Water Management in Horticultural Crops (1+1)

Water management for problem soils

a. Water logged soils

• Water logging is caused in a location when the inflow of water into it exceeds the outflow resulting in progressive rise of water table. The inflow may be due to excessive and high intensity rainfall, seepage from canals, reservoirs, flood and over-irrigation. The outflow declines with impaired drainage, lack of adequate drainage, rise of water table owing to construction of reservoirs and rise in water level in rivers.
Signs of bad drainage

• There may be a number of indications by which a land can be identified as badly drained land. Soil is very soft and wet. It sticks to farm implements and tools and feet of animals and shoes of farm labourers. Occurrence of spots or pools of free water and may be flowing out of the field from side of ditches or over the soil surface. Presence of good growth of bright green grasses or weeds in some places and aquatic and water loving plants are seen growing. When crops are sown, seedlings grow slowly. Many seeds may not germinate as there is excess water in the soil. Plants look usually yellowish or pale colour and unhealthy and are stunted in growth.
Effects of water logging and excess soil water on crops and soils

• Water logging condition and presence of excess water in soil have various harmful effects on crops, soils and farm animals. The first and foremost effect is on the aeration of soil, which is essential for plants to carry on various vital activities. Root growth, availability of nutrients and their uptake, escape of carbon dioxide and other harmful gases produced in the soil, optimal activity of useful bacteria do not take place properly.
Methods of drainage

• Two methods of land drainage are adopted and they are: (i) surface drainage and (ii) subsurface drainage. Besides, pumps may be used to drain out water from lower lands and to lower the high water table affecting crop growth.
Surface drainage

• Surface drainage consists of disposal of surplus water by gravity flow from accumulating on the land surface and getting into the soil profile raising the ground water table to a problematic level.
Component drains of surface drainage system
• A drainage system consists of main, sub mains, laterals and field drains for effectively disposing drain water. Besides, there is an outlet at the end of the main drain which is located outside the farm. Outlets are often provided with covers that prevent cattle getting into the farm.
Subsurface Drains

• These drains are laid below the soil surface and are covered. They do not interfere with normal movement of farm implements and cultivation practices and no area is wasted for constructing drains. Different types of materials are used for construction of subsurface drains. These may be short clay, concrete or plastic pipes, fibrous wood materials, covered stone drains and bituminous fibrous materials.
Various other types of drainage devices

• Setting up various other drainage devices can lower water table in an area. They may be mole drain bamboo or wooden pole drain, stone drain and drainage wells.
Mole drains

• This type of drain is a continuous round passage at a depth of 75 cm ,10-12 cm in diameter and spaced 4-5 metres apart in the soil profile to drain water from the crop field in a grade of 0.05 to 0.10%. They are made with a mole plough. The suitable soil type is clay and may last for two to three years.
Bamboo or pole drains

• A cheap way of making temporary underground drains is by using bamboo or wooden poles in the shape of triangles with 60-90 cm deep and 30-40 cm wide. It is enveloped with leaves, small twigs and pebbles and then covered with soil and connected to the main drain.
Stone drains

• Small drains at 30-40 cm depths are dug and stone pieces are laid in a fashion to construct a continuous rectangular channel. Drains are covered with leaves, twigs and small pebbles and soil at the top allowing regular farming practices. The drains are more durable than bamboo or pole drains.
Drainage by wells and pumps

• Construction of wells and then draining water can lower high water table. The well may be gravity wells located in an unconfined aquifer to remove water directly from the crop root zone, or well to tap an aquifer containing water under pressure.
• Pumps can be successfully used to drain out accumulated water in lower lands or to lower the water table by pumping water out to another area or to a natural drainage passage. This water may be used for irrigation to crops in the surrounding area, if the quality of water is good or safe for use in crops.
b. Salinity

• A soil may be rich in salts because the parent rock from which it was formed contains salts. Sea water is another source of salts in low-lying areas along the coast. A very common source of salts in irrigated soils is the irrigation water itself. Most irrigation waters contain some salts.
• After irrigation, the water added to the soil is used by the crop or evaporates directly from the moist soil. The salt, however, is left behind in the soil. If not removed, it accumulates in the soil; this process is called salinization. Very salty soils are sometimes recognizable by a white layer of dry salt on the soil surface. Salty groundwater may also contribute to salinization.
i. Water Salinity

• Water salinity is the amount of salt contained in the water. It is also called the “salt concentration” and may be expressed in grams of salt per litre of water (grams/litre or g/l), or in milligrams per litre (which is the same as parts per million, ppm) .However, the salinity of both water and soil is easily measured by means of an electrical device. It is then expressed in terms of electrical conductivity: millimhos/cm or micromhos/cm. A salt concentration of 1 gram per litre is about 1.5 millmhos/cm. Thus a concentration of 3 grams per litre will be about the same as 4.5 milliomhos/cm.
Irrigation water quality

• The suitability of water for irrigation depends on the amount and the type of salt the irrigation water contains. The higher the salt concentration of the irrigation water, the greater the risk of salinization.
  
