Tải bản đầy đủ - 0 (trang)
III. Green Manuring in Pedalfers

III. Green Manuring in Pedalfers

Tải bản đầy đủ - 0trang

154



J.



S. JOFFE



None of the reports on green manuring in the South cited by Pieters

offer valid proof of having increased the organic matter content of

soils. Neither do later reports from this geographic area provide evidence of successful attempts to accomplish this. Thompson and Smith

(1947) and Thompson and Robertson (1953) in Florida, and Ware and

Johnson ( 1951) in Alabama find it “difficult to maintain and even more

difficult to increase the organic matter in southern soils.” Ware and

Johnson have battled with this problem since 1941. Even though they

have recognized the difficulty of increasing the organic matter content

of soils, they show no reluctance to repeat their futile attempts. Were

they aware of the zonality principles governing the constancy of the

organic matter component in the respective zonal soils, they would

have given up their vain efforts in this direction.

To bolster their waning hopes of increasing the organic matter of

soils in Alabama, Ware and Johnson find solace in reporting increases

when animal manures, with or without green manure, are added year

in year out. As shown presently (Section 111, le) residual effects of

green manure are of short duration in the zone of laterization. Organic matter “burns up” in this zone. The differences in the effects of

animal manure and green manures are discussed by Joffe elsewhere

(1949).

Bonnet and Lugo-Lopez (1953), after a period of 13 months, found

no increase in organic matter after plowing under 5 to 10 tons of velvetbean green manure crop followed by two corn crops. When 25 tons of

the green manure crop was plowed under, some increase in organic

matter was noted.

Theron (1936) questions the necessity of increasing the soil organic matter of the semiarid soils of South Africa where 5 to 6 per cent

(a little over 2 inches) of the total precipitation (about 32 inches) falls

during the dry season.

It is clear that in the humid tropics and subtropics and in other

areas of this climatic belt where green manuring is beneficial to the

succeedmg crop, the chances for accumulating organic matter above

the constant are nil. Attempts to gain such increases by green manuring are doomed to fail. Favorable effects of green manuring in the zone

of laterization are to be looked for not in the accumulation of organic

matter.

In the experiments conducted in the tropics of South Africa, as reported by Scherbatoff, “it made very little difference whether the green

manure was cut at the end of the rains and buried green immediately,

cut and allowed to decompose gradually before burial or allowed to

grow until it died of drought before being dug. It was even found that



GREEN M A N U H I N G



155



there was little or no loss in the succeeding maize crop if the green

manure was burnt in situ. The only thing which seemed to make any

substantial difference to the result obtained from green manuring was

the complete removal from the field of the whole tops of the crop. This

was due presumably to the loss of mineral matter.” Sen and Baine

(1952) in India and Orchard (1952) in South Africa substantiate the

findings as summarized by Scherbatoff. Shepherd ( 1952) suggests beneficial effects in Rhodesia due to formation of antibiotics.

b. Supply of Nutrients. Green manure crops utilize the nutrients

which without them would be leached, and mobilize other nutrients

from the soil. As to how green manures make nutrients available, two

viewpoints are current. The older one is that organic matter, as it decomposes, releases nutrients from the insoluble soil minerals. The more

recent viewpoint does not exclude this source of nutrients, but it stresses

the nutrient supply mobilized by green manures from the soil and released in the processes of humification and mineralization. All agree

to the dictum that green manure is of no value unless decomposition

takes place.

The release of plant nutrients from soil minerals is influenced by the

specific characteristics of the respective zonal soil types which determine the intensity factor in the mode of decomposition of organic matter. This factor finds its highest expression in the zone of the humid

tropics and subtropics, where the organic acids formed are readily converted by microbes and animal forms of life into end-products of decomposition, carbon dioxide, water, and mineral salts. The organic

acids formed in the decomposition process have little chance to react

with the soil minerals for any length of time. The inorganic acids,

nitric, phosphoric, sulfuric, and carbonic, recombine with the bases released to form neutral salts and some bicarbonates. The copious rainfall in the tropics rapidly removes these decomposition products.

In the zone of laterization there is a paucity of primary minerals

arid an abundance of R203, the end-products of lateritic weathering.

The latter serve as a protective coating on the few remaining primary

minerals against an attack by the reagents of humification and mineralization. Thus, the circulation of nutrients received from this source i s

limited, and the burden of supply falls on the mobilized nutrients of the

green manure crop itself.

