There have been several suggestions concerning the stabilization of

soil aggregates by means of irreversible -colloidal cementing agents.

Williams (1935), Sideri (1936), and Kubiena (1938) give the general

impression that freshly formed hydrated organic substances are sorbed

on mineral surfaces, and when these organic substances become dehydrated, they will not again hydrate on later contact with water. Thus,

these irreversible organic substances serve as cementing agents between

mineral particles. Lutz (1936) has proposed a somewhat analogous role

for the hydrated iron oxides.


Clay mineralogy and clay physicochemistry have developed almost

entirely during the last two decades. I n this short period, remarkable

progress has been made towards a better understanding of the clay

minerals and their intricate activities. There still remains much to be

accomplished, however, in the way of more refined and more precise

studies of these minerals.



The reactions of the clay minerals are determined by the amount and

nature of their external, internal, and voided surfaces. More refined

methods are needed to determine these properties of the clay mineral


The clay minerals vary in the vigor with which they enter into

physicochemical reactions. They also vary in their capacities to hold

various sorbed substances. More precise methods for the quantitative

estimation of the clay minerals are needed in order to be able to predict

the nature of the react*ionsof the clay mineral mixtures commonly found

in soils.

Combinations between the clay minerals and sorbed substances have

properties which are often greatly different from the properties of the

components. Further data is needed to reveal the importance of sorbed

substances in changing the properties of the clay minerals.

The clay minerals have been shown to be extremely important in

plant-soil relationships. More studies are needed to reveal the nature,

mechanism, and extent of the physicochemical reactions of the clay

minerals which influence plant growth. Some of these reactions are desirable and others are undesirable. Studies need to be extended to show

how the undesirable reactions can be siippressed and how the desirable

reactions can be enhanced.

Past developments in clay mineralogy have very closely paralleled

the development of physical methods of analysis. Some of the newer

techniques involving x-ray and electron diffraction can be expected to

be helpful in furthering research on the clay minerals. The use of radioactive and mass “tagged” isotopes in studying the physicochemical reactions of the clay minerals promises ta give results which will permit

more definite interpretations than have been possible with some of the

older techniques.


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Alfalfa Improvement


Dominion Forage Crops Laboratory. University of Saskatchewan]

Saskatoon. Saskatchewan



I . Introduction . . . . . . . . . . . . . . . . . .

I1. Seed Setting and Production . . . . . . . . .

. .

1. Tripping and Its Necessity

. . . . . . .

. .

2. Self- and Cross-Pollination and Seed Setting . . . .

3 . Tripping and Cross-Pollinating Agencies . . . . . .

a . Rain, Wind. Antomatic and Mechanical Tripping

b . Tripping Insects . . . . . . . . . . . . .

4 . Factors Influencing Bee Visitation . . . . . . . . .

5 . Soil. Climatic and Vegetat.ive Growth Factors . .

6. Injurious Insects . . . . . . . . . . . . . .

a . Lygus Bugs . . . . . . . . . . . . . . .

b . Control of Lyguw Bugs . . . . . . . . . .

c . Other Insects . . . . . . . . . . . . . .

I11. Progress in Methods of Breeding . . . .

. . . . .

1. Breeding Characteristics . . . . . . . . . . .

2 . Utilizing Hybrid Vigor . . . . . . . . . . . .

3. Methods of Testing for Combining Ability . . . . .

4 . Selection Procedures for Certain Characteristics . . .

IV . Conquering Some Diseases

. . . . . . . . . . . .

1. Bacterial Wilt. . . . . . . . . . . . . . . .

2. Black Stem . . . . . . . . . . . . . . . .

V . Summary and Conclusions . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . . . . .



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. . . . . . 208

. . . . . . 210

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. 212

. . . . 215

. . . . . . 218

. 219

. . . . . . 219

. . . . . . 222

. . . . . . 224

. 225








. 227



. 231












. 232

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Medicago sativa L., known by its Arabic name. alfalfa. in the United

States and Canada but commonly called lucerne in other parts of the

world. is generally regarded as one of the world’s most valuable cultivated forage crops . Few if any crops are equal to it in capacity to

produce heavy yields of highly nutritious palatable feed . The excellent

soil-improving ability of the crop is also generally recognized . A combination of desirable attributes as a forage plant and adaptation to a

wide diversity of soil and climatic conditions has led to the use of alfalfa

in the world to an extent probably exceeding th a t of any other single




legume or grass species. It is utilized as a cultivated crop on every

inhabited continent and in many countries extending from near polar

regions to the tropics.

