It’s not only recently that farmers all over the world have been using what they call green manure—making use of the biomass of one kind of plant as fertilizer for another plant—but few scientific studies have been made to explain how this arrangement functions and to quantify its results. It was precisely the search for those answers that inspired agronomist Edmilson José Ambrosano, a researcher at the São Paulo Agency for Agribusiness Technology (APTA), affiliated with the São Paulo State Department of Agriculture and Supply, to conduct two projects supported by FAPESP. The research showed that sunn hemp (Crotalaria juncea) can completely replace chemical nitrogen fertilization in sugarcane fields, resulting in a 35% increase in productivity and a financial gain of about 150%.
Originally from Asia, sunn hemp grows very quickly, and vigorously. It is the species that produces the highest volume of biomass in the shortest period of time. In addition, it is a fibrous plant and also finds use in the manufacture of special kinds of paper.
One of the principal advantages of using sunn hemp as fertilizer comes from the fact that it is leguminous. Legumes are members of a family whose species are able to fix, i.e., incorporate nitrogen from the air into an organic molecule. “With rare exceptions, legumes are the only plants in the vegetable kingdom that can accomplish such fixation from atmospheric air. They do it with the help of bacteria found in their roots,” Ambrosano explains. “Sunn hemp not only supplies nitrogen but can be used to restore degraded soils as well.”
Sugarcane is one of Brazil’s most important crops. A kind of semi-perennial plant, it can be grown from four to eight years on the same tract of land, and harvested every year. At the end of that period, the cane field is renewed by destroying the old plants and planting new ones. In Brazil, 1.9 million hectares of cane fields are renewed every year. “It is in those areas, or in new fields, that green manure is sown in order to restore the soil by incorporating nitrogen,” Ambrosano says. “We have been doing this in Brazil since 1934.”
The purpose of the study, which began in 2003 and continued until very recently, was to study the effect of green manure on sugarcane. “We already knew that sunn hemp was a good fertilizer that functioned well as a source of nitrogen,” he recalls. “What we wanted to find out was how much of that element comes out of the plant and enters the sugarcane. We used the opportunity to verify the transfer of nitrogen found in ammonium sulfate, a very popular kind of chemical fertilizer. Our idea was to compare the efficiency of the two types of fertilizer, the green one and the chemical one.”
To conduct the study, Ambrosano designed an experiment that used a technique known as isotopic labeling of nitrogen. Nitrogen is the most abundant element in the Earth’s atmosphere, accounting for about 78% of all the gases that surround our planet—oxygen accounts for 21%. In air, it takes the form of N2 molecules that are composed of two atoms in a triple covalent (electron-sharing) bond that is extremely strong. That is why animals and plants are unable to metabolize nitrogen.
Nature’s way of using nitrogen is through leguminous plants, with the aid of bacteria, especially bacteria of the genus Rhizobium. The microorganisms associate with the plants in a symbiotic relationship, forming nodules on their roots from which they capture gas from the air—the soil is porous—and convert it into nitrogen compounds such as amino acids that can be used by plants in their metabolism. Another way of converting nitrogen from nature into an element that plants can exploit is the method used by fertilizer factories. The problem is that the manufacturing process uses a lot of energy, which makes the product the most expensive fertilizer a farmer can buy.
In nature, nitrogen is found as two isotopes, nitrogen-14 (14N), which represents 99.634% of all nitrogen in the atmosphere, and nitrogen-15 (15N), which corresponds to the remaining 0.366% of all nitrogen in the atmosphere. Isotopes are variants of a single chemical element that have the same properties and the same number of protons, but a different number of neutrons. And so 14N has seven protons and seven neutrons, but 15N has an extra neutron, which makes it heavier. “That is why we had to think of a way to label what is present in sunn hemp, so that we could verify how much of it would be used by the cane,” Ambrosano explains.
The study, conducted at the Center for Nuclear Energy in Agriculture (CENA) at the University of São Paulo (USP), produced a nitrogen compound that contained 70% 15N and 30% 14N. The next step was to prepare two plots of land, one measuring 2.80 m by 2.0 m and the other measuring1.40 m by 1 m. Sunn hemp was planted in both. The first plot was sprayed with urea rich in 15N. The crop in the second plot received ammonium sulfate, also rich in 15N. The plants were then allowed to grow to a height of about 2 m, after which they were cut down and sugarcane planted in their place. The cane was cultivated for five years and harvested three times. The amount of 15N recovered was measured at the time of the first two harvests.
To carry out the assessment, Ambrosano selected leaves from the sugarcane and took them to the laboratory where, using a mass spectrometer, he verified the volume of labeled nitrogen, i.e., the amount of 15N from the sunn hemp. “The transfer of those elements from the sunn hemp to the cane in the first two (consecutive) harvests varied from 19% to 21%, and the transfer of nitrogen applied with ammonium sulfate was 46% to 49%,” says Ambrosano. “We concluded that the nitrogen from the application of fertilizer met the needs of the sugarcane, equivalent to using 70 kg of that element per hectare.”
Although the ammonium sulfate transferred more nitrogen to the sugarcane, the green manure has other benefits that make up for the difference. “Besides being cheaper, sunn hemp protects the soil from heavy rains and decompacts it, thereby improving the infiltration of water,” says Ambrosano.
1. Dynamics of nitrogen in sugarcane after green manure fertilization with Crotalaria juncea (nº 2006/59705-0); Grant mechanism Regular Line of Research Project Award; Principal Investigator Edmilson José Ambrosano (APTA); Investment R$36,860.00 (FAPESP).
2. Dynamics of nitrogen in sugarcane after green manure fertilization with Crotalaria juncea (nº 1998/16446-6); Grant mechanism Regular Line of Research Project Award; Principal investigator Edmilson José Ambrosano (APTA); Investment R$26,309.10 and US$701.02 (FAPESP).
AMBROSANO, E. J. et al. 15N-labeled nitrogen from green manure and ammonium sulfate utilization by the sugarcane ratoon. Scientia Agricola. v. 68, n. 3, p. 361-8. Jun. 2011.