The primary importance of the nutrient supply effect of the green

manuring practice gains very strong support from investigations on

burning the crop instead of incorporating it by plowing or disking.

Faulkner and Mackie (1933), Faulkner (1934) , Freize ( 1939), Mehta

(1950), and Vine (1953) have shown that burning was just as effective,



156



J. S. JOFFE



and in some cases even superior, to the succeeding crop as the conventional methods of handling green manure crops. Vine’s data cover a

period of 20 years of experience in Nigeria. On the basis of these findings it was logical to conclude, as stated by Scherbatoff, that in Nigeria

“the efficiency of green manure crops in accumulating readily available minerals as well as nitrogen is the primary function of green

manuring.’’

Why ashing green manure crops is as effective as the conventional

method of handling these, or why the latter practice is not superior to

the former is not explained by Vine, nor by his predecessors. A rational

answer to this and other questions lies in spheres thus far not explored.

It is suggested that other factors, individually or in combination, are

associated with the effect of supply of nutrients by green manure: ( I )

probability of nonsymbiotic nitrogen fixation; (2) role of roots; (3)

disturbed balance of nutrients as a result of green manuring; (4) specificity of nutrients supplied by green manuring. Of these factors, nitrogen fixation may be tied in with the efficacy of burning the green

manure crop.

( I ) Nitrogen fixation. Even though ashing the crop, as reported in

the Nigerian experiments, eliminates the nitrogen, the succeeding crop

is not affected, in spite of the low nitrogen content in these soils. An

analysis of the facts on hand suggests the reasonable deduction of effective nonsymbiotic nitrogen fixation in the zone of laterization. The

probability of such a postulate finds support in correlating the newer

knowledge of nitrogen fixation by Azotobacter, especially Azotobacter

indicurn, as pointed out by Starkey (1939), with the mode of organic

matter decomposition in the soils of the tropics. With a sufficient moisture supply, organic matter in the tropics and subtropics decomposes

very rapidly, sending into circulation a supply of bases and other

mineral substances essential for Azotobacter metabolism. In the case

of ashing the tops, the mineral substances are made available to the soil

surface. This mode of supply is apparently more effective than the

gradual release by decomposition. Undoubtedly the rise in pH due to

ashing is a factor favoring Azotobacter. In addition, the reduced rate

of decomposition of roots, in the absence of plowed-under top growth,

affords a more extended period of chelating effects to capture the unavailable iron and phosphorus of the soil and retention of minerals by

sorption. These elements are of importance to the nitrogen-fixing organismsFindings in recent years on nitrogen fixation by blue algae add

credence to the postulate advanced. These algae thrive luxuriantly in

the tropics, especially when the soil has an abundance of moisture. The



GREEN MANURING



157



claims of Dhar (1942) and others on nitrogen fixation in the tropics

by photochemical action present another angle not to be overlooked.

Some investigators maintain that the nitrogen reported by Dhar is probably associated with the algae factor. Vageler (1938) refers to extra

nitrogen supplied by the copious precipitation in the tropics.

While on a visit at the Yalta Botanical Gardens, Crimea, Russia, in

1928, Vil’yams, the biochemist in charge of investigations on cultural

practices of vineyards, showed the writer data on the vigorous fixation of nitrogen by Azotobacter in the soils of the Crimea which had a

very low nitrogen content and did not respond to mineral nitrogen

fertilization.

(2) Role of roots. Ashing the tops of a green manure crop influences

the mode of decomposition of the roots and of the native humus, the

so-called soil organic matter left in the soil following humification (see

paper by Russel, 1929, for a bibliography on this subject). Ashing

eliminates some of the negative effects of plowing under green tops,

such as the acceleration of the otherwise slow rate of decomposition of

roots and of native soil organic matter referred to above. Broadbent and

Norman (1946) and Broadbent (1947) have demonstrated that the

carbon dioxide released and mineralization of nitrogen (20 and 60

per cent, respectively, according to Broadbent) originate not exclusively from the green manure crop but in part from the native

soil humus. Lohnis (1926) had shown this to be the case long before

the isotope technique made it possible to trace the reactions quantitatively. In tracing the movement and translocation of nutrients in the

profile by the lysimeter method (data not published), it was found by

the writer that a green manure crop of rye accelerates the decomposition of the native humus, as shown by the nitrogen balance in the soil

inferred from the nitrate in the leachings.