With such a wide distribution and use under extremely diversified

environmental conditions the problems of production and utilization are

many and varied. Some problems are more or less local or regional in

nature, such as cold resistance or soil nutrient deficiencies, while others,

of which seed sett.ing is a good example, are more universal in occurrence.

Diseases and insect pests are universal problems.

Solution of some of the problems by cultural or management practices

or by breeding better varieties has already resulted in expanded utilization of the crop. Further expansion and increased production will undoubtedly follow as research on the factors limiting production and

utilization establishes ways and means of elimination or control of the


Investigations involving alfalfa cover a wide diversity of subjects and

the literature is indeed voluminous. Consequently in the preparation of

this review limitation of space necessitated a choice between a sketchy

coverage of many topics or a more comprehensive consideration of a few

selected phases. The latter alternative was chosen. The subjects selected

are those on which there has been rather extensive investigation and

noteworthy advances in the past decade, but by no means does the selection of subjects represent only those fields in which recent advances have

been made. Other recent or fairly recent reviews, however, have dealt

with topics not covered in this review. Atwood (1947) has summarized

the cytogenetic literature on the crop. Klinkowski (1933) has reviewed

the early and modern history of the distribution and utilization of the

crop in the world. An abstract review of the alfalfa literature for the

period 1925 to 1930 covering several subjects has been presented by the

Imperial Bureau of Plant Genetics: Herbage Plants (1931). Tysdal and

Westover (1937) have dealt with earlier improvement work.


Alfalfa is notoriously erratic in respect to seed production. I n many

extensive areas where the crop is widely utilized for hay and pasture

the yields of seed are so low and undependable that practically no acreage

is devoted to seed production. Thus dependence for a large portion of

the seed requirements of a region, nation, or continent generally falls

upon relatively few, often rather restricted areas where for some reason

or reasons yields are comparatively dependable. Stewart (1926) emphasizes this fact by stating that from “80 to 90 per cent of all alfalfa

seed in North America is grown in eleven areas. Six of these are small



and concentrated; the remaining five are more extensive but production

of seed is less intensive.” Even within major seed growing areas violent

interannual and interfield yield fluctuations occur. I n Utah, for example,

in 1926, the production amounted to 20,000,000 lbs., but in each of

several recent years it has only been about 4,000,000 lbs. (Tysdal, 1946).

Investigations over the past several decades have served to reveal the

multiplicity of factors influencing seed setting and seed yield, and contributing to the variability from area to area, field to field, and year to

year. T o understand and interpret the role of various factors it has

been necessary first to gain a knowledge of the biology and functioning

of the alfalfa flower. To this fundamental information the influences of

soil, climate, beneficial and injurious insects, disease, and management

practices can be added.

1. Tripping and I t s Necessity

The anthers, anther filaments, stigma, style and ovary, collectively

called the staminal or sexual column, are enclosed by the two keel petals

which are united along one edge and held firmly together along the other

two free edges. The filaments of nine of the ten anthers are united t o

form a tube which practically surrounds the ovary and style and exerts

a strong forward pressure. Whenever a force separates the two keel

petals even slightly along their free edges, the restraining mechanism is

released and the staminal column is violently snapped (tripped) forward

from the pressure exerted by the tube. Upon tripping the upper end

of the staminal column makes a strong impact with the standard (banner) petal and comes to rest on it several degrees from the original

upright position. The process of release of the staminal column from

the keel is known as tripping.

Although tripping has been observed for many decades the fundamental nature of the process to seed setting has been a matter of controversy even fairly recently. Carlson (1935) and Brink and Copper

(1936) maintained that a considerable proportion of flowers set seed

without tripping. Recently Tysdal (1946) drew attention to the fact

that t.he procedure used by Brink and Cooper was open to question.

Ufer (1932), Armstrong and White (1935), Hadfield and Calder (1936),

Knowles (1943), and Tysdal (1940, 1946) concluded that a t most only

a very small percentage of flowers set pods without first tripping. Both

Tysdal (1940) and Knowles (1943) report on detailed observations

covering many individual flowers on large populations of p1ant.s over

extensive periods of time and a variety of soil and climatic conditions.

Their data show that about 1 per cent of untripped flowers may set pods.

These observations and conclusions are further supported by the high

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VIII. Functions of the Clay Minerals

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