Notes on soils, an outline for an elementary course in soils

Preface

The aim in the pages that follow has been to present a brief outline of work in Soils, adapted to the needs of students pursuing short courses in Agriculture. The present volume has been hastily prepared, and therefore many errors may have been overlooked. Much valuable data, not included in these pages, will be found in the bulletins listed in the appendix, and in the larger text books upon the subject of Soils. Acknowledgment is due Mr. Stuart L. Clark for his careful preparation of the line drawings.

A. R. W.

H. L. W.

The University of Wisconsin,

College of Agriculture,

December, 1909.

Contents

Chapter Page

I. Conditions essential for Plant Growth 1

II. The Origin of Soil -Material 19

Chapter I. Conditions Essential For Plant Growth

In studying the growth of the plant it is convenient to divide its life history into three periods: First, the period of germination ; second, the period of vegetative growth, and third, the period of fruition. It is, of course, true that these periods shade one into the other or overlap to a certain extent, but it is nevertheless helpful to study the effect of conditions on plants from the standpoint of this threefold division. Some conditions are essential to the life of the plant at all times while others apply to only one or two of its periods of development.

Conditions Necessary for Germination.

( 1 ) Absorption of Water. The first act in the germination of seed is the absorption of moisture. The factors which influence this process are : Temperature, closeness of contact between the soil and seed, amount of moisture in the soil and the amount of soluble salts in the soil.

(2) Temperature. By placing beans or peas in warm and in cold water the greater rapidity with which

the warm wain- is absorbed can be readily determined. It is for this reason, in part, that seeds germinate in warm soil faster than in cold soil. But, besides influencing the rate at which water is absorbed, the temperature also influences the chemical changes which take place in the seed during germination.

The influence of temperature on germination has been studied by a number of experimenters and the averages of their determinations of the best and of the lowest temperatures, for the germination of the seeds of a few crops, are given in the following table.

Best. . . .

Lowest .

Wheal and Barlej .

Red

( lover.

Corn.

Rape.

75 deg. 40 deg.

70 deg. lo deg.

90 deg. is deg.

90 deg. to deg.

There is usually little gained by sowing seeds in the spring too early and while the ground is loo cold for the respective crops, although for some seeds, as for instance Red Clover, the moisture conditions may be better earlier.

The ways in which the temperature of the soil can be influenced will be studied in ;i later chapter.

(3) Contact of Seed and Soil. The ease with which water can lie absorbed from the soil by the seed will also depend on the number of points of contact between the soil and seed. This will depend on the fineness of the soil and on the firmness with which it is pressed down on the seed.

It is for this reason that so much depends on the care taken in the preparation of the seed lied and in firming the soil by rolling after the seed is planted. And the fineness depends very largely on the condition of the soil when cultivated, as will be shown in a later chapter.

The degree of fineness necessary for good germination depends also on the kind and size of seed. The smaller seeds, such as those of the grasses and clovers, require a finer seed lied than is essential for corn and peas.

(4) Influence of Salts. On account of the attraction between salts and water the presence of a consideral Le amount of soluble salts in the soil will prevent the seed from absorbing water. This can be shown by placing 1 cans or peas in two cups of water — one of which contains a small amount of common salt while the other contains none. The salt will partly or entirely prevent tie absorption of water by the seed. It is in this way that the large amounts of salts in the soil of arid regions often prevent the germination of seedi Some salts are actually poisonous to plants even in small amounts while others simply act to prevent the water from entering the seed and are not injurious except when present in considerable quantities.

( 5 ) Oxygen. The work of constructing new tissues and of forcing the newly formed root into the soil is done by the energy resulting from oxidation of food stored in seed and therefore, the germinating seed requires a supply of available oxygen, just as the energy which a man uses in doing "work comes from the food which he consumes when oxidized iii the muscle tissue. It is therefore necessary that the soil iii which the seed is germinating allow the air to reach it. otherwise the seed will rot without germination. The access of air to the seed is frequently prevented by loo much water in the soil and by puddling of the soil as will he described later.

(6) Mineral Elements not Needed for Germination. It is often noticed that seed will germinate and grow for a short time better in poor, sandy soil than in fertile soil and even better in sawdust or other loose material. This shows that nothing hut water and air need he taken in by the seed in germination and in the early growth of the seedling.

The amount of growth made by the seedling developing in sawdust or in the air alone under suitable conditions of moisture, temperature and light depends on the amount of material stored in the seed. Small seeds such as those of most grasses will allow but little growth while larger seeds such as those of pea and hean may allow the seedling to attain considerable size; in some e;ises half the size usually attained in ordinary soil, and even produce flowers.

