Precise Potassium Fertilization for Double Crop Rice

The highly productive double rice cropping in the hilly regions of Hunan, China, is more likely to be sustained by fine-tuning the potassium and nitrogen fertilizer strategy.

Hunan Province has a humid monsoon with lots of sunshine, warmth, and rainfall. Almost a third of the province is hilly and this is the farmland famed for its rice cultivation, usually as a double season crop grown on paddy soils.

Unsensitive Farming has led to Inadequate Supply of Soil Potassium

The increasing intensification of rice production in Hunan region has not been without its problems, particularly for potassium levels in the soil. Choosing rice varieties with higher yields, greater use of inorganic fertilizers (particularly nitrogen and phosphorus), and an increased tendency to remove the rice straw for fuel or forage have all contributed to the declining soil potassium level.

Added to these issues, imbalanced fertilizer strategies combined with high temperatures and very heavy rain events have also diminished the level of soil potassium. This has been proved by studies which have shown that most rice-rice production systems have a negative potassium imbalance. Moreover, farmers continue to focus on heavy nitrogen applications rather than consider the rice crop’s potassium requirements. As a result, it is a sad fact that the rice crop performance tends to be poor.

The priority has been to show Hunan rice farmers and those who advise them, how important potassium fertilizer is and what constitutes proper use for the crop, for the soil and for farm profitability.

A team of specialists set to work. Drawn from the Soil and Fertilizer Institute of Hunan Provence with support from Agro-scientific Research in the Public Interest of China and the International Potash Institute (IPI) they had a clear aim: to study the effect of potassium application on yields and soil potassium through using two nitrogen application levels.

Investigating the right amount of Potassium and right Combination with Nitrogen

The experiment took place in successive early and late rice crops in two counties in Hunan which had different soil types, red-yellow, in Changsa, and yellow soil in Hengshan. There were six treatments in total (each had four repetitions in a random block design) as shown in the table. Most of the nitrogen fertilizer was applied pre-planting for both the early and late rice crops and the rest was applied at tillering. Phosphorus and potassium fertilizers were applied at pre-planting.

Table showing detailed description of fertilizer treatments examined in rice-rice research

There were several analyses to measure the results. Firstly, soil samples were collected from the top 20 cm of soil, before and at the end of each crop cycle, to establish the level of available potassium. At harvest, grain yield and aboveground biomass was measured. Then some of that biomass was processed to determine potassium content. In addition, the cost of the various fertilizer regimes was measured against the return on the investment required.

Effects of Combined Nitrogen and Potassium Application on Rice Yields

In both soil types, the highest grain yields were obtained under the highest combination of nitrogen and potassium application. In the red-yellow soils of Changsha county, on the early crop, lower rates of potassium under the lower nitrogen rates made little difference. But on the later crop the yield increase was much greater at over 13%. There was a similar, but slightly better, yield increase result with the late crop.

Meanwhile yields on the yellow soil of Hengshan county were generally higher by about 600 kg/ha per season thank the red-yellow soil of Changsha county. The yield response to the lower nitrogen and first level of potassium rate was over 12% (early crop) and over 17% (late crop), but increasing the potassium rate further did not have any further significant effect on the yield.

Economic Considerations in Optimizing Fertilizer Balance

The research team were mindful of the economic context for fertilizer practice and its influence on the likelihood of potential change, and evaluated the costs and financial returns from the various treatments. Farmers need to recoup in sales more than the cost of the nutritional inputs, otherwise they incur a loss.

In the early crop on the red-yellow soil, the results showed that the highest yield of revenue did not necessarily indicate the highest profit. In fact, the late crop and the lower fertilizer combination yielded the highest profit. Revenues and profits were significantly higher on the yellow soil, especially for the late crop.

The Right Fertilizer Recommendation takes careful Preparation

Potassium is an important plant nutrient and must be present in adequate levels for high and stable rice yields. However, this study in China reveals the complexity of the relationship between potassium and rice yield, and potassium and nitrogen use efficiency, as well the effect of different varieties and their nutritional needs at different times.

Rice yields were consistently higher in response to the basic potassium application compared with the non-fertilized control. But if nitrogen levels were kept low, at 150 kg/ha, raising the potassium level made no further yield increase.

However, when the rate of nitrogen applied was increased to 195 kg/ha the yield jumped, if the necessary potassium was also applied. It seemed that at lower nitrogen the need for potassium is met by the basic level. But adding more nitrogen prompted further crop development and reignited the crop’s need for potassium.

The economic analyses in the study showed farmers’ maximum profit from the early rice crop came from high nitrogen and potassium applications while a lower combination was sufficient for the late crop.

All in all, the study illustrates that significant rates of K application are a must for rice-rice cropping systems on paddy soils, while demonstrating the complexity of interactions between nutrients, soils, varieties, weather, and farm business needs.

The full research paper Effect of Combined K and N Application on K Use Efficiency and Balance in Rice-Rice Cropping Systems in the Hilly Regions of Hunan Province, China can be found in the e-ifc published by the International Potash Institute (IPI).