Humus in Its Relation to Soil Fertility

HUMUS IN ITS RELATION TO SOIL FERTILITY.

By HARRY SNYDER, B. SC, Professor of agricultural chemistry in the College of Agriculture of the University of Minnesota, The term humus is applied to a large class of compounds derived from the decay of former animal and plant life. The animal and vegetable materials (organic matter) undergo decomposition in the soil, the final result of which is the disappearance of these substances, leaving only a few gases and a small amount of mineral matter. When the organic matter is in its intermediate stages of decomposition, and mixed with the soil, it is known as humus. Opinion as to the fertilizing value of humus has swung, pendulum like, from one extreme to another. The alchemists taught that the spirits left the decaying animal and vegetable matters and entered plants. By many of the earlier chemists, humus was considered as supplying the larger part of the materials necessary for the develop- ment of the crop, but when the combined labors of De Saussure, Bous- singault, Dumas, and Liebig demonstrated that the air supplied plants most of their food, particularly that part which was supposed to come from humus, scientists, as a rule, assigned a low value to humus. From the very earliest times, however, farmers have assigned a very high value to humus as a factor of soil fertility, and this belief was strengthened by the observed facts that soils rich in humus were, as a rule, highly productive, and that such materials as animal excrement or barnyard manure, which supplied the soil with an abundance of humus, possessed a marked fertilizing power. Although many of the old theories which were supposed to account for the value of humus are no longer tenable, recent experiments have shown that there are sound scientific reasons for ascribing to humus a high value as a factor of soil fertility, and have demonstrated that ''farmers are wholly right in attaching great importance to the preservation of humus in their soils." As the following pages will show, humus performs a number of different functions in the soil which are of the highest importance in crop production. It influences the temperature, tilth, permeability, ab- sorptive power, weight, and color of soils, and directly or indirectly controls to a high degree their supply of water, nitrogen, phosphoric acid, and potash. 131 132 YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE.

