Soils support life. And without soils, many of the world’s living organisms will find it difficult to survive and thrive. Besides forests and grasslands, this includes economically important plants like rice, which feeds more than half of the world’s population.
Soils also allow plants like rice to grow upright and turn towards the sun. They also provide needed nutrients to ensure enough yields, and store and supply water to plants. It is estimated that 99% of the food and fiber we produce grows on soils and only 10-12% of the earth's surface is covered by soils available for agriculture. Soils also provide many more essential services for humans. They help filter water; they immobilize many toxic substances, they mineralize crop residues and store carbon, as well as exchange gases with the atmosphere.
Soil is made up of air, water, mineral particles, organic matter, and organisms. About half of it is pore space that can be filled equally with water and air while most of the solid portion is made of mineral particles. Even though organic matter usually makes up about 2% of a top soil’s weight, it binds soil particles together, stores nutrients, and feeds soil organisms. Organic matter is created by tiny, living organisms that decompose dead plants and animals, creating nutrients that plants can use.
Although these nutrients already come from the soil, some plants like rice may still need supplemental nutrients (those added to the soil with fertilizers) especially when higher yields are required for a growing population. IRRI works on four areas that encompass soil and rice, including managing of nutrients.
As early as 1962, IRRI began the Long-Term Continuous Cropping Experiment (LTCCE) where at first, two rice crops of rice per year are grown. Then from 1966 onwards, it went up to three rice crops. LTCCE is Asia’s most intensively cropped experimental site and it has proved that with appropriate fertilizer management, not only can yields be maintained, but soil health can be improved as well.
In this field, the time between harvesting one crop and planting another has been minimal (2-3 weeks), and crop residue is removed after harvest, rather than incorporated into the soil. This experiment has shown that organic matter in the soil and soil nitrogen-supplying capacity were maintained. Over a 15-year period (1983-1998), the study also found no decline in the ability of soil to supply nitrogen to rice plants.
Another important message that LTCCE communicates is that the unique properties of flooded soils make rice different from any other crop. Because of prolonged flooding in rice fields, farmers are able to conserve soil organic matter and also receive free input of nitrogen from biological sources. This biological fixation amounts to around 25 kilograms per crop, enough to help ensure a stable yield of near 3 tons per hectare per crop in the absence of applied nitrogen fertilizer. This has been sustained for more than 50 years of doing LTCCE.
More than a decade of research in Asia’s major rice-growing areas has led to the development of site-specific nutrient management (SSNM). SSNM is an approach that allows rice farmers to tailor their nutrient management to the specific conditions of their field and provides a framework for best management practices for rice. It aims for efficient nutrient use by rice, and hence, help the farmer obtain high rice yields, translating to high cash value of the harvest per unit of fertilizer invested.
SSNM was developed by researchers in the mid-1990s through a number of partnerships forged between IRRI and many national organizations. From 2001 to 2004, the initial SSNM concept was transformed to provide farmers and extension workers with a simplified plant-need-based management. This included the leaf color chart for nitrogen management.
Since 2005, dissemination of SSNM was intensified through expanded partnerships with research and extension organizations, non-government organizations (NGOs), and the private sector. In 2008, emphasis was placed on providing extension workers, crop advisors, and farmers with appropriate tools to quickly develop and implement best management practices to suit specific rice-growing conditions.
This led to the development of Nutrient Manager for Rice and more recently Crop Manager, a web-based decision support tool that provides farmers with field-specific recommendations for profitable crop and nutrient management.
Organic materials are widely recognized for improving the physical, biological, and chemical properties of soils. They include crop residues, plant biomass, green manure, farmyard manure, compost, household waste, industrial and urban waste, and commercial products produced from plant and/or animal materials.
In some countries, especially produced “Organic fertilizers” are sold on the market. They are a specific group of organic materials produced from decomposed plant or animal materials, used as a source of nutrients for crops.
Organic materials can reportedly improve a soil’s physical properties leading to better structure, aggregation, improved water-holding capacity, and better drainage. However, these changes may not do much to the flooded rice soils in Asia where fields are typically flooded during land preparation by plowing or rotovating and then tilling at soil saturation (called puddling) which eventually destroys soil structure.
Incorporated or surface-applied organic materials could potentially improve the physical properties of rice soils in cases where soil is prepared without puddling like direct dry seeding. In these cases, the potential effects on soil physical properties will depend upon tillage practices and the decomposition rate of the added organic material.
On flooded soils, the most probable benefit of organic materials would be as a source of essential nutrients. Slow-decomposing organic materials which have stayed longer in the soil are more preferred when it comes to improving the physical properties of soils. But those which have a higher carbon-to-nitrogen content or those with high recalcitrant components like lignin, would be less desirable as nutrient sources for crops.
Those that are most effective as nutrient source for crops would have high concentrations of essential nutrients and relatively rapid rates of decomposition which can lead to an almost synchronized release of plant-available nutrients to coincide with the needs of the rice plants.
Micronutrients are elements that are essential for plant growth, but are only required in small quantities. They include zinc, manganese, iron, copper, and several others. There is at least a small amount of these elements present in all soils, and often there is no need to add them to achieve healthy plant growth. However, there are two general cases in which it is helpful to add them: 1) when other soil components interact to make a micronutrient unavailable for plant uptake; or 2) when plant yields are very high (due to optimum management of macronutrients and mitigation of common crop growth constraints) so that large plants are using up the small amount of micronutrients available in the soil.
For irrigated rice systems, zinc is the most common micronutrient deficiency while iron is the most common micronutrient toxicity. This is because flooding the soil causes biochemical changes that make zinc less available, while iron becomes more available to rice plants. It is possible to partially mitigate both of these micronutrient problems by carefully allowing the soil water level to go about 10 cm below the soil level for a short amount of time (5-10 days) prior to re-flooding. Another strategy to deal with these micronutrient problems is to breed varieties that are tolerant of their constraints.
A third strategy to deal with zinc deficiency is to add zinc-containing fertilizers to the soil. However, it is very difficult for rice growers to identify or predict zinc deficiency in their fields because it tends to be very specific to the site and the season. Because zinc fertilizer is much more expensive per kilogram than NPKS fertilizers, it is not profitable for growers to apply zinc if they are not sure it is necessary. Also, some of the same biochemical processes that cause soil zinc to become less plant-available after flooding also occur with applied zinc fertilizers. In other words, a grower may apply a zinc-containing fertilizer to a rice field without having any effect because the zinc from the fertilizer turned in to an unavailable form before the plant could use it. Our research is focused on helping growers identify when it is necessary to add zinc and to provide recommendations about the best way to apply it so that it stays available to the rice plants.
Zinc is also an important micronutrient for humans. Read more about our work on high-zinc rice varieties and how we help improve human nutrition.