When the Green Revolution in Asia started to take off in the late 1960s, a group of Japanese soil scientists had the foresight to go out and sample rice fields all over Southeast Asia. Led by Professor Kazutake Kyuma from Kyoto University, they felt that intensifying rice farming would significantly change soil fertility – or soil health as we call it today.
During the 1960s and 1970s, they sampled 410 paddy fields in nine tropical Asian countries and found that the soil fertility status varied widely. About 50 years later, researchers went back to re-sample 142 of these locations in Malaysia, Thailand and the Philippines. This allowed them to assess the changes that may have occurred as a result of about 100 rice crops grown since the original surveys.
The most striking result from the re-survey is that, in all three countries, the content of available soil phosphorus (P) increased substantially over 50 years – more than 8-fold on average. While prolonged flooding of soil increases the availability of P, this large increase indicates that farmers may have been using too much P fertilizer over time.
They also found that the soil pH increased modestly. Changes in soil carbon ranged from an increase in Thailand to a decrease in Malaysia, probably because the latter mostly represented more recently cultivated land with high initial soil organic matter content. Overall, this study confirms that the intensification of rice systems causes no major negative changes in soil fertility, but may in fact improve soil nutrient status along with increased productivity.
