Plants need water. It's a simple enough statement, and if they didn’t get it in the amounts that they need, they would not have evolved into the wide array of flora that exist today.
Managing cultivated plants so that they receive just the right amount of water when they need it is as much an art as it is a science. Growers have struggled with this for centuries and will continue to do so. Every situation is unique and no two years provide the same amount of precipitation or temperatures.
Beyond providing plants with the water they need, controlling the rate at which it leaches through the soil or drains away is critical for successful growing.
Other than selecting plants based upon their moisture needs and native habitats, there are a few factors that can be controlled, but first let’s look at what contributes to drainage.
Probably the most important attribute of the soil and its ability to drain or retain moisture is what it is comprised of. Soils the world over are comprised of various amounts of rock or gravel, sand, silt, and clay.
In general, and there are other factors that affect the drainage, some of which are described below, soils with a high gravel content drain faster than sandy soils, which drain faster than silty soils, which drain faster than soils high in clay.
It is rare when a soil is purely one type or another; rather almost all soils are a blend of all of the above mentioned types, with higher or lower concentrations of the individual parts. Understanding which soil type you have is the first step in understanding drainage in your particular situation.
Unless your soil is sterile, it is also home to all manner of life and nutrients. Healthy soils have organic matter, which is sometimes referred to as humus. It is derived from decaying plant and animal matter and add nutrients that are available to the soil.
The origin and the amount of organic material in the soil however do also affect the drainage. If the organic matter is comprised of densely packed roots, the soil will not drain well. If it is the result of decaying leaves and insects, it should not negatively impact the drainage. However, too much of anything is detrimental.
Beyond organic matter, your soil should be teeming with worms, beneficial fungi, healthy bacteria, and microbiotic species too numerous to mention.
If all of these exist in healthy ratios, they will create natural aeration throughout your plant’s root system and aid in proper drainage. A lack of these due to poor drainage does not bode well for the survival of most plant species.
This is not relevant if you are growing in a field or garden, but for the millions of people that do garden and grow plants in containers it becomes highly relevant.
Plastic containers dominate the container market and without drainage holes do not allow any water to escape regardless of soil type. Wood containers without drainage are not much better, unless they are constructed with slats or in a way that excess water can escape.
Clay containers, as long as they do not have an additional sealer or glaze on them, will allow for excess moisture to weep or ooze out through its pores.
Without supplemental drainage holes, however, it may not do so quickly enough to be of benefit to the plant. In general, any container that does not have allowances for excess moisture to escape through the bottom is not conducive for proper drainage.
What to do When Your Soil Doesn't Have Enough Drainage
Plants that don’t like to be too wet will let you know. Their leaves will often turn yellow and the entire plant may begin to collapse. Too much moisture can also invite fungal pathogens which will in turn wreak havoc on your plants.
If you are growing under cover and are providing all of the plants water, then reduce the amount of water, first by half and then monitor closely to see if the soil is sufficiently draining and adjust accordingly. If growing outside, there are other factors that you can change if you can’t control the amount of water your plants are getting. For example, soil aeration.
If you are growing in containers, the fix is relatively easy. First identify if there are proper drainage holes on the bottom and if not, grab a drill and add them yourself.
You can remove the plants and soil and add gravel, bricks, broken pottery pieces or anything that will take up space in the bottom of the container and allow for water to pass through. Then if possible, replace the media with a more porous one, or add perlite to the mix.
If growing in soil, the fix is not so easy. Where possible, aerate with a shovel, tiller or gas powered aerator around the plants, being careful not to disturb the roots if possible.
Work in as much gravel or other drainage material as you are able to. Other solutions include starting over and building raised garden beds that tend to drain better or digging drainage ditches with gravel and drainage pipe alongside your garden beds.
What to do When Your Soil Has Too Much Drainage
This is a preferable situation than having not enough drainage; it is almost always easier to add water than to take it away. The solutions to combat too much drainage are essentially the reverse of not enough.
For container-grown plants, see if there are too many drainage ports and try to fill or patch some of them if water is running out too quickly.
If the problem is seemingly with the media, you can add any number of amendments such as vermiculite, peat moss, coir, or any organic matter that will absorb and trap moisture to keep your plant’s root wetter, longer
For garden or field raised plants, the solution if not to add additional irrigation, lay in how they are planted. As opposed to raised beds, sunken beds may be the solution. Just as in container media, incorporating moisture-holding amendments can be a solution so long as you can do so either between crops or in such a way as to not disturb root systems.