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Abe J. and Morita S. eds. 1997. Root system management that leads to
maximize rice yields. JSRR, Tokyo. 34-35.

Effects of Nutrients Management on Roots
and Its Possible Contribution to Stabilize
Growth and Yield in Upland Rice

Introduction

Upland rice is comprising 11.6% of total rice land and producing 3.9% of total rice production in the world as a staple food in considerable area in tropical Asia (IRRI 1993 ). The average yield in upland remains at 1.2 t ha-1 which is much lower than in irrigated system at 3.5t ha-1 and rainfed lowland at 2.3 t ha-1. The main reason for this low and unstable production is considered to be water and nutrient stress and combination of those. The soils used for upland rice are generally low in available N and P and farmers hardly input any nutrients. With increasing population pressure, intensive and highly productive upland rice culture system is desired. Understanding the effect of nutrients on water use efficiency and rice response to the drought is essential to improve productivity by introducing appropriate nutrient management. In genotypic comparisons, the deep root system was suggested as a important trait for drought tolerance (Yoshida and Hasegawa, 1980). However, nutrient effect on roots development in soil profiles is not well understood. In this study, the effect of nutrient management on root and consequent water extraction was investigated to seek for the possible contribution of nutrients to stabilize rice growth under upland condition.

Objectives

1. To understand root development in soil profile along growth stages in upland rice.
2. To analyze effect of N application methods (timing, placement, controlled release-N) on root development.
3. To compare soil types in the effect of N and P on root development.
4. To clarify the effect of N on water extracting characteristics under mild and severe water stress.

Methodologies

Experimental Site:

• Objectives 1 , 2 and 4: IRRI (volcanic deposit, moderately acid) .
• Objectives 3: IRRI, Batangas (volcanic ash soil, moderately acid), Cavinti (acid Ultisol), Leyte (P deficient acid Ultisol),

Nutrients and Water Management:

• N: Urea in basal or 3 split application and controlled release urea (POC urea) in basal band application (90-110 kg ha-1).
• P: SSP (60kg P ha-1) as basal incorporation.
• Water: Rain fall except in Objective 4 in which mild (regular, but limited irrigation) and severe stress (no irrigation) was imposed at PI stage in dry season.

Root sampling:

• Core (5cm diameter) sampling to determine root length density (RLD) and root mass density (RMD).

Results

1. Roots developed both horizontally and vertically until max. tillering. After tillering stage, roots tended to increase more in shallow layer till PI. Roots developed again both horizontally and vertically after PI till flowering, especially in RMD. RLD and RMD decreased drastically after flowering.
2. Basal N application stimulated root growth in surface layer at initial stage and in sub-surface layer at flowering stage. It is assumed that this effect is related to tillering patterns. Basal N stimulate the development of early tillers which have deeper root at later stage. Split application starting from basal application or controlled release N is most suitable to stimulate root development and N recovery. No clear localized stimulation was observed in a comparison between band application and incorporation.
3. Both N and P were effective to increase RLD in deep layer below 30cm in acid soils. However, the effect ws not consistent across the soils.
4. Fertilization of N increased water extraction even below 40cm depth under mild stress. However, there was no clear stimulation under severe stress. Roots below 40 cm could not sustain whole demand by shoot.

Implications

1. Tillering characteristics is important factor to control nodal root formation and root distribution up to reproductive stage.
2. Expanding rooting zone by N and P input can be advantageous to exploit water in deeper layer under mild stress. However, there is probably limited benefit under severe stress. There is a need to test in different soil types.

Future research direction

1. Water extracting characteristics in rice root system.
2. Quantify the beneficial effect of N and P input in water uptake and yield in representative soils in tropical Asia using growth modeling approach.
3. Understand systematically of root reaction to N and P fertilization with consideration on the factors affecting root elongation.
4. G x E interaction in root system with emphasis on nodal root formation and elongation and genetic control of root traits using double haproid lines.

References

• IRRI (1993) IRRI Rice Almanac. International Rice Research Institute, Los Banos, Philippines.
• Yoshida, S and Hasegawa S (1982) The rice root system :Its development and function, In "Drought resistance in crops with emphasis on rice" International Rice Research Institute, Los Banos, Philippines, pp97-114.

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