Probably the most unusual thing about how I approach growing plants is the absence of routine watering. Pouring water on plants makes them grow. That is one of the first things we learn as children and it happens to not be entirely true. This post seeks to explore the alternative approach I have (re)discovered.
Irrigation is a technique that can increase growth and boost yields, but it also consumes a lot of resources and allows people to achieve the illusion they are producing when in fact they are consuming overall. Modern irrigation systems require vast amounts of energy intensive materials and energy to keep the water purified and pumped under pressure. The industrial water systems were mostly constructed during the post war economic boom and in recent decades have suffered creeping neglect around the world. Scenarios like the contamination debacle in Flint, Michigan, are unfolding around the world as the non-renewable resources needed to maintain these complex systems become less available. Droughts in Australia are making tap water more expensive and less reliable. When water comes out of the tap at almost zero cost it is easy to ignore the price of the whole system, and its vulnerabilities.
On my farm I am in the slightly more challenging situation of not having a town water connection, instead relying on rain water from tanks. The house and my small greenhouses run off 60 000 L capacity, split between one large concrete tank and several smaller plastic ones. Without industrial infrastructure this approach is also unsustainable. Before reticulated water people spent a good part of their time carting surface water or drawing it from wells and consequently household use was much lower. We recently lowered our pump pressure dramatically and our daily water use has dropped to about 50 L per person. We could imagine carting a bit less than this amount from the creek by hand if we had to, though would need to manage the lower water quality. The animals have a further 10 000 L capacity, mostly for the goats who have never come close to emptying the tanks.
Using any of this tank water for irrigating plants would be foolish. Pouring water on the ground to grow plants is extremely wasteful, with only a tiny percentage ending up in the final crop, and it is commonly used to grow such luxuries as lettuce that are 99% water anyway. Just drink the water instead. When plants wilt during dry spells here they either endure on their own or die. I only hand water recent transplants of species that will be difficult to replace, and only then if I make a mistake and transplant before the season turns unexpectedly dry. From experience plants that are unable to quickly establish and fend for themselves are almost always a lost cause anyway. During prolonged droughts I prefer to change my habits and expectations. The volume of vegetables we eat steadily declines after the rains stop and I shift my energy to culling animals as the amount of their available feed declines. This means we eat less veggies and more meat. I also tap into reserves of preserved and frozen produce, and eat through dry stores and tubers. Droughts normally arrive in spring and summer when more fruit is available as well. In many ways a dry season in the subtropics is analogous to a snowy season in the temperate zone. While people might do a few tricks to extend the season and hold off frosts, for the most part people give up on expecting fresh lettuce in the middle of a blizzard and put their energy elsewhere. With cheap electricity they might consider providing plants with artificial heating and lighting but if they couldn’t hide the costs of all that industrial support the activity would be sheer folly.
The water we store in our tanks is extremely expensive in real terms, and also extremely limited in the impact it can have on the landscape. Our 40 acre farm receives about 1000 mm of rain each year, equivalent to 160 000 cubic meters of water. By contrast our tanks only hold 70 cubic meters of water total, a tiny 0.044 % of the water the farm receives. Irrigating with tank water during a drought is like spitting in a fire. People might then say I should be pumping water from our dams. But their total volume is about 1000 cubic meters, a mere 0.6 % of the total rainfall, which would be pumped back up against gravity using enormous amounts of fossil fuel energy (to mostly leach and evaporate if done so during a drought anyway). So where does all the rest of the water go?
When rain falls it interacts with vegetation first, with a large amount being absorbed directly by intact tree canopies. That which reaches the soil can be absorbed into the spaces between the soil particles, though different soil types differ in the maximum rate at which they can absorb water. As most soils become saturated the rate at which the can absorb more water decreases until eventually water starts to run along the surface. This often leads to erosion of surface sediments, especially in soil that lacks a healthy microbial population which normally secretes a host of glue like molecules that hold soil together. Bare soil isn’t the problem here per se and many healthy ecosystems feature patches of bare soil at various times that are often essential to allowing new seeds to establish. Instead a persistent and large scale lack of vegetation can deplete the soil microbiota until the glue like secretions are gone.
Soils with the largest pore sizes are able to absorb water the most rapidly and resist erosion. At the most extreme would be a field of boulders, then gravel, then the more common growing medium of sand. At the other extreme soil composed of the tiniest particles is fine clay, where water takes a very long time to percolate. If this was the only factor that mattered then gravel or sand would be the ideal growing medium, and indeed in places where irrigation water and chemical fertiliser are cheap and plentiful industrial vegetables are grown in sand almost as hydroponic plants. Unfortunately a soil that absorbs water rapidly also loses it rapidly and plants growing in pure sand will wilt rapidly when the irrigation is cut off. There is an inescapable trade-off between absorption and retention of water.
