Soil Testing in My Garden
Hello friends,
As part of the Floret Flower Farming workshop, it was suggested that we get soil testing done in our gardens/flower farms to check what our growing conditions are like.
So I dug out my horticulture course book on Soil Properties, and got to work. If you are ever thinking about studying horticulture, I do really recommend Open Polytechnic's "New Zealand Certificate in Horticulture", which is a free online course for anyone in New Zealand. I completed this course over a period of two years from 2016, and it was really worth it in my opinion.
First up was looking at the soil profile in my back garden by cutting a vertical profile through the soil with a spade. The organic horizon was 3 cm deep, followed by the topsoil (A horizon) which was 9 cm deep. Next, was the subsoil which was at 12 cm (B Horizon), and only was 3 cm deep. And lastly was horizon C, which is the weathered parent material.
Things to note in the soil profile: the top soil was dark brown in colour, but contained no earth worms, and the roots from plants only penetrated 10 cm down into the ground. The subsoil layers were yellow/orange in colour and formed very dense clods that were sticky when wet. This means there is not a deep topsoil layer for plants to grow in, and the lack of earthworms means that they are not bringing stuff down into the soil to improve the soil structure. This can be amended with compost and mulch, and hopefully with time earth worms will come into the garden.
Next up was the soil composition tests, which are the thread test, the ribbon test, and the soil settling tests. The thread and ribbon tests are qualitative, and the soil settling test is quantitative. Both the thread and ribbon tests (not shown), predicted the soil type was silty clay. This wasn't surprising because the suburb of Mosgiel sits on a flood plain.
The soil settling test was next, and it's a very simple experiment to do. First, dig down to 10 - 15 cm in the garden, and get a long bottle or cylinder with a lid, and fill the container half fill with soil at that layer. To the soil add 1 tablespoon of dish washing soap (this will help break the soil down into its components). Then fill to the top with water. Shake for 30 minutes, or until the soil is completely broken down to it's individual particles and no clumps remain. Let the particles settle at the bottom of the container, and as a warning, this may take weeks to happen if there is a high clay content in your soil.
Here is the result for my backyard soil, and my sample did indeed take weeks to settle because of the clay content.
The bottom layer in the bottle is the sand layer (usually only takes about a minute to settle). The middle layer in the sample is the silt layer (which takes 1 - 2 hours to settle). And finally the top layer is the clay layer (and this took about 3 weeks to settle). Qualitatively the silt layer is the thickest, followed by clay, and then sand. To measure it quantitatively you just need to measure the total depth of the sample, and then each of component layers, and then work out the percentages of the sand, silt and clay in the sample compared to the total soil measurement.
The quantitative results show that silt makes up 61.5% of the soil, clay makes up 23.1% of the soil, and sand makes up only 15.4% of the soil. This indeed makes it a silty clay soil.
The good thing about silty clay soil is that there are a lot of nutrients, but the bad thing about silty clay soil is that it dries as hard as a rock in summer, and is sticky and wet in winter. This means that in summer it's hard for the plants to take up water. And in the winter, the wet and sticky soil means that the plants can very easily rot, especially dahlia tubers...
The best ways to improve silty clay soils is by doing the following: add organic matter in the forms of compost and mulch, add a clay breaker (to help break up the clay and silt), avoid compaction of the soil by not walking on it wherever possible, and plant strategically with plants that will tolerate the current conditions. I plan to do all of these things in my garden beds in the coming years. I am already adding Tui Mulch and Feed to some of my front garden beds (I've been picking up some bags whenever I go into Mitre 10), but this will take a long period of time over the coming year as I can afford to buy the 40L bags. The thing I like about Tui Mulch and Feed is that it contains "A two-in-one combination provides the benefits of mulching whilst the healthy additions of blood and bone, mulching straw and sheep pellets replace vital nutrients used by your plants."
The next soil testing I wanted to do was using the soil test kit "Soil Check Now Kit" (which I purchased from Gubba when they had a sale) to check pH levels, along with Nitrogen, Phosphorous and Potassium levels. Each kit comes with a bottle of demineralised water, which is enough to do 10 tests each for pH, nitrogen, phosphorous, and potassium.
After reading all the instructions, and making sure I had the materials needed (like paper cups, measuring spoons, and mixing spoons), it was time to take the soil samples. I decided to test a sample of back garden soil, and also some glasshouse soil. I dug down to a depth of 15 cm of soil in the back garden and the glass house, and took soil samples of each equivalent to a large handful, before laying each of them out on some newspaper, crushing any lumps with the back of a spoon, and leaving them to dry overnight. After this, it was time to begin soil testing...
Glasshouse results:
The pH in the glasshouse was 6.5, making it slightly acidic, and it is the perfect pH for growing most plants.
The phosphorous levels in the glasshouse is high, at around 7 g/m², which is good for plant growth. This high level means I do not need to amend the soil in the glasshouse for my tomatoes, cucumbers, and capsicums.
The nitrogen levels in the glasshouse are high, at around 36 g/m², thanks to compost addition in the glasshouse each spring.
Potassium levels in the glasshouse are also high thanks to compost addition, at around 7 g/m². No amendments need to be done for potassium in the glasshouse at the moment.
Overall in the glasshouse has had significant amendments done to the soil each spring, with the addition of compost before tomato, capsicum, and cucumber plants are planted into the ground. At the end of this growing season I will repeat the soil testing to see if the levels have changed, and then I can make a decision whether more compost needs to be added to the soil in the glasshouse next spring.
Back garden results:
The pH of the back garden is pH 6.75, making it very slightly acidic. Most plants prefer it slightly more acidic than this, so I'll think about how to amend the pH in the back garden at the end of this growing season.
In the back garden nitrogen levels are at a medium level, around 18 g/m².
Phosphorous levels in the back garden are at a medium level, which is around 2 g/m².
And finally, potassium levels in the back yard are high, at around 18 g/m².
For the back garden, the medium levels of nitrogen and phosphorous in the soil, along with the high levels of potassium in the soil, means that the soil only requires moderate levels of fertiliser added over the season as plants take up nutrients from the soil. I will also be adding compost and mulch to all the garden beds in my garden over the coming year to build up the top soil and organic soil layers. Hopefully these soil amendments will improve the soil structure and bring earth worms into the soil.
In the meantime I have bought two organic fertilisers from Seacliff Organics (which is near Dunedin), to test their products on my garden.
First up is their Seacliff Opulent Bloom Mix, which is a topdressing powder to sprinkle on soil around flowering plants, and I plan to use this on my Floret zinnia beds this summer. It contains Fish Bone Meal, Langbeinite, Gypsum, Potash, TM7, BioPhos, and has an NPK of 1.7 : 9 : 5. It also contains a good dose of magnesium which is needed for flowering plants.
I also bought some horticultural coconut water to try, which contains "Naturally occurring bioactive
enzymes, including acid phosphatase, catalase, dehydrogenase, diastase,
peroxides, and RNA polymerases. Coconut water is rich in b-vitamins, sugars, minerals and traces of
calcium, iron, manganese, magnesium and zinc. Coconut Water Powder is
fairly innocuous, making it an awesome candidate for regular watering to
aid general plant health."
Overall, I can't wait to get started in improving the soil health in my garden over the coming years.
Have a wonderful day
Julie-Ann
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