Relevant links:
http://www.evworld.com/view.cfm?section=article&storyid=1107
http://www.solovivo.com/
Depending on how a soil is managed along the successive culture cycles, it may degrade severely, losing progressively its capacity to hold water, air and LIFE. Consequent destruction of its aggregates results in compaction and/or erosion.
Sugar cane and soil erosion
Milton Maciel
Depending on how a soil is managed along the successive culture cycles, it may degrade severely, losing progressively its capacity to hold water, air and LIFE. Consequent destruction of its aggregates results in compaction and/or erosion.
If this happens in the sugar cane industry, all depends on how cane is cultivated and how its processing residues are returned to the culture land. For best results, it is mandatory that cane isn't burned for making harvest easier. At the origin of all is the large biomass that not-burnt cane leaves on field.
Stalks, taken to the mill, are just a part of what the cane plant produces yearly. A large mass of tops, green leaves and dry leaves (trash) remain over the soil after the cane is harvested. This may exceed 15 tons/year as organic DRY matter. Aerobic digestion takes a part of that to the air as carbon dioxide and water vapor. And the remaining material results incorporated slowly to topsoil, contributing with crude organic matter, then humic and fulvic acids as it is turned into humus and then mineral nutrients as it finally mineralizes. During the process, that layer serves also the functions of protecting soil against heavy rains and winds and making emergence of weeds much more difficult.
In round numbers, each plot of integral harvested sugar cane results in
15% sugar
15% bagasse (dry matter)
15% trash (dry matter)
Trash is the combined result of dry leaves, green leaves and tops, all left over the harvested plot or burned, meanwhile the stalks are taken to the mill.
To burn or not to burn?
This mixed residue is burned in order to make manual harvest easier, safer and more yielding. The combustion of trash has a tragic record for soil fertility and is a clear squandering of a very rich source of organic matter and nutrients for this soil. Most minerals remain at place as ashes, but nitrogen and sulfur are lost to the air, contributing to pollution, as their oxides are harmful to environment and human health. Of course, burning cane is also a process of liberating CO2 to atmosphere, contributing to global warming, instead of making a considerable part of this Carbon available to soil life and fertility. As sugar cane burning fumes have very harmful effects on the health of people in the cities near by, pose dangers to aerial navigation, and is also harmful to the manual harvest workers themselves, it is to be phased, by force of law, until the year 2018.
But we have to consider not only the loss of matter (sulfur, nitrogen, carbon), but also the absurd loss of ENERGY that goes with the hot gases of trash combustion in the fields. Some studies point to the possible use of trash as a source of energy in the factory, for feeding its boilers, replacing a large part of bagasse. This surplus bagasse could then be employed to produce more ethanol (since cellulosic ethanol process may become economically feasible) or other byproducts. For the moment the main obstacle is HOW to harvest and transport this trash to the mill.
Organic sugar and ethanol mills say that they would never prefer not to take trash to the factory, as they use the whole mass of trash to protect soil, combat weeds and, more important, to recycle nutrients. They affirm that this is one of the principal reasons why their stalks yield is so much larger than that of conventional farms (plus 30%) and why the useful life of the cane plantation is also much larger (plus 50%, average).
Anyway, both in organic or conventional sugar cane farming, mechanical harvest is the best way to avoid burning and of preserving the best soil protective and nutritive qualities of trash.
When managed so, especially under organic cultivation, sugar cane can contribute to a regenerative effect on soil fertility. To explain this may be somewhat long and somewhat technical, but is extremely simple. I’ll start considering the most severe depletion that cultures cause in topsoils, EROSION. The nutrients issue may come in a following post, if you wish.
TYPICAL EROSION RATES FOR SUGAR CANE AND OTHER CROPS IN BRAZIL
Let’s consider first the worst case design, when sugar cane is produced by conventional chemical agriculture and is submitted to the use of fire at the moment of harvest. Typical measured erosion is 12 ton/hectare of top soil/year (#1). When cane is not burned and the remaining shopped pieces of tops, green and dry leaves are left standing over ground, erosion rate is 5.7 ton/ha/year (#2). This is the typical result of harvesting green crude cane with a combo mechanical harvester.
The first number seems to be high; but as we compare it with erosion rates for other cultures, we find, under the same conventional agriculture methodology, in Brazil:
CROP…………………EROSION rate (in ton/ha/yr)
Castor oil beans..…………..40
Beans………………………..38
Cotton…………………….....25
Soybeans..………………….20
Sugar cane (#1)…………….12 (burnt before harvest)
Sugar cane (#2)……………...6 (biomass of chopped tops, green and dry leaves left over ground)
Sugar cane (#2.0), ………….0 (organic: actually there is a net gain of material in topsoil)
[For all crops, for sake of comparison, we consider a standard soil slope of 10%, limiting value for mechanical harvest of sugar cane].
