SOIL MOISTURE MEASUREMENT METHOD IN AGRICULTURE /2020

SOIL MOISTURE MEASUREMENT METHOD IN AGRICULTURE

INTRODUCTION-

Moisture content of the soil is a major factor determining plant growth1, especially in irrigated systems. Currently there are many and varied methods for determining soil water content on a volume basis (qv, m3m-3) or a tension basis (kPa or bar) as described by Gardener2.

The basic objective of irrigation scheduling is to minimise water stress of the plant, that of over irrigation, and under irrigation. The manager aims to manipulate the biological process of cell elongation and cell reproduction for improved plant yield3 and maximum use of available effluent.

In optimising plant cell reproduction and growth (cell expansion), the ability to monitor the soil moisture content is the principal facet of developing good water management programs. A tendency to over or under-irrigate results due to the absence of information about the soil moisture status down the soil profile. The result of over irrigation is poor utilisation of natural rainfall because of high surface run off, and production problems associated with excessively wet soil such as waterlogging, leading to recharge of underlying aquifers, leaching of nutrients, increased incidence of plant disease and reduced daily water use. The reduced daily water use of plants increases the area of irrigated land required to dispose of a given volume of water increasing the capital cost of land based waste water disposal systems.

The decline of soil water content will result in a decrease of photosynthesis and cell expansion of the plant. Under-irrigation will result in stress being placed upon the plant root water uptake mechanism to maintain transpiration rates. A subsequent reduction in daily water use and cell production will occur with decreasing soil moisture content. Ludlow et al. 3 showed that stem elongation rate declined ( in Cajanus cajan) at a linear rate after 10% of available water was utilised by the plant until elongation was 40% of the maximum rate as the plants approached wilting point.

To develop an irrigation scheduling program the basic requirement is the ability to regularly obtain objective data. The ability to accurately measure soil water content, plant size and condition is an integral mechanism in the process of developing an irrigation scheduling program that allows a better understanding of plant and soil water relations. From this basis, an understanding of plant agronomy is developed with an appropriate computer interface giving the manager a better working knowledge of what is happening to the applied irrigation and its relation to plant water use and soil moisture status.

Soil moisture

Instruments Available for Objective Measurement of Soil Moisture Content

Objective soil moisture measurement can be undertaken with simple tools, such as a shovel, or complex tools that record measurement of soil moisture on a volumetric basis. The method of measurement is simply a device allowing moisture determination in an objective fashion. It is important that measurements are made regularly and recorded systematically to allow improvement in irrigation scheduling and soil/plant management decisions.

There are different methods available for development of an irrigation scheduling program using different tools to collect relevant information and present the data to the irrigation manager.

Soil moisture method mainly two types-

1- Direct method

2- Indirect method

1- Direct method-

Direct method of soil moisture measurements are:

. Feel and appearance method

. Gravimetric method.

. Alcohol burning method

Hot air drying

Gypsum sorption plugs

Infrared balance.

1-Feel and appearance method :

Soil samples are obtained with soil probe or auger from rientative depths in root zone and observed for colour, plasticity and cohesiveness (by eezing). These parameters are a function of soil texture, structure and moisture content. With ence, accuracy is + or - 5 to 10 per cent available soil moisture.

2-Gravimetric method : 

Soil samples are obtained as indicated above in tight moisture cans cpl in shade to avoid evaporation water losses. Moist sample is weighed and oven dried at to constant weight. The ratio of weight loss in drying to dry weight of the soil multiplied

100 gives the soil moisture percentage (Aw).

Merits

 Ease of handling
 Low cost.
Minimum technical skills required.

It is the standard method of soil moisture determination with which other methods are compared.

Demerits •

 It requires relatively longer time for soil moisture determination

Sampling and weighing errors limit the accuracy.

• Soil sampling is destructive and may disturb experimental plots to distort the results.

3-Alcohol burning method :

Soil moisture from the sample is evaporated by adding alcohol and igniting Provided the sample is not too large, the result can be obtained in less than 10 minutes. About 10 ml of alcohol per g of soil sample at FC and 0.5 ml at PWP is adequate for evaporating the soil moisture. This method is not recommended for soils with high organic itter.

4-Hot air drying :

Hot air around 110°C is passed on a screen with weighed sample of moist soil. Hot air vaporises the moisture and sends it out. Soil samples must be pulverised for using this method It needs relatively expensive equipment

5-Gypsum sorption plug :

Gypsum plugs placed in soil comes into equilibrium with surrounding soil moisture. They are removed and weighed to determine soil moisture content. It is necessary to calibrate the weight of porous cup with soil moisture content for different soils.

