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Monday, December 31, 2012

Getting to Know Your Garden’s Soil: Improving Soil Drainage

Over the past few months we’ve discussed urban soils (September 2012), soil testing (October 2012) and soil amendments (November 2012).  We end the year by considering soil drainage.  Next month we’ll present one final soils topic - the soil as a living, productive ecosystem.

If you’ve done a soil perc test you now know whether your soil drains quickly, slowly or somewhere in between.   Depending on your results, you may decide to  select appropriate plants based on their drainage needs.  Alternatively, you may choose to alter the drainage of parts of your garden. As always, your gardening choices are unique to your own situation. Our goal at Mother Nature’s Backyard is to help you make informed choices.

Image courtesy of http://www.salinitymanagement.org/
Soil drainage is determined by pore spaces.  Soil pores are spaces that are filled with either air or water (see picture above).  Most soils have a combination of large and small pore spaces.  Under normal conditions, larger pores are filled with air and smaller pores with water.  This is good because plant roots and most soil organisms need ready access to both air (oxygen) and water.

Being composed of relatively large soil particles, sandy soils have more large and fewer small pores than soils with more silt and clay particles. The abundant large pores fill quickly, allowing sandy soils to take up water more easily than clay soils.  On the flip side, large pores don’t retain water as well, so sandy soils also dry out more quickly.  

Soils that drain quickly present several challenges.  First, they require more frequent watering, particularly during hot, windy weather.  These soils also leach out soil nutrients more quickly.  Nutrient leaching occurs for two reasons: 1) water percolates through sandy soils more quickly, taking with it dissolved minerals and 2) clay, silt and humus (organic) particles attract soil minerals (via a small electro-magnetic charge) and hold them better than do sand particles. 

As a result of leaching, sandy soils may have lower levels of key nutrients like nitrogen, phosphorus and iron.   Growing plants with high nutrient requirements (like many vegetables and citrus trees) in sandy soils requires routine application of fertilizers or compost.   Fortunately, many Southern California native plants have low nutrient needs and are well-suited to sandy or rocky soils, even without fertilization.   For a list of native plants suitable for sandy soils see: http://www.nbs.csudh.edu/biology/projectsound/habitat/documents/Sandy_Soils-3-11.pdf

On the positive side, very well drained soils hardly ever become saturated (water-logged).  Clay and silty soils have fewer large pore spaces (and more small pores), so they take up water more slowly – but retain soil moisture longer.  These soils also have positive and negative characteristics with regards to drainage.

Slowly draining soils require less irrigation and retain nutrients better.  But they have the potential to become water-logged during wet winters or with over-watering.  Water-logged soils deprive roots of oxygen; plants growing on wet soils tend to be shallow rooted.  Prolonged exposure to water-logged soils can be hazardous to plant health and even lead to death.  Plants that need well-drained soils – including many California native plants – are particularly vulnerable.  These plants often do well in clay soils until their first El Niño year, when they suddenly yellow and die.   They’ve literally ‘drowned’ in the water saturated soil.

On the other hand, slowly draining soils are perfect for a wide range of interesting native plants.  These include plants that normally grow around lakes or streams as well as those from our local seasonal (vernal) marshes and vernal pools (see                http://www.nbs.csudh.edu/biology/projectsound/habitat/documents/Clay_Soils-3-11.pdf and http://www.nbs.csudh.edu/biology/projectsound/habitat/documents/Riparian_Palette-3-11.pdf ).  If you have a slow-draining soil, consider yourself lucky!  People with quickly draining soils can only grow such plants in containers

Compacted soils can be a challenge in local gardens.  Compacted soils (see above) often lack sufficient large pores.  This explains several characteristics of compacted soils.  First, they tend to be low in oxygen compared to non-compacted soils.  This usually only becomes a problem during periods of prolonged heavy rains or when compacted soils are over-watered.   Compacted soils also tend have slow water uptake, leading to runoff.  If you have badly compacted soils – or have a hardpan layer beneath the topsoil - you may want to try one of the methods suggested below.

Our approach to gardening at Mother Nature’s Backyard emphasizes respecting the forces of nature and working with existing conditions.  We’ve come to that philosophy in part for scientific reasons;  we believe it’s important to conserve and restore natural landscapes for their beauty, habitat value and sense of place.   But we’re also pragmatic.  It’s often easier to work with what you have – by choosing appropriate plants and watering methods – than to change your soil dramatically.  That being said, there are a few, relatively simple things you can do that will allow you to grow plants that normally require soils that have faster (or slower) drainage than your garden’s soil.

Garden in Raised Beds, Containers & Planters

One of the easiest ways to provide soil with specific characteristics is to garden in raised beds, containers and planters.  These contained environments allow you to create exactly the right soil mix.  You can create soils with better drainage if you want to grow cacti or plants that require sandy or rocky soils. You can provide slowly draining soils for marsh or bog plants.  Or you can create a light, well-drained mix with high organic and nutrient content for growing garden vegetables.  

Container gardening is fun and can be done in almost any garden.  There are a few tricks to gardening in containers and raised beds.  Fortunately, there are many good books and internet resources to get you started.