This is where clay soils really shine. That sticky clay that was so sluggish about absorbing water is also extremely reluctant to let it go. As a result a clay soil can become highly charged with water during wet seasons and slowly trickle it out to plants over many months without rainfall. Clay soils such as mine can hold about 50 L of available water per square meter in the top 30 cm of soil, meaning my farm can hold 8000 cubic meters of water in the top foot of soil, equivalent to a non-negligible 5% of our annual rainfall. And the clay doesn’t stop 30 cm down. It is several meters deep in most places, so more like 30% of our rainfall is stored by the soil at any one time. The rain doesn’t arrive all at once so this buffering capacity is enough to hold several months’ worth of water, enough to tide plants over through droughts which normally last at most for six months. Increasing organic matter modifies water absorption and holding capacity, but more important is the presence of a diverse soil ecosystem which constructs a range of different sized tunnels through the soil profile. Live plants outperform dead mulch any day. Deeper plant roots extend this soil life further into the soil profile, further increasing effective water absorption and holding capacity. When I see my sticky clay soil cracking heavily during a drought I don’t despair. Instead I notice that all those gaps were once filled with water during the wet season. The cracks also provide a convenient channel for rainfall to percolate into the subsoil, along with organic matter from the surface that falls in.
Soils with finer soil particles have more of the property known as capillarity, the ability to wick water upwards against gravity, much like a piece of paper dangled into a glass of water. As the soil surface dries out water stored in deeper layers can make its way back to the surface in a soil with sufficiently narrow channels between the particles. I often find plants growing in more compacted soil with lower organic matter levels and less fluffing up from cultivation do better during dry spells for this reason. The dream of having a fluffy, high organic matter soil that is meters deep only makes sense in a world with constant cheap irrigation. The variation in soil composition with depth can be extremely useful as well. Often in sandy soils there will be a subsoil clay layer, above which a water table will tend to accumulate in places. This depends on the hidden underground contours of different soil types across the landscape. Just focusing on surface contours to design structures such as swales totally overlooks this unfathomable reality. This explains why often even apparently flat and superficially uniform growing areas will have weird patches that never seem to grow plants properly. In a natural ecosystem different plants would dominate those areas to take advantage of the varying subsoil niches.
One final major consideration is the matter of how plants actually receive water. Their roots do not usually absorb the water directly. Instead the water is provided for the most part by symbiotic microbes that extend in a delicate web much further out into the soil. This relationship mirrors the other species of symbiotic microbes that deliver different soil nutrients. It is now known that providing artificial fertiliser to plants causes them to neglect these crucial relationships. Why would the plant spend precious sugars for a nutrient it already has plenty of? The problem arises when those artificial fertilisers suddenly run out or are leached from the soil (another problem made worse by irrigation). The plant then finds itself without a functioning microbial relationship and hits a sudden nutrient deficiency, leading to stunted growth and pest/disease problems. In the same way a plant that is artificially irrigated will neglect to develop its microbial water delivering partners and develop a weak and shallow root system in general. When the grower inevitably forgets to water, or the weather suddenly turns drier, the plants cannot adjust and suffer a shock. By contrast I very rarely see wilting in my plants despite never watering them. Growth slows gradually as the soil moisture level decreases but plants usually endure and resume growth when rain returns. When a human thinks of water they think of a cup of concentrated liquid that they need to sustain their bodies that are otherwise disconnected from the wider environment. This is much the same how we view food as something concentrated and discrete like a baked potato. We naively transfer this perspective when caring for plants, despite them being very different organisms. Food for a plant is the highly diffuse sunlight that falls on the earth and the even more diffuse carbon dioxide drifting on the breeze. Water for a plant is the humidity and ground water that is invisible and useless to us. That form of water is everywhere and always flowing. The river is everywhere, in the sky, under the ground, as well as the tiny amount on the surface where we can see it. By understanding all this we can help our plants tap into this vital resource, and free ourselves from the burden of irrigation forever.
3 thoughts on “Everywhere is a River”
Very interesting. I’ve noticed that my pepino plants will wilt alarmingly after a day in the high 30’s, but next morning they’re back up to turgor. If we have a hot spell with temps consistently in the 30’s for several days, they’ll do this until eventually they don’t come back up by next morning and that’s when they get a good soak. I’m going to try this approach with more plants, especially those I can propagate easily, so losses are no problem. Self-sown also does better than propagated in tubes and planted. That first root gets as deep as it can and sustains the plant in a dry spell. The roots in a tubed plant are only as deep as the tube and the plant will wilt very quickly. Experimenting is a great deal of fun as well as learning!
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Water demand varies a lot throughout the day, with lower temperature and higher humidity allowing wilted plants to catch up again overnight. Wilting is actually a useful mechanism in plants as it causes them to release hormones from their growing tips that travel down and stimulate greater root system growth. If you irrigate a plant perfectly then it never stresses and ends up with a weak root system, vulnerable to the day you forget or the pump breaks.
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A very informative and great article.