Dense, low albedo clay textured soil with blocky peds. Not much grows there!
While I did not think that soil was a major contributor to the microclimate, it turns out that it does have an important part to play and so should be taken into account when identifying, modifying and creating new microclimates. The impact of soil on the microclimate is due to its –
- Thermal conductivity
- Albedo (solar radiation reflectivity)
- Texture and structure
- Water content
Thermal conductivity
Impact - a dense soil (such as clay) will heat up more slowly, but retain solar heat for longer than a less dense sandy soil. The sandy soil will heat up more quickly, but also give up that solar heat more quickly when the sun goes down or is hidden by clouds or foliage. Soil thermal conductivity declines with increasing organic matter in the soil but increases with increasing soil density and soil moisture.
Modification – The thermal conductivity of soil can be modified by cultivating the soil (tillage) to introduce air and reduce bulk density in a heavy soil, although the downside of tilling the soil is the negative effect it will have on soil biota. Adding water to the soil (irrigation) will also improve the thermal conductivity of the soil and adding organic matter will reduce thermal conductivity.
Albedo
General – Albedo is the reflectivity of a surface (in this case soil) to solar radiation (light and heat). High reflectivity is referred to as high albedo and low reflectivity is (strangely enough) referred to as low albedo.
High albedo organic mulch
Impact – A light coloured soil (high albedo) will reflect more solar radiation and therefore heat up more slowly than a dark coloured soil (low albedo) which will absorb the solar radiation.
Modification - A soil rich in organic matter will, in general terms, be darker than one deficient in organic matter so adding organic matter to your soil will also increase its ability to absorb solar heat and reflect less light. Reflectivity can also be reduced by applying charcoal to the soil (like biochar), which will also aid in carbon capture as well.
A low albedo is a great thing in spring when plants are starting to come on after winter, but can obviously be a problem in a hot summer. This is where mulch comes in, being raked back from plants in early spring to allow more heat to be absorbed by the soil, but re-applied to beds in late spring/early summer depending on the macroclimate in your area to reduce the heat input to the plant’s root zone.
Also, in a hot area a light coloured rock mulch will reflect light and heat away from the soil, keeping it cooler, but allowing rain to percolate down to keep the soil moist.
Texture and Structure
Glass Jar Test
General - Soil texture is generally described in terms of the percentages of clay, silt and sand present in the soil and can be determined by a glass jar test to determine percentages and then referring to a phase diagram to identify your soil texture eg clay, clay loam, sandy loam etc. A soil composed of 20% clay, 40% silt and 40% sand is referred to as being ‘loam’, an ideal soil for growing vegetables. Soil structure is how the individual soil particles of clay, silt and sand group together to form aggravates (called peds). These aggregates will vary in size from very small crumbs to large blocks.
Impact – A clay soil will hold more water for longer than a sandy soil, which will be more free-draining. Clay soils will also swell when wet and crack when drying out, allowing water to penetrate when it rains. Plants which don’t tolerate ‘wet feet’ such as citrus, will not be happy in a clay soil and thirsty plants like bananas will require extra watering if planted in a sandy soil.
Our soil has a high clay content, and it cracks pretty dramaitcally when drying out
Modification – in general terms if you want to improve water holding capacity of a sandy soil and improve drainage of a clayey soil, the fix in both cases is to add organic matter such as compost. The structure of a clay soil can also be improved (in sodium clays) by adding calcium salts, gypsum (calcium sulphate) if the pH is where you want it to be or agricultural lime (calcium carbonate) if the soil is too acid.
Soil moisture
Impact - Damper soil ie, higher soil moisture has a cooling effect on the soil surface and lower wind speed near the soil surface whereas dry soil tends to be warmer and have a higher soil surface wind speed. High levels of soil moisture also encourage growth of soil biota and the resultant benefits to plant growth and biodiversity.
Modification – soils, particularly sandy soils can become hydrophobic if they have been dry for some time, it is easy to check, just pour on some water and if it runs off or pools rather than sinking in, it may be hydrophobic. Wetting agents are a short term fix but a better, longer term fix is by adding organic matter to your soil and mulching.
Measuring soil moisture
In general terms, adding organic matter and using an organic mulch will improve the water holding capacity of just about any soil, as well as all the other advantages listed above.
Conclusion
The soil aspects that affect the microclimate do not have their impact in isolation. For example modifying the soil moisture content by irrigation will also impact its thermal conductivity and adding organic matter to a soil can impact its thermal conductivity, albedo and structure. It is therefore important to understand what changes to the soil that need to be made to achieve the desired microclimate and what side effects these changes may have in other ways.
Needless to say the soil characteristics mentioned above (Thermal conductivity, albedo, texture and structure and water content) may also be impacted by changes made to other aspects of the microclimate.
Previous Articles
What are microclimates and why are they important?
Identifying your microclimates
Modifying and creating microclimates - Temperature and light
Modifying and creating microclimates - Air circulation and water