LOSS OF SOIL HUMUS AND DECLINE IN FERTILITY. A virgin soil or one recently cleared may show a high state of productiveness for a number of years after it is brought under cultivation. Gradually, however, a decline in fertility is observed, which is slight at first, but more marked after a lapse of fifteen or twenty years. Experiments have shown that the decline in fertility is not entirely a result of the removal from the soil of the essential fertilizing elements—nitrogen, phosphoric acid, potash, or lime—but is due in many cases to getting the land out of condition through a loss of humus. Experiments conducted by the Minnesota Agricultural Experiment Station on different types of soils worn by continuous grain cropping have shown that when a fertilizer was used containing nitrogen, phosphoric acid, potash, or lime, or when any one of these materials was applied alone, there was '4n no case an increase of over 3 bushels per acre of wheat and 2 of flax. * * * "With soils that have been cropped for twenty years, the largest increase was 4 bushels per acre." The difference between the grain- producing power of new soils and of worn soils of the same original character was about 15 bushels per acre. These results, as well as many others which could be quoted, make it clear that the decline in fertility of the soils was not entirely due to a loss of the essential elements of fertility, and that we must seek the cause elsewhere. The most important difference, physical or chemical, between the composition of old, worn soils and new soils of the same character is in the amount of humus which is present. That the loss of humus is an important factor in the decline of fertility is also indicated by the fact that with methods of farming in which grasses form an important part in the rotation, especially those that leave a large residue of roots and culms, the decline in productive power is much slower than when crops like wheat, cotton, or potatoes, which leave little residue on the soil, are grown continuously. Under grass and similar crops the soil humus increases from year to year, while the continuous culture of grain, cotton, or potatoes gradually reduces the original stock of humus. Grass and grain crops in rotation result in alternately increasing and decreasing the humus of the soil and keep the land in a higher state of productiveness, although more nitrogen, phosphoric acid, and potash is removed from the soil .than when grain, cotton, or corn is raised continuously. In no case, however, do those systems of farming which return humus-forming materials to the soil reduce the land to so low a state of productiveness as do those systems in which there is a continual loss of humus from the soil (seep. 141). Agriculturally considered, the two most important points regard- ing the composition of humus are (1) the presence of nitrogen as a HUMUS IN ITS RELATION TO SOIL FERTILITY. 133 constant constituent, and (2) the chemical union of the humus with potash, lime, and phosphoric acid, forming humâtes. NITROGEN IN HUMUS. Humus, as ordinarily obtained from the soil, contains from 3 to 12 per cent of nitrogen. According to Professor Hilgard, the soils from arid regions are poor in humus, containing from 1 to 2 per cent, but this humus is correspondingly rich in nitrogen, in many cases containing 14 per cent. In many of the prairie regions the soil contains about 5 per cent of humus, and this humus contains about 10 per cent of nitrogen. Since, therefore, nitrogen is one of the prominent constituents of humus, it is easily understood how a loss of humus has also resulted in a loss of nitrogen. This decline in the nitrogen content of the soil is one of the most serious results of the loss of humus from the soil. A virgin soil containing 4 per cent of true humus and 0.35 per cent of nitrogen will after twenty years of grain cropping show about 2.5 per cent of humus and 0.2 per cent of nitrogen. In the twenty years, therefore, there has been a loss of 1.5 per cent of humus, equivalent to about 3,500 pounds per acre, and 0.15 to 0.2 per cent of nitrogen, which is equivalent to 3,000 to 5,000 pounds of nitrogen per acre. Since 50 pounds per year of nitrogen is a large quantity for any ordinary grain crop to remove, the 20 crops have at the most removed 900 pounds of nitrogen. At least 2,500 pounds have, therefore, been lost by the decomposition of the humus, the nitrogen being lost either in the free state or in the drainage waters. For every pound of nitrogen remQved in the crops during the twenty years of cultivation there has been an additional loss of 3 or 4 pounds of nitrogen from the soil by the decomposition of the humus. We know that most if not all of the changes that organic matter undergoes are the result of the action of microscopic organisms. Such changes as nitrification, or the transformation of organic nitrogen into nitrates and its opposite denitrification, or the reduction of nitrates to gaseous nitrogen, besides many others which might be mentioned, are illustrations of the work of these minute organisms. Humus furnishes a medium peculiarly adapted to the activity of these organisms. The decomposition of humus, by which it loses its nitrogen, is due chieñy to the combined action of the organisms of nitrification and denitrification. The nitrifying organism feeds upon the humus, break- ing down its organic nitrogenous constituents and producing nitrates which may be washed out in the drainage, and the denitrifying organism completes the work by feeding upon the nitrates, producing free nitrogen gas, which escapes into the air. Nitrification is one of the most important natural provisions for rendering the inert fertility of the soil available to plants, and a certain amount of it is necessary to plant growth, but it can be readily seen that under injudicious management or cultivation of the soil 134 YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE. it may work a positive injury by causing unnecessary waste of tlie nitrogen, or, in case of rich soils, it may supply the growing crop with too much nitrate and thus produce a rank growth of straw and leaves. Summer fallowing,—Bare summer fallowing is widely practiced, and has been very beneficial to the succeeding crop by increasing the available nitrogen of the soil, but frequently more nitrogen is rendered available than is necessary for the following crop, and whatever the crop is unable to utilize is lost by leaching or else escapes into the air. The available nitrogen is thus increased, while the total nitrogen is greatly decreased. Experiments at the Minnesota Agricultural Experiment Station indicated that one year of fallowing caused a gain of 0.0022 per cent available nitrogen and a loss of 0.0114 per cent of total nitrogen in a soil containing originally 0.1536 per cent of total nitrogen and 0.0002 per cent of available nitrogen. For every pound of nitrogen rendered available by the fallow treatment there was a loss of over 5 pounds of nitrogen from the soil. Bare summer fallowing is, therefore, only temporarily beneficial at the expense of the total humus and nitrogen of the soil. When a soil is poor in humus and nitrogen the loss of nitrogen is much smaller, but even then it is doubtful whether bare summer fallowing is a wise practice. In no case should summer fallowing be practiced on a new soil. Fall ploiüing keeps the humus and nitrogen of the soil in better condition than late spring plowing. Nitrification goes on in the soil until quite late in the fall, and in the South the process goes on the entire year. The change is most rapid near the surface, where there is plenty of oxygen from the air. In early faU plowing the available nitrogen formed from the humus is near the surface, where it does the sprouting seeds and the young crops the most good. With late spring plowing this available nitrogen is plowed under, and inert organic nitrogen is brought to the surface. In old soils the process of nitrification does not go on rapidly enough to furnish available nitrogen to the crop. In a new soil the process of nitrification is liable to go on too rapidly. Deep plowing and thorough cultivation aid in nitrification. Hence the longer the soil is cultivated, the deeper and more thorough must be its preparation. Plowing must be done at the right time, preferably in the fall, so as not to interfere with the next year's water supply. The application of lime and wood ashes aids in the reduction of nitrogen of humus to available forms and prevents the formation of sour mold. Good drainage is also necessary to nitrification in the soil. In water-logged soils the humus does not decompose normally, but peat is produced on account of the absence of oxygen. We thus see that nitrification, although sometimes a serious source of loss, may be largely controlled by careful management of the soil. HUMUS IN ITS RELATION TO SOIL FERTILITY. 135