Conditions Essential for Vegetal ire Growth.

(7) Mineral Elements Taken from the Soil.

While tin' seed will germinate readily and the seedling grow rapidly for a short time in pure sand or sawdust il soon begins to lose its healthy green color and finally stops growing. On the other hand, while the field soil may not allow the seed to germinate as rapidly as the pure sand or saw-dust, it does allow it to continue its growth to maturity.

The reason for this is that it furnishes material which is necessary for continued growth although not necessary for germination.

When plants are analyzed the following elements taken from the soil are usually found: sodium, potassium, calcium, magnesium, iron, silicon, chlorine, sulphur, phosphorus and nitrogen. With the exception of the nitrogen these are almost entirely left in the ash on burning. Besides these elements there are a number of others which are often found.

Part of the above list of elements are not entirely necessary to growth, as has been shown by growing plants to maturity in a solution not containing them. Those which do not seem to be essential are sodium, siliccn and chlorine. All of the remaining ones must be present or growth will cease before maturity.

(8) The Amount of Salts Taken up by Plants. The amount of the various elements taken up by plants depends on three factors: first, the relative amount present in the soil in a form which can be taken up by the plant; second, on the combination it makes in the plant, and third, on the kind of plant.

More salts will be taken up from a soil rich in soluble salts than from one poor in them. If a given element combines with other substances in the plant much more of it will be taken up, than if no such combination is made.

There is therefore quite a range in the amounts of different elements found in the plant, and also in the

<i NOTES ON Si ll I. S.

total amount of ash. Again, different plants growing dti the same soil will take from it different amounts of i he various elements.

(9) Variation in Fertilizing Constituents at Different Stages of Growth. Different plants not only use different amounts of plaid food, but the same plaid lias a varying composition at different stages of growth. This matter lias received careful attention at the Ducal Agricultural Experiment Station in Bernburg, Germany, and some of their results will he given here.

The amount of potash in the crop from one acre of wheat was SS lbs. at a very early stage, 123 lbs. when jusl heading out. 122 lbs. when fully headed out but green, and decreased to 72 lbs. at the ripening stage, less than at a very early stage, showing that the plant had taken up large amounts of potash, used it in life processes, and then returned it to the soil, either by way of the roots or by excretion on the surface of the leaves and stems to be washed off by rain and dew. The same general relation holds for nitrogen in the case of the wheat plant, while phosphorus, on the other hand, reaches its maximum when the plant is fully headed out. and does nut materially decrease ;it maturity. Analyses of another cereal, barley, at different stages in its growth showed the same general tendencies, although the absolute amounts are different.

Quite different results were obtained with the potato. The amount of potash in the tubers, leaves, and sleius from one acre of potatoes was 47 lbs., when tubers were just beginning to set (June 17). 78 lbs. one month l;tcr (July 16). 112 lbs. two months later (Aug. 18), and when the potatoes were fully ripe (Oct 5), 143 lbs. showing a continual increase in amount of potash absorbed. Similarly phosphorus increased from 8 lbs. at an early stage to 28 lbs. at ripening stage, and nitrogen from 45 to 111 lbs. This great difference between the cereals and the potato is probably due to the fact that the potato does not dry out on maturing as do the cereal crops.

It should be remembered that the experimental work here discussed was carried on under a particular set of conditions and with a particular soil. The results obtained might have been quite different under different conditions, for we have pointed out in the previous paragraph that the amount of an element taken up by a plant depends upon the relative amount in the soil in an available form. This general conclusion can be drawn, that the composition of a plant at maturity does not necessarily show the amount of plant food it may have absorbed during its growth.

(10) Average Amount of Salts Removed by Crops. While, as stated in previous paragraphs, the amount of various salts absorbed by plants from the soil varies greatly during different stages of growth still the amount of the three most important elements, Nitrogen (N), Phosphoric Acid (P 2 5 ), and potash (K 2 0), found in mature crops grown on ordinary clay loam soil is quite constant. Since these are the amounts removed from the soil by crops when harvested, they are very important to bear in mind.

The following table, taken from Bulletin 47 of the Minnesota Experiment Station, shows the amount of the b NOTES ON SOILS.

more importanl elements taken by different crops from clav loam soil.

Tdbli showing the Plant Food material removed by the crops in pounds per acn .

Crop.

( tress Weight.

Nitrogen.

Phosphoric Acid.

Potash.

Lime.