Now let’s first understand the architecture of the sugar cane plant. STALK is the part of sugar cane plant that is taken to factory, in order to be pressed in the mill for extracting the sugary juice. The other parts of the plant stay on the culture plot.
As I can’t use a picture here, please just imagine (or make a simple draw yourself) you have a 3 meters (~10 feet) high plant. For making things easy to understand, suppose you cut this plant at a height of 2 meters and also cut it close to the ground. This piece is the STALK. The remaining upper part is the TOP. Top’s juice doesn’t have enough sugar and has undesirable components in it; that’s why it is uninteresting to the mill. Attached to stalks and tops there is a lot of leaves. Most of the stalks leaves are old and die as the plant grows. These are DRY leaves; a great part of these leaves fall spontaneously (deciduous leaves) and cover the ground with a protective layer. There are also many newer leaves, alive and making photosynthesis, attached to stalks and tops. These are the GREEN leaves.
When cane is burned, fire propagates very easily because of the great mass of those dry leaves over the soil and dry leaves still attached to stalks. Then green leaves are dehydrated and burn too. Part of the tops still remains and all stalks resist the high temperature, because of its high (70) % moisture content. Under such conditions, a large part of biomass is lost, but most mineral nutrients remain in the resulting ashes, made available to the new cycle of growing through water of rains – except for nitrogen and sulfur, that are lost to the air as oxides, in fumes.
The only partial protection that this culture land receives is that proportioned by fallen dry leaves during the second half of culture cycle of one year. During the first months after harvest, when plants are growing again, leaves are new and of low height. With a standard distance of 1.4 meters (~5 feet) between rows, soil is bare, with limited protection from the trash that remains over soil, after burning and after cut tops are left on ground. Soon after harvest, starts the rain season and, under severe tropical rains, most of the erosion occurs. Under these circumstances, weeds thrive and cultivation is required, mechanical or chemical (herbicides). More erosion! Then, along the rain season, as the plants grow and grow, emitting ten or more stalks per plant and a lush of new leaves, they eventually get to cover the whole area, leaving conditions for weeds development and for erosion very unfavorable.
You must consider that I’m not mentioning anything about organic cultivation until this moment. So, the better score for conventional sugar cane (#1), when compared to other crops, is to be credited only to the sugar plant itself - its architecture, its cycle, its characteristic of semi-perennial culture, its deciduous (=falling) dry leaves, its very high photosynthetic conversion, its very high biomass production. As I always say, this plant is a kind of miracle of Mother Nature. You could never compare this plant culture with a culture of corn, aimed to the same objective of producing ethanol.
Now, before I can proceed and talk about organic cane’s soil regenerative possibilities, I would like to put those numbers on erosion under a frame of reference, so you can evaluate what they mean. When I say that a soybeans’ plot shows a typical erosion rate of 20 tons/ha per year, you have to consider that:
1 hectare = 10 000 square meters
Then suppose you cut a slice of that whole hectare, with a 20 cm depth. Your volume would be
10 000 m2 x 0.2 m = 2000 m3
This two thousand cubic meters is the volume of this 20 cm slice. It is the typical volume of this area TOPSOIL (Horizon A). Now let’s see what would be the weight of this topsoil layer, in tons:
Well, now we have to remember that the average DRY density of soil is somewhere between 1.52 grams/cubic centimeter for pure sand and 0.9 for a forest topsoil (it’s so light that it would float on water, if we could seal it against leakage).
Let’s say we have a SILT LOAM, whose dry (or bulk) density is 1.28 g/cm3. Evidently, the corresponding REAL soil would be lighter, because it would have to include a water and an air phase too. But let’s only consider that dry density:
We have 2 000 m3 of topsoil, with a dry density of 1.28 g/cm3
As 1g/cm3 = 1 000 000 grams / m3 = 1 ton/m3, we may now calculate:
2000 m3 x 1.28 ton/m3 = 2 560 ton/hectare is the weight of our silt loam topsoil
Then consider erosion rates. Divide them by 2 560 tons/ha and multiply by 100:
Beans……………..…..38 ton/ha/yr………..1.48% per year
Soybeans………….….20 ton/ha/yr………..0.78%
Sugar cane (#1)……...12 ton ha/yr………..0.46%
Sugar cane (#2)……….6 ton/ha/yr………...0.23%
12 tons is the weight of a slice of topsoil that is:
2 560 tons - 20 cm
12 tons - x cm Then x = 0,1 cm DEEP = 1 MILLIMETER
A ONE INCH (2.5 cm) layer of one hectare of this silt loam would weight:
20 cm – 2 560 tons
2.5 cm – x tons where x = 320 tons
Then, 320 / 2560 x 100 = a 12.5% yearly erosion would be needed, to loose one inch of this silt loam topsoil.
Of course no good professional farmer would loose such a foolish amount of soil in one year.
In short, these are the reasons why sugar cane is so much more efficient, at the culture field, than any other plant used for producing ethanol, including its marvelous low rate of soil erosion and high yield of both biomass of cellulosic material and sugar.
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