6- Infrared balance:

 It gives fairly reliable moisture estimates in about 5 minutes. It consists of a 250 Watt infrared lamp. sensitive torsion balance and auto transformer, all housed in an aluminum cabinet. The radiation emitted by infrared lamp quickly vaporise the soil moisture. The Instrument is directly calibrated in per cent.

7-Volumetric Method -

Soil sample is taken with a core sampler or with a tube anger whose clume is known. The amount of water present in the soil sample is estimated by drying it in the The volumetric moisture content can also be estimated from the moisture content estimated n dry weight basis.

Indirect Method -

1-Tensiometer

2- Gypsum block 

3-Neuton probe,

  -4Pressure plate and pressure  membrane appartus.

1-Tensiometer-

Tensiometers is also called irrometer since they are used in irrigation scheduling . A irrometer consists of a long tube with a porous ceramic cup ce and The other end of the tube is closed with rubber crk and a vacuum gauge is fitted to the side of the tube. The h of the tube varies from 30 sm to 100 cm depending on be depth at which moisture is to be estimated. With the help of a crow har, a hole is made in the soil to the desired depth Tensiometer is placed in the hole and firmly pressed on all es for the soil to be in close contact with the ceramic cup.

The tube is filled with water slowly so as to avoid formation of air bubbles. As soil dries from the porus cup enters the soil. Due to depletion of water in the tube, vacuum which is measured with vacuum gauge. Refilling of the tube is necessary after each irrigation rainfall Tensiometers are sensitive up to 0.9 bars of soil moisture. Hence, tensiometer suitable for sandy soils as most of the available water in the sandy soil is within one bar potential. 

  2-Gypsum Blocks-

 Small porus blocks of about 5.5 cm length, 3.75 cm width and 2.0 cm thickness are made with gypsum. The blocks can also be made from nylon or plaster of paris. Two electrodes are embedded in the block and are connected with electric wires which are in connected to a resistance meter or Bouycos Water-Bridge Resistance to transmission of electricity between the two electrodes is indirectly related to water content which is measured in resistance meter. Gypsum blocks are buried at desired depth in the soil. The moisture content of gypsum blocks comes to equilibrium with soil moisture. Whenever required, the resistance is measure with Bouycos Water Bridge and moisture content is known from the calibration curve. Gypsum blocks are not suitable for saline soils as salts in the soil increases conductivity.

3- Neutron Moisture Meter-

 Soil moisture can be estimated quickly and continuously w neutron moisture meter without disturbing the soil Another advantage is that soil moisture ca be estimated from large volume of soil. This meter scans the soil to about 15 cm diameter round the neutron probe in wet soil and 50 cm in dry soil. Neutron moisture meter consists of a probe and a scalar or rate meter. The probe contains fast neutron source which may be a mixture radium and beryllium or americium and beryllium. Access tubes are aluminium tubes of 50 100 cm length and are placed in the field where moisture has to be estimated. Neutron probe lowered into access tubes to the desired depth. Fast neutrons are released from the probe which scatter into the soil. When the neutrons encounter nuclei of hydrogen atoms of water, their sleep is reduced The scalar or rate meter counts the number of slow neutrons which are direc proportional to water molecules. Moisture content of the soil can be known from the calibration curve with counts of slow neutrons. The two drawbacks of the instrument are that it is exponent and moisture content from shallow top layers cannot be estimated. The fast neutrons are slowed down by other sources of hydrogen (present in the organic matter). Other atoms such chlorine, boron and iron also slow down the fast neutrons thus overestimating the soil moisture content.

4- Pressure Membrane and Pressure Plate Apparatus-

Prasure membrane and press plate apparatus is generally used to estimate field capacity, permanent wilting point and me content at different pressures. The apparatus consists of a air-tight metallic chamber in porus ceramic pressure plate is placed. The pressure plate and soil samples are saturated placed in the metallic chamber. The required pressure, say 0.33 bar or 15 bars is applied the a compressor. The water from the soil sample which is held at less than the pressure trickles out of the outlet till equilibrium against applied pressure is achieved. After that samples are taken out and oven-dried for determining the moisture content.

Conclusion-

Soil moisture Meseaurement method most important to agricultural practices in  growing of crop field .

Scheduling irrigation and time of irrigation maximum crop production etc.

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