Organic Farming: Irrigation Water

Introduction

We know, water is crucial to life and existence. It is equally important for crops as well. Water management is an important aspect of cultivation. It refers to irrigation, drainage water harvest, and storage of water as well as conversation of water. Irrigation is very important in improving the yield and quality of crops. In waterlogged soils, the drainage of excess water is equally important. We should harvest and conserve the rainwater for irrigation during dry seasons. Many water management technologies have been evolved over the years. Some of them are adopted by the farmers.

Significance and approaches to water management vary widely in different climatic zones. It also gets affected due to the differential moisture retention capacity of soils. The importance of irrigation for crops varies differentially in sandy soils and deep loam soils. Similarly, the importance of water management varies between deep-rooted crops and shallow-rooted crops.

In organic farming, the irrigation water should be drawn from a safe source. Also during conveyance, the water should not pass through any hazardous materials. India’s NPOP [National Program for Organic Production] advocates that soil and water resources should be used in a sustainable manner.

 

Functions Of Irrigation Water In The Soil

Besides nursing the plants, water has important functions in the soil as well. The irrigation water softens the tillage pans. It acts as a solvent and carrier for the nutrients which is absorbed by the crop plants. The irrigation water supplies moisture that is essential for the beneficial microbes and for the chemical reactions within the plant responsible for plant growth. Some salts present in the soil get dissolved in water and help nourish the plants. Irrigation water is also helpful in washing out or diluting the unwanted salts from the soil.

 

Quality Of Irrigation Water

The quality of irrigation water depends primarily on its silt and salts contents. Silt may add fertility to the soil, but tends to clog channels and canals; it is deposited in some parts of the field, necessitating periodic land leveling.

There are many factors that determine water quality. The important factors are the total concentration of the salt, the proportion of sodium to other cations, and the presence of toxic ions such as borate, chloride, sodium, or bicarbonate. As salt concentration increases, the problem of drainage and prevention of excessive accumulation become important. In the medium and fine-textured soils exchangeable sodium to the tune of 12 to 15 percent reduces their permeability. The availability of plant nutrients is also disturbed. Excess sodium is more difficult to manage because of its equilibrium relationship between the exchangeable cations of clay and cations in the soil solution.

All irrigation water contains sufficient salt to damage crop plants if it is accumulated. Careful attention is required for managing the salt content. Good drainage creates conducive conditions for prevention. A second preventive condition is the application of additional irrigation water periodically to leach out excess salts from the soil.

In some situations, water of low quality can be mixed with higher quality to form acceptable irrigation water. In addition, crop plants vary appreciably intolerance to salt. So selection of crops may be another step towards reducing salinity effects. In sandy soils, drip irrigation with water often considered too saline for irrigation has been effective. For irrigating an organic farm, the irrigation water should be obtained from a chemical-free source.

 

Standards For Irrigation Water

Irrigation water may be unsatisfactory for organic farming, if it contains chemicals / metals that are [I] toxic to plants or for the persons using the plants or their products as food; [II] react in the soil to produce unsatisfactory moisture characteristics and [III] facilitate development of bacteria which may harm plants or their produce or are harmful to human beings or animals. In an organic farm, appropriate steps should be taken to avoid such contamination.

 

Scheduling Of Irrigation

When available soil moisture is reduced to a point at which a significant reduction in plant growth rate occurs, the crop should be irrigated. In practice, the farmer decides the appropriate time of irrigation based on plant appearance, stage of crop development, soil characteristics, soil moisture content, and climatic factors. The period between two irrigations is determined by the water holding capacity of the soil, the depth of rooting of the crop, and the quality of the irrigation water. Evapotranspiration [ET] is an important factor in determining the time and frequency of irrigation. The scheduling of irrigation includes: [I] how to irrigate [II] how much to irrigate, and [III] when to irrigate.

 

Preparation Of Land For Irrigation

Land preparation is an important aspect of cultivation. For proper irrigation, we need a well-leveled field. The land preparation for irrigation should be done by removal of bushes, roots of trees, and leveling the land. The land should be provided with the proper slope in the direction of the falling gradient. For it, the land may be divided into small plots depending on the method of irrigation to be practiced. Supply and drainage water courses should be kept for ensuring proper flow.

 

Methods Of Irrigation

The manner in which the irrigation water is applied to land is called methods of irrigation. You know, generally, crops are irrigated by surface and sub-surface methods. These are the common practices utilized whether the crop is grown organically or not. The surface irrigation methods include uncontrolled or wild flooding, controlled flooding, contour laterals, border strip method, check flooding, basin flooding, zigzag method, furrow method, corrugation method, and contour irrigation. In sub-surface irrigation, method water is introduced in the root zone directly. This method includes the use of perforated pipes, tiles, sub-surface drip, etc. Let us examine some of the important irrigation methods in detail.

Surface Irrigation Method

• Uncontrolled Or Wild Flooding

In uncontrolled or wild flooding methods, water is applied in flat land without much preparation. This method is used in the inundation irrigation system, in which water from high streamflow is led to spread over the entire area. It is practiced largely where irrigation water is abundant and inexpensive. But, it is a faulty approach to managing the water. It is discouraged in organic farming.

