<P> Humidity is slightly more difficult to control than temperature . The more commonly used houseplants have established that they can survive in low humidity environments as long as their roots are kept properly irrigated . Most plants thrive in 80% relative humidity while most homes are usually kept around 20% to 60% relative humidity . Besides buying a humidifier, there are a few things that can be done to increase humidity around houseplants . The most popular methods used to raise the ambient humidity are misting and pebble trays, which are shallow trays covered with pebbles and filled with water that evaporates to increase humidity . Other methods of raising humidity include grouping plants closely together and not placing plants in drafty areas . Misting has become controversial among gardeners and horticulturists, some of whom will point out that it seldom provides moisture in the form of water vapor, and that, instead, by wetting the leaves it increases disease problems . </P> <P> In a potted environment, soil nutrients can eventually deplete . Adding fertilizer can artificially provide these nutrients . However, adding unnecessary fertilizer can be harmful to the plant . Because of this, careful consideration must be taken before fertilizing . If a plant has been in the same potting mix for a year or more and is no longer thriving, then it may be a candidate for nutrient replacement done by using a complete fertilizer at half the recommended label dilution rate . </P> <P> Fertilizers are usually marked with a number such as 20--20--20 . These numbers indicate the percentages of nitrogen, phosphorus, and potassium respectively, the three elements that are needed in the most quantity for plant growth . Nitrogen is essential for green, leafy growth . Phosphorus is essential for flowering or fruiting plants . Potassium is essential for strong roots and increased nutrient uptake . Numbers higher than 15 are usually man - made, chemical fertilizers . Organic fertilizers have a much lower ratio . A 4--2--2 ratio of these elements is usually good for green foliage plants, while a 2--6--4 ratio is usually better for flowering plants . A complete fertilizer will also include the minor and trace elements, such as calcium, magnesium and iron . </P> <P> While variation may occur between brands, a general rule is to mix 1 tablespoon to every four liters (one imperial gallon) of water . In all cases, it is better to under - fertilize than over-fertilize . The diluted mixture is then used to water the plants . The growth of the plants should be monitored to determine if the fertilizer is helping or harming, and how often (if at all) it should be used . Schedules can range from every other week to every three months . For convenience, granular, time - released fertilizers are also available . </P>

Why do not the leaves of these plants rot in water