Is completed by bacteria. The carbon of the organic

matter, by its decay and union with oxygen in the forma-

tion of carbon dioxide, furnishes food energy to the micro-

organisms. In addition to carbon dioxide, many other

compounds are formed, some of them being rather com-

plex. Many of the compounds resulting from organic

decay act as solvents in making mineral matter more

available to growing plants. The humus remaining in

the soil as a result of decay is usually lower in carbon and

higher in nitrogen than the plant residues from which it

was formed. This is particularly the case in arid climates

where decay has gone on with but small quantities of

moisture.

163. Relation to nitrogen. — Of all the plant-food

142 The Principles of* Agronomy

elements of the soil, nitrogen is probably the one needing

most attention. It must constantly be worked over and

changed from one form to another. A part is lost from

the soil as free nitrogen and ammonia, which escape into

the air, or as soluble nitrogen salts which are leached out.

To prevent- these losses and maintain in the soil a supply

sufficient for the needs of crops, is one of the greatest

problems of agriculture.

The atmosphere contains a vast store of nitrogen, but

this is in an uncombined form and is, therefore, not in a

condition to be used by plants. The supply of combined

nitrogen in the soil, on the other hand, is limited. It was

thought for some time that, on account of losses which

occurred, this supply would in time be entirely exhausted

and that it would eventually be impossible to raise crops.

This was before the action of bacteria was understood.

We now know that, under proper conditions, these or-

ganisms are able to combine the nitrogen of the air with

other elements in such a way that it can be used by plants.

The discovery of this process known as nitrogen-fixation

is responsible for a change of ideas regarding soil fer-

tility.

Other kinds of bacteria are able to change the nitrogen

contained in dead animal and plant bodies into a form

that can be used by living plants. This general process

which takes place in a number of distinct stages is known

as nitrification. When available forms of nitrogen,

like the nitrates, are transformed- into non-available

ammonia or free nitrogen the process is known as denitri-

fication.

154. The fixation of nitrogen was first found to occur

in connection with little nodules which are found on the

roots of legumes such as peas, beans, alfalfa, and clover.

It was observed that where these plants grew, the nitrogen

Organisms of the Soil 143

content of the soil was increased. Investigation showed

that the nodules were caused by bacteria working on

the roots. The bacteria living in these nodules are able

to use free nitrogen of the air and combine it into the

organic compounds of their bodies from which it may later

become available to the higher plants. The fixation of

nitrogen in connection with the growth of legumes makes

these plants desirable in all crop rotations. They make

it possible to maintain the soil nitrogen. It was later

found that certain bacteria and fungi working independ-

ently of plants are also able to fix nitrogen from the supply

in the air. The quantity of nitrogen they fix in the soil

is large in some cases, though fixation by means of legumes

proceeds more rapidly.

155. Nitrification and denitrification. — Most of the

soil nitrogen has once been held in plants where it was

one of the important constituents of protoplasm. When

plants die, their nitrogen returns to the soil as complex

protein compounds and, as such, it cannot again be used

until the compounds are broken down. Some bacteria

and fungi attack dead plants and cause decay, during

which at least a part of the nitrogen is converted into

ammonia compounds. Ammonia is then attacked by a

group of nitrous bacteria which change the nitrogen into

nitrites, which are in turn converted into nitrates by the

nitric bacteria. In the form of nitrates, the nitrogen is

again available to crops. Thus the nitrogen cycle is

carried on by a number of different forms of organisms.

In this cycle, nitrogen is taken up as nitrates by the

higher plants. In their bodies it becomes a part of the

complex protein compounds. When the plant dies, these

compounds are broken down into ammonia, which by

the process of nitrification, is converted into nitrites

and finally into nitrates, when it is again ready to be

144 The Principles of Agronomy

used. Nitrification requires a good supply of oxygen, a

proper amount of soil moisture, a favorable temperature,

and a number of other conditions.

In the soil there are denitrifying organisms which

change the nitrates back into nitrites and ammonia.

