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IX. Water Penetration Under Prolonged Submergence
materials that exert a favorable effect on soil structure under one set of
conditions may exert an unfavorable effect under other conditions,
and emphasizes the importance of factors other than the presence of
binding substances in favorable structure formation in soils.
Microorganisms influence the physical properties of the soil by aiding in the process of water-stable aggregate formation. The influence
may be direct or indirect, the latter acting through the compounds produced during decomposition. The favorable effect of microbes is contingent upon the decomposition of organic residues in the soil. During
the decomposition process, substances synthesized by the organisms
and products of decomposition undergo chemical and physical interactions with the soil particles which may increase aggregate stability.
During periods of intense microbial activity, the cells and filaments of
the organisms themselves may mechanically bind soil particles together.
The soil-binding substances produced through microbial activity are
slowly or quickly destroyed by subsequent microbial action. In order
to maintain aggregate stability at a high level, a continuous supply or
periodic additions of organic residues are necessary.
Microbial species and different types of organic residues vary in
their soil aggregate stability effects. Some fungi or bacteria are very
effective, whereas others have little influence. In general, complex
organic residues containing relatively large amounts of easily decomposable constituents bring about greater aggregation than substances
which are relatively resistant to decomposition. Low or moderate temperatures and moisture conditions are more conducive to stable aggregate formation than are higher temperatures and excessive moisture.
Exchangeable cations in the soil influence soil aggregation.
Polysaccharides synthesized by soil organisms are effective soilbinding substances but other organic compounds are undoubtedly active.
More work is needed to determine the nature of the active substances
produced through microbial decomposition of organic residues.
The effects of organic substances on soil binding appear to be associated with OH, NH,, and COOH groups, and with the length and
special characteristics of the molecules. Hydrogen bonding through the
alcoholic hydrogen may be the mechanism by which polysaccharides
bind soil particles. Different types of organic compounds may bind soil
particles through different mechanisms. More work is needed further
to elucidate this aspect of soil aggregate formation.
Some of the synthetic polymeric soil conditioners currently available for use are markedly resistant to decomposition and can, when
SOIL A G G R E G A T I O N
properly applied, effect marked improvement in the physical properties of the soil. They are not likely to replace organic matter in soil
management procedures but can be an important supplement thereto.
One important advantage of certain synthetics is that they can be used
as a research tool to elucidate the importance of soil structure without
introducing fertility factors, and at exceedingly low concentrations.
The action of organic compounds in affecting aggregation may be
very complex and at best is little understood. Many have postulated that
soil particles are held together in aggregates by the organic compounds.
There is, however, considerable evidence that another important role of
these compounds may be in modifying the expression of cohesive forces
between clay particles through adsorption on the surfaces of the clay.
Thus, active organic materials may be thought of as acting both to hold
soil particles together and in other cases to hold the clay particles apart.
The exact mechanisms involved in formation and stabilization of soil
aggregates need to be studied intensively if a full understanding of this
important soil characteristic is to be obtained.
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Recent Changes in Swedish Crop Production
Institute of Plant Husbandry,
Royal Agricultural College, Uppsala, Sweden
I. Swedish Crop Production-Background
1. Sweden-Location, Climate, and Soils.
2. The Present Pattern of Crop Production-Background
3. Developments during the Last Two Decades . .
11. Crops and Special Measures
. . . . . . . .
1. Wheat . . . . . . . . . . . . . .
. . . . . . . . . . . . . .
3. Barley . . . , . . . . . . . . . .
. . . . . . . . . . . . . .
5. General Remarks on the Cereals . . . . . .
6. Temporary Leys and Natural Grasslands . . . .
. . . . . . . . . . . . .
8. Fodder Roots and Sugar Beets . . . . . . .
9. Legumes . , , . . . . , . . . . .
10. Oil Crops . . . . . . . . . . . . .
11. Miscellaneous Crops . . . . . , . . . .
12. Seed Production and Seed Testing . , . . . .
13. Black Fallow and Weed Control . . . . . .
111. Summary and Outlook for the Future . . . . . .
. . . . . . . . . , . . . .
I. SWEDISHCROP PRODUCTION-BACKGROUND
I . Sweden-Locution, Climate, and Soils
Sweden is located far to the north, between 5 5 O and 69" N. The
country has a length of about 1000 miles from south to north. With its
widest part stretching over about 300 miles from east to west between
loo and 2 4 O E., Sweden is a rather large country, particularly with regard to length. With its location far to the north this means that there
are great variations in climatic conditions. There are correspondingly
great variations in the types of agriculture existing in different parts
of the country. Typical of the climate in the northern part are very
short summers with extremely long days, and long winters with a
heavy snow cover. Characteristic of the climate in the middle and
southern parts of the country are short summers with long days and
rather cold and windy winters. In the southernmost provinces in
Sweden the snow cover does not usually stay on the ground very long.
The cold winds can therefore be hard on the overwintering crops. Conditions like the ones just mentioned require an extremely good winter
hardiness in the overwintering plants.
Under the conditions indicated above, there must exist marked differences between temperatures in different parts of the country, ranging from a mean annual temperature of 7 O C. in the southernmost parts
of the country down to -3O C. in the northernmost parts. Although
these temperatures are low, they are still higher than in most countries
at the same latitude. This is due to the Gulf Stream, which brings warm
winds from the Atlantic across the country. These winds warm up
mainly the areas in the south. The mountain ranges on the border between Norway and Sweden prevent the winds from giving northern
Sweden a more genial climate. Differences in climate are, however,
not due merely to the influence of the Gulf Stream. Sweden has a varying topography, and the elevation above sea level has a definite influence on the climate in local areas. Typical plains are found in the
extreme south; across the central part there are lowlands bordering on
the great lakes Vanern, Vattern, Hjalmaren, and Malaren, and along
the coast line and the rivers there are coastal plains and valleys. These
plains, lowlands, and valleys are, however, broken off by mountain
areas and by forested uplands, mixed with small farmland areas. Mountain areas are found primarily in the northern parts of the country,
but the forested uplands are found in most parts of the country except
in the extreme south and along the coast lines. In such areas the climate
is more severe than in the lowlands and the plains, even though they
are located at the same latitude. Consequently there are different climatic conditions for crop production all over the country. This must
be borne in mind when crop production in different districts of the
country is discussed below.
The danger of night frosts during spring and fall confronts the crop
producer with another problem. In southern Sweden late spring frosts
are most likely to cause crop damage, and especially to those which are
now at their northern growing limit. These crops are sensitive to a night
frost after they have emerged. In northern Sweden, on the contrary, the
fall frosts are most dangerous, as they often appear before the cereal
crops are ripe. In the upland areas in different parts of Sweden these
night frosts can occur during every month but July. It occasionally
happens that not even the month of July is free of night frosts. But on