McVickar et aL. (1947) report roots and tops of a ryegrass green

manure crop supplying 156 pounds of nitrogen 123.5 and 32.5 pounds,

respectively, in the roots and tops. Assuming 5 per cent nitrogen in

native humus, the probable source of nitrogen for the ryegrass crop, a

total of 3100 pounds of it had to decompose to meet the demand. Allowing a high 50 per cent transfer efficiency for the 156 pounds of nitrogen

from the native humus to the ryegrass crop, 6200 pounds of native

humus had to be decomposed. The fallow plot contained 40,000 pounds

of organic matter (presumably in the Ap) at the end of the four-year

rotation. Thus, more than 15 per cent of the native humus had to decompose to deliver the necessary nitrogen for the ryegrass crop.

Of course, the last ryegrass crop in the rotation had at its disposal

the residues of the previous cover crop as well as the crop residues of



158



J. S. JOFFE



the harvested corn crop (nothing is mentioned as to how the corn stalks

were disposed of). If we examine the status of organic matter in the

ryegrass plot, we find that it yielded in the spring of 1946 (apparently

the last year of the experiments) 9653 pounds of roots and 2441 pounds

of tops, in all 12,094 pounds of dry matter. In April of 1946, the total

organic matter content of the ryegrass plot amounted to 46,000 pounds.

Assuming that the plots were uniform (a dangerous assumption for

coastal plain soils) and that the residual effects of four years crop rotation were of minor consequence (a plausible assumption for the coastal

plain), it is reasonable to conclude that the bulk of the extra 6400

pounds of organic matter came from the last ryegrass cover crop. It

indicates that the last crop of ryegrass tops (2441 pounds), equal to

about 25 per cent of root growth, enhances the decomposition of the

roots. This condition puts a demand for nitrogen which has to come

from the stable native humus. The newly formed humus contains readily available NPK contributed by the flora responsible for the decomposition of the old native humus which is ordinarily not touched by

the normally prevailing flora. This process is probably responsible for

reports of decreases in soil organic matter after green manuring.

Ashing green manure tops favors a balanced supply of nitrogen, inasmuch as the C:N ratio is not disturbed by this practice. Since roots

decompose more slowly than tops, even in the zone of laterization, but

still at a higher rate than in the other zonal soils, the rate of supply

oE the succeeding crop compares favorably with that when the crop is

dug in. Vine’s (1953) data on the nitrate supply under the methods of

handling green manure (ashing or digging in) are of interest in this

connection.

During the early stages of the maize crop grown after ashing or

digging in green manure, the soil of the latter treatment had 80 to 90

p.p.m. of nitrate-N during May (presumably in the plowed layer).

This amounts to 1000 to 1150 pounds of NaNO, equivalent per acre

( 2 million pounds in 6% inches of soil). The burnt plots (ashing) had

182 to 75 pounds of NaNO, equivalent. Thus, the maize crop in the dugin plots was at a disadvantage in its early stages of growth because of the

excessive supply of nitrates (Vine does not give data on the NH, content). Besides, the rate of supply of phosphorus, potassium, and other

nutritional elements may have caused unbalanced ratios, let alone the

other disturbances which are to be mentioned presently. On the other

hand, the burnt plot was in good balance. The roots kept up a slow but

steady rate of supply of nitrogen and other mineral nutrients, and the

succeeding crop thrived well. I n June, the nitrate content dropped.

Heavy rainfall undoubtedly had caused some leaching of the nitrates.



GREEN M A N U R I N G



159



By then the crop had put a higher demand on the supply of nutrients,

and it is probable that the supply was not there. Unfortunately, Vine’s

compiled data give no figures on the movement, translocation, and distribution of nitrate in the profile. The low nitrate content in the surface

soil is no criterion of the loss of nitrates. Lysimeter studies by the writer

supplemented by analyses of soil-water extracts, show that the nitrates

removed from the A, horizon or the Ap layer do not always indicate

a total loss, if the soil is heavier than a loamy sand. Even in this soil

texture the retention of nitrate and other ions is possible, if there is a

normal B horizon. Very frequently the nitrates are translocated to the

B horizon. In his case, deep rooting conditions, which are better in the

zone of laterization than in the zone of podzolization, are helpful to

the crop. This phase of the subject can stand some fruitful investigations.