These work in conditions just the opposite to those favor-

able for the nitrifying bacteria. Poor drainage and a

lack of soil air are among the conditions favoring their

action. In ordinary well-tilled soils these nitrate de-

stroying organisms have but little effect. Only where

large quantities of nitrate fertilizers are applied to poorly

aerated soils do they have great economic importance.

156. Bacteria and the farmer. — Soil bacteria will go

on doing their work in spite of anything the farmer does ;

but he may, by proper methods, increase their usefulness

to him. By the introduction of leguminous crops into

his rotations, he is able to keep up the nitrogen supply,

and by the plowing under of organic matter, he furnishes

carbon for the forijiation of humus which assists in mak-

ing available the various mineral plant-foods. By drain-

ing wet lands, by adding limestone to soils that are acid,

by the liberal use of barnyard manure, and by proper

tillage methods, the farmer is able to get the greatest

good out of these invisible, but powerful, workers in his

behalf.

SUPPLEMENTARY READING

Any textbook of bacteriology.

Agricultural Bacteriology, H. W. Conn.

Bacteria in Relation to Country Life, J. G. Lipman.

Soils, Lyon, Pippin, and Buckman, pp. 421-474.

Cyclopedia of American Agriculture, Vol. I, pp. 441-453.

Agricultural Analysis, Vol. I (Soils), H. W. Wiley, pp. 519-572.

CHAPTER XIV

TILLAGE AND CROP ROTATIONS

New reasons for cultivating soils and rotating crops

are constantly being found; but tillage and rotation

were practiced long before any reason was known except

that the yields of crops were increased by these prac-

tices. Before any modern implements were made, the

soil was stirred with bent sticks and rude devices of vari-

ous kinds. These methods served the purpose of the

time ; but as knowledge increased and better imple-

ments were invented, the tillage of the soil was completely

transformed. To-day, instead of being confined to a

mere scratching of the land, it may include the intelligent

use of a number of specialized implements during a sea-

son. Although there are many reasons why cultivation

of the soil is desirable, the following are probably the

most important: (1) to improve the structure, or tilth

of the soil, (2) to control the growth of weeds, (3) to cover

manure, stubble, and other plant residues, and (4) to

conserve soil moisture. Almost every tillage operation

effects one or all of these.

157. Improving soil structure. — Every plant requires

for its best growth a looseness of soil that permits a free

passage of air and an easy penetration of roots. When

left undisturbed for a number of years, the soil becomes

compact and is not in the best condition for crop growth.

It is necessary, therefore, to loosen it by the use of some

tillage implement. In cultivating the soil to improve

L 145

146 The Principles of Agronomy

tilth, attention must be given to the amount of moisture

present. When a very wet soil is stirred, its particles

are wedged together and the result is puddling, which is

much more unfavorable to plants than is the merely com-

pact condition of virgin land.

Plowing should mean more than the mere turning over

of the soil. If well done, every clod will be shattered

and every particle have its relation to every other particle

Fia. 41. — Field in good condition for crops,

changed through the shearing action which should take

place when the plowed slice is turned over. As the soil

falls into the furrow, it should be a granular, mellow mass

of loose particles. The kind of implement that will best

produce this condition varies with each soil. Sand or

loam may be made mellow with almost any kind of plow,

but a heavy clay without organic matter can be given a

good tilth only when everything is favorable. Soil in

good condition is shown in Figs. 41 to 43.

TiUage and Crop RotaMons 147

168. ControUing weeds. — Weeds are a menace to

every farm. They thrive under all conditions that pro-

duce crops, and it is impossible for ordinary crops to com-

pete with them without the farmer's aid. Weeds are

injurious, since they consume available plant-food and

moisture needed by crops; they shade and crowd out

Fid. 42. — A Bood seed-bed.