Burning over of soils.—Another source of loss of liiimus in the prairie and forest regions is the frequent burning over of the land. Soils covered with x>ine, in which sand largely predominates, frequently lose half or three-quarters their total nitrogen when visited by forest fires. The sand, being of an open and porous nature, aids in the more complete combustion of the humus. In the timbered regions of the Northwest the great forest fires of 1894 resulted in the average destruction of over 1,500 pounds of humus nitrogen per acre, to say nothing of the nitrogen lost in the burning of the timber. Analyses of soils, before and after the fire, made by the Minnesota Agricultural Experiment Station showed a loss in some cases of 2,500 pounds per acre of nitrogen, equivalent to a loss of 75 per cent of the total amount in the soil. The prairie fires have not been so destructive upon the humus as the forest fires, because the burning has been confined more to the surface. An average prairie fire, however, will remove more nitrogen from the soil than five ordinary crops of wheat. MINERAL MATTER IN HUMUS. Besides being a great reservoir of nitrogen, humus is an indirect means of supplying the plants with other fertilizing constituents. Humus as it occurs in the soil is combined with potash, lime, phosphoric acid, and other compounds which are essential as plant food. The decaying animal and vegetable matters form various organic acids, which combine with the potash, lime, iron, and alumina, as well as with other elements, and form a series of compounds known as humâtes, of which but little is definitely known. By some, the potash, lime, and other mineral constituents of the humus are regarded as simply associated with the humus and not organically combined with it, but there are a number of facts which indicate that the union is chemical and not simply mechanical. The mineral matter combined with the humus is characteristically rich in phosphoric acid and potash, two compounds which are of great value agriculturally. The mineral matter combined with the humus from different soil types, however, is not always of the same nature, and the amount of plant food thus combined with humus has not been extensively investigated. In the case of rich prairie soils over 1,500 pounds of phosphoric acid and 1,000 pounds of potash per acre to the depth of 1 foot have been found to be in combination with the humus. In the case of soils poor in humus and worn by cropping, the amount may be reduced to 100 pounds per acre. The average of analyses of the mineral matter of the humus from samples of productive prairie soils yielding 25 per cent of humâtes showed 7.50 per cent of potash and 12.37 per cent of phosphoric acid. In these soils, which were well supplied with humus, 1,500 pounds of phosphoric acid per acre out of a total of 8,750 was combined with humus, and 1,000 pounds of potash out of a total of 12,250 pounds. According to Hilgard, the amount of 136 YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE. phosphoric acid usually found associated with humus varies from 0.1 to 0.5 of the total amount in the soil, indicating in many cases the amount of this element available to plants.

VALUE OF HUMATES AS PLANT FOOD. The value of these various forms of humâtes as plant food has been the subject of extensive investigations and many of these experiments indicate that the humâtes, when acted upon by the proper microorganisms, are very valuable forms of plant food. At the Minnesota Agricultural Experiment Station oats and rye have been successfully grown when the only forms of mineral food were humâtes of potash, lime, magnesia, iron, and humic phosphate and sulphate. Húmate material obtained from rich prairie soil was mixed with pure sand, which contains practically no plant food, and gypsum was added to prevent the formation of sour humus. The mixture was watered with Teachings from a fertile field, so as to introduce the organisms which usually carry on the work of humus decomposition. Oats seeded in the soil thus prepared finally produced fertile seeds, the entire plants containing fifty times more potash than was in the seeds sown, and over sixty times more phosphoric acid. The only source from which the plant could obtain these substances was the humâtes added to the soil. In experiments in which the soil leachings were omitted the oat plants made only feeble signs of growth, plainly showing that unless the potash, phosphoric acid, etc., combined with the humus is set free by the action of microorganisms the plant is unable to use them. There are a number of facts in field practice which also indicate that plants are capable of feeding on humâtes. The roots of plants, particularly those of grains, will always be found clustering around any decaying vegetable matter that may happen to be present in the soil. When wheat or oats follow a corn crop the roots of the grain will be found in many cases to completely incase any decaying pieces of cornstalks that are present. The cornstalks are not rich in plant food, but they decay in the soil and combine with the soil potash, phosphates, etc., forming humâtes which the grain feeds upon. Large piles of sawdust many feet in height and circumference are frequently left around sawmills, or the sawdust is used for filling in low places. The sawdust is very slow in decomposing, but in time it is covered with vegetation which must obtain most if not all of its mineral food in the form of humâtes.