Some advantages of ashing versus digging in the green manure crop

are also possible and even probable by virtue of supplying the desirable

scjurce of energy (slow decomposition of roots) and the suitable mineral

salt medium for Azotobacter and Clostridium (the latter operating

more effectively during the wet periods of the growing season). This

phase of the problem also offers a fertile field for study.

In evaluating the relative importance of tops and roots in the zone

of laterization we are to remember the results obtained in Nigeria,

which show that removal of tops did reduce the yield of the succeeding

crop. This adds more evidence that the effect of nutrient supply is the

most important one in the tropics. Whether the loss of nutrients by

removing the tops can be made up with mineral fertilizer has not been

established. Since ashing is helpful, additions of mineral fertilizer should

act the same way.

( 3 ) Disturbances caused by green manuring. A generally recognized

disturbance manifests itself in a n upset balance of the C:N ratio when

straw or a nonlegume crop is plowed under. Heavy crops of legumes

frequently cause the same disturbance, especially in heavy soils. In the

zone of laterization, this undesirable feature does not last as long as in

the zone of podzolization, where the time period for organic matter decomposition is much longer. Of course, the negative effect of a wide

C:N ratio can be easily offset by adding mineral nitrogen. With exceptions, this remedy is not a must in the zone of laterization. The crop

should not be allowed to grow big, whereby more difficultly decomposable organic constituents form. The rapid decomposition of young

plants and their higher nitrogen content adjusts the C :N ratio before

damage is done to the succeeding crop. The probability of nitrogen fixation referred to earlier should be recognized in C:N ratio adjustments.



160



J . S. JOFFE



From the point of view of C:N ratio disturbance, burning the crop

(ashing) in this zone, although it involves a loss of nitrogen and other

desirable elements, appears to be a positive factor. Grazing the crop

and harvesting it for hay, grass silage, or mulching materials are also

positive expedients in adjusting the C:N ratio.

A disturbance not generally recognized is the accumulation of carbon dioxide and simultaneous reduction of oxygen in the rhizosphere.

This condition may become serious during the period of active humification of the green manure. During this period, the ensuing crop is subjected to a very uncomfortable environment. It has been shown by

Lundegardh (1931) that above a concentration of 0.2 per cent carbon

dioxide most crop plants begin to suffer.

Tomatoes, celery, and other crops in New Jersey planted soon after

plowing under a heavy green manure crop or fresh manure have been

ruined by hydrogen sulfide and methane. Such cases have been encountered by the writer even in the sands or loamy sandy soils of the

coastal plain of South Jersey, where decomposition reactions during the

summer resemble those of the humid subtropics. Excessive precipitation

during the period following the plowing accentuates the formation of

the sulfide and methane. An effective remedy against this disturbance

is shallow plowing, especially the heavier soils, where an abundance of

rainfall is expected. However, in the zone of laterization this disturbance is mitigated by the highly favorable conditions of humification

and mineralization, but it should be recognized.

Ware and Johnson (1951) report that rye was not as effective as

vetch for green manuring even when the nitrogen factor was adjusted

by adding mineral nitrogen. Harvesting the rye has not adversely affected the succeeding crop. It is evident that disturbances other than the

C: N ratio have entered the picture.

It may not be amiss to reflect at this point and compare the cultural

methods in the periods before and after mechanization of tillage operations. The writer remembers well in the days of 1909 to 1911 plodding behind a plow drawn by a fairly good team of horses, trying to

plow under a dressing of 30 to 40 tons of cow manure. Seldom did we

succeed in accomplishing it. Most of the time the manure would be

plowed in, or rather mixed with the surface 3 to 5 inches of soil.

This manner of incorporating manure had the advantage of excellent aeration. Changes of air through the surface 3 to 4 inches are more

frequent than in layers below that depth. A high redox potential is

easily maintained for decomposition of added organic matter, with no

danger of hydrogen sulfide and methane formation. Besides, the rate of

decomposition is reduced by virtue of rapid drying of the surface 2 to 3



GREEN M A N U R I N G



161



inches of soil. This mode of decomposition offers a steady rate of supply

of nutrients, the cardinal effect of manuring, throughout the growing

season, with fewer chances of loss by leaching.

When the tractor appeared on the scene to turn a furrow 90° to

180° to a depth of 6 to 10 inches, large quantities of manure found their

way in the layer below the surface 3 to 4 inches, producing anaerobic

conditions and bringing about the disturbances described. Plowing under a green manure crop or barnyard manure to such depths and turning a furrow 180° are therefore to be avoided. Again, precautions are

essential. Even in the zone of laterization these disturbances may become a problem.

