Properties of Lowland Rice Soils | Rice Cultivation | Agronomy

In this article we will discuss about the properties of lowland rice soils.

Land submergence for lowland rice brings about various changes in soil, which distinguish it from its upland counterpart. One of the most outstanding characteristics of flooded (lowland) soils is the absence of oxygen in the root zone. The diffusion of oxygen into water is only 10^-4 of that in soil, so that exchange of oxygen from the atmosphere to soil is extremely small. This brings about a reduced condition in the soil.

A few mm of soil beneath the soil water boundary does contain small amounts of oxygen and it remains in an oxidative condition. Beneath this thin oxygen sink, the soil remains largely in a reduced state through the metabolic activity of aerobic and facultative anaerobic microorganisms, which either act directly as electron acceptors in anaerobic dissimilation reactions or by forming various organic decomposition products.

The main chemical changes occurring in flooded soils are reviewed by Patrick and Mikkelsen (1971) and Ponnamperuma (1972).

These include:

1. Accumulation of gases such as CO₂, CH₄, N₂ and H₂ from microbial decomposition products.

2. An increase in pH of acid soils and a decrease in pH of calcareous and sodic soils. This change in pH of acid soils occurs when Fe²⁺ iron and Mn²⁺ manganese begin to precipitate and equilibrate with CO₂ and HCO₃– in soil solution. In alkaline soils, the acidifying effect of CO₂ lowers the soil pH, which is buffered by Ca and Mg carbonates.

3. The change of NO^–₃, Mn⁴⁺, and Fe³⁺, and SO^2- to their reduced forms is controlled established thermodynamic redox sequence found to occur in flooded soils.

4. Increased electrical conductivity as correlated with HCO₃-, Fe²⁺ and Mn²⁺ ion concentrations in acid soils and with Ca and Mg bicarbonate concentrations in alkaline soils.

5. Increased supply and production of NH⁺₄-N both from native soil nitrogen and nitrogen fixing organisms.

6. Increased availability of P, Fe, Mn, Si and Mo as a consequence of reduction, dissolution and desorption reactions.

7. Accumulation of toxic substances and the associated anaerobic decomposition of soluble carbohydrate materials may lead to the formation of gases such as CO₂, CH₄, N₂ and H₂, organic acids such as acetic, butyric, propionic and formic acids and H₂S from reduced sulphates in flooded soils.

These changes are of practical significance in soil management for lowland rice. Loss of nitrogen from decomposed organic material, if the soil is dry, can the minimised by flooding the field either just before or after incorporation of green manure or any other organic material.

Presence of oxidised layer on the soil surface of lowland rice leads to nitrogen loss since ammonium fertilisers will be readily converted into highly mobile nitrate (NO₃) nitrogen, which will be lost through leaching.

It is, therefore, desirable that applied ammonium fertilisers be incorporated with the soil or placed deep in the soil to prevent the nitrogen losses. In acid soils with high iron, addition of large amount of fresh organic matter could be detrimental since soluble iron in high concentration is toxic to plants.

One of the most outstanding characteristics of flooded soil is the absence of oxygen in the root zone. This brings about a reduced condition in the soil, which is measured by the Oxidation – reduction potential. Oxidation-reduction potential is a measure of the degree of reduction or oxidation of the medium.

Redox potential influences:

a. Oxygen concentration in the soil.

b. Soil reaction (pH).

c. Availability of P and Si.

d. Concentration of Fe²⁺, Mn²⁺, Cu²⁺ and SO₄^2- directly and those of K⁺, NH₄⁺, Ca²⁺, Mg²⁺, Zn²⁺, B (OH₄-) and MoO₄^2- indirectly.

e. Generation of organic acids, ethylene, mercaptans, organic sulphides and hydrogen sulphide.

Ranges of redox-potential (Eh) of submerged soils at different levels of aeration are given below:

Presence of organic matter, a low nitrate and manganese dioxide content and a temperature of 35°C or above favour the decrease in Eh and a value as low as -250 mv may be attained within two weeks after flooding. Decrease in Eh due to land submergence has positive and negative effects on rice growth.

The benefits are increase in supply of nitrogen, phosphorus, potassium, iron, manganese, molybdenum and silicon. The disadvantages are loss of nitrogen by denitrification, decrease in availability of sulphur, copper and zinc and production of substances that interfere with nutrient uptake or that poison the plant directly.

The pH of acid soils increases, while that of calcareous and sodic soils decreases on flooding. Thus, whatever the initial soil pH, on submergence it stabilises at a value between 6.5 and 7.5 and this makes it possible to grow rice on these problem soils. Soils rich in organic matter such as peat soils, however, may not show a change in pH appreciably.