Sustainable soil management and novel inputs help reduce use of mineral fertilizers in agriculture

Introducing more modern agricultural practices in Brazil could save farmers more than USD 20 billion in coming decades via a reduction in the use of phosphate fertilizer alone, a study by the University of São Paulo shows.

By Elton Alisson  |  Agência FAPESP – Implementing more sustainable soil management strategies, such as no-till with crop rotation and use of novel biological inputs based on organic waste or microorganisms, among other solutions, can help increase the efficiency of the mineral fertilizers critical to Brazilian agriculture, and hence reduce the volume of these fertilizers used. This is one of the main findings of studies supported by FAPESP and conducted by researchers affiliated with several universities and research institutions in Brazil.

Adoption of such practices could save Brazilian farmers more than USD 20 billion in the coming decades by reducing the use of phosphate fertilizer, Paulo Sérgio Pavinato, a professor at the University of São Paulo’s Luiz de Queiroz College of Agriculture (ESALQ-USP), told Agência FAPESP.

The volume of phosphate fertilizer used in Brazil has risen 43.4% in the last ten years, and more than 67% is imported from countries in North Africa, especially Morocco.

“Leaving straw and other plant residues on the ground between harvests, as in no-till, and rotating crops so as to keep the land continuously in production instead of lying fallow are ways to make cycling of phosphorus and other nutrients by plants more efficient,” Pavinato said.

Nitrogen, potassium and phosphorus are the macronutrients most used to fertilize crops in Brazil. Native soil phosphorus is considerably underused, he explained, because tropical soils are typically clayey and rich in iron and aluminum oxides, which bind strongly to phosphorus. As a result, much of the phosphorus applied as fertilizer stays in the soil in forms not highly accessible to plants.

“Crops in Brazil have used up only 50% of the phosphorus fertilizer applied in the past 20 years on average. Half of it goes away with the crop when it’s harvested. The other half remains in the soil. For this reason, farmers frequently apply more than double the amount of phosphorus plants need,” Pavinato said.

Through a project supported by FAPESP, he collaborated with colleagues at Bangor University in the UK to make an inventory of the residual phosphorus that has accumulated in the soil since the 1970s, when large-scale use of the fertilizer began in Brazil. The results of the study are detailed in an article published in Scientific Reports.

Calculations based on estimates of the average phosphorus input via fertilizer application and the output via harvested crops pointed to an accumulation since the 1970s of some 33.4 million metric tons of the mineral in Brazilian agricultural soil. Areas that have been farmed for a long period, located in São Paulo, Paraná and Minas Gerais states, had the largest amounts of phosphorus stored in the soil. 

“Areas with more recent agriculture, such as parts of Mato Grosso, Mato Grosso do Sul and Goiás, as well as MaToPiBa [known as the new Brazilian agricultural frontier, comprising parts of Maranhão, Tocantins, Piauí and Bahia states], have much less phosphorus accumulated in the soil precisely for that reason, but even in these areas there’s much more total phosphorus in the soil than in areas of the UK, for example,” Pavinato said.

Appropriate land-use management, including crop rotation and growing cover crops such as signalgrass or millet after soybeans, for example, can make more efficient use of the stored phosphorus and increase crop resistance to drought, he added, as plant roots enhance their ability to absorb nutrients from the soil.

“Farmers with properly implemented production systems, including crop rotation for several years, don’t need to fertilize their crops between harvests because the soil has a sufficient reserve of nutrients, especially phosphorus,” he said. “Farmers who follow the conventional system will suffer more during a crisis like the present, when fertilizer is scarce, because the soil lacks this reserve.”

Cover crops

In an ongoing study, also supported by FAPESP, Pavinato and collaborators are evaluating the use of cover crops such as vetch, oilseed radish, lupin and ryegrass in winter, before corn is planted in summer, to improve soil phosphorus uptake.

They performed experiments between 2008 and 2015, applying soluble and natural phosphorus to plantations in Paraná in which corn was grown in rotation with these cover crops. Fertilization of the areas ceased after this period.

Preliminary results showed that the corn harvested in later years, with a water deficit, was twice the amount compared with areas not fertilized with phosphorus. “The cover crops that promoted higher corn yields in the areas fertilized with phosphorus were black oat and ryegrass. These annual forage grasses are able to cycle more nutrients in general, but it should be stressed that these responses can only be obtained in the long term,” Pavinato said.

Organomineral fertilizer

An organomineral fertilizer developed by researchers at EMBRAPA Soils, a unit of the Brazilian Agricultural Research Corporation (EMBRAPA), could also help increase the availability of phosphorus for crops and reduce the environmental damage done by fertilizers.

In the past 11 years, the researchers developed a granulated organomineral phosphate fertilizer produced from poultry litter – a mixture of excreta, spilled feed, feathers and bedding material (wood shavings, rice husk, hay, straw, ground corn cub, and sawdust).

This agricultural waste used to be a supplementary source of cattle feed in Brazil but was banned in 2004 in response to the emergence of “mad cow disease” (BSE).

Its use as crop manure is commonplace. However, there are no standard technical specifications, noted Joaquim José Frazão, a professor at the Federal Institute of Roraima (IFRR).

“The lack of specific technical recommendations has caused improper use and superficial application, with inadequate doses, low agronomic responses, and risk of environmental contamination by the large amount of nitrate poultry litter contains,” Frazão said.

