Spatial distribution of phosphorus in the Ap horizon soils of paddy rice fields after rice harvest: Effect of machinery localized fertilizer application at transplanting of rice [Oryza sativa] seedlings
2006
Akahane, I.(Tohoku Univ., Sendai (Japan)) | Nanzyo, M. | Takahashi, T. | Sekiguchi, O. | Saigusa, M
Ap horizon soils of Japanese paddy rice fields, in general, are completely mixed by plowing and puddling before transplanting. In these situations, the possible factors to cause heterogeneous distribution of P in the Ap horizon soil are machinery fertilizer application at the time of transplanting rice seedlings, P diffusion to the redox front within the Ap horizon, and so on. High values of modified Bray-2 P and total P were found at the depth of 2-3 cm from the soil surface after rice harvest in the Ap horizons of 4 soils with different colloidal properties. Two, Utsunomiya and Kawatabi, have high content of active Al (allophane, imogolite or Al-humus complex) and the others, Furukawa and Maeda soils, crystalline layer silicates and low active Al content. The position of the high P values is close to the depth of the machinery applied fertilizers. The vertical distributions of exchangeable K, Ca and Mg content were rather different from those of P within the Ap horizon. Phosphorus availability to rice increases under flooding and the P sorption capacity of the Furukawa soil somewhat decreased to 2.8 g P2O5 kg(-1) when the soil was strongly reduced. However, the amount of P application (1.8 g P2O5 kg(-1)) was still less than the P sorption capacity of the soil under the strongly reduced conditions. Thus, P fertilizer did not move from the applied sites in Ap horizon soils. We must be careful in soil sampling for P availability evaluation of the paddy rice field where fertilizer application was done by the rice seedling transplanting machine.
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