Master the art of watering and you’ll be on your way to growing great plants
STORY BY JOSH VOLK
So often I see gardeners, and even farmers, trying to improve crop yields by looking for new ways to fertilize and feed. Or trying to solve pest and disease problems through sprays and other controls. But simply changing the way they water can have a much bigger impact and eliminate most of the problems.
Water in the Soil
Good garden soil has two basic components: mineral particles and organic materials. The mineral particles come in various sizes, from large gravel and sand to small clay particles. If your soil is mostly gravel and sand, water drains through it very quickly because there is plenty of space between the large soil particles and relatively little surface area for the water to stick to. Clay, on the other extreme, has very little space between the particles and much, much more surface area for water to stick to. Water moves relatively slowly through clay soil.
Organic material is usually a relatively small fraction of the soil, but it plays a very big role. In clay soil, it helps create larger spaces between the particles, allowing water and roots to travel through the soil more easily. In sandy soil it helps hold the sand together, offers more surface area for water to stick to, and more water-holding capacity. The benefits of adding organic matter is why practices like leaving plant roots to rot in the soil and adding compost are so important. Adding small quantities of quality compost — one-quarter inch or less, once or twice a year — will make a big difference over time. As the soil improves, you will see it absorb water faster, meaning less pooling. And the soil will be able to store more water overall.
Water and Fertility
One reason water is so important is that it delivers most of the plant’s nutrition. Even if your soil has plenty of the food the plant needs to grow, the plant can’t access it without water. The mineral and organic particles in the soil hold the elements the plants need, but they aren’t released until there is enough water to do so. Plants then take up the water that contains the food they need to grow. The plants also interact with all sorts of micro-organisms that are in the soil, and some of those help feed them, too. But without water, those micro-organisms either go away, go dormant, or die.
How much to water
On average, plants use about 1 inch of water per week. When it’s cool and the plants are small, they use less than 1 inch per week. When the plants are full-sized, and it’s hot and windy, they use up to 3 inches or more per week.
In my garden and on the farms I’ve worked on, we’re usually watering plants of all different sizes, and lots of different kinds of plants all at once. Most weeks, I make sure the plants get 1 inch of water, either from rain or irrigation. When it’s very hot and dry, I might put on 1 ½ to 2 inches, but I find it’s better to put on slightly less, rather than slightly more, to prevent disease and pest problems. Don’t be too skimpy though; much less is even worse than too much.
For most soils I’ve worked with, I’ve found that watering two times a week, one-half inch each time, works well. Soil can only hold a limited amount of water at one time, so if you put on too much, the soil won’t be able to hold the extra. Very well-drained soils, especially raised beds and containers, will probably be better if you water four or more times a week, with one-quarter inch per watering. If you have heavy clay soil with deep-rooted crops, you can water as little as once a week, putting the full inch on all at once.
Too much water can be just as bad as too little water. Between the soil particles, there are pore spaces for both water and air. Plant roots and soil micro-organisms need both to be healthy. Soil can only hold so much water. Put too much on it, and it will either drain down so far that the roots can’t reach it, taking valuable nutrients with it, or it will start to pool and run off the surface.
Rain and irrigation water are usually measured in inches. With rain and sprinkler irrigation, it’s easy to measure how much water the soil has received by putting a rain gauge or a straight-sided container on the ground to collect water. The amount of water is measured by the depth.
You can easily convert inches to gallons if you first calculate the area of soil you are watering. 1 inch equals 1/12 of a foot and there are 7.5 gallons of water in one cubic foot (hard to believe, but if you had a box that was 1 foot on each side you could pour 7.5 gallons of water into it without spilling any!). To put 1 inch of water on a garden bed that is 10 square feet (e.g., 2 feet by 5 feet), you would need 5/6 of a cubic foot, or in other words you would put 6 1/4 gallons of water on to equal an inch.
How To Apply Water
Water slowly and gently to minimize soil compaction and prevent any run-off. If you are watering by hand, use a water breaker to soften the flow, and move quickly back and forth from one part of the garden to the other, making many passes — and maybe even taking breaks once in a while to let the water soak in.
Sprinklers with small nozzle sizes are an effective way to water slowly. Mine usually take at least an hour or two to put down one-half inch of water. I find lawn sprinklers usually put out water too quickly for good irrigation in the garden, but there are lots of good mini-sprinklers out there, and a good irrigation supplier can help you design a system for your space for free. I’ve gotten most of my irrigation supplies from dripworks.com for many years, but there are many other suppliers, both locally and online.
With both sprinkler and hand-watering, I expect as much as 25 percent of the water that I’m putting out to evaporate before getting to the plants, so I put on more than 1 inch to make up for the loss.
Drip tape, or other styles of drip irrigation, are another way to water slowly and can be extremely efficient with your water application. Well-designed drip systems can be nearly 100 percent efficient, using 25 percent less water. I prefer drip tape for straight runs. If you want to go around curves, I suggest tubing with built-in emitters. You’ll want a filter and pressure regulator with your drip system. These last a long time and aren’t very expensive. I don’t recommend soaker hoses, as they don’t water as evenly as other drip systems.
Master the art of watering, and you’ll be well on your way to growing healthy, thriving plants!
Josh Volk is the author of the book Compact Farms. He writes from a desk that looks out over his garden, farms in Portland at Cully Neighborhood Farm, and travels around the country speaking to farmers and gardeners. You can find more about Josh at slowhandfarm.com.