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VII. Contribution to Plant Nutrition
THE SOIL ORQANIO FRBOTION
Nitrogen occupies a unique position among the nutrient elements in
that it occurs almost entirely in the organic form in soils. Since the
supply of available nitrogen is very closely related to crop yields, the
factors governing the conversion of the organic reserve to the available
form are of grea,t importance. The soil organic fraction furnishes the
raw material for many of the important processes in the nitrogen cycle
and provides the fuel for most of the microbial transformations involved. This close relationship between the carbon and nitrogen cycles
makes it possible to relate certain nitrogen transformations to the carbon :
nitrogen ratio. Thus, when the ratio is large, an unbalance exists in the
proportion of energy material available to the soil population to the
quantity of nitrogen which can be used for synthesis of new cells. Under these conditions no net increase in inorganic nitrogen can occur ; in
fact any available nitrogen present is rapidly assimilated by microorganisms. The well-known nitrate depression which follows plowing under
large quantities of wheat straw or stubble is a typical example of the
situation just described. Nitrogen mineralization does not occur until
the unbalance has swung over to an excess of nitrogen, corresponding to
a low carbon: nitrogen ratio. This is accomplished by the conversion of
excess energy material to carbon dioxide and water. The ordinary circumstance in the soil organic fraction is that the carbon : nitrogen ratio
is low, but the supply of readily available energy material is limited and
ammonification is rather slow. This arrangement is advantageous in
several respects. The main reserve of soil nitrogen is held in an insoluble
form which cannot be leached away and is slowly converted to forms
which can be utilized by plants. In many cases, however, mineralization
is too slow to supply the current needs of a growing crop. I n this apparently anomalous situation two or three thousand pounds of nitrogen
may be present in an acre of soil, and yet a crop requiring perhaps 70
pounds in a season may suffer from nitrogen deficiency. The mineralization process can be speeded u p by increasing the activity of the soil
population; this may be accomplished through the addition of decomposable organic material, provided the nitrogen requirements of the
microbial population are also met. Thus, plowing under a green manure
crop stimulates microbial activity, supplies nitrogen for synthesis of
microbial cells, and accelerates mineralization of soil nitrogen.
When inorganic nitrogen fertilizers are applied to a soil, some of the
soluble nitrogen is assimilated by soil organisms and becomes part of the
organic nitrogen fraction, although it may be subsequently mineralized.
A large portion of the nitrogen is therefore derived from the soil organic
P. E. BROADBENT
fraction, even though the soil on which it is grown may have been fertilized with green manures, farmyard manure, or inorganic nitrogen
As Ensminger and Pearson (1950) have pointed out, our knowledge
of the nitrogen cycle is seriously lacking in certain respects. Most of
the processes are understood in a general way and can be explained in
terms of over-all changes taking place, as has just been done in the case
of ammonification. More detailed information concerning spec,ific reactions and factors controlling them is urgently needed. Provided environmental conditions are suitable for microbial activity, the controlling
factor is usually the available energy supply. However, expression of
the energy status in terms of carbon : nitrogen ratios is an oversimplification not justified by the great diversity of substrates and of microbial
populations which occur in soils. A long step forward will have been
taken when the availability of a given type of organic matter can be
accurately predicted and a good estimate of cell synthesis obtained. The
availability of the nitrogen in organic materials has been assessed in
greenhouse pot tests, such as those conducted by Parbery and Swaby
(1942) and by Rubins and Bear (1942). A procedure which places
emphasis on the rate of liberation of nitrogen from the organic form in
humus has been developed by Harmsen and Lindenbergh (1949). Their
technique involves depletion of inorganic nitrogen reserves to a very low
level by cropping the soil to be tested with a fast-growing crop, which is
allowed to grow until nitrogen deficiency symptoms are manifest. The
crop plants in their entirety are carefully removed, root fragments are
sieved out, and the soil sample is then incubated under controlled temperature and moisture conditions. Determination of water-soluble nitrogen in the soil samples a t intervals permits calculation of the rate of
mineralization. Estimates of the capability of soils to supply mineral
nitrogen based on incubation procedures have also been obtained by
Pritchett e t aZ. (1947) and Cornfield (1952). The latter author concluded that ammonia as well as nitrate accumulation should be considered in assessing mineralizable nitrogen. He found that total nitrogen
may be used as a rough indicator of nitrogen availability, provided no
recent additions of fertilizers or organic material have been made,
whereas in the soils he studied carbon: nitrogen ratios were of no value
in predicting nitrogen availability.