MEANS OF INCREASING THE HUMATES OF THE SOIL. Inasmuch as both experiments and observations in the field appear to strongly indicate that plants have the power of feeding upon humâtes, it becomes important to determine to what extent the HUMUS IN ITS RELATION TO SOIL FERTILITY 137 addition of animal and vegetable matters to the soil is capable of affecting the amount of available plant food. Experiments conducted at the Minnesota Agricultural Experiment Station have an important bearing upon this question. To a box holding 100 pounds of loam soil 20 pounds of cow manure was added. The contents of the box were kept moist and well mixed. At the end of twelve months the amount of mineral matter combined with the humus was determined, and the amount found compared with that originally in the box. Another box containing an equal amount of the same soil to which no manure was added was treated in the same manner. In the first case the mineral matter originally present in the manure was deducted, as well as the amount which was only sol- uble in the solutions used in the analysis. The results were as follows: Increase of humâtes in the soil due to applications of manure.

Total hu- Total at Gain of Total hu- Loss mâtes in the end of humâtes mâtes at when no 100 pounds 13 months from soil the end of manure of original in ma- through 13 months, was soil. nured box. manure. no manure. added.

Oraras. Grams. Gh'ams. Grams. Grams. Potash 7.25 9.14 1.89 6.93 0.33 Soda 7.84 10.11 3.27 7.50 .34 Iron 2.44 4.13 1.69 3.46 Magnesia ._ .35 .54 .19 .27 .08 Alumina 2.96 4.64 1.68 2.75 .21 Phosphoric acid 11.97 13.99 3.03 11.50 .47

As will be seen, the cow manure increased the amount of mineral matter combined with the humus to the extent of 15 to 25 per cent of the original amount present in the soil. In addition to adding new elements of fertility to the soil, it has also resulted in changing a part of the potash, magnesia, and phosphoric acid, as well as other solid elements, into forms more valuable as plant food. The manure, therefore, not only has a direct fertilizing value, but is also useful in making the inert plant food of the soil more available. A number of facts in field practice also point to the same conclusion. It is well known that barnyard manure is among the most lasting in eifect of any of the fertilizers which can be applied. This is undoubtedly due to the power which the manure possesses of uniting with the soil potash, phosphoric acid, etc., to produce humâtes. It has been frequently observed that when potatoes are cultivated on new prairie land for three or four years in succession, both the yield and the size of the potatoes decrease. When the land is seeded to a grass crop, the sod plowed under, and potatoes again planted, the yield and size of the potatoes are often nearly the same as when the land was new. This result has been attained without the addition of any manure to the land except the vegetable matter in the sod which has furnished materials for the formation of humâtes. In the same 2 A 95 5* 138 YEARBOOK OF THE Ü. S. DEPARTMENT OF AGRICULTURE. way, wheat grown continuously on prairie soil will gradually decline in yield, but if grass is alternated with the wheat, nearly the original yields are restored. Besides performing the useful functions just discussed, which are essentially chemical in character, humus profoundly modifies the physical properties of soils. This inñuence is most marked in relation to the water content and temperature of the soil.

HUMUS AND THE WATER SUPPLY OF CROPS. A soil rich in humus not only absorbs more water, but holds it more tenaciously in time of drought than a soil poor in humus. In fact, this is one of the most important differences between soils rich in humus and those poor in humus. A soil which by long cultivation has lost half of its total humus will show a loss of 10 to 25 per cent of its water-holding power. These differences are well illustrated in the following table, compiled from data obtained in the examination of two typical Minnesota soils : Water capacity of soils containing different amounts of humus.

Water. After 10 In hours' original exposure Loss. soil. to the sun.

Per cent. Per cent. Per cent. Soil richer in humus (3.75 per cent). 16.48 6.13 10.36 Soil poorer in humus (3.50 per cent). 13.14 3.94 8.30

Humus is also an important factor, especially in sandy soils, in assisting the capillary rise of subsoil water to the roots of crops. In a mixture of sand and humus, water will rise to the surface by capil- larity much more rapidly than in pure sand. As is well known, soils which are properly manured and thus supplied with abundant humus retain more water and yield it up more slowly and evenly to growing crops than unmanured soils. The part which the humus takes in the water supply of crops is sufßicient in itseK for placing a high value upon the humus of the soil.

