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Conserving Water in the Soil


Conserving Water in the Soil

What perfect timing that 2015 has been designated the “Year of the soil”! This is perfect timing because soil not only feeds us and much of the life on our planet, but it can be one of our most important and accessible reservoirs of water. Thankfully, we’ve gotten a few more inches of rain in early February. Our weather pattern has shifted towards exactly what the experts were suggesting a few years ago – long dry spells, then bigger storms dropping large amounts of rain in a short time period. It is more important than ever that when the rain comes, we “slow it, spread it and sink it”, in the soil.

Increasing water infiltration into the soil needs to be done at all scales. The Water Agency is setting up some projects to do this, and our Resource Conservation Districts are encouraging rural land owners to use management practices that help the soil absorb rain. For example, creating berms and swales can help capture water along contours so it sinks down rather than racing off a slope.

On a home scale, there are many ways that we can increase water capture in our soil and keep it there. When possible, reduce impermeable surfaces, ie. solid paving, to allow rain to sink in and reduce run off. Tree roots can go sideways up to 5 times the height of the tree, so moisture under your driveway could help adjacent trees in summer. Break layers of compaction and add compost to your soil, which will act like a sponge, holding significant amounts of water.

It’s also very effective to implement practices to keep moisture in the soil. Water is lost from soil through evaporation and through “transpiration”. Transpiration is the water that is actively taken up by plants for their own daily functions. Plant roots pull water from the ground and moisture is given off to the air through pores or “stomata” in the leaves. Since all plants transpire, limiting unwanted plants, ie. weeds, will help conserve soil moisture. Sometimes mowing alone is adequate to limit weeds, especially grasses, but hoeing or pulling is sometimes needed. Mowing and hoeing are best done when the surface is a bit dry but pulling weeds is often easier in moist soil, (but take care to not compact soil by working in it when too wet). The sooner weeds are controlled the more benefit to water conservation.

There is some controversy regarding weeds and plant spacing. Dry farmed crops are always planted much farther apart so the plants don’t compete with each other for moisture and roots can colonize a large area. Most references say to increase plant spacing and mulch between plants to conserve water. But there are those who advocate close spacing and leaving weeds to shade the soil. Perhaps this works when the soil’s “water table” as well as organic content and fertility are high? But for most of us, I think the wider spacings are best, especially for deep rooted crops like tomatoes and squash.

To reduce evaporation on a home scale the most obvious and beneficial practice is mulching with organic material. Mulching works well with drip irrigation when material is added over the tubing. Mulch is also valuable in to protect soil from erosion and compaction. In vegetable beds, straw, alfalfa hay, old leaves, aged manure or rough compost will slowly decompose and enrich the soil. These materials can be home to slugs, snails, and earwigs, and can prevent the soil from warming in the spring, so should not be applied until plants are well established. Wood chips and sawdust should not be used in vegetable beds but are fine for paths and for covering a layer of compost around fruit trees and other perennials. Always leave an unmulched ring of space around the base of perennial plants, trees, and shrubs. 

When it’s time to start watering, check the soil first by digging down at least a few inches. Most likely only light watering will be needed for newly transplanted or seeded crops for several weeks after rains stop. Many gardeners over-water, so it’s a good idea to use a shovel to check soil moisture periodically through the season. Group plants together by their water needs, so those which need more frequent, shallow watering like most vegetables, can be watered on different schedule and with different emitters than established perennials. If you don’t have a drip system yet, now is the time to install one! Plants can be watered more effectively and you’ll spend less time watering and weeding.