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Relative Humidity Learn More About Relative Humidity
Relative Humidity
- What is the relative humidity and how it affects your plants

Is my crop properly absorbing nutrients from the soil? One of the environmental factors that can directly affect the fertilization of your plants is relative humidity. Why do we measure relative instead of absolute humidity? What problems can I detect on my crops? And more importantly, how can I control it? All the main doubts you may have in this regard are solved in this article.

- Why do we use the term “relative”?

Since early times, the influence of the moisture content in the air on the plants' growth has been observed. Originally only the actual amount of water vapour in the air was measured, obtaining the absolute humidity value. It is expressed as grams of vapour per cubic meter of air (g/m3) and does not take into account any other factor. Due to this, another descriptor was defined and nowadays is the most commonly used. It is the relative humidity (RH), expressed in percentage, that defines the ratio of the water vapour to the total amount that can be held at saturation at a given temperature and atmospheric pressure. However, the temperature affects directly the amount of water that can be dissolved in the air. This calculation makes a big difference since cold air can hold far less moisture than warm air, which would give a higher value of HR than warm air.

Related to humidity in agriculture, there is another term that you should know which is more accurate in determining the plant’s water loss. It is the de vapour pressure deficit (VPD) and expresses the difference between the vapour pressure inside the leave compared to the vapour pressure of the air. It affects directly the opening and closing of the structures encharged of regulating the gas exchange during respiration, called stomates. If the DPV drops, the stomates will close reducing the plant water consumption and consequently the absorption of the nutrients. That is why is crucial for your plants to grow healthy the combination of the best fertilization program, that you can find in the i-Plant Nutrition, with an accurate watering according to the environmental conditions.

- How does the relative humidity affect your crops?

Especially in extreme situations, you would see dramatic effects on your crops that may vary between species and plant stages.

When the RH is insufficient, besides wilting, the most common observation is that plants take longer to reach a suitable size for sale. It produces a loss of fruit quality and less fruit set. The crops growth is compromised together with the fall off of the lower leaves. Observe the leaves because they could present a smaller size and curly edged. The leaves also should present dry and burned tips. In addition, when the RH is too low, the chances of suffering spider mite infestation increases.

On the other hand, when the RH is excessive the crops lose quality and it would cause a reduction in your profits. It leads to weak plant growth and favours the infection of pathogens responsible for leaf and root diseases. The excess humidity also reduces perspiration and the absorption of nutrients. Besides, it can cause oedema (a type of abnormal water retention) and reduce successful pollinations.

To measure the relative humidity in your cultures, the conventional hygrometer indicates an approximation of the amount of moisture in the air expressed in percentage. Although when more precise information is required, there are more sophisticated instruments. A psychrometer provides an accurate calculation based on two thermometers (one exposed to ambient air and another whose bulb is permanently humid).

- Give your plants the humidity they need

If you detect abnormal RH in your crops, you can correct the situation in favour of your plants. When the air in your field or greenhouse is too hot and dry you may need to increase the relative humidity. In this case, keep in mind that your greatest precaution will be to prevent the water from being puddled on the ground or condensed on the leaves. In a glasshouse, the simplest and most economical way is to place damp sponges or towels in the grow room. However, the use of humidifiers is more accurate, with evaporation devices such as nebulizers, mist units or sprinklers in both indoors and outdoors cultures. These devices add water vapour to the air to cool it down and reduce the DPV we referred to earlier. You will notice a special improvement in the health of your crops, especially when combined with the personalized fertilization program you can find in i-Plant Nutrition. Remember that increasing the RH should reduce the frequency of irrigation, especially in greenhouses.

On the contrary, if the air retains too much humidity, you need to reduce the relative humidity. For this, proper ventilation is recommended in outdoor crops, being very effective in the use of airflow devices that not only ventilate but also increase the air temperature. However, also for open fields, exists dehumidifiers devices that perform this function. If your culture is in a glasshouse, you can find in the market highly sophisticated systems, for example, the use of infrared radiation to reduce condensation. This method increases the temperature of hard surfaces, such as plant and substrate surfaces, but not air, so that there is less temperature variation between the indoor and outdoor air. Other systems, like the use of fans and heat shields, avoid sudden temperature fluctuations and reduce condensation.

We strongly recommend having a precise tracing of the environmental conditions, to adjust them according to the requirements of your crops for higher productivity. Proper fertilization along with favourable environmental conditions ensure great success and healthy development of your plants.
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