Article and Photos by Mark Godlewski

A good understanding of your soil is one of the keys to successful gardening, especially if you are committed to following xeriscape principles. In Part I, I provided some general information about soils in the Okanagan. Here in Part II, I will suggest some methods for evaluating your own particular soil.

From a gardener’s perspective there are three key soil parameters: pH, nutrient level, and texture.

The pH of soils: in the Okanagan basin soils are almost invariably alkaline. This is typical of areas of low to moderate rainfall. Even if you somehow had neutral or acidic soils, our water is alkali so the soils will soon change. This alkalinity is not generally a problem. It gets reduced to some degree by the addition of organic material. The plants that really like acid soil such as azaleas and blueberries also love moisture and so are not good candidates for a xeriscape garden.

The nutrient level of soils: in the Okanagan is variable. The glacial silts and sands of many of our subsoils are low in nutrients and organic content. As a result, most gardens have a fair bit of variability within themselves with respect to nutrients and, in fact, while some xeriscape plants like rich soil, others really prefer poor soil. For plants that like rich soil it is a good idea to constantly add organic material. More about that below.

Soil texture: is an important topic for gardeners. Examples of soil textures are sandy loam, silty clay, etc. There are 13 of these terms and they are shown on the soil texture triangle Fig 1. It is worthwhile for every gardener to know their soil texture because it has a strong impact on how you handle the soil, how you irrigate the soil and how well your plants perform. Generally, you would like your garden soil to plot in the parts of the texture triangle that contain the word “Loam”. For example the soil you see in the cover photo of this blog is a sandy loam (Gammil type) in Kalamoir Park. 

Fig. 1 – Soil Texture Triangle

To gauge your soil texture, one of the first things you can do is to select a couple sample areas that are representative of your garden. For example, I live in a house with a walkout basement. The elevation of my back garden is lower than the front and the soils are quite different between the front and back because of this elevation difference. Also, after the foundation was finished some sandy fill was brought in and used around the foundation in the front garden. Often backfill just uses the subsoil from digging the basement but it is not uncommon for some of the backfill to be brought in. Although I requested a thick layer of topsoil, it is a bit of a rare commodity in these parts. There are quite a few areas where the landscaper only put in 4-6 inches of topsoil over the subsoil. For my representative sample areas, I have chosen a location in the front garden and one in the back. They are located far away from the foundation, from the hardscaping, and away from any large trees or shrubs that required a lot of digging when they were put in.

In each sample area you want to dig a hole at least 8” in diameter. Do your best to dig down to a depth of about 12”. As you dig note the thickness of your topsoil. There is usually a marked difference in colour and texture between the topsoil and subsoil. The topsoil is almost always darker and more organic. Take a representative sample of the topsoil and subsoil and put it in a plastic bag marked with the location.

Glass jar method for texture – If you look on the web you will find several different sites that explain how to find your soil texture by mixing soil and water in jar and watching how it settles. Although this method is clean and effective for some soils, it has a couple of serious problems. If there are a lot of fine particles (fine sand, silt, and clay) and if they are all similar in colour it can be exceedingly difficult to see the sediment/water contact or the silt/sand contact in your jar at any given time. The bigger problem is that in parts of our valley the glacial clays tend to easily mix with and coat the glacial silts so that they settle together as a single unit in the mixing jar. This means you cannot use this jar method to reliably distinguish between clay and silt and therefore you cannot determine the true texture of your soil. The jar method is somewhat useful, however, in terms of providing a measurement of the quantity of sand and organics in your soil. The sand sinks to the bottom in about 30 seconds and the organics form the darker material that floats on top or settles gently on sediment after a day or so.

Handheld method for texture – This is a better method for estimating soil texture, and it involves getting your hands dirty by playing your soil. The method is well demonstrated in this link from  Australia . It is quite effective at estimating relative clay and silt percentages which people often struggle with. Silt can be mistaken for clay, but it is generally more benign. You can get an idea of what the results from this method look like from Fig 2. If you are careful with your observations, you can locate your topsoil and subsoil quite nicely on the texture triangle.

