Having a good understanding of the soil on your permaculture plot is key to making it productive. The soil, as the medium in which plants grow, is the bedrock of your site. Soil can be changed over time, and plants can adapt to those changes, but having the knowledge about the make-up of your soil will make your planting and soil treatment more effective.
The properties of the soil were originally determined by the material that went into its original manufacture – the rocks which in the past were weathered down into soil particles. However, agricultural and permaculture practices can also have an impact. Here are the major properties of soil.
The texture of the soil refers to the proportions of three major materials within it: sand, clay and silt. Particles of these materials are of differing sizes, and their presence in the soil will affect the soil’s ability to retain nutrients and moisture. Sand particles are the largest of the three, so a soil with a higher proportion of sand in it has more space through which water moves, meaning it dries out quickly. Clay has the smallest particle size of the three, and so a soil heavy in clay will retain water well, but have a slower rate of infiltration. Silt particles are somewhere in between. As well as affecting moisture retention, the soil texture directly influences the soil’s fertility, because they provide surface area to which nutrients can adsorb (the process by which molecules adhere to a surface). Clay particles provide more surface area for this to occur than sand particles. Soil that has a good proportion of each of the three materials is an ideal growing medium, and is called loam.
To get a basic idea of the texture of the soil on your plot, simply pick some up and feel it. A sandy soil will not adhere into a ball and have a gritty feel. A clay soil, by contrast, feels smooth and almost plastic to the touch. Loam will have a springy feel.
Linked to the texture is the structure of the soil. This refers to the way the particles of sand, clay and silt aggregate together (form clumps) and the spaces that form between and within them (called pore spaces). The spaces between aggregates are larger than those within, and as such are the key to the soil’s drainage ability and aeration. The spaces within aggregates, being smaller and more tightly packed hold water. As with many things concerning soil, a good balance is required to create a decent soil. Stable aggregates combined with plenty of pore spaces are best. Adding organic matter will help do so.
The biological property of the soil refers to the amount of microorganisms that are active within it. These include everything from earthworms and insects to fungi and bacteria. In a healthy soil, the number of organisms living in the soil will be high, which in turn helps to keep the soil healthy, by processing nutrients and improving soil structure, which helps plants to grow. Healthy soil biology also means that populations of species are kept in balance, with some organisms predating on others so they do not get too high. The amount of organic matter available, adequate soil moisture, as well as good drainage and aeration of the soil affects the biological property of the soil.
Here’s the science bit. Cations are positively charged ions of nutrients that are in solution. Basically, for plants to access nutrients in the soil (such as nitrogen, phosphorous and potassium, those nutrients need to be in a soluble form, mixed with water. When the atoms of the element are in water they become electrically charged, either negatively (called an anion) or positively, the cations. The positive charge allows the cations to adhere to soil particles (the process of adsorption) from where plant roots can access them.
A bit more science. The pH level of the soil refers to the concentration of positively charged hydrogen ions in the soil moisture. With more of these ions, the soil becomes acidic, while fewer indicate an alkaline soil. The pH of a soil is measured on a scale of 0 to 14 (the lower the number, the more acidic the soil), and the position on the scale is likely to affect levels of essential nutrients in the soil. For instance, a soil with a pH level lower than 6 is likely to be deficient in potassium, calcium and phosphorous, while a soil with a pH of higher than 7 will probably lack sufficient iron, zinc and copper for healthy plant growth. The best way to ensure a healthy pH in your soil is the addition of organic matter. This will provide nutrients that the soil may lack, as well as protect plants from the effects of excess acidity or alkalinity. If you have a very acidic soil on your site, the addition of agricultural lime can help lift the pH level. However, you can also plant species that require certain pH conditions to thrive. Blueberries, for example, grow well in acidic soils, while the brassica family of vegetables (such as cabbage) prefer alkaline soils.
The water within a soil, combined with the nutrients in solution within it, affects the salinity of the soil. If moisture levels in the soil are low, particles of sodium, calcium, magnesium, and potassium increase, meaning the soil becomes more saline. This can be exacerbated if, due to lack of moisture at the surface, plants draw up moisture from the groundwater that, in turn, evaporates, leaving the salts behind. If the salinity of the soil increases too much at the surface, it will negatively affect the growth of plants within it. Ensuring soil receives and retains sufficient moisture (through diverting runoff to dry areas, mulching to prevent excess evaporation, etc.) ensures salinity does not become a problem.
As with many things in permaculture, none of these properties stands alone; all are impacted by changes in the others, and all are in an ever-evolving relationship with the plants that are growing in the soil. By understanding the properties of the soil on your site, you can make sure that you are planting species that suit the soil conditions, and that you are aware of how making changes impacts upon the soil system.