HUMUS AND THE HEAT OF THE SOIL. Humus soils are generally considered cold or sour, but this is not always true of them. In humus soils decomposition or oxidation of the organic materials is constantly taking place, and this oxidation is accompanied by the evolution of a certain amount of heat. A portion of this heat is used up in warming and evaporating the additional water stored up in the soil on account of the humus, but even after this is provided for there is still some heat left from the oxidation of the humus to aid in warming up the soil. HUMUS IN ITS RELATION TO SOIL FERTILITY. • 139

It should be observed also that humus, as a rule, imparts a darker color to the soil, and thus causes it to absorb more of the heat of the sun. In autumn humus soils are not affected by sudden changes of temperature to the same extent as soils poor in humus, the difference frequently being sufficient toward off an ' early frost and to enable corn in the Northern States to reach its full maturity. Applications of humus-forming materials, such as manure, have frequently been observed to raise the temperature nearly a degree, and this in colder climates is often sufficient to prevent the growth of a crop from being checked. In the colder regions soils which are poor in humus freeze much deeper than soils which are richer in humus. In the preceding pages the attempt has been made to demonstrate that the chemical action of humus in providing available plant food in the soil makes it of the greatest value as a fertilizer; that it assists materially in bringing about the physical conditions in the soil best suited to the growth of plants; that it furnishes a medium peculiarly suited to the activities of such organisms as those of nitrification, which are useful in plant growth ; and that loss of humus from the soil is always attended by a marked decline in its productiveness. It is now important to discuss the means by which this valuable constituent of soils may be conserved and increased.

MEANS OF MAINTAINING THE HUMUS OF THE SOIL. On account of the variable composition of humus it is difficult to state the definite amount which should be present in all soils. A large amount of humus, containing a very high per cent of carbon, approaching in many cases the composition of charcoal, is not as valuable as a smaller amount of humus which is capable of readily undergoing decomposition. With an excessive amount of water, and in the absence or scarcity of the proper soil elements, like lime, potash, etc., humus-forming materials may produce sour soils, but in good soils well stocked with lime there is but little danger of this result. It is safe to conclude, therefore, that soils as a rule will be benefited by those systems of culture w^hich conserve or increase the humus content. The liberal use of well-prepared farm manures, green manuring» and a judicious rotation of crops are the three most important means of maintaining the humus of the soil. The preparation and use of farm manures and green manuring have already been discussed in som.e detail in bulletins from the U. S. Department of Agriculture,^ and it is only necessary to briefly refer to these subjects here. In the arid regions, and in many of the prairie sections, the proper preparation of farm manures is a problem which has not as yet been satisfactorily solved. On account of the slowness of decomx^osition

^ Farmers' Bulletins Nos. 16 and 21. 140 YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE. of the straw in the manure, many farmers in the regions named have begun to look upon manure as a detriment rather than a benefit to the land. In these regions, however, the soil is in greater need of humus than in the regions of uniform summer rains, and it is of the highest importance to devise some system of preparing the manure produced on the farm so that it may be utilized to the fullest extent. The humus materials of the soil may be increased by the use of well-prepared muck. It is best to draw the muck during the summer. After drying, it can be used as an absorbent in stables, for which pur- pose it is very valuable, many mucks having the power of absorbing more than their own weight of liquid. When muck is mixed with urine, it readily undergoes fermentation, which increases its fertilizing value. The brown mucks are much quicker in their action than the black. A little marl or land plaster mixed with the muck keeps it from forming sour mold. Clover and plants of the leguminous family are more suitable for green manuring purposes than any other class of farm crops, because, in addition to supplying an abundance of humus-forming materials, they add to the soil large amounts of nitrogen drawn principally from the air. In the South the cowpea is extensively used for this pur- pose with good results, and crimson clover has proved valuable on the sandy coast soils of the Eastern States. Where land is cheap and fertilizers and labor are expensive, green manuring will doubt- less prove to be the most economical way of maintaining fertility. Where land has a high value and labor is cheap, better returns will be obtained from feeding the crop to stock and using the manure rather than resorting to green manuring. Rotation of crops.—Another means of maintaining the humus of the soil is the practice of proper systems of rotation of crops. The general laws which apply to the rotation of crops are in perfect accord with the conservation of the soil humus, but definite rules can not be given on account of the variations in soil and climate of different parts of the country. The methods of farming which are the most destructive to the soil humus are continuous grain cropping without manures and the continuous cultivation of cotton, corn, or potatoes, while the methods which increase the soil humus are the growing of grass crops and dairy and stock farming, which result in the production of large quantities of manure. These statements are by no means intended to discourage grain, potato, or cotton growing, but they are intended to encourage a definite course of rotation in the culture of these crops, and the use of more well-prepared farm manures, so as to keep up the humus of the soil. The influence of different systems of farming on the humus content and fertility of soils is illustrated by the four examples, HUMUS IN ITS RELATION TO SOIL FERTILITY. 141 selected from a large number of similar import, given in the following table : Influence of different systems of farming on the chemical and physicial properties of soils.