Wheat, 20 bu

1200 2000

25 10

12.5 7.5

7 28

1 7

Barley, 40 bu

11)20 3000

28 12

15 5

8 1 30 ' 8

38 1 9

( >al s 50 bu

12 6

10 35

1.5

Straw

9.5

Corn, 65 bu

Stalks

40 15

15 80

1 •jo

Total .

Peas, 30 bu

St raw

1800 3500

;i

GO

Flax, 15 im

900 1800

39 15

1!)

Straw

18 27

1(5

Potatoes. 300 bu

Mangels, 10 tons. . . .

2000 1000

20000

20 28 10 35

15 12

66 75

150 50

150 30

(11) Distribution of Elements in Plant. Study -of the foregoing table will show that nitrogen and phosphoric acid go chiefly to the seed or grain of crops, while potash and lime are largely found in the stalks or straw. From this we are able to determine the probable losses when either the grain or straw or both are sold.

(12) Food Constituents of Plants. The chemist divides the chief food compounds produced in plants into three classes : carbohydrates, fats and proteids. The carbohydrates, such as starch, sugar and a large part of crude fibre and the fats and oils of the plant are made up of only three chemical elements : carbon, oxygen and hydrogen. These are derived from carbondioxide of the air and from water absorbed by the root. The proteids. however, which form the most valuable part of the food contain considerable nitrogen, small amounts of sulphur and in some cases phosphorus. These three elements are taken from the soil.

(13) Variation in Composition During Growth. Since young plants and growing parts of older plants have thin walled cells filled with protoplasm, they have a high per cent of protein and a low per cent of crude fiber or cellulose. Crops cut very green are therefore richer in protein than mature crops. Moreover, if crops are prevented from making full growth, the stunted crop is usually richer in protein than the crop making a larger growth. There is an exception to this when the failure to grow is due to lack of available nitrogen as mentioned in paragraph 15.

(14) Function of Elements. Although it is comparatively easy to determine which of the elements found in the plant arc essential and which are unessential to the growth of the plant it is very difficult to find out just what the use of the essential elements is to the plant. There are specific functions which each of the essential elements have to perform and there are more general functions which various elements can perform. While a certain minimum amount of each of the essential elements must be present for' the specific functions, there must be larger amounts of some of the essentia] elements or of the non-essential elements to produce the best growth. A soil which is poor in all the essential elements will often be helped to a certain extent by an addition of any one while it is much more benefited by the addition of all those lacking.

One of the special functions of potash seems to beto aid in the process of starch formation. Corn growing on the marsh lands of our state has been greatly benefited by potash fertilizer and in all cases the increase of grain — containing a large amount of starch — has been greater than the gain in the stalks. The formation of starch takes place in the leaves, it being then carried to the seed and the potash is chiefly found in the Leaves and stems of plants.

The phosphorus is necessary to the formation of some proteids, and since the proteids form a larger per eon of the seed than of the stalk and leaves, the phosphorus is found in larger amounts in the seed or grain than in the stalk and leaves.

Calcium or lime seems necessary for the development of leaves, for plants grown in solutions free from calcium do not develop leaves readily.

CONDITIONS ESSENTIAL FOR PLANT GROWTH.

Nitrogen is absolutely essential for the formation of prot cids and the relative amount of protein found in the plant depends partly on the relative amount of nitrogen in the soil in an available form, as well as on the nature of the plant.

(15) Relation Between Composition of Plant and Amount of Food Material Available to it. The composition of the plant is to a certain extent dependent on the relative amount of the necessary elements available to it. When the amount of available nitrogen is small the plant cannot produce as much protein as it does ordinarily. The crop grown on soil very low in available nitrogen is therefore often low in proteid, the most valuable food material.

Experiments made by growing corn, oats and rape on three sand plots to which had been added different amounts of nitrates showed the following per cent of protein where plot one received no nitrates; plot two, a medium amount; and plot three, double this amount.

Oats.

Corn.

Rape.

Plot 1

Plot 2

Plot 3. . .

12.00 per cent. 15.81 per cent. 16.63 per cent.

S.44 per cent.

S».94 per cent.

1 1.25 per cent.

12.56 per cent. 14.00 per cent. 14.25 per cent.

These crops were cut green, as they would be used for soiling.

These experiments, with others, show that not only is the crop on fertile soil larger than that on poor soil ; but it is richer in the most valuable food constituent.

The composition of plants is also influenced by their rate of growth and varies at differenl stages of growth. This is discussed in paragraph 20.

The influence which climate and soil have on the per cent of protein in wheat has been investigated by Deherain in France and by Sonic in Tennessee.

During the period of most rapid growth of the cereals comparatively little starch is transmitted to the seed, most of it collecting in the upper part of the stem during this period and being carried to the seed during the ripening stage.

