In soil gardens, which can easily get either too wet or too dry, how much and how often you water your garden is vital to plant vigor and growth. Although several things affect a soil garden’s required watering frequency and cycles, there are some basic guidelines that can help direct a gardener towards success, starting with the parameters affecting typical watering recommendations.
Air temperature – Air temperature has a big impact on the amount of water plants transpire. Transpiration is the amount of moisture a plant releases into the air and needs to replace. There are tiny openings in plant leaves called stomata that allow oxygen and carbon dioxide to enter the plant and water vapor to exit. This essential process is the main reason plants run out of water or moisture. Air temperature and direct radiation play a large role in this process.
Soil temperature – Soil temperature is another big factor that affects water demand, primarily through increased levels of evaporation. Ambient air temperature is generally what establishes soil temperature, but artificial and natural light will warm the soil as well. Shaded soil will stay cooler. Being aware of this factor is key to understanding how your watering schedule may need to be adjusted.
Retention rates and more – You also need to get to know the water retention characteristics of your soil, the surface area of that soil, and any thermal control there might be for the soil. The transpiration rate of the specific plants you are growing and the size of your containers are also key in determining how often plants will require water.
A good place to start is by taking a look at a few typical situations and roughing out a general schedule. Here are some general watering frequency recommendations for various stages of plant growth:
- New transplant – Daily, being sure to avoid overly wet soil
- Root development (first 10-20 days after transplant) – 2-3 times a week, allowing for adequate surface drying to encourage deeper roots
- Vegetative growth – Every other day
- Fruiting – Daily to every other day
- Mature vegetative – Daily to every other day
These suggestions are intended only as a reference point. Evapotranspiration—the combination of evaporation and plant transpiration—will be the sole factor for determining watering frequency while your crop is growing. The watering frequency will change considerably throughout your crop’s life cycle, whereas the amount of water (minutes on the timer) for each watering will not.
Know Your Soil
A key part of maintaining a proper watering schedule is knowing your soil. This means determining how much water your soil holds when it is fully saturated. Any more water added past this point will simply drain off, and any less will produce poor root growth at the bottom of your containers.
Determining how much water your soil holds when it is fully saturated is easy. Weigh a sample of your soil when it is completely dry, and then determine the empty weight of the container you are going to put it in.
Fill that container with the dry soil and slowly water it until the water starts to run off. Wait half an hour and water again. After thoroughly draining (another half an hour or so), weigh the container with its water-saturated soil, and then do the math to find out how much water it is capable of holding, while factoring in the weight of the empty container. This is known as the soil weight capacity. As a point of reference, 1 pt. of water weighs about 1 lb.
The goal is to keep your container’s soil water content equal to or greater than one-third of the soil water capacity for most plant types. Drought-tolerant plants can certainly go lower than this without stress, but not vegetables. Knowing how long your soil will retain appropriate levels of moisture with only evaporation as a factor is crucial.
The ratio of container volume to surface area will affect the length of time your soil takes to dry out. Small, flat containers will dry faster than large, deep containers. To keep things simple when weighing your soil, use the same size test container you plan to grow in.
Using the practical weight examination mentioned earlier is a good way to predict how long the soil retains water when only evaporation is a factor. Simply control the temperature, wait 24-48 hours, and weigh the sample again.
Using the same formula to find out the remaining water capacity will give you an idea about the maximum interval for watering. The surface of the soil should have dried by 50% or so, allowing adequate air into the roots for respiration. The bottom of the soil should not have dried this much. Once you have checked and logged the moisture levels, you can use these parameters to time future waterings.
There are many moisture-measuring devices available to help keep an eye on your plants. Their effectiveness is somewhat affected by soil type and the presence of salts. Compare the readings from your meter with your soil moisture calculations to see the correlation between the meter and the moisture weight.
You will also notice that when you measure the soil moisture with a meter after 24 hours of drying, the deeper into the soil you go, the wetter the soil is. This is because evaporation occurs mostly at the surface, depending on the type of your pots or garden beds.
Amount vs. Frequency
When watering your soil-based garden, the goal is to saturate the entire soil structure from top to bottom, as well as completely across and throughout the soil. Watering only the surface, or missing entire areas within the soil structure, will lead to a reduction in the size and health of the root systems.
As roots replace soil during growth, the amount of water applied before saturation and runoff will change. Most organic materials will retain more water than roots by themselves, so your watering run time may need to be reduced slightly. As this process occurs, the transpiration rate for plants increases as they get larger, so watering frequency also needs to change accordingly.
The amount of water your plants require is determined by the characteristics of the soil and by the volume your containers can hold. Some plants prefer more moisture than others, but the amount should always be enough to saturate the entire soil structure. Again, as the plant root systems grow and expand, the amount of water will likely change to some degree. Just pay attention to the amount of runoff in the saucers, and adjust accordingly. The goal is to have a little runoff, but not too much.
Water should be applied at a slow rate, which allows for a more complete, uniform moisture level throughout the soil and root system. Rapidly applied water will run through the soil, leaving much of the root system dry. Slowly applied water will spread out to the sides, enabling equal moisture throughout the soil structure.
Watering should ideally be done at two different times, staggered by about an hour. Watering too often, or not enough, tends to build surface roots, not deep roots. Water applied slowly, with staggered application times and in sufficient amounts, tends to build large, deep roots.
Watering is a science. Study your soil and your plants, and enjoy the results.