Its phosphorus content varies, however, and researchers at EMBRAPA Soils, in partnership with Frazão, spent several years testing mixtures of the material with minerals for use as fertilizer. Results of tests involving application of the organomineral fertilizer in Rio Verde and Goiânia (Goiás state) and Piracicaba (São Paulo state) during Frazão’s PhD research with a scholarship from FAPESP showed it to be comparable in terms of agronomic effectiveness to traditional mineral fertilizers such as monoammonium phosphate (MAP) and triple superphosphate in the first soybean or corn crop. An article on the study is published in the journal Sustainability.

“We also observed in other studies that the product has a residual effect on soil,” Frazão said.

Because organomineral fertilizer is released more slowly than other available sources of phosphorus, which are water-soluble, it supplies the macronutrient required by the plant and at the same time reduces the risk of loss due to adsorption (fixation) by iron and aluminum oxides. “Traditional phosphate fertilizers are water-soluble and almost immediately released into the soil, whereas the organomineral fertilizer we developed is released more slowly, remaining available in the soil for longer,” he explained.

According to Frazão, EMBRAPA Soils has patented the technology used to produce the organomineral fertilizer.

Several other organic sources besides poultry litter can be used to produce organomineral fertilizer, such as poultry or cattle manure, and rice husks. “The agronomic effectiveness of organomineral fertilizer based on these other sources, however, may not be as good as the product made from poultry litter owing to variations in chemical composition,” he said.

Organic fertilizer based on sewage sludge

Another promising source of raw material for fertilizer production is a compost based on the residue that accumulates in sewage treatment plants, according to studies conducted by researchers at São Paulo State University (UNESP) in Ilha Solteira.

Sewage sludge is rich in organic matter, and a source of plant macronutrients and micronutrients such as nitrogen, phosphorus, copper, iron, manganese and zinc. It has been shown to be a potential by-product for use as agricultural fertilizer since the 1980s, but concern about the risk of soil and plant contamination by heavy metals, viruses and other pathogenic microorganisms has limited its application for this purpose, said Thiago Nogueira, a professor at UNESP and principal investigator for the studies.

“Even though the beneficial effects of sewage sludge as a crop fertilizer have been proven, most Brazilian states have passed laws establishing criteria that hinder the use of this urban waste. A very small amount has been used on a large scale in agriculture not just in São Paulo but also in other parts of Brazil,” Nogueira said.

Through a partnership with a company based in Jundiaí, São Paulo, the researchers composted sewage sludge to eliminate pathogens and reduce metal content so that it could be used in agriculture. They are now studying its use as an organic source of nutrients for soil in the Cerrado, Brazil’s savanna-like biome, which is naturally poor in nitrogen, phosphorus, boron, manganese and zinc, to fertilize such crops as rice, dry beans, soybeans, corn and sugarcane.

Preliminary results of the study, conducted as part of the master’s research of Adrielle Rodrigues Prates with a scholarship from FAPESP, showed that application of the compost increased the amount of copper, manganese and zinc in the soil and in the leaves of soybean plants.

“We also observed a 67% rise in soybean yield and residual effects of application of the compost, with corn yield rising above the national average and very similar results to those obtained solely with mineral fertilizers,” Nogueira said.

The sewage sludge compost clearly increased the soil availability of nutrients such as nitrogen and phosphorus, as well as some micronutrients, boosting crop yields.

Other studies are in progress to investigate use of the compost with no-till cover crops in the Cerrado, emphasizing soil health surveillance, Nogueira added.

Phosphate solubilizing microorganisms

In addition to soil management, improved plant varieties and more efficient fertilizers, another strategy that is being implemented to boost plant uptake of nutrients is the use of phosphate solubilizing microorganisms such as bacteria and fungi.

These microorganisms help plants access unavailable soil phosphorus, for example, said Antônio Pedro da Rocha Camargo, a researcher affiliated with the Genomics for Climate Change Research Center (GCCRC), an Engineering Research Center (ERC) set up by FAPESP and EMBRAPA at the University of Campinas (UNICAMP).

“Microorganisms can help plants obtain nutrients in various ways,” Camargo explained. “One of the most familiar is mycorrhizae, a symbiotic association between plant roots and fungi that increases the volume of soil available for absorption of nutrients and water. There are also bacteria that help plants obtain soil nutrients they wouldn’t normally be able to absorb, such as insoluble phosphorus.”

During his PhD research, conducted with a scholarship from FAPESP, Camargo investigated microorganisms associated with plants in campos rupestres, a type of rocky montane savanna located in the central region of Brazil. These biodiversity hotspots have soils that are extremely poor in phosphorus owing to the geological conditions, and highly acidic, but nevertheless are home to a striking diversity of plant species, most of which are endemic (occurring only in the region).

“The physiology of these plants has been studied for years with the aim of understanding how they grow in such conditions,” Camargo said.

He and his collaborators discovered that while the soil in campos rupestres is very poor, it also has a great diversity of equally endemic bacteria and other microorganisms associated with the plants. When they analyzed the microorganisms, they found that bacteria near plant roots had an expectedly large number of genes associated with phosphorus solubilization.

“Several functions associated with plant phosphorus uptake were enriched in these bacteria,” Camargo said.

A comparison of their genomes with those of evolutionarily close bacteria found elsewhere also pointed to more genes associated with plant phosphorus uptake. “This shows that the functions of supplying plants with phosphorus are probably being selected for in this environment. Plants may release compounds that serve as nutrients for the bacteria that solubilize phosphorus in order to recruit them and thereby obtain the mineral,” Camargo said.

The main aim of the study is to construct a basis for selecting and growing these bacteria on a large scale so that they can be applied to crops as an inoculum designed to increase phosphorus absorption.