If therefore the weather becomes dry during the ripening stage less starch is carried to the seed and it has a relatively high protein content.

(16) Chemical Requirements of Different Crops. Since different plants produce different relative amounts of carbohydrates, fats and proteids, we should expect that they would need somewhat different relative amounts of the various elements. Those plants which produce a relatively large amount of starch, such as corn and potatoes, require considerable available potassium while those which produce a relatively large amount of proteid should require considerable available nitrogen. This is found to he home out by experience, but in addition to these facts which we can explain in this way. there are other cases where certain plants require relatively large amounts of certain elements for reasons we do not yet know. It will be helpful in studying the relation of the various crops to the soil and of their tieatment with fertilizers to lirst study some of the most important cases of this varying requirement.

Cereals, such as wheat, oats and barley, need relatively large amounts of nitrates and phosphates, while not a very large amount of potassium is necessary. Corn iu addition to available nitrogen and phosphate must have considerable available potassium, possibly on account of the large amount of starch which is produced. Potatoes also, both white and sweet, require considerable potassium. Timothy and most grasses require also large amounts of available nitrogen. Turnips do best when the amount of available phosphorus is large, while beets and carrots require more nitrogen. These are some of the facts which have been learned from experience, but our knowledge of this subject is still very incomplete.

(17) Uses of Water to the Plant and Amount Required. There are a number of Afays in which water is of service 1 in the growth of plants, among the most important of which are the following: (1) to keep the cell Avails of the leaves moist so they can absorb carbon dioxide; (2) to evaporate from the surface and so prevent the plant from getting too hot, just as the evaporation of perspiration from the human body tends to keep it cool; (3) 'to furnish the small amount of water needed for building the various tissues in the plant; (4) to carry the salts from the soil into the plant and to the leaves where they are chiefly used in the chemical changes taking place there.

This current of water into the plant roots, up the stem and out of the leaves is called the transpiration current.

The rate of transpiration depends chiefly on the amount of moisture in the air, on the temperature, on

14 xo'i i:s ox soils.

the strength of Lighl and on the character of the plant. Moisture in the air lessens it; warmth increases and cold decreases il : strong Lighl increases and darkness greatly

retards it: and some plants transpire nmeli less than others under the same conditions because they are protected in various ways or have fewer stomata.

A aumber of experiments have been made by different men to determine the number of pounds of water lost by the plant for each pound of dry matter formed by the plant. An average of these would be approximately as follows: Barley, 465; Oats. 500; Corn. 27-"); Clover, 575; Peas. 47-"); Potatoes, 385.

A single determination of soy bean shows that it used 527 pounds for each pound of dry matter. From these figures we would conclude that if no water ran off the surface or drained away a crop of 2(1 bushels of wheat would require only six inches of rain during the growing season ; or a crop of 50 bushels of oats only eighl inches; 60 bushels of corn, ten inches; 300 bushels of potatoes, six and one-fourth inches; two tons of clover hay. nine inches.

(18) Relation of Light to Plant Growth. In the development of the plant there are two processes going on; one is the process of forming starch, fats and prot< ids either for the plant itself or to be stored in some part of the plant, such as the seed or root. The other process is that of growth or the development of new tissue and parts. This latter process is carried on by the use of some of the material produced in the first process. Prom this we see thai the plant differs from the animal in that the plant produces as well as uses food, while the animal uses food only and must be supplied with it from outside

Now the production of starch, fats and proteids requires light, while their use in growth and movement does not require light. We have all noticed the growth which potato vines will make from material stored in the tuber in the entire absence of light and not only is light not necessary but to a certain extent it retards growth. The tendency of plants to turn towards light .is due to the fact that the side away from the light grows more rapidly than the other and pushes it over towaids the light. The question as to the amount of light necessary for producing the greatest amount of Pood is not entirely understood, but it is known that the strength of light received on the earth's surface in our latitude during mid-day hours of the summer is as great as can be used by most plants. Moreover in all probability the intensity of light at five o'clock during clear days in the summer is as great as can be used by our crops.

It will therefore be seen that the intensity of light is not of so much importance as its duration. During the long summer days of Northern latitude much more rapid growth is possible than in the regions near the « quator, because more food material is produced. The starches and sugars in particular require light to produce them and hence relatively larger amounts of these substances are produced in those regions where there are few clouds to interrupt the sunshine, than in those regions where there is much cloudiness. This is probably, in part, the reason why the sugar beet produces a relatively larger amount of sugar in Colorado where there is much more sunshine than in the Mississippi Valley where there is more cloudiness.

