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II. Green Manuring in Zonal Soils

II. Green Manuring in Zonal Soils

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result from the increase in the nitrogen and humus contents of soils

that comes from plowing in the upper parts of plants in a green and

succulent state. Other advantages ascribed to green manuring are improved physical properties of the soil and conservation of soil moisture.

I n most writings of green manuring attention has been mainly concentrated on the decomposition of the green manure and the resulting effect

on the composition of the soil.” He cites investigations in South and

West Africa by Faulkner (1934), Faulkner and Mackie (1933), and

Haylett ( 1943), who claim that “‘green manuring under the light soil

conditions at Ibadan, Nigeria, benefits the crop not by virtue of its improving the nitrogen or organic matter content of the soil, by modifying

soil moisture relationships, but by the mobilization of nutrients.”

Introducing the divergence of views, Scherbatoff compares results

“of most writings on green manuring” (that is, on soils of divergent

zonal soils) with results from the tropics. In a pedologic analysis of the

problem, it is brought out that the effects of green manuring operate

differently in the respective soil zones of the world. For example, in the

zone of laterization the effect on soil structure is negligible, whereas

in the zone of podzolization and in transition zones this effect is of considerable importance. In the humid tropics and subtropics the nutrient

supply effect follows a different pattern than in other soil zones with

reference to source of supply, top growth or roots. The moisture effect

is the determining factor in the total failure of green manuring in most

of the pedocals. In the light of the zonality principle, with reference to

the respective zonal soil groups, the apparent divergence of views, contradictions, and confusion prevailing in the literature disappear.

2. Applying the Zonality Principle

The excellent summary of Pieters, made more than 25 years ago, on

the status of green manuring in some parts of Europe, Asia, the East

Indies, and the United States, offers an opportunity to demonstrate the

application of the zonality principles to clarify the problems of green

manuring. Unwittingly, Pieters has applied this principle in compiling

his data on the adaptability of specific green manure crops and their effectiveness in the different climatic regions of the country. He assembled the facts available in a climatic-geographic setting: The southern states, the North Atlantic seaboard (from Virginia to New Jersey),

the northeastern United States, and Canada (including the Corn Belt),

the Great Plains, and the Pacific Coast. This setting lends itself to a

pedologic analysis.

By and large, the effectiveness of green manuring, as may readily

be inferred from Pieters’ data, varies as we move from the zone of



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laterization, the southern states, through the transition zone from laterization to podzolization (the North Atlantic seaboard), and through the

soils in the zone of podzolization proper (the northeastern United

States, Canada, etc.). I n the northern soil subtypes of the zone of podzolization reports appear on the ineffectiveness of green manure and

even damage, or loss of revenue, from practices of green manuring.

Pieters includes the Corn Belt in the region of the northeastern

United States. An examination of the scanty reports on green manuring cited by Pieters and those published since in Indiana, Illinois, and

Iowa shows that the practices in this belt do not conform to the conventional system of green manuring practices of the southern states or

North Atlantic states. In the Corn Belt, the so-called green manuring

system calls for a legume sod crop in the rotation, usually clover. The

objectives of sod, while related in some aspects to those of the conventional green manure crops, are chiefly to replenish the nitrogen and

readily available phosphorus and perhaps to improve the soil structure.

This system of sod culture aims to avoid the extensive use of mineral

fertilization. In other words fertilization of crops in these areas is an

economic problem.

Pedologically, the soils of the Corn Belt are a transition zone (referred to also as interzonal, not intrazonal) located west of the redyellow soils in the southeast and of the gray-brown podzolic soils in

the northeast, using Marbut’s (1935) classification scheme. The organic matter constant of these soils is high. Green manuring or additions of other sources of organic matter, advocated by the alarmists on

the exhaustion of organic matter even in these soils, are a risky practice, as shown in part 111. The regeneration of the native organic matter

in the soils under consideration comes about by way of roots and stubble. This source of native organic matter can be enhanced by judicious

use of mineral fertilizer, chiefly nitrogen and phosphorus and perhaps

also some sulfates of potassium and magnesium-a system now becoming standard practice in many areas of grain culture in the United

States. Of course, here and there the introduction of grass sod will find

a place, but far from the economically ruinous program advocated by

the organic matter faddists and neo-Malthusians.

In the Great Plains region, Pieters found very little application of

the green manuring practice. He states that, “as a rule, green manuring is not practiced in the Great Plains and repeated trials have shown

that the practice does not pay on dry land. The green manure crop

itself takes so much water that the following crop suffers and yields

less than after fallow.” The soils in this region are of the chernozem

type. I n it, the organic matter factor, as discussed in connection with



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the Corn Belt, asserts itself more strongly. All in all, there is some justification for the accepted belief that green manures in these soils are

generally ineffective.

For the region of the Pacific Coast, the discussion of Pieters offers

little for pedologic analysis; it is devoted primarily to types of green

manure crops rather than their effects. Still, Pieters has separated this

area unwittingly from the point of view of the zonality principle. The

Pacific Coast with its Mediterranean type of climate affords good

chances for growing winter green manure crops, especially in the

northern region. In southern California the paucity of total precipitation introduces a new factor, irrigation.

Soil management under conditions of irrigation in the arid and

semiarid region of the temperate and tropical-subtropical climates is

beset with many problems and difficulties. However, the general principles of the pedologic approach are applicable in solving the problems

associated with the practice of green manuring in irrigating farming.

The naturally favorable temperature factor of these regions, chances

for the control of the moisture factor, and coordination of irrigation

with natural precipitation offer an opportunity to regulate in a large

measure the all-important soil organic matter phase. The dependable

desiccating effects of the hot sun may be directed to simulate conditions approaching those of the chernozem zone. This speculative suggestion has its drawbacks, primarily in areas where the crops are grown

exclusively with irrigation. The specificity of the profile constitution of

these soils, the ever-present specter of salinization, and the necessity to

flush the soils present specific problems. Outside of these areas, there is

a chance for simulating in some measure conditions of the chernozem

zone.



111. GREENMANURING

IN PEDALFERS

I . Zone of Laterization

In the zone of laterization green manuring has become a recognized

beneficial practice. Its value is being questioned in areas of distinct intermittent dry and wet seasons. With some exception, most of the

investigations in this zone deal not with the mechanism of reactions of

green manuring. Efficacy of green manuring is evaluated in terms of

yield increases.

a. Organic Matter Supply. No other soil type than those in the zone

of laterization reflects more forcibly the validity of the proposition and

corollary, mentioned under Section I, 4c, on the futility of trying to

increase the organic matter content of soils by green manuring.



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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



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