The chief disadvantages of tests such as have been used to estimate
mineralizable nitrogen lie in their time-consuming nature and in the
fact that the information obtained is usually applicable only to conditions similar to those under which the test was conducted. The situation
is further complicated by differences i n behavior of the soil population
THE SOIL ORGANIC FRACTION
between cropped and fallow soils. Clark (1949) has pointed out that
nitrogen mineralization is usually less in cropped soils. Goring and
Clark (1948) attributed this decrease to immobilization in the soil, owing
probably to the increase in numbers of microorganisms that occurs with
The contribution of phosphorus in the soil organic fraction to plant
nutrition has been the subject of intensive research in recent years. Considerable evidence has been accumulated which indicates that under
some conditions a substantial part of the phosphorus taken up by a
growing crop may be derived from organic forms. I n many soils organic phosphorus compounds may account for more than half the total
reserve (Bower, 1949). The mineralization process is not well understood but appears to be analogous to nitrogen mineralization in many
respects. Thompson and Black (1949) and Kaila (1949) have obtained
evidence of a relationship between carbon : phosphorus or nitrogen : phosphorus ratios and the occurrence of immobilization or phosphorus release. Kaila (1950) estimated the critical phosphorus content of natural
materials a t 0.2 per cent. I n materials having a higher content, phosphorus in excess of microbial requirements is present, and mineralization
would be expected to occur. It is sometimes pointed out in objection
to the view that soil organic phosphorus supplies part of the crop needs
for this element that peat soils having almost all their phosphorus in
organic form frequently are deficient in available phosphorus. This objection can be met satisfactorily by Kaila’s (1950) explanation that the
phosphorus content of peat soils is often below the critical value and
the mineralization which would follow a decrease in the carbon : phosphorus ratio is limited by a lack of available energy material. This is
essentially the same situation as with nitrogen, except that the critical
carbon : phosphorus ratios appear to be somewhat more variable than the
corresponding carbon : nitrogen ratios.
3. Other Elements
As a direct source of plant nutrients other than nitrogen and phosphorus the soil organic fraction is of relatively minor importance, although evidence of organometallic complexes has been obtained by
Bremner et al. (1946), and other nutrients are, of course, retained in
exchangeable form by the organic colloids. On the other hand the availability of some nutrient elements, notably iron, manganese, and sulfur,
may be profoundly influenced by organic matter and the activities of
the soil population. I n this connection the soil reaction and oxidation-
F. E. BBOADBENT
reduction status are of particular importance. Robinson (1930) haa
called attention to the changes which accompany submergence of soils.
Owing to the rapid depletion of oxygen through absorption by microorganisms, reducing conditions develop quickly ; combined oxygen in
various chemical compounds may then be utilized with resultant alteration in chemical composition of the soil solution. Sulfates may be reduced to sulfides, and insoluble ferric and manganic compounds reduced
to the relatively more soluble ferrous and manganous forms. Microbially produced carbon dioxide helps to keep in solution ions which
might otherwise be precipitated as carbonates or as the more soluble
bicarbonates. Toxic quantities of suMdes, iron, manganese, and aluminum may soon accumulate under the conditions described. Mann and
Quastel (1946) have shown that the addition of an available substrate
such as glucose to soil will cause an increase in soluble manganese under
aerobic conditions; a decrease in manganous ion follows the disappearance of glucose.
Under oxidative conditions, particularly in soils of neutral or alkaline reaction, iron and manganese are oxidized to insoluble forms, and
deficiency of these elements may result. Leeper and Swaby (1940) reported that microbial oxidation of manganese occurred over the pH
range 4.8-8.9, with the most rapid oxidation occurring between 6.0 and
7.5. Gerretson (1937) pointed out that manganese deficiency usually
occurs between the p H limits 6.5-7.8, the range in which he found the
manganese oxidizers to be effective.
The activities of chemosynthetic autotrophes capable of oxidizing
ferrous iron to the ferric form are well known; soil autotrophes also can
convert sulfur or sulfides to sulfates. Little is known concerning the importance of such groups of organisms under field conditions, but certainly
they exert some influence on nutrient availability,
The indirect effects of the organic fraction, particularly the living
portion, on soil fertility have received relatively little attention and deserve much greater emphasis in future research.