• Controlled Flooding

In the controlled flooding, water is allowed to spread over the land which has been bunded properly to control the depth of water. Controlled flooding can be achieved by free flooding, contour laterals, border strips, or by checks or levees.

This method is also known as irrigation in plots. The field is divided into a number of small-sized properly leveled plots. Water is allowed into these plots at the higher end and the supply is cut off as soon as the lower part of the plot has received the sufficient depth of water. Oblong plots are preferred to square ones. The size of plots depends on the porosity of the soil.

• Contour Laterals

This method is useful in the steeper terrain. The field is cut into a relatively dense network of small contour laterals. The spacing of the laterals depends upon the slope gradient of the field between two adjacent laterals, the uniformity of slope, and the soil type.

• Border Strip Method

In the border strip method, the farm is divided into a series of strips of 10 to 20 meters width and 100 to 300 meters length. These strips are separated by low levees or bunds and run down the predominant slope. Water is turned on from the supply ditch onto the head of the border for irrigation. The advancing water is confined and guided towards the lower end of the strip. The leveled surface facilitates the advancing of water to cover the entire strip. This method is suitable for forage crops. The advantages are : low investment, low labor requirement, and high irrigation efficiency. The length of the border strip depends on the infiltration rate of the soil, longitudinal slope of the land, and size of irrigation stream available. The following lengths of border strip are suggested for moderate conditions:

Types Of Soil And Border Strip Length:

Sandy soil or sandy loam: 60 to 90 m [Border Strip Length].
Medium silt loam: 90 to 150 m [Border Strip Length].
Clay loam or clay soil: 150 to 300 m [Border Strip Length].

• Check Flooding

In the check flooding method, a large stream discharges water into a leveled plot surrounded by checks or levees. The levees are generally 2 to 3 m wide at their bases. They will not be more than 25 to 30 cm in height. In a leveled farm, the plots are generally rectangular in shape. If the ground has some slope, the checks or levees may follow the contours. The method is suitable for both permeable and impermeable soils. The permeable soil has to be quickly covered with water to prevent excessive percolation losses and the impermeable soil has poor infiltration which should be covered with a slow-moving water stream.

• Basin Flooding

The basin flooding is suitable for the trees. The basins are usually formed for each tree; in some cases one basin may be formed for two or more trees. The basins may also be interconnected. Water is supplied to these basins through a supply ditch.

• Zigzag Method

In the Zigzag method of flooding, water takes a circuitous route before reaching the dead end of each plot. The whole area is divided into a number of square or rectangular plots; each plot is then subdivided with the help of low bunds or levees. This method is suitable for leveled plots.

• Furrow Method

The furrow method of irrigation is used for row crops like maize, jowar, sugarcane, cotton, tobacco, groundnut, and potatoes. In this method, only one-half to one-fifth of the surface is wetted; thereby the evaporation loss is reduced. A furrow consists of a narrow ditch between rows of plants. The length of furrows varies from 3.0 m or less for garden crops to as much as 500 m for field crops. If the furrows are too lengthy, losses due to deep percolation and soil erosion near the upper end of the field may occur. The general slope for furrows may vary from 0.2 to 5 percent.

For maize, potatoes, sugarcane, and other row crops, one irrigation furrow is usually provided for each row. The common size of furrows for row crops such as cotton, tobacco, and potatoes is about 25 cm wide and 8 to 10 cm deep. The spacing of furrows is kept from 1 to 2 m in case of orchard irrigation. It is essential to have deeper furrows for irrigating the crops like sugarcane.

• Corrugation Method

The corrugations are small furrows that are used for grain and forage crops. The corrugations are appropriate when the available irrigation streams are small. These are also suitable for lands of uneven topography.

• Contour Irrigation

Contour farming is the practice of carrying out field operations, such as ploughing, planting and cultivating land across the slope. Contour farming is practiced in hilly areas. The farm area is divided into contours. The irrigation water is stored in the depressions between the bunds.

 

Sub-Surface Irrigation

The sub-surface irrigation method consists of supplying water directly to the root zone of the plants. Water is supplied to a series of ditches, half to one meter deep and 25 to 50 cm wide with vertical sides. These ditches are located 50 to 100 cm apart. Water flows at a slow rate and seeps into the ground. The sub-surface irrigation method is suitable for impervious sub-surface at reasonable depth or high water table and for permeable soils in the root zone. Uniform topographic conditions, moderate slopes, and good quality irrigation water are required for sub-surface irrigation. Precautions should also be taken to prevent salt accumulation or waterlogging. This method is preferred because of the economical use of water and low labor cost in the land preparation for irrigation.

 

Pressurized Irrigation

This method includes sprinkle and drip systems of water application. In the sprinkler method, water is sprayed over the crop through the nozzle. The method is useful where soil topography is uneven. Soluble plant nutrients can also be applied along with sprinkler irrigation.

Drip irrigation is also called trickle irrigation is the method of applying water slowly and directly to the root zone of the crop plants. Due to pressure, water oozes out from the emitters and trickle into the root area of plants. This system of water application was originally developed in Israel by Simca Blass, a hydraulic engineer in 1959. We may also apply plant nutrients along with water directly to the root zone. It saves about 70% of water in comparison to the surface method of water application.

There are many more methods of water application such as pitcher method, sub-soil injection, suction irrigation, etc. These methods are location specific, hence, not practiced widely.