The following table gives an idea of the risk of salinzation


Salt concentration of the irrigation water in g/l	Soil salinization risk	Restriction on use
Less than 0.5 g/l	No risk	No restriction on its use
0.5-2 g/l	Slight to moderate risk	Should be used with appropriate water management practices. Not generally advised for use unless consulted with specialists.
More than 2 g/l	High risk


• Higher the concentration of sodium present in the irrigation water (particularly compared to other soils), the higher the risk.


ii.Soil Salinity

• The salt concentration in the water extracted from a saturated soil (called saturation extract) defines the salinity of the soil. If this water contains less than 3 grams of salt per litre, the soil is said to be non saline. If the salt concentration of the saturation extract contains more than 12 g/l, the soil is said to be highly saline.


Salt concentration of the soil in g/l	water (saturation extract) in millimhos/cm	Salinity
0 -3	0-4.5	Non saline
3 -6	4.5-9.0	Slightly saline
6 -12	9.0-18.0	Medium saline
more than 12	more than 18	Highly saline

Crops and saline soils

• Most crops do not grow well on soils that contain salts.One reason is that salt causes a reduction in the rate and amount of water that the plant roots can take up from the soil. Also, some salts are toxic to plants when present in high concentration.
• Some plants are more tolerant to a high salt concentration than others.
  
Some examples are given in the following table

Highly tolerant	Moderately tolerant	Sensitive
Date palm	Tomato	Peas
Sugarbeet	Potatoes	Beans
Asparagus	Carrot	Pear
Spinach	Onion	Apple
	Cucumber	Orange
	Pomegranate	Prune
	Fig	Almond
	Olive	Apricot
	Grape	Peach


• The highly tolerant crops can withstand a salt concentration of the saturation extract up to 10 g/l. The moderately tolerant crops can withstand salt concentration up to 5 g/l. The limit of the sensitive group is about 2.5 g/l.
Improvement of saline Soils

• Improvement of saline soils implies the reduction of the salt concentration of the soil to a level that is not harmful to the crops.
• More water is applied to the field than is required for crop growth. This additional water infiltrates into the soil and percolates through the root zone. During percolation, it takes up part of the salts in the soil and takes these along to deeper soil layers, the water washes the salts out of the root zone. This washing process is called leaching. The additional water required for leaching must be removed from the root zone by means of a subsurface drainage system.
Prevention of salinization

• Soils will become salty if salts are allowed to accumulate. Proper irrigation management and adequate drainage are not only important measures for the improvement of salty soils, they are also essential for the prevention of salinization.
c. Soil Sodicity

• Salty soils usually contain several types of salt. One of these is sodium salt. Where the concentration of sodium salts is high relative to other types of salt, a sodic soil may develop. Sodic soils are characterized by a poor soil structure and they have a low infiltration rate. They are poorly aerated and difficult to cultivate. Thus, sodic soils adversely affect plant’s growth.
Improvement of sodic Soils

• Improvement of sodic soils implies the reduction of the amount of sodium present in the soil. This is done in two stages. Firstly, chemicals (such as gypsum), which are rich in calcium, are mixed with the soil; secondly the calcium replaced sodium is leached from the root zone by irrigation water.
Irrigation management and drainage

• Irrigation systems are never fully efficient. Some water is always lost in canals and on the farmers’ fields. Part of these seeps into the soil. While this will help leach salt out of the root zone, it will also contribute to a rise of the water table: a high water table is risky because it may cause the salts to return to the root zone. Therefore, both the water losses and the water table must be strictly controlled. This requires careful management of the irrigation system and a good subsurface drainage system.
Management of poor quality waters

• The poor quality water when used continuously the following management points are considered for a sustained crop production.
1. Dilution with good quality water
2. Flooding with good quality water once or twice to flush out salts beyond root zone
3. Gypsum mixing with water to reduce sodium hazards and also to improve soil structure
4. Providing drainage to remove salts
5. Using poor quality water in sandy soils
6. Growing salt tolerant crops
7. Adopting drip irrigation method for poor quality water