(19) Relation of Temperature to Plant Growth. The most favorable temperature for formation of food by the plant is usually the most favorable for growth also. When the amount of material produced by the plant is studied at different temperatures, it is found that it increases with increase in temperature up to a certain point and then decreases; that is. the production of food in the plant is lower at too high as well as too low temperatures. Probably the most favorable temperatures for our field crops art 1 between seventy and seventy-five degrees Fahrenheit. Even corn, which is often supposed to need much heat, will make its very best growth without rising higher than seventy-five degrees. Since growth goes on during the night as well as day. the influence of warm nights is very great.

(20) Relation of Character of Plant to Rate of Growth. It is frequently noticed that plants which are growing very rapidly lack in stiffness of stem. This is because the cell walls of the tissues are thin in rapidly growing plants, as was remarked in a former paragraph, and the growth is more rapid in either complete or partial absence of light than where it is intense. The result of this is' that those plaids or those parts of plants which grow in partial darkness are softer and less rigid than those which grow in strong light. The succulence of good vegetables is. therefore, largely caused by their rapid growth and so in the production of vegetables it is essential to hasten the growth in every way possible by supplying them with the best condition of moisture, light, temperature and food material.

(21) Lodging". The weakness of the stem of grain which causes it to lodge, is due to the rapid growth made when there are large amounts of water available and when the light is partly excluded either by cloudiness or by thickness of planting. It has been supposed that the degree of stiffness of the straw was determined by the amount of silica present, but this is probably not true There is very little tendency in grain to lodge in dry regions even when large quantities of water are used in their irrigation, probably because of the continuous sunshine, although it may in part lie due to the larger amounts of salts such as potash and phosphates in the soil.

Conditions Influencing Fruition.

(22) Translocation of Material in Plant. The object of the plant in the production of seed is to secure the reproduction of itself. As a general rule plants tend to form seed when the conditions become unfavorable to continued rapid growth of vegetative parts. On the other hand if the conditions for vegetative growth remain very favorable, the formation of seed is retarded and the seed when formed is often not so well matured. The conditions most favorable to the formation of seeds are, therefore, different in some respects from those most favorable to vegetative growth, although, pf course, a certain amount of growth is necessary to allow the plant to produce the seed in full maturity. It is very important for the farmer to realize the difference in the conditions necessary to the production of the largest amount of stalk and leaves on the one hand and of

18 NOTES ON Soll.s.

seed mi the other. In growing fodder and hay. a heavy growth of stalk and leaves is wanted, while in raising grain, a heavy yield of mature seed is the object.

The formation of material stored in the seed and fruit is largely the result of moving it from the stalk and leaves where it is produced to the seed or fruit where it is stored. In most of our farm plants growing under favorable conditions there is nearly enough of this material in the stalks and leaves at the time of tlowering l<i reproduce the mature fruit and seed. Some water must, of course, be available to the plant during the ripening period, hut little else is accessary. It' now. there is an excess of water and especially of available nitrogen, there is a strong tendency to continue vegetative growth which retards and in some cases practically prevents the formation of seed. It is essential. therefore, to the besl development of fruit and seed that the amount of these substances available to the plant be limited during the period of maturity. This principle is very important in planning a rotation of crops and will he referred to later when that subject is discussed.

(23) Relation of Growth to Soil. On account of the many ways in which the soil affects the plant's growth the importance of its study is very great. It supplies it with those chemical elements necessary, and with water. Then since a considerable part of the plant must grow under ground for these materials the temperature of the soil and its ventilation are important factors.

THE ORIGIN OF SOIL MATERIAL. 19

Chapter II. The Origin Of Soil Material

In order to understand the distribution of the various kinds of soils and also the processes by which the elements in them become available to plants it is necessary to study their origin.

If we examine any soil carefully we find that it is made up of two different kinds of material. The first kind includes gravel, sand and clay and is derived entirely from rocks. The second, which forms the black or organic material and is called hum us, is derived from the decomposition of animal and plant organisms which have grown in the soil.

(24) Soil-forming Rocks and Minerals. There are three chief classes of rocks in Wisconsin : granite and related rocks, limestone and sandstone. The granite occurs in the North Central part of the state. It is a crystalline rock composed of various minerals, the chief of which are quartz, feldspar, mica and hornblende. The quartz is white, the feldspar has flat surfaces and is pink or flesh colored. There are two kinds of mica, one white and the other black, but both are characterized by being in very thin* layers which can be easily separated with the point of a knife blade. The hornblende is black, very hard, and usually occurs as more or less needle-like crystals.