Fig. 2  – Soil Extrusion Textures

Percolation tests for texture – Percolation tests are strongly correlated to soil texture. In many ways, they are the most useful measurement because they are simple and direct. You can perform one easily using the same holes you dug to get the topsoil and subsoil samples. Make certain that the hole has relatively straight sides, is about 12” in diameter and 8”-12” deep. Carefully fill the hole with water and leave it overnight to saturate the soil. The next day fill the hole again and lay a straight edge across the top (see Fig 4). Measure the level of the water down from the straight edge in inches. Come back in an hour and measure how much the water has gone down. You can come back again after two hours and measure again to get the average number of inches per hour for your Perc test. Generally, a rate of between 1 and 4 inches per hour is considered acceptable for most plants.

One thing to keep in mind about percolation tests is that they are affected by shallow water tables. If the water table is within about 20 inches of the bottom of your hole, then it can slow down the percolation flow appreciably.

The effect of texture on irrigation – Your soil texture controls how water permeates your garden. A nice illustration of this (Fig. 3) comes from a particularly useful publication from the University of California Santa Cruz

Fig. 3  – Water Movement in Soil

UnH2O garden – It is interesting to apply these texture analyses to the UnH2O garden. I performed this analysis in two different locations in the garden (Fig 4 and Fig 5) and got similar results. We know from Part I of this blog that the garden is in an area where the subsoil texture should be a loam (sandy loam, loamy sandy, silty loam) which is popular with most plants. On the other hand, the percolation tests in the UnH2O garden gave a drainage of just 1.5 inches per hour which is on the low side of the preferred range. While the texture of the soil is reasonable the problem is that the water table is quite shallow in this part of the city. You can contrast that with my “vegetable garden” in West Kelowna 100m above lake level where I measured a drainage rate of 11 inches per hour. I clearly have a problem with my subsoil in that part of the garden and have plans to fix that immediately.

Fig. 5 – Water filled percolation hole

Improving your soil – It turns out that the recipe for improving your soil is remarkably similar for most of our soils in the Okanagan. In almost all cases you should be adding organic matter. If the soil is too sandy add composted organic matter to slow down drainage and improve the nutrient level. If the soil is low in nutrients, again adding organic matter will help considerably. It is a good idea to work some of the organic matter into the soil and lay 3” of organic mulch on top.

Clay soils also benefit from the addition of organic matter to improve the drainage. These soils are difficult to work with, however, because it is hard to effectively incorporate materials such as coarse sand and organics into clumps of clay. Clay soils will hold moisture for longer, but it is hard for many plants to access. Irrigation water can run off instead of penetrating, and walking on wet clay soil leads to over compaction.

If you suspect that you have a problem with a high water table then the best solution may be to build raised beds of loam. This will allow your plant root systems to penetrate as far as they want to get the moisture level that they prefer.

Get to know your own soil characteristics and you will find that your knowledge will pay big dividends in success of your gardening.

Article by Mark Godlewski

A good understanding of your soil is one of the keys to successful gardening especially if you are trying to follow xeriscape principles. In this first article I will provide some general information about soils in the Okanagan. In the second I will suggest some methods for evaluating your own particular soil.

Anywhere you live in the Okanagan, a good part of your soil likely comes from the last major glacial episode that occupied the valley. From 30,000 to 13,000 years ago this valley was filled up with ice to a level at or above the tallest mountains that surround us. That ice sheet slowly melted leaving behind all the boulders, gravels, sands and rock flour (silt) that form the sediments in the valley. The soil may have been reworked and moved around since that time, but it originated from that Fraser Glaciation episode.

One of the most interesting features of the Okanagan from a soil point is the presence of a series of benches that rim the present-day system of lakes including Kalamalka, Wood, Okanagan, and Skaha. These benches attest to the presence of a significantly larger lake (Lake Penticton) that filled this valley to a level about 115 m above the current lake level about 10,000 years ago. The lake extended down to the Okanagan Falls, Vaseux Lake area. Here the valley narrows and was filled up with a dam of ice and glacial debris for an extended period. It is quite likely that the dam was periodically broken and reformed depending on the vagaries of ice movement.