th^ more desirable plants; and they often reduce the

market value of crops. In arid regions where crop pro-

duction is limited by lack of moisture, successful farm-

ing cannot be practiced unless weeds are kept in check ;

indeed, the quality of farming in any region may be judged

by the thoroughness with which weeds are controlled. •

Some one has said that weeds are a good thing for the

148 The Principles of Agronomy

farm since they keep the farmer cultivating. Be this

as it may, it is probable that a large part of the tillage

operations are performed in order to kill weeds ; but the

soil receives other benefits at the same time. Much

energy is wasted in trying to control weeds which are

allowed to grow and begin seed production before the

cultivator is used. It takes a great deal of work to kill

big weeds, and if their seeds have been scattered a new

crop of troable may be expected. The best time to kill

weeds is just after they have germinated and before they

have become well established in the soil. A mere stirring

of the soil at this time is all that is necessary, but if they

are allowed to get well established, a number of hoeings

or cultivations are often required.

The implement used to kill weeds depends on the crop

grown, the kind of land, and the kind of weeds. On

fallow land, an implement covering considerable area can

be used to advantage. The spike-tooth, disk, and spring-

tooth harrows, and implements with blades running just

beneath the surface of the soil are effective. For tilled

crops such as corn and potatoes, some sort of cultivator

is used to advantage ; while in crops like alfalfa, the spring-

tooth harrow is a good implement to eradicate weeds. The

great secret of weed control with any tool lies in doing

the work at the right time.

169. Covering manure and plant residues. — Organic

matter accumulates on the surface of any soil that is

cropped. In the orchard, leaves fall to the ground; in

the grain field, stubble is left after harvest ; and in mead-

ows that are to be followed by another crop, a sod must

be disposed of. These plant residues cannot decompose

readily if left at the surface. They need to be turned

under and mixed with the soil in order to decay and give

up their plant-foods as well as to assist in making available

TiUage and Crop Rotations 149

the mineral matter of the soil. Farm manure is constantly

being applied to the land, and must be covered and mixed

with the soil if it is to do the most gopd. Practically all

of this covering must be done with some kind of plow,

although the disk harrow finds occasional use where the

land has recently been plowed.

160. Ccmserving moisture. — One of the most im-

portant reasons for cultivating the soil is the conserva-

tion of moisture. Even in regions of abundant rainfall,

there are times when it is necessary to save soil moia-

Fia. 43. — Orchard soil m good tilth.

ture; and in arid regions, the very life of agriculture

depends on conserving the scant supply of water (see Fig.

44).

If the soil is compact and hard, rain water will nin off

the surface rather than penetrate the soil where it can be

used by plants. The soil must, therefore, be loosened in

order that it may absorb moisture. The water that is

in the soil moves by capillarity from particle to particle,

and if the surface particles are pressed tightly together,

the water will rise to the surface where it is lost by evapora-

tion. This loss can be prevented by stirring the surface

and forming a loose, dry mulch of earth which does not

150 The Principhs of Agronomy

allow moisture to escape readily. This mulch may be

preserved by many implements, such as harrows and

cultivators of various kinds.

Rolling the land is often practiced to make the surface

smooth and to break clods. Compacting the surface

soil by the roller increases capillary movement toward

the surface and thereby the loss of moisture. The fact

that the soil seems more moist after a roller is used often

misleads farmers who think they are actually saving water.

161. Tillage of various crops. — The implements of

tillage may be divided into three main classes — (1)

plows, (2) cultivators, and (3) crushers and packers.

The primary purpose of the plow is to loosen and pulver-

ize the soil and make it more fit for the growth of plants.

Plows are of nimierous designs; no one kind is suitable

for all conditions. The disk plow has given good results

in many places, but the moldboard plow is doubtless

Tillage and Crop Rotations 151

suited to a much wider range. The old walking plow is

rapidly giving way to some form of riding plow.

Many different kinds of cultivators are used in pre-

paring the seed-bed, in eradicating weeds, and in tilling

crops during growth. Every farm should be equipped

with several kinds of cultivating implements. A very

useful and simple device used to smooth the land and to

break clods without compacting the soil is made by at-

taching a niraiber of planks together with their edges

overlapping. This planker, or float, is especially useful

to precede the grain drill, since it scrapes off little eleva-

tions and fills depressions, thus insuring a more uniform

depth of planting.