Soil cum sand mini plot technique:
In this method, one cubic meter pit is dug in the middle of field. About five percent of sand by volume is added to the dug soil, mix well and pit is filled in the natural order. Crops are grown as usual in the entire area of the field including the pit area. The plants in the pit show wilting symptoms earlier than the other plants in the remaining area. Irrigation is scheduled as soon as wilting symptoms appear on the plants in the pit.
Sowing high seed rate:
In an elevated area, one square meter plot is selected and crop is grown with four times thicker than natural seed rate. Because of high plant density, plants show wilting symptoms earlier than in the area indicating the need for scheduling irrigation.
Feel and appearance method:
Moisture content can be roughly estimated by taking the soil from root zone in to hand and making in to small ball. It requires lot of experience to estimate the soil moisture by this method.
Irrometers or tensiometer:
Tensiometer is also called irrometers since they are used in irrigation scheduling. Tensionmeters provide a direct measure of tenacity (tension) with which water is held by soil. It consist of 7.5 cm porous ceramic or clay cup, a .0protective metallic tube, a vacuum gauge and a hollow metallic tube holding all parts together. At the time of installation, the system is filled with water from the opening at the top and rubber corked when set up in the soil. Moisture from cup moves out with drying of soil, creating a vacuum in the tube which is measured with the gauge. Care should be taken to install tensiometer in the active root zone of the crop. When desired tension is reached, the soil is irrigated. The vacuum gauge is graduated to indicate tension values up to inch atmosphere and is divided in to fifty divisions each of 0.2 atmosphere value.
Merits of tensiometer:
1. It is very simple and easy to read soil moisture in situ.
2. It is very useful instrument for scheduling irrigation to crops which require frequent irrigations at low tension.
Sensitivity of a tensiometer is only up to 0.85 atmospheres while available soil moisture range is up to atmosphere and hence is useful more on sandy soils wherein about 80% of available water is held within 0.85 ranges.
As the plant is the user of water, it can be taken as a guide for scheduling irrigation. The deficit of water will be reflected by plants itself such as dropping, curling or rolling of leaves and change in foliage colour as indication for irrigation scheduling. However, these symptoms indicate the need for water. They do not permit quantitative estimation of moisture deficit.
Growth indicators such as cell elongation rates, plant water content and leaf water potential, plant temperature leaf diffusion resistance etc. are also used for deciding when to irrigate. Some indicator plants are also a basis for scheduling irrigation e.g. sunflower plant which is used for estimation of PWP of soil is used in Hawaii as an indicator plant for irrigation sugar cane.
Infra red thermometer:
Canopy temperature is measured with infrared thermometer. It also simultaneously measures canopy temperature (Tc) and air temperature (Tq) and displays Tc-Tq value. Tc-Tq values can be used for scheduling irrigation. When transpiration is normal, due to its cooling effect canopy temperature is less than air temperature. The negative values of Tc-Tq indicate the plants have sufficient amount of water. When Tc-Tq values are zero or positive, which indicates stress irrigation is scheduled. Stress degree days (SDD), useful for scheduling irrigation are summed in a manner that is analogous to growing degree days SDD = (Tc-Tq) canopy temperature is measured during midday when air temperature is maximum. Yield reduction is maximum, when total number of cumulative SDD’s exceeds 10 to 15 between irrigations.
In projects areas, where a single crop is grown on large area, irrigation scheduling can be done with the help of remote sensing data. Reflectance of solar radiation by the plants with sufficient amount of water is different from that of stressed plants. This principle can be used for scheduling irrigation. The following methods can be recommended to farmers for scheduling irrigation.
Soil -Cum-Sand Mini Plot Technique
Increased Plant Population