Burning the crop eliminates the dangers enumerated. It should also

be possible to accomplish similar results by harvesting the crop for hay,

grass silage, or some other purpose and to replenish the loss of nutrients

by adding mineral fertilizer.

( 4 ) Specificity of nutrients supplied by green manures. Except for

nitrogen which legumes add to the soil, green manure crops do not add

nutrients directly, but contribute some indirectly by retarding or obviating losses by leaching. Another nutrient added by green manures,

but generally overlooked, is carbon dioxide released for the succeeding

crop to utilize in its photosynthetic activities. No experimental data are

available to evaluate this possibility. From evidence on supplementing

the supply of carbon dioxide to a growing crop, the probability of benefits from the release of carbon dioxide by green manuring is justified.

The principal beneficial effect of green manuring is the mobilization of the conventional nutrients from the soil and their return to the

succeeding crop in a balanced form, readily available. Besides, the crop

also returns the known and unknown so-called minor elements and

other vital substances synthesized by plants, such as vitamins and hormones.

It is true that mineral fertilizer can furnish substitute for the nutrients green manure crops may supply. Experiments have proved this.

Still, one may speculate on the other known and unknown benefits

green manures contribute which mineral fertilizers cannot supply.

c. Soil Structure Improvement. A careful perusal of the literature

reveals vagueness if not misconceptions on the effect of green manuring

on soil structure. This circumstance and inadequate methods of determining quantitatively respective size aggregates or structural units

have added to the prevailing confusion.

Contributions on soil structure in general and more so with reference to green manuring display poor orientation on the mechanism of

the two elements or agents of structure formation: (1) binding, and



162



J. S. JOFFE



(2) stabilizing. To form structural aggregates, the individual units of

the soil separates-sand, silt, and clay-have to be bound and then

stabilized. It is known, but not thoroughly appreciated in certain circles

dealing with the subject, that clay is the principal binding agent and

organic matter in association with divalent (primarily calcium) and

trivalent cations is the principal stabilizing agent. It is not uncommon

to find reports of experiments showing improvement of soil structure

by the addition of organic substances to sandy soils containing practically no clay, or not enough clay, to bind the individual particle-size

units. Sand grains temporarily mechanically bound by some everpresent gluelike substance in decomposition products of organic matter

are misconstrued as soil structure. No one as yet has shown that organic

matter itself can play the part of binding agent in soil structure formation.

In the zone of laterization, the mode of decomposition, as pointed

out earlier, dictates a low organic matter constant. On the other hand,

R,O, is in abundance. With the low level of calcium and the abundance

of moisture in these soils, very little of the possible Ca-humates formed

may persist. At the same time, this reaction retards the immobilization

of humates by the R,O, because of their low dissociation constants. This

does not preclude the formation of some humates of iron and aluminum,

and perhaps also of manganese. However, the paucity of organic matter

in general also puts a low limit on the formation of such humates.

The lack of plasticity of soils in many areas in the zone of laterization is attributed to the coatings of R,O, over the clay-bound particles.

It was therefore suggested by Joffe (1949) that the structural stability

of the soils in this zone is due to the R,O, gels which upon dehydration

reduce the plasticity of the clay.

Evidence is meager on improved soil structure by green manures in

the zone of laterization. The same holds true for evidence on the ineffectiveness of green manures to improve soil structure in other soil

zones. Martin (1944), working in Uganda, reports no improvement of

soil structure by green manures with or without lime. These results,

in his words, are “in direct opposition to those of Tyulin and Ilmenev.”

Martin, not aware of the zonality principle, did not realize that Tyulin

and Ilmenev conducted their experiments in zonal soils in which organic matter is the principal stabilizing agent of soil structure. Bhowmick and Raychaudhuri (1953) from India conclude that there is no

convincing evidence that organics have improved soil structure.

I n the tropics and subtropics, the over-all specific processes of soil

formation, apart from the broad fundamental processes inherent in all

soil zones, introduce factors not recognized thus far or the significance



GREEN M A N U R I N G



163



of which is not appreciated with reference to the effects of the organic

component of soils. These specific processes undoubtedly tend to modify

the course of reactions. I n the tropics, these processes and conditions

tend to lower the over-all negative charge of the soils and even impart

a positive charge. I n such a soil system the protective (or chelating)

action of soluble organic colloids on the movement of R,O, is at a minimum or ceases to function. Besides, the electrokinetic potential in the

soil system of the tropics precludes the movement of R,O, and no organic matter accumulation takes place in the lower horizons as it does

in the zone of podzolization.