The ancient lake benches comprise deposits of mainly fine sand and silt (glacial rock flour) with smaller amounts of coarser sand and clay laid down by multiple glacial streams from the melting ice sheet carrying a heavy sediment load into a large, turquoise glacial lake.

The distribution of soils in the Okanagan can be understood in a general way by looking at surficial geology maps. There is an excellent one from the Geological Survey of Canada for the greater Kelowna area and one for the Vernon area.

Kelowna Surficial Geology
Vernon Surficial Geology

Fig. 1 – Surficial Geology of Kelowna, Geological Survey of Canada

To get a sense of what these maps mean we can look in detail at the Kelowna area example. The areas coloured red are the rock outcrops of the surrounding hills and mountains. The green areas are generally mixed glacial till often derived directly from the local rocks. The light-yellow area is a broad flood plain formed by creeks and rivers flowing from the north and east down the valley. These were much larger than the current Mill Creek, Mission Creek etc. and they have reworked and redistributed the glacial sediments. In some parts of the light-yellow area there are patches of organic rich soils from swamp type environments, but these are relatively rare. 

The purple areas on the map show the Lake Penticton bench sediments. These are the same sediments that form the light grey bluffs of fine sand and silt on the west side of the Bennett Bridge and also the prominent bluffs further south in the Summerland and Naramata areas. These benches are the source of some of the most consistent soils in our area and home to extensive vineyards and orchards. These Lake Penticton bench sediments are dominantly fine sand and silt but contain some pockets of coarser sand and clay. 

While these surficial geology maps give a good general picture of the types of sediment on the surface there is a much more detailed soil map dataset available on the web. The data come from a major soil mapping project completed in 1986 titled “Soils of the Okanagan and Similkameen Valleys”. This mapping is oriented principally towards agriculture, but it can be quite useful to gardeners and there is other environmental data there like frost free days. 

There are a few things to keep in mind about the government soil mapping: 

• The central urban areas such as central Kelowna  have been left out of the mapping but sometimes you can get an idea of what to expect by looking at adjacent polygons
• Much of data is quite technical in nature and contains many elements that are not particularly useful to a gardener
• Gardens inside housing developments often contain a certain amount of soil that has been moved in from somewhere else. This is particularly likely if your house is on a lot with a steep slope nearby.

 To see how to use this soil mapping data go to: 

BC Government Soil Map

 and type your postal code into the search box on the top left-hand side. The map will zoom to your area and then you can click inside the yellow polygon that covers your garden. A text box comes up.

Here is what you will see in the text box if you were to check our UnH2O garden location:

Fig. 2  – Text box for the soil polygon containing the UnH2O garden

The area in and around the UnH2O garden contains a mix of 60% “Tanaka” type soils and 40% “Rumohr” type soils. The part highlighted in yellow is some basic soil information for those types of soils. The blue highlights are links that provide more detail on the soil types and how they can be used in agriculture. The sections highlighted in green are useful environmental information. It is interesting to compare the 3 environmental estimates with data from a similar polygon in near Vernon and one near Osoyoos.

If you take the time to cruise through the soil descriptions in the valley you quickly realize that there is really an exceptionally large variation. Soils are generally silty or sandy but there are some areas of heavy clay. Soils are usually alkaline and low in organic material but there are some acidic, high organic soils often located in lower areas. Soils are generally well drained but again in the lower areas there is some very poor drainage.

Once you have done what you can to determine the likely soil textures and drainage for your area you can move on to some specific tests for your garden in Part II of this blog. The general description from the mapping is probably accurate for your subsoil and is a good starting point for what was originally in your garden.

Take the time to get to know where your soil came from. You may have brought the top few inches from somewhere else but what is below greatly effects your trees and shrubs.