World’s First Biosensor to Measure Activity of Soil Microorganisms
- Predicting Soil Disease Outbreak -

The same disease risk threatening human beings, such as tuberculosis and severe acute respiratory syndrome (SARS), also occurs in the plant world, including vegetables and flowers. “Soil disease” refers to infectious disease of crop plants caused by the infection with soil micro-organisms, such as viruses, bacteria, and fungi.

However, not all of these microorganisms are responsible to disease risk. Among hundred millions of microbes living in 1 gram of soil, most are not causal agents of diseases in plants (non- pathogenic-good microbes), while some are harmful (pathogenic-bad microbes). The balance between good and bad microbes in soil plays an important role. Healthy soil contains a lot of good microbes. The situation with the intestinal micro-flora applies to the soil environment where vegetables and flowers are cultivated. Farmers have been making efforts to make up healthy soil inducing the proliferation of good microbes by putting in compost or soil conditioner on the basis of their experience. In practice, however, soil has often been degraded continuously through repeated cultivation of same crop, unfavorable weather conditions, and excessive use of chemical fertilizer, decimating habitat of good microbes and leading to soil diseases.

Once soil disease breaks out, crop roots begin to perish, spreading the disease to surrounding areas, and ultimately leading to total wipeout of harvests in the region. A historically famous case is the epidemic of potato blight around 1845, which started from a part of Europe and spread to England and Ireland. The disease propagated further in the next year, causing serious harvest failure. More than a million of people living on potato were starved to death. With this as a turning point, nearly four millions of Irish emigrated to North America and other areas, to break fresh ground, from 1851 to 1905. The emigrants included forefathers of now-famous families, such as John F. Kennedy’s and Ronald W. Reagan’s. In this way, the soil disease often devastated expansive areas producing a certain kind of crop in the past, posing crucial issues in food supply to human beings. It is often very difficult to predict the epidemic until the day of serious outbreak. In the event of severe symptoms, soil improvement by using agrichemicals may be required. However, when chemicals were resorted too heavily, soil pathogens evolve resistance to them, repeating vicious cycles between new strains of pathogen and development of new kinds of agrichemical.

The basic principle of soil diagnosis with the biosensor is to estimate the relative activity of “good microbes” and “bad microbes” in the soil on the basis of quantitative measurement of differential oxygen consumption in the respiration of tow types of soil microorganisms.

The measurement proceeds through the following steps: two sensors impregnated with “good microbes” and “bad microbes”, respectively, are immersed in a suspension of soil sample in buffer solution. Half an hour later, the oxygen consumption data by two microbes are displayed on a PC screen. By comparing two data it may be possible to quantitatively decide which microbe favors the soil. It is feasible, therefore, to predict whether or not soil disease is ready to break out in the tested soil beforehand. It is to be emphasized that the biosensor offers an innovative technique of diagnosing soil condition based not on experience but on numerical data.

In case of human disease, knowing the constitutional predisposition of a patient for adult disease in advance will be highly helpful for preventing the disease well before its onset, by taking nutrition, rest, exercise or preventive medicine appropriately. However, if you do not know your own constitution and fail to take preventive measures, the risk of falling ill will rise up. In this event, not only your health is deteriorated, but also the administration of preventive drug and/or the surgical operation may be needed to increase your economic burden.

The same applies to the agriculture. If you understand the “constitution” of your plot of field previously, you may be able to avoid a tragedy of losing entire harvest of vegetables or flowers through soil disease immediately before marketing. Preventive measures include early eco-friendly control, such as selective activation of good microbes to suppress bad ones in adaptation to particular soil conditions, and selective use of crop cultivar, cultivation method, cropping pattern, and resources used, in adaptation to particular soil nature and expected soil disease. While it was possible previously to diagnose soil conditions by chemical analysis of fertilizer composition, pH measurement, and physical determination of porosity (relative volume of air space in soil), none of biological approaches were available up to now. The newly developed “biosensor for soil diagnosis” makes it possible to make overall characterization of soil through biological diagnosis. This approach allows quantitative soil conditioning by putting in effective microbial resources without resorting to experience and gut feeling. It is expected that the biosensor reduces soil disease and use of chemical fertilizer to implement eco-friendly agriculture, ensuring “safety” and “security” requested by consumers.

The “biosensor for soil diagnosis” may be regarded, therefore, as a useful diagnosing tool for providing agriculture friendly both for global environment and for human health.