Air humidity

This climate factor has a strong influence on the activities of the plant. The trans-evaporation is part of the ‘pump system’ which requiresplants to take up water and nutrients and distribute this through all parts. At high humidity and low air movement levels the water vapor released by the plants cannot disperse and hence the transpiration is reduced. With low humidity level (and high temperature) the transpiration is very fast and the plant will close the stomata to avoid excessive water loss/dehydration. Correct air humidity will allow the plant to keep open the stomata, take up appropriate water and nutrients, cool itself and take up CO2 for the photosynthesis. Plants that are for a long period in conditions of high temperature and low humidity, may go in to stress, resulting in stunted growth, deficiency and susceptibility to diseases. Low temperature and high humidity reduces growth rate and may result in development of fungal diseases.
To be able to control air humidity, first of all we need reliable hygrometers. The better the quality of these tools, the more reliable they will be, without too frequent equilibrating. Somehow the simple wet/dry bulb thermometers are rather accurate, using the table for interpretation. Having two of them they can be checked against each other and equilibration is not required.
Once we know the level of humidity we have to take action when it is out of range.

Humidification systems
The most simple and old fashioned way to increase moisture is by sprinkling water with a hand hose. This can be done very precisely depending on the worker. Open spaces, like pathways, in the green house are made moist; the quantity of water is regulated by discharge of water and speed of walking. Plants can be kept dry and if done well, there are no dry or wet places in the green house. Wherever it is dry, more water is added, less in moist places.
With the immobile systems (fixed outlets) the discharge points will become wetter than the areas in between the discharge points, unless the density of the emitters is very high. For systems on soil level, it is recommended to place the misters/foggers/mini-sprinklers in the open spaces (like pathways, sides of the beds) and not between the plants. Within the crop, the humidity is maintained by the trans-evaporation of the plants themselves. On soil level the discharge of the emitters may be vertical, blowing the mist in the air or circular horizontal (umbrella type), covering a certain area. The drip size is not very important, since time is too short for evaporation in the air. A low discharge is relevant to avoid adding too much water to the soil and become additional irrigation (without fertilizer!). 10 to 15 liter per hour would be a suitable discharge; even with frequent use this is not more than 100 ml/m2, since most of the water will evaporate from the soil.
Distribution of water in the green house is better in the elevated system, the higher the better, say 4 m above the soil. The cooling effect is much higher with the aerial system, because water is evaporating in the air instead of settling on plants and soil. The system is operated very shortly (seconds) and very frequent (10 to 12 times per hour); this has the best effect and results in a stable humidity and also a cooling effect. All ways of increasing humidity in the green house should be stopped after 3 pm, so the green house can dry up, avoiding high humidity build up during the night.
In case of airflows in the green house the moisture will drift to one side and leave certain parts dry.
It is recommended to have more emitters at the periphery of the green house, where humidity is lowest due to side vents. Emitters shall be provided with an anti-‘after dripping’ device to avoid wet spots in the crop.
The quality of the system is demonstrated by the drip size and avoidance of clogging of the emitters. High speed of the water drops at discharge points reduces the size. That speed is created by high pressure, but also by an internal channeling system that increases the speed of water flow in the emitters, sometimes even with relatively low pressure. High water speed also has self cleaning effect on the emitters, however good quality of water and a good filter system is a must to avoid obstructions. Good quality water is low in suspended particles and other impurities. Water from a reversed osmosis system is superb, but very expensive.
Metal emitters will have a long lifespan and have a largely hassle-free performance. Obviously using metal material allows working with higher pressure than the systems assembled with pvc or polythelene. The high-pressure systems are used in poultry and cattle stables. Because of large extensions, the cheaper systems are used in green houses.
Another mist or fog system, semi-static, with the same characteristics as overhead systems, is the mist blowing fans.  With the swiveling movement of the fan a large area can be covered (outblow up to 40 meters). The fans can be operated for longer periods of time and because of mobility, distribution is selective.
Most sophisticated in green house air humidification is the pad and fan system. By pulling the air through the pads, water is added and drawn through the crop. Even more high-tech humidity control is in place in some glass green houses in Europe and other places. These are not workable for the semi open green houses, we work with in the sub-tropics.

Maintenance of all systems is an important aspect of good performance. Regular checking and unblocking of emitters, cleaning of filters, repairing leakages (also in hand hoses), etc. is a necessary practice. Further, when wet or dry spots are seen, adjustments have to be made.

Reduction of humidity.
Excessive high humidity as encountered during certain periods of the year,at night and especially early morning, is more difficult to manage than low humidity. The only way to reduce humidity really is by heating by which the air expands and becomes relatively drier. In semi-open green houses this is economically not feasible.The other, less perfect method is by air movement. There is a difference between ventilation and circulation.
By ventilation we make use of the vents in the green house, top and sides or sometimes forced ventilation by fans and extractors. When the outside air is less humid or is warmer this can be brought into the green house, sometimes supported by fans for further distribution. Even if the outside air is as humid as the inside air it still helps in the circulation of air.
Air circulation mixes the micro climates that exist in green houses, not only different humidity levels but also small differences in temperature. In general this will avoid a humidity level of 100%. In the crop the humidity builds up to high levels because of the trans-evaporation. By circulation this accumulation can be removed.6-8 fans per acre will provide the necessary ventilation in a closed green house.An airflow of 0.5 to 1.0 mtr/second is sufficient.

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