As we face increasingly dry springs and summers, ecologist and author Becky Searle explores the simple changes we can make in our gardens to help soil retain more moisture and plants thrive with less watering. Discover why organic matter, mulching and growing more densely can make a real difference to your garden:
In late May I sat, my weather app open on my phone, glancing between it and the window, praying for rain. My water butts were empty, my IBC (Intermediate Bulk Container) tank on my allotment also empty, the plants I had so lovingly raised from seeds drying out before my eyes. But rain had been promised, and it couldn’t come soon enough. It felt like I was awaiting the arrival of some exciting delivery.
Living where I do in Exeter, we are in something of a rain shadow. Where other parts of Exeter experience a lot of rain, it often passes us by, so I’m used to these anxious waits. While plants are establishing themselves in spring it is particularly important to water deeply, ensuring that the roots can penetrate into the soil, anchoring them and prospecting for more water. While the root balls are small from being raised in pots, or being transferred out of a seedbed, they are still incredibly vulnerable. This means that dry spells during late spring can often leave us in something of a pickle if we don’t have running water.
Certain soils will dry out much faster than others. Where I live we have heavy clay, which holds onto water well, but when it dries out it turns solid, restricting root growth and causing water to run off the surface. Sandy soils dry out quickly, but because they are light and uncompacted, plants find it easier to put out deep roots.
There are certain plants, like carrots, that develop long taproots. These are well suited to soils with better drainage. Other plants however, like lettuce, will put out lateral roots, to take advantage of the lighter, upper layers of the soil. This means that not all plants are able to grow deep roots, so we do have to sometimes adjust our watering strategies to suit different plants, and different soil conditions. The old adage of watering deeply rather than often is still true, but it’s only a small part of the story.
There is one universal thing that we can do to help our plants, however, whether they have deep roots or shallow roots, and that is to increase the amount of organic matter in your soil. Don’t worry, this isn’t complicated, but it is important that you understand how and why it works so that you can trust the process and not panic that you’re getting it wrong.
Dead organic matter in the soil is what holds onto water. Building the amount of this in your soil will help it hold more moisture and therefore reduce how much you need to water your plants. This is usually in the form of dead plant matter, in other words, compost.
You can do a simple experiment to see this in action. Pick up a pot in your garden before watering it, then pick up the same pot after watering it. A dry pot of compost is very light; a wet pot is heavy. This is because the compost is holding water. Most of you will have already experienced this in action in your garden or houseplants. A pot of pure sand, silt, gravel or even clay (sediments) will not perform in the same way, it may weigh slightly more after watering, but most of what you put in the top will run off the surface or through the drainage holes in the bottom.
The obvious solution to many of us, and the prevailing gardening wisdom for many years knowing this, is to mix organic matter in with the soil. After all, if you mixed compost into your pot of sand or gravel, its water-holding potential would increase significantly.
However, with the soil in our gardens we need to be more careful. It’s a process that can take years.
The first thing to know is that when we dig, we release organic matter from the soil. This happens with the sudden introduction of a lot of oxygen, which causes a huge spike in microbial activity. The microbes in the soil process the organic matter really quickly, releasing a lot of it as carbon dioxide into the atmosphere. Alongside this, many of the organisms in the soil, which themselves are needed to mix and break down organic matter, are exposed to conditions that they simply can’t survive, and they die. Incidentally, this causes a spike in nutrients being released into the soil which is partly why we learned to dig our soils in the first place. However, with a healthy population of organisms within the soil plant nutrients are released steadily throughout the year.
If we want to build organic matter content in our soils the first thing we must do is stop digging. If your soils are compacted, you may want to insert a fork into the ground then pull it back by a few inches. This will introduce a little oxygen into the system and help decompact without causing major disruption to your ecosystem. Do this little by little if you have to, and not at all if you don’t.
The next thing we need to do to build organic matter content in our soils and help us water less, is to mulch. Adding a layer of mulch on the surface of the soil helps water to be absorbed into the soil rather than running off, and traps a layer of moisture, reducing surface evaporation. I mulch with Lakeland Gold because I’m on clay soil. It’s important to add a mulch every so often, every year or even every six months if you can, but less if you can’t, to replenish the organic matter that is naturally removed during tidying and harvesting in our gardens.
A compost mulch will soon start to disappear down into the layers of your soil. This gradual and subtle magic is performed by the creatures in the soil, slowly pulling it underground, breaking it down as they do and turning it into forms that our plants can extract nutrients from.
The modern style of gardening often advises digging your compost in, and well-meaning gardeners will follow this advice, feeling as though they are doting on their plants. Compost around the plant roots still needs to be broken down by creatures like earthworms before plants can extract the nutrients from it. These organisms will naturally come to the surface to feed.
What this means is that laying the compost on the surface is actually a more efficient way of getting the nutrients to your plant roots, as instead of having to hunt around for it, worms can easily find it when they visit the surface to feed. By allowing this to happen, we incorporate more organic matter into the soil, with less effort on our part, and create a nice layer on the surface to lock in moisture. We also don’t risk reducing organic matter content by digging. In fact, even if you don’t have any compost to lay on the surface, it’s still better not to dig your soils.
The next thing we must do is to grow plants. Plants and the soil have a symbiotic relationship. In order for both to survive and thrive naturally, they need each other. Having live plant roots in the ground helps to build organic matter content. This works because plants pump sugars and carbohydrates into the soil, which are then turned into organic glues, helping to stick together the structure of the soil. The sugars and carbohydrates help to act as billions of tiny sponges in the soil, hanging onto water as it flows from the surface down.
What plants also do is cast shade on the ground. This is very important when it comes to reducing watering. Plants will harvest the sunlight and turn it into plant matter. While they do lose some water through their leaves, they lose less than what would be lost from the surface of bare soil. There is an excellent paper by Zhang et al. 2017 where they studied this and found that a denser canopy over the soil resulted in less water loss.
Utilising spaces between plants such as planting lettuce or carrots between garlic or onions can not only be beneficial to the plants as companions and help you grow more in the same areas but also results in less watering. You can use crops like radish and rocket that grow fast to fill spaces when needed too.
The last thing you may want to consider for particularly thirsty plants is a drip system. These are surprisingly efficient, delivering water directly to the base of the plant. I use a small solar powered one in my polytunnel that takes water from my water butt and delivers it directly to my plants, using a lot less water than I would with a watering can, and doesn’t require me to do anything!
Watering less means creating conditions where water can be used more efficiently. It means cultivating soil that holds onto water, rather than letting it drain away or evaporate. It means using natural process to help lock moisture into the soil.
Don’t forget that you can implement any of these strategies at any point in the year. Mulching in summer will help significantly, especially around particularly thirsty plants, and with several years of mulching and practicing no dig, you will find your soil starts to noticeably improve.
Becky Searle is a garden writer and author of Grow a New Garden; Plan, Design and Transform any Outdoor Space. She trained as an ecologist and specialises in soil health and ecosystem gardening. You can follow Becky online at @Sow_Much_More.