Phosphoric Weight Nitro- acid com- Water- Character of soil. per cubic Humus. gen. bined with holding foot. humus. capacity.

1. Cultivated 35 years; rotation of crops and Pounds. Per cent. Per cent. Percent. Per cent. manure; high state of productiveness 70 3.32 0.30 0.04 48 2. Originally same as 1; continuous grain cropping for 35 years; low state of productiveness .. 72 1.80 .16 .01 39 3. Cultivated 42 years; systematic rotation and manure; good state of productiveness _ - 70 3.46 .26 .03 59 4. Originally same as 3; cultivated 35 years; no systematic rotation or manure; medium state of productiveness _ 67 2.45 .21 .03 57

Soils ]N^os. 1 and 2 are from two adjoining farms, and originally had practically the same crop-producing power. No. 1 has received regular and liberal dressings of manure, and has produced wheat, corn, oats, timothy, and clover in rotation. There has been no apparent decline in fertility. No. 2 has been under continuous grain cultivation and has never received any farm manure or other humus-forming materials. During the first few years heavy crops of wheat were raised, but during the past few years the yield has been very low, especially in dry seasons. The land has been reduced in wheat- producing power from 25 to 8 bushels per acre. The main difference between the two soils at the present time is in the amount of humus and nitrogen and phosphoric acid. Soils Nos. 3 and 4 are from the same farm. No. 3 has been cropped forty-two years, timothy and clover, wheat, oats, and corn having been raised in rotation. Every five years the land has received 10 tons of stable manure per acre. No. 4 has been cropped only thirty- five years, producing mainly wheat, oats, and corn, with an occasional crop of timothy. It has not been cropped continuously to one crop, neither has it been under a regular system of rotation. The soil which has been cropped forty-two years shows more humus and nitrogen than the one which has been cropped thirty-five years. SUMMARY. (1) The decline in the crop-producing power of many soils is due to a loss of the partially decomposed animal and vegetable matters known as humus. (2) The humus of the soil is decreased by the continuous cultivation of grain, cotton, potatoes, or any crop with which the land is 142 YEARBOOK OF THE U. S. DEPARTMENT OF AGRICULTURE. kept constantly under the plow without the addition of any humus- forming materials. (3) The loss of humus involves a loss of the nitrogen, which is one of the elements composing humus. The loss of nitrogen from the soil is not always due simply to the nitrogen removed by the crop, but is frequently caused by waste of the humus by improper methods and systems of cultivation. (4) The humus of the soil is increased by the use of well-prepared farm manures, green manures, and by a systematic rotation of crops in which grasses, or preferably clover, form an important part. (5) The loss of humus from the soil results in decreasing its power of storing up and properly supplying crops with water. Soils with a liberal amount of humus are capable of more effectually withstand- ing drought than similar soils with less humus. In arid regions the loss of humus from the soil is more serious than in the regions of continuous summer rains. (6) In sandy soils the loss of humus is most severely felt. In poorly drained soils, where there is a deficiency of lime, potash, and other similar materials, the humus may form sour mold, but this can usually be corrected by a dressing of lime, marl, or wood ashes. (7) Humus-forming materials, like the decaying animal and vegetable matters in farm manures, have the power of combining with the potash and phosphoric acid of the soil to form humâtes which are readily assimilated by plants when acted upon by the proper soil organism. These humâtes thus increase to a marked extent the available plant food of the soil. (8) Farm manures and other humus-forming materials are not only valuable for the elements of fertility which they contain, but also for the power of making the inert material of the soil more available to plants. (9) In soils where there is a good stock of reserve materials it is cheaper to cultivate fertility through the agency of humus than it is to purchase it in the form of commercial fertilizers.