Japanese Journal of Soil Science and Plant Nutrition

The rising prices of inorganic fertilizer have led farmers to consider alternatives. Composted animal manure is the most readily abundant alternative source of soil nutrients. Molding compost into pellets reduces the bulk and makes it more suitable for storage, transport, and mechanical application. I examined the effect of compost pelletization on the availability to plants of phosphorus (P) in the compost at various application rates in pot experiments using an andisol showing a high P sorption capacity. A mixture of cattle and poultry manure compost was pelletized by means of a disc pelleter and used as the sole source of applied P for komatsuna (Brassica campestris L.) and buckwheat (Fagopyrum esculentum Moench). The pelletized compost resulted in a superior P recovery rate by plants compared to that from ordinary compost. The P recovery rate from pelletized compost was 12.5% for komatsuna and 24.5% for buckwheat, while the corresponding values from ordinary compost were 8.6% and 15.9%, respectively. The P recovery rate from pelletized compost was comparable to that from superphosphate. Pelletization therefore appears to be beneficial to the phytoavailability of P in compost and may help avoid problems with P fixation in certain soils. Further investigations are necessary under various soil and cultivation conditions.

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In a particular high-altitude area of Sasebo city, Nagasaki, a unique disorder in paddy rice has been observed since the 1960s. This disorder is called suitou hakare shou in Japanese, and we refer to it here as rice hakare symptom. Despite decades of research on rice hakare symptom, the particular environmental factors and mechanisms causing rice leaf damage are unknown. We therefore conducted a metabalomic analysis with the aim of clarifying the environmental stresses inducing this disorder. We collected healthy and damaged uppermost leaves from rice plants at different developmental stages and analyzed the various stress-related metabolites by means of gas chromatography-mass spectrometry. We found that glucose and fructose, early metabolites in photosynthesis, tended to be greater in rice leaves collected from paddy fields displaying signs of damage from rice hakare symptom, especially in late August. Furthermore, non-targeted metabolic profiles derived from mass spectral data led us to select γ-amino-butyric acid, trehalose, sorbitol, and serotonin as stress-related metabolites. Sorbitol and serotonin, minor components in healthy leaves, were accumulated specifically in damaged leaves at any growth stage, and further increased with development of the leaf lesion. These results indicate the significance of sorbitol and serotonin in rice leaves as biomarkers of rice hakare symptom. Both compounds are generally known to accumulate in response to drought stress or desiccation of plant tissue, suggesting the possibility that changes in leaf-water relations are an important part of the mechanism that causes rice hakare symptom.

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We assessed plant nutrient problems in rice paddy soils in southern Okayama and based on results, developed guidelines for potassium fertilizer application. (1) We investigated the soil in 135 parcels of rice paddies located around Kojima Bay in southern Okayama. The soil pH was <6.0 in 83% of the fields and the potassium saturation level (exchangeable K/total exchangeable cations × 100) was ≥5% in 67% of the fields. (2) The potassium saturation level tended to be higher in double-cropped rice paddies in which rice and wheat were cultivated in rotation. Presumably, the soil pH was low in these rice paddies as a result of reduced base saturation caused by leaching of the exchangeable calcium, and this was exacerbated by an excess of potassium in the soil. (3) To prepare guidelines to reduce potassium fertilizer application based on the chemical conditions described above, we conducted a potassium fertilizer application test in rice paddies with various soil exchangeable potassium contents. The results show that the rice shoot sodium content was greater in the test group when the potassium saturation was ≤4%. We therefore established a target level for the improvement of potassium saturation level in the soil based on the increased sodium content resulting from potassium shortage. (4) Based on the target improvement level, we designed a fertilizer application guideline in which potassium fertilizer was applied with the target of 4% when the potassium saturation level before fertilizer application was ≤4%, whereas potassium was not applied when the level was >4%. This guideline would result in lower application rates of fertilizer than if a fixed rate for potassium fertilizer application was prescribed.

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We investigated the effect of rice husk charcoal (RHC) as a cover or mixed with plug medium on nitrogen (N) availability when plugs were fertilized by corn-steeping liquor. We measured the amount of nitrate-N and ammonium-N in the medium and in the water that had percolated through the medium (percolate) as well as the diurnal temperature of the medium and the growth of lettuce seedlings in a greenhouse experiment with three treatments: RHC as a surface cover (RHC-cover treatment), RHC mixed into the medium (RHC-mixed treatment), and no-RHC control. We also used an incubator experiment to investigate the effect of daily maximum temperature and the presence or absence of light period on the amount of inorganic-N in the medium in RHC-cover and RHC-mixed treatments. In the greenhouse experiment, the amount of nitrate-N in the percolate and remaining in the medium at the end of the experiment was higher in the RHC-cover treatment than in the other treatments. The amount of ammonium-N in the medium did not differ between the treatments. The lettuce seedling growth was greatest in the RHC-cover treatment. The diurnal temperature in the medium did not differ between the treatments, but the temperature on the medium surface increased more in the cover treatment than in the other treatments when watering was withheld. In the incubator experiment, the amount of nitrate-N in the percolate increased in the RHC-cover treatment when daily maximum temperature increased, but no effect was measured in the RHC-mixed treatment. The amount of nitrate-N in the percolate and in the medium of the RHC-mixed treatment was higher in the absence of light period than in the presence of light period. The RHC-cover treatment was only slightly affected by the presence or absence of a light period; thus, in the absence of a light period, the amounts of nitrate N in the percolate and in the medium of the mixed treatment were equal to or more than those of the cover treatment.

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