System of Rice Intensification (SRI)

M Gyanendro Singh *

Rice cultivation is a water intensive enterprise. About 3000-5000 litres of water is consumed for producing one kg of rice. SRI method is the technology to increase water use efficiency in rice.

The SRI developed in Madagascar during 1983-84 by Fr Henri de (Laulanie, on agriculturist, is an altered practice of rice cultivation which includes:

1. transplanting single seedlings rather than planting them in clumps of 3-6 plants,
2. transplanting young seedlings (8-14 days) which preserved a potential for tillering and root growth,
3. planting the single seedlings in a square pattern with wide spacing (25X25 to 50X50 cm) between plants, rather than in rows, so that weeding can be done with the simple 'rotating hoe' perpendicularly in two direction instead of just one;
4. keeping the paddy soil moist and not continuously saturated during vegetative growth phase of the plants.
5. using compost, decomposed biomass rather than manure for best results but not necessary for raising the yeild.

The success of SRI methods, as currently understood, depends basically upon:
Enabling plants to grow larger root system and promoting larger and more dirverse populations of soil micro-organisms.

SRI practice has two main outcomes that lead to hight yeilds:
1. Transplanting young seedlings, single and with wider spacing (25 or 30 cm), and growing them in soil that is not continuously saturated produces larger root systems. Root system of rice plants conventionally grown remain close to the surface, and they begin degenerating after a few weeks, so that by the time reproduction begins, rice plants may have lost much of their root system.

2. With larger root systems and larger canopies (SRI pheno types can have 30-50 tillers or more) there is more root exudation. This substrate augmented by the addition of compost to the soil for getting the higest SRI yields and water menagement practices that create a combination and flug of aerobic and anaerobic soil conditions, supports larger and more diverse earthworms, etc, which contribute to both plant nutrition and derive:

• from synergy between root and and shoot growth, with each enabling the other to grow larger and function better and
• from symbiosis between the plant and the various soil microflora and fauna, particularly earthworms, that improve the soil both physically and chemically.

One indication that SRI produces a different phenotype is the positve correction seen with SRI grown rice plant between:

• the number of tillers/plant and
• the number of grains/panicle.

Reason for physical changes in plant and soil in SRI:

SRI plant management practices of transplanting young seedlings (less than 15 days) carefully and 'sparsely,' only one per hill and wit wide spacing among them in a square pattern lead to more productive phenotypes, which provide conducive soil conditions for getting expression of the plants ful genetic potential for tillering root growth and grain filling.

By having only one plant per hill and by spacing hills widely SRI methods introduce 'the edge effects' over the whole field. It is well known that plants on the edges of fields grow better and more productive, because of their greater exposure to sunlight an moving air, than plants in the whole field average when planting close.

Where soil quality assessed in terms of microbiological diversity and activity as well as chemically available nutrients is poor, higher yield may be obtained initially from 2 or even 3 plants/hill.

But with SRI, fewer plants on soil where quality has been improved over time, can give higher total yield. The plants have both more tillers (upto and some times over 100/plants and more grains per panicle (average can reach 250-300; the largest SRI panicle could have upto 900 grains on it).

Mechanisms contributing to higher yield in SRI practice:

In addition to greater tillering and root growth result with SRI practices, there are factors, which contribute to higher yields:

1. Photosynthesis:
The edge effect is a matter partly of more tillers and roots growing into unoccupied space, but increased exposure to solar radiation is probably an important factor. When plants are densely spaced, the upper part if the canopy shade the lower parts and reduce photosynthetic potential.

2. Root Functioning:
SRI root systems are much larger and root pulling resistance (RPR)/clump is more than double for SRI plants. SRI plants take up N, P and K more efficiently or more effectively than do conventionally-grown plants.

Whether this is due to a larger or better functioning root system or due to increased supply of nutrients in the rhizosphere because of the activities of soil microbs. SRI rice plant roots is more and the accumulation of carbohydrates, N and dry matter is greater in the vegetative organs and the absolute partitioning rate of stored matter from the vegetative organ is remarkably higher. Total translocation from leaves, stems and sheathes is also greater.

The corresponding root growth, promoted by avoiding contiuous soil hypoxia, is also supported by wider spacing.. The recommended distance between plants is 25X25 cm; but possibly greater distance (50X50cm) found to be more productive once soil quality improves due to the practice of SRI, having increased soil organic matter, more root emdation and better soil aeration from different water the rotating hoe?

Polycropping:
SRI is developed for monocropping with target yield of 6-12 tha-1, but the highest yields are recorded in rotation with another crop, in both cases with potatoes (21 tha-1 in Madagascar and 16 tha-1 in Siehuan, China).

The alternate crop enriches the soil by putting into it a different profile of root exudates (sugar, amino acids hormones, etc) thereby supporting more diversified population of soil microbes that enhance plant nutrition.

Creating a better soil enviroment:

Application of the compost as soil amendment yields more than cheminal fertilizers SRI practice of keeping paddy fields moist but phase, contributes to the growth of larger roots inaerobic soil enviroments and avoids the inhition and degeneration of root growth.

The application of inorganic N fertilizer can inhibit biological nitrpgen fixation (BNF) processess nitrogenase enzyme by plant roots and soil organisms. BNF is greatly increased when aerobic and anaerobic soil horizons are mixed together. SRI water management practices and the recommended eeding with rotating hoe also enhance BNF.

On better day soil and without any soil amendments yield under SRI practice can be enhanced from 1.8 to 6.1 -1 Use of chemical fertilezer with SRI practices can boost yield still further with 9.0-1. But there is significant reduction in the soil microbial population, so that the yield obtained is more dependent on inorganic N.

Use of compost with SRI method raise yield even higher upto 10.5 tha-1. a ebar boost by microbial N-fixation though the evidence is indirect. The soil aeration promoted by SRI methods is beneficial for the growth of oearobic microbes including particularly mycorrhizal fungi.

Mycorrhizae enhance the variety to the plant. In addition, they stimulate hormone production in plants improve soil structure supress plant diseases including nematods infection, enhance leaf chlorophyll levels, and enable plants to tolerate various kinds of stress.

SRI has water saving potential and can reduce water requiredment for irrigated rice by 25-50% while raising yield 50-100% or more. SRI methods here water is not an abundant and effectively free good.

Wherever water is scarce even it cannot be reallocated through intra-or inter-basin transfers, benefits from lowering water requirements can be passed on to downstream user, if only in reduced costs of management. One social benefit hard to quantify is the advantage of reducing the amount of conflict over water.

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* M Gyanendro Singh wrote this article for The Sangai Express. This article was webcasted on November 26, 2008.

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