In the zone of laterization where a distinct dry season prevails, the

R,O, gels, together with the soluble organic colloids that might be associated with them, upon desiccation attain cementation properties which

give rise to new formations. It is suggested that the buckshot structure

noted in the zone of laterization is one of these new formations. Leaching effects that follow the dry season and intermittent desiccation and

wetting periods may have something to do with the smooth surface of

the buckshot.

d . Moisture Relationships. Ever since the inauguration of modern

practices of green manuring it has been recognized that soil moisture is

one of the factors determining the expediency of these practices. If a

green manure crop is plowed under when the soil is deficient in moisture, its slow decomposition and accompanying disturbances extend the

unfavorable conditions for the succeeding crop over a long pefiod of

time. This quality prevails in soils of the humid temperate zone of

forest and adjoining grass country, of which more is to be said presently, but it is not apt to arise in the zone of laterization where natural

rainfall or irrigation provide an ample supply of moisture. Favorable

temperatures always prevail in this zone for rapid decomposition, thus

shortening the period of disturbance to a minimum.

Rapidity of organic matter mineralization in the humid tropics and

subtropics makes the nutrient supply effect the outstanding positive

feature of the green manure practice. However, this positive feature is

accompanied by the negative one of leaching. To reduce the danger of

loss of nutrients by leaching, a reduction in speed of decomposition of

the green manure may prove a desirable expedient, providing it does

not increase the havoc of disturbance entailed by slow decomposition.

By scheduling the plowing operations after the end of the rainy

season, if such does occur, and if the timing does not conflict with the

cropping schedule, the decomposition may be retarded by the paucity of

moisture. During this period the organic colloids age, the soluble colloids

become insoluble, and the insoluble become still less soluble, by virtue



164



J. S. JOFFE



of the reactions of irreversibility of such colloids due to desiccation. It

should also be possible to accomplish the same effect by cutting the green

manure crop, allowing it to dry, and only then digging it into the surface 3 to 4 inches. Short periods of wetting followed by extended periods

of desiccation enhance the irreversibility of the colloids and reduce still

more the rate of decomposition. Incorporation of a green manure crop

so conditioned into the surface 3 to 4 inches should improve the friability

of the soil and gain for it improved physical conditions.

Speculations of this kind lend themselves to experimental test. They

stem in part from deductive reasoning on the chemical and physical

reactions associated with the specific soil-forming processes in the

chernozem type of soil formation. These speculations also find support

in the friability of the surface soil attained by tillage practices in the

pretractor days of farming with manure, as discussed earlier.

e. Residual Value. In general, residual values vary with the type of

green manure crop grown or type of organic material added and crop

following. As to lasting effects, the zonality principle governs, being

long in the humid temperate climate and short in the tropics and subtropics. The time period varies not only with the zonal but also with

texture and intrazonal characteristics: hydrogenic, orogenic, and lithogenic.

Reports from various sectors of the tropics and subtropics are scanty

on this phase of the problem. Those from Nigeria and Uganda, as reviewed by Scherbatoff, are emphatic on the “burning up” of green

manure in the tropics, with very little residual effect. Bonnet and LugoLopez (1953) found no residual effect after the first crop of corn was

grown following the plowing under of a velvet bean crop.

Reports on the red-yellow soils in the United States, which represent

soils of a transition zone of laterization-podzolization, also indicate no

green manure residual effects. Thus, Ware and Johnson (1951) in

Alabama state that turning under a summer crop for fall vegetables has

very little residual effect on the next spring vegetables. “On plots on

which cowpeas had been turned under annually for 9 years, the average yield of spring vegetables, 3 years after the last crop was turned

under, was practically the same as the check.”

In Puerto Rico soil areas were examined which had received annually for several years hundreds of tons per acre of filter press cake

(a by-product of the sugar-cane industry) besides the natural stubble

and roots of the crops grown. As much as a thousand tons of this cake

per acre in one year was incorporated into a sandy soil. This means

about 30 to 40 per cent of organic matter in the surface 6% inches of

soil. Within two to three years, practically all organic matter was

“burned up” and no residual value could be detected.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

III. Green Manuring in Pedalfers

Tải bản đầy đủ ngay(0 tr)

×