Wednesday, August 24, 2011

Feed your soil, not your plants

A simple detergent test enabled me to determine
what the composition of my proposed topsoil was.

If you’ve been gardening for any length of time at all you will know that your plants are only as good as your soil. Yet what does that mean? What is ‘good’ soil?

Soil is made up of three basic components; sand, silt and clay plus organic matter. Not all plants want the same amount of each; for example drought tolerant Russian sage (Perovskia sp.) needs sandier soil while Japanese maples need a well balanced mix plus plenty of well rotted organic matter to thrive. Understanding what you have can drive your plant choices, help you diagnose problems or enable you to amend the soil in such a way as to promote healthier growth.

The aim of this post is not to present a definitive essay on soil biology. Rather I want to show you how thumbs of all colors can do two simple soil tests to gain a better understanding of their soil type.

Preparing the soil sample.

Scrape away the top 1-2” soil, dig down 6” and remove approx 1 cup of soil. Do this in at least three different spots in the area you are testing, mix them together and use this as your sample. Remove any stones or twigs – sift if necessary.

If your soil sample looks like a piece of
potters clay - it probably is!
Photo credit; The Nature of Framingham
The Squeeze Test.

Squeeze together a handful of soil for a few seconds. If it makes a solid shape which does not easily fall apart when prodded this indicates your soil is easily compacted and will have poor aeration; typically this suggests a clay soil but could also have a high proportion of silt. If the soil does not hold a form at all, it is mostly sand which will drain very freely but has little moisture retentive properties. Finally if the soil holds a form which crumbles when prodded then you have a good mix – commonly called loam.

The Detergent Test.

Add 1 cup of soil to a straight sided jar such as a Mason jar. Add 1 tspn of dishwashing detergent and fill the jar with water. Shake thoroughly for 3 minutes then leave undisturbed for at least 24 hours. The detergent acts as a surfactant, keeping the particles separated to allow sedimentation.

The largest sand particles will drop first followed by silt, then clay with organic matter floating at the top of the water. There are some interesting calculations you can do using a soil texture triangle (see references) to determine exactly what soil type you have, but for general purposes it is enough to simply know the ratios of each component. If the sand and silt are each approx. 40% with 20% clay and a decent amount of organic matter you have great soil! As this ratio shifts you may want to consider amending your soil.
What we'd all like; dark, crumbly loam.

Easy solutions

If you have predominantly sandy soil, add 2” of compost or other organic matter onto the soil surface each year. As the earthworms mix it into the native soil, the compost will improve the moisture and nutrient retention properties of the soil. You will slowly see your soil change from a light brown to a richer color as the worms work their magic over the course of a few years.

If your soil is either clay or predominantly silt it will not drain easily and be poorly aerated. Clay particles are very small so pack tightly together forming an impervious barrier in winter and cracking like a hard, dry river bed in summer. By adding organic matter these clay particles are pushed apart and the water can percolate. Some gardeners like to add coarse sand to clay soil to improve drainage but be careful; clay + sand = CEMENT! Always add plenty of compost. This is often all that is necessary but you can add some coarse sand in addition to compost.

So if your plants are ailing do some investigating rather than throw fertilizers at them or spray ‘just in case’ there is an infection or hidden bugs. If several plants in one area are doing poorly this could be a strong indicator that the soil is the problem. Perhaps plants are drowning in winter  but suffering from summer drought? If possible check the roots at this point to see if they are healthy and white or brown and mushy from rotting which would confirm the suspicion that saturated soil is the culprit.
An example of root damage caused
by voles.
Photo credit

Looking at the roots may also reveal a different problem; I have lost four hyssop (Agastache sp.) recently to voles chomping away! They must like the herbal smell. (Voles will often use mole tunnels to find plant roots or vegetables to eat, whereas moles are hunting primarily for grubs. It seems we have an abundance of both!) At least the voles will have fresh breath...

There is another use for these tests. I need to bring in a LOT of topsoil for our new garden borders but have to be very careful not to tip the balance of our already poorly draining land. I therefore ran these tests on topsoil samples to see if I needed to add more or less of any component. I have elected to go with a mix that has a well balanced sand:silt:clay ratio  and just a small percentage of moisture retentive compost. I’m going to amend the planting holes with additional compost as I go rather than adding an excessive amount to the topsoil itself. Understanding what my soil needs and what the new topsoil offers enables me to make considered choices and provide optimum growing conditions for my new plants.

Just to be clear, I always advocate the ‘right plant, right place’ approach and wouldn’t amend a large bed of solid clay soil just so I could grow lavender for example. However, when previously healthy plants steadily decline or new plants never seem to get going, knowing how to analyze your soil could save you a lot of frustration and money.

Additional references and resources;

'How is your soil texture?' by Wayne Cahilly; a great article which includes the soil texture triangle.

'Improving clay soils' by Keith Baldwin; another first rate article from Fine Gardening. Well written and with accurate scientific information in everyday language.

pH test kits - widely available at nurseries if you want to determine how acidic/alkaline your soil is. If the pH is too low or too high the minerals in the soil are not readily available to the plants which causes its own set of problems.

Professional soil testing - to get accurate information on the levels of minerals, trace elements, pH and more. For WA State residents this is provided by the Washington State University Cooperative Extension Service, as well as private laboratories. Each State has its own cooperative extension service, usually found in the government section of the white pages. Test results take 1-3 weeks.

1 comment:

  1. I really appreciate this information. Never knew how to test or check soils and this is very helpful.
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