Mary, Mary quite contrary how does your garden grow?

I don’t know about Mary or why she’s contrary, but I am a self-confessed pandemic gardener! That is to say, it takes a pandemic and a period of social distancing to turn my attention to the garden. But with all garden centers closed, what materials science skills can I draw upon to sort this mess out?

Figure 1. Social distancing opportunities in the garden
Figure 1. Social distancing opportunities in the garden

Soil analysis of course! If you want your grass, shrubs, bushes, and trees to grow, they’ll need the right kind of soil. For most gardening applications a loamy soil is required, but what makes it so? Particle size distribution, naturally! Soils are composed of clays, salts, and sands. Each of which differs in particle size, whereby clays are the finest at < 2µm, sands are the coarsest at >50 µm and silts are in between. This composition will impact aeration, water drainage, and nutrient retention. A plant of any kind will need a balance of all three properties, hence the percentage of each component is sought. The ternary phase diagram in Figure 2 is from the Soil Survey Field Handbook: Describing and Sampling Soil Profiles by Hodgson in 1974 and shows that loam is approximately 20% clay, 40% silt, and 40% sand.

Figure 2. A ternary phase diagram of soil
Figure 2. A ternary phase diagram of soil
Soil Survey Field Handbook: Describing and Sampling Soil Profiles by Hodgson in 1974

Most experienced horticulturalists will estimate particle size distribution, or texture, by rubbing it between their fingers. But for novices like myself who want a more quantitative approach, laser diffraction is the obvious choice. With a statistically relevant sample measured in minutes, the Mastersizer 3000 can reliably differentiate between soil types. Figure 3 shows the size distributions of two different types of soil with differing proportions of clay, silt, and sand. The X-axis uses Phi units, whereby higher positive values represent finer particles and lower or even negative values represent coarse particles. Using these size distributions and the Soils report function on the Mastersizer 3000 software, the volume percentage in the relevant size bands is calculated to determine what soil type they are.

Figure 3. Particle size distributions of loamy sand and silty loam and their size fractions.

Great! I can now start tilling with the confidence of having the right soil type. Ah that’s better…

Figure 4. The result of robust soil analysis, hard work… and a picture of someone else’s garden
Figure 4. The result of robust soil analysis, hard work… and a picture of someone else’s garden

Okay, so you won’t find the Mastersizer 3000 on sale in your local garden center and I imagine most gardeners will stick to traditional methods for texture analysis. But there is a greater concern than the bloom of your begonias which relates to particle size, and that’s soil erosion. England and Wales lose 2.9 million tonnes of topsoil, annually. The implications of this to agriculture are stark – no soil, no food. Additionally, there are secondary factors relating to the environment including carbon storage, pollution and flooding. To mitigate these risks the British Geological Survey mapped the soil texture (Earthwise Magazine, Issue 25, pages 22-23) in a region in the East of England using laser diffraction.

Figure 5. Proportion, as a percentage, of silt (particles between 2 and 60 micrometers in size) in topsoil across Eastern England (A. Tye, B. Rawlins, R. Lawley, Mapping soil erosion risks, Earthwise Magazine, Issue 25, page 22)

Subsequently, linear correlations were made between this particle size dataset and elemental analysis using X-ray fluorescence (XRF), where a much larger dataset is used to create an atlas for soil texture across the UK. This atlas is hosted by the UK Soil Observatory with the intention of giving guidance to land management practices for sustainable agriculture. But the data is also summarised to the wider public in the app; mySoil, for quick identification of the soil type in a given area and improved horticulture.

Happy gardening folks!