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Vapour Pressure Deficit (VPD) — A Powerful Weapon for Smart Farming

Vapor Pressure Deficit (VPD) is a function of temperature and humidity which represents the combined effect of both factors. It is a powerful weapon to drive the plant’s growth and development. VPD shows how dry the atmosphere is. It measures the pressure created by water vapor in the atmosphere. If the volume of water vapor is less, then it is more deficit, the atmosphere sucks more water from the surrounding. The surrounding can be an open farm or closed structures like polyhouse and greenhouse. The VPD values lie in the range of 0–10 kPa or more based on the environment.

Vapour Pressure Deficit

The above picture shows a different range of VPD. The Red shadow represents the VPD range where the plant does not grow at an optimum level when such a condition exists for a longer time. The Green shadow represents the most favorable range of VPD for crop growth while yellow-shadow shows plant can be in slightly stress if this range exists for a longer time.

Transpiration is the most important physiological phenomenon of plant that drives all the physiological and morphological changes of plants. It is simply the loss of water from stomata. Transpiration drives the uptake of water and nutrients from the soil. The transpiration is regulated by a number of factors like stomata density, pore size, soil water availability, plant hydraulic capacity, and environmental stress temperature, humidity, sunlight, and VPD. So here, we are focusing on the VPD which drives the transpiration and influence growth and development of plants.

VPD Threshold

0.7–2.5 kPa is the optimum VPD range for the growth of most of the crops which drive the transpiration, photosynthesis, and uptake of water and nutrients. VPD below 0.5 kPa shows that there is sufficient humidity in the atmosphere. When there is sufficient humidity in the atmosphere, then the atmosphere exerts lesser pressure on the leaves, which reduces the transpiration and in the end, it reduces the uptake of water and nutrients. VPD above 3.5 kPa can be lethal for plants. It stops the stomatal opening and reduces transpiration and photosynthesis. There is a certain limit above which stomatal opening completely stops and plants show wilting symptoms. This mostly happens during the midday in summer. At this stage, plants are in severe stress and cannot uptake nutrients or water. The longer duration of this period leads to slow growth, lower biomass, flower dropping, fruit aborting and shriveling.

Fasal VPD Graph
Fasal VPD Graph

We know that transpiration and photosynthesis happen during day time which drives the water and nutrient uptake. So VPD during the day can be important for daily operations. But VPD value varies every hour in the day which can be more complicated to draw the inference from hourly VPD. So to avoid this complexity, Fasal has designed the concept of daily hourly sum VPD. For this, Fasal uses VPD data from sunrise to sunset which is more important than night time VPD data. Based on VPD, Fasal provides the management practices of crops to its farmers.

VPD Based Crop Management

Low VPD — When VPD is low, the stress in the environment is not enough to trigger the optimum transpiration and uptake from roots. In this condition, there is enough moisture in the atmosphere which hinder the transpiration. This leads to the persistence of water droplets in the leaf and increases the leaf wetness. Leaf wetness is responsible for different fungal and bacterial diseases in crops. In closed structures, we can manipulate the temperature and humidity to bring optimum VPD based on requirements while it is not possible in open cultivation. During low VPD, the irrigated water and nutrients remain in the root zone and there is a high chance of leaching out of water and nutrients and repeated fertigation causes accumulation of nutrients in the soil. This can increase soil EC which ultimately causes mineral toxicity and imbalance in nutrients uptake. Thus, it is not a good decision to do irrigation and fertigation in such conditions. The overall efficiency of water and fertilizers will below.

High VP — When VPD is high, the water vapor level in the atmosphere is low which creates more stress in the atmosphere. Till a certain limit, the high VPD drives the transpiration but after certain limits (3.5 kPa), plants close their stomata partially and reduce the transpiration and photosynthesis. In the end, the uptake of water and nutrients from the root zone is reduced. This causes wilting in the plants and damages the flowers and fruits. Plants show stress symptoms. The growth of plants slows down and flowers drop down and fruits get abort and shiver. In this condition, the uptake of water and nutrients from the root zone increases at first then it reduces and stops. So nutrients won’t be uptake from the root and it will accumulate in the soil or leach out or fixes into some stable compounds which are not accessible for plants. This fertigation is not recommended at these conditions.

Optimum VPD — Optimum VPD is required for the proper growth of plants and better flowers and fruit set. The applied irrigation and fertigation will be taken by plants timely and more efficiently.

Disease and Pest Management — The fungal spores and mycelia are the causal agents of many fungal diseases that cause severe crop loss and reduce the quality and quantity of fruits and vegetables. These fungal spores exist in a moist environment. Low VPD range means the environment is moist and favorable for spore germination, survival and spread. Thus, if we can manipulate the environment towards a higher VPD range where spores get dried, then there will be a low risk of diseases. In open cultivation, based on the VPD range, we can calculate the risk of the disease and based on that we can design the more precise spray schedule to control the diseases timely and effectively with lesser number sprays. Even for pests like thrips, mites, and aphids, Fasal calculates the risk and manage the smart spray schedule.

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