Ten years ago Norman (1942), in discussing the chemistry of soil
organic matter, expressed the belief that the time was ripe for a new
assault on the problems related to the fundamental nature of this important soil constituent, one which would bring our knowledge of it to
a level comparable to our present knowledge of clays. Progress in the
intervening decade has been hampered by a world war, but some real
THE SOIL ORGANIC FRACTION
advances have been made, although the number of workers in this particular field is still quite small.
The goal of a fairly complete characterization of the soil organic
fraction is still f a r from realization, but it may confidently be expected
that intensive application of new techniques already a t hand, such as
chromatographic and electrophoretic separation, use of isotopic tracers,
and ultraviolet and infrared spectrophotometry, will greatly accelerate
progress in that direction.
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F. E. BROADBENT
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THE SOIL ORaANIC FRACTION
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Progress in Agricultural Engineering
L . W. HURLBUT
Nebraska Agricultural Experiment Station Lincoln. Nebraska
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I11. General Characteristics of Modern Farming . . . . . . . . . . . 187
I1. Concept of Agricultural Engineering
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1 Increased Power per Worker
2 Comparisons of Crop Yields and Per Capita Production
3 Impact of a 20 Per Cent Increase in Total Farm Output
4 A Few Important Developments, 1900-1950
I V Future Trends
1 Farm Power
a Tractor Power
b Electric Power
c The Airplane
2 Soil and Water Management
a Surface-Mulch Farming
b . Runoff-Water-Control Systems
3 Harvesting and Storing Grain
a Some Harvesting and Storage Losses
b. Moisture Limits for Some Harvesting Machines
c Conditioning Wheat and Shelled Corn for Storage
4 Harvesting and Storing Hay and Forage
a Main Sources of Loss
b Sun-Cured vs Dehydrated Hay
c Silage vs Dehydrated Green Corn
d Need t o Define Quality
e New Developments
5 Cotton Production
a Weed Control and Defoliation
b New Harvester Developments
c New Ginning Developments
6. Equipment for Applying Fertilizers
7 Mechanization of Special Crops
V Concluding Statements
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Agricultural engineers and agronomists find many areas of mutual
interest throughout the agricultural industry . Both groups have helped
to establish the fact that the production per agricultural worker is di186
L. W. EUBLBUT
rectly related to the investment in farm machinery and farm power,
which in turn depends upon the development of certain characteristics
in crops. The widespread use of farm power and machinery in agricultural production has been one of the most significant features in the
development of both American agriculture and American industry.
All developments stem from basic interests of the general public.
Viewed from a national standpoint, the public’s interest in the agricultural industry includes four fundamental features : first, adequate quantity of production ; second, satisfactory quality of products ; third,
production a t low cost ; and fourth, the well-being of the people engaged
in the industry.
11. CONCEPTOF AGRICULTURAL
It seems appropriate to review briefly the various phases of agricultural engineering. Among these are :
1. Development and application of farm power and machinery.
2. Farm buiIdings and rural housing.
3. Farm electrification, involving the application of electric energy
as heat, light, radiation, and power.
4. Mechanical processing of farm products.
5. Engineering phases of soil and water usage, including erosion control, drainage, and irrigation.
6 . Methods engineering.
Engineering may be defined, generally, as “the art and science of
utilizing the forces and materials of nature for the benefit of man and
the direction of man’s activities toward this end.” This definition implies two activities, namely, art and science. The science of engineering
is based upon the pure sciences, physics, chemistry, and mathematics.
This is the phase of the field which is exact and rational. The a rt of
engineering refers to the ability to judge, estimate, and manipulate the
uncertainties of engineering to a satisfactory solution of a problem. It
refers to a procedure which has been found by a series of trial-and-error
events, carried out in as logical a sequence as possible, to produce a desired result without a knowledge of all of the basic principles involved.
It is well known that a t the present time the field of agricultural engineering encompasses more uncertainties than do the more common engineering fields.
The use of basic science in research in agriculture is already very
great, but there are good indications that it will continue to expand.
The work of Bu rr (1942, 1943, 1945, 1947), Burr and Sinnott (1944),
and Nelson and B u rr (1946) indicates the possibility of a search for in-