How Much Water?
Once a schedule (see the last post) is in place that allows you to maintain better constancy in your soil moisture, you can now tailor the amount of water you apply to different plant species. There is no plant I know of that does not appreciate water, unless it is in dormancy. The issue is with the amount of water, and thus the amount of air, that is in the soil at any given time. Most cacti, for example, are evolved for long periods of drought, but this doesn’t mean that this is their preference. So often certain plants are described as ‘needing to dry out between watering’, but the truth is that this is not the case: rather, the plant needs to be prevented from ever becoming too moist for too long. Maintaining a very light but consistent level of soil moisture will allow xerophytic plants like cacti to maximize their growth without causing damage to the roots. This requires something of a steady hand in order to not give your plants too much water, and adapting your soil for different species can help a lot to get things perfect (in the case of cacti and other succulent species, I recommend adding something with a large particle size that improves the aeration porosity of the soil). It is worth noting, though, that with careful watering alone, you should be able to keep widely different species in a standard peat/perlite mix.
Knowing which species you are caring for is the first part of knowing how much water a given plant will need. There are countless resources online and elsewhere that recommend care for the majority of species in circulation. If you’re more inclined to nerdiness like me, you’ll also probably look into where the plant originates in order to learn more about its natural habitat, and take cues from this towards the plant’s care. (I will write more on this at some point; I promise.) All this, though, needs to be tempered with your own experience of the plant and how it grows in your environment, and this requires that you pay attention to what your plants are doing, and how their environment changes throughout the year. Some species are known to rest a little through the winter, for instance, and typically these do not require much moisture at that time. Conversely, the air generally becomes much drier indoors during the winter, when we run our furnaces, which may contribute to the plants’ increased respiration and thus need for additional water. Your growing situation is unique, and it is up to you to find what works best.
Some plants prefer a moist soil, and yet do not take up too much of the water, so after obtaining the correct level of moisture, one can apply small amounts to compensate for the plant’s uptake and any loss through evaporation. One the other hand, some plants which prefer moist soil use a tremendous amount of the available water, and so will need to be watered heavily on the regular. Plants receiving more light will use more water than those in shadier spots (and will require more feeding, but this is best left for another time). Larger plants will obviously use more water than smaller ones, though larger plants are often more drought-tolerant than smaller ones.
A quick note on watering until water flows from the bottom of the pot: unless your water is of very poor quality or you are feeding large amounts of fertilizer, this practice is not really necessary. It is a good way to ensure that your soil becomes evenly moist (read saturated), and it is definitely helpful to have drainage in the event that you do overwater, but it is also a waste of water. In our industry we keep plants for many, many years in containers which do not drain, and they fare quite well. Another aspect to this is that if soil is completely dry, often the applied water will travel straight through it and out the bottom of the pot without wetting anything along the way, which can give a false sense of accomplishment if all we`re looking for is water pouring out from the bottom. If your soil is already a tiny bit moist, simply water carefully (read slowly) until you bring the soil to where you want it, moisture-wise, and you won’t need to waste any water.
Quantifying (sort of) Soil Moisture
So I just spent a whole bunch of paragraphs explaining that different plants prefer different levels of soil moisture, often at different times. But what does barely moist or nearly wet actually look and feel like?
Enter the squeeze test. In most quality peat-based media (not the off-the-shelf stuff), the soil will dry to a conspicuous pale tan colour. If the soil is completely dry, take a handful and squeeze it and it will not hold together. A slightly moist soil will be slightly darker and will mostly hold its shape if you squeeze it as above. Moderately moist will be darker still, and should hold together and even release a tiny bit of moisture if squeezed. Moist and saturated soil will be as dark as the soil can get, with some and quite a bit of water, respectively, released upon squeezing.
This squeeze test is a good way to familiarize yourself with how much water is in the soil in relation to its appearance. Once you get a feel for it (ha ha) you will be better able to eyeball the moisture level of the soil more accurately, though relying on appearance alone can sometimes be deceiving (more on this in the third part).
If a plant wants to stay moist but not wet (and many do, but please don`t accept the care instructions that come with purchased plants as gospel, for there are quite a few variables I've already mentioned [and more that I haven`t] that can affect how much water a plant uses), keeping it between moderately moist and moist will likely keep it happy. Keeping a soil barely moist, as I mentioned above, is a great way to optimize the soil environment for succulents or similar plants. And then there's all of the in-between.
You will need to make your best judgement as you work with soil moisture levels based on your growing environment and the species you're working with. Don't be afraid to experiment, but make your changes incremental as you pay close attention to the plant in order to draw useful conclusions and avoid issues.
One more thing I think I should note before I leave off: you'll notice that I keep (almost exhaustingly, probably) using the word soil, and I should clarify that when I say this I am referring to whatever you've got your plants growing in (which I hope is a high-quality peat-based growing medium), and not that heavy mineral stuff from the back yard. These high-quality growing media are referred to as soilless mixes for the fact that they do not actually contain proper soil, but for our purposes here the term soil will be used (in part because it's shorter and I seem to be typing it an awful lot).
I'm going to follow this post with one on Ways to Tell if a Plant Needs Water next week.