Create Micro-berms to Increase Soil Drainage

 Another trick for creating faster drainage is to use small soil berms.  A berm is simply a mound of earth. In the garden, berms are usually planted.   Berms need not be massive or require heavy earthmoving equipment to create.  You can make them with a little effort and a hand shovel in most soils.    
Central berm before planting (view looking south).  Note gentle slope.

Micro-berms are 1-2 feet taller than the surrounding soil and of any size or shape.  In general, rounded curved shapes with gently sloping sides are most attractive. The photo above shows our central berm in Mother Nature’s Backyard.  It’s about 20 feet across and gently curved in shape.  The central berm is about 2 ½ feet taller than the rest of the garden at its highest point and slopes gently down to the north and east.  We grow local Salvias (Sages), California Sagebrush, native Buckwheats and Penstemons on our central berm.  These plants like good drainage and we have clay-loam soil with moderate drainage.   We provide better drainage with our central berm.   The plants that need the best drainage are planted at the top.

Central berm as it looks today - view looking west

Micro-berms needn’t be large.  Below is a photo of our small ‘Salvia Berm’ which looks more like a pitcher’s mound than a large berm.  Our ‘Salvia berm’ is only about 1 ft. tall and perhaps 6 feet across. When the plants mature you won’t even know it’s there.

The small 'Salvia Berm' is circled

If your soil drains slowly, you may be tempted to create micro-berms using imported topsoil.  Imported soils may improve drainage and often can be delivered (and dumped) in place in your garden.    But you’ll need to insure that the better-draining imported soil doesn’t just sit atop the slower-draining native soil, creating an impermeable layer.

When creating a berm with imported soil, mix some of the imported and native soils and create a mixed-soil layer between the berm and native soil.  Ideally, this layer should be at least 12 inches deep, though even 6 inches will be of benefit.  If you cannot create a mixed layer, at the very least you should break up the top layer of native soil before creating your berm.  Taking these steps – or using soil that is the same or similar to the native soil – will insure that your berm drains properly.

Create Small Depressions to Increase Water Retention

You can also increase the water-holding capacity of a soil by creating shallow depressions where water can collect.  This is particularly helpful in well-drained soils.  Such depressions are sometimes called swales,  although  true drainage swales and rain gardens require a little more thought and engineering.  Simple depressions don’t have to be deep – several inches below the rest of the garden will do.  Be sure to gently slope the sides so you don’t create a safety hazard.  Then  grow plants that need a little more water in these depressions.

Add Organic Materials (unless the plants need nutrient-poor soils)

Consider adding compost to your vegetable garden or non-native flower beds.  Compost adds nutrients while increasing the soil’s ability to hold water.  Organic particles (humus) create small soil pores and also decrease the leaching of nutrients.   They are a normal part of most mature soils.    Even organic mulches (see July, 2012 posting) contribute organic materials to the soil as they break down.  At least a 1 percent increase in organic matter is required to have an effect on the water- and nutrient-holding capacity of a soil.

Drill Holes Through a Hardpan Layer

If you have a clay or hardpan layer, you may want to consider drilling holes through it.   First test your soil to be sure that a hard, compacted or impermeable layer is the cause of your drainage problems (see October 2012 posting on ‘Testing Your Soil’).  This will also indicate the location and depth of the compacted layer.   Use a soil auger to drill a series of holes through the hardpan.  See the following for more: http://ceventura.ucanr.edu/Gardening/Coastal/Landscape_578/Hardpan/ .


Frequently Asked Questions
Do I need to change my soil’s drainage properties?   No.  If you have a well-drained soil,  no action is needed.  If your soil drains quickly or slowly, you can select plants appropriate for your sandy or clay soil.   Most local soils support a wide range of native and non-native plants. 
Should I rototill or double-dig my garden before planting to improve drainage?    In most cases, the answer is no.  While these methods have long been used for intensive vegetable gardening, they may actually do more harm than good for many local soils.  We’ll discuss this more in our January 2013 posting. These methods also bring up buried weed seeds. 

Can I improve drainage with ‘lasagna gardening’?  ‘Lasagna gardening’ involves double-digging then layering cardboard/newspaper strips and green materials that decompose over time.  The method is sometimes recommended for gardens with thin/no topsoil – usually in places with far more precipitation than we get.   The ‘lasagna’ method does not improve drainage in most local soils – in fact it may decrease it.  Like any double-digging method, it also destroys the natural soil layers that build up over time.  We don’t recommend this method.  See https://sharepoint.cahnrs.wsu.edu/blogs/urbanhort/archive/2011/03/30/is-quote-lasagna-gardening-quote-really-worth-the-effort.aspx for more.

Do I need to ‘loosen’ my soil every year?   Once again, the answer is usually no.   Soil around trees, shrubs and other permanent plantings is better off left alone.  Use a layer of mulch if appropriate, but don’t dig to improve drainage.   You may want to loosen the soil in your vegetable garden or garden beds containing non-native flowers that are routinely replaced.    Adding compost will also help to improve drainage in soils that are used intensively.


Additional Resources




Send your garden soils questions to mothernaturesbackyard10@gmail.com  or post a comment below.

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