Picture credit: MesserWoland licensed under CC BY-SA 3.0.įascinatingly, small pores act to draw groundwater up through the soil, providing a source of water in the absence of rain. Hence, macro and micropores complement each other, allowing air and water to reach plants’ roots. Micropores retain water, available for use by plants. Macropores drain too quickly to be of much use to plants, providing little water, but allowing flows of oxygen to plants’ roots. The smaller the pore, the more tightly the water is held. Micropores remain filled and are unaffected by gravitational flow, the water held by electrostatic attraction. Larger pores are the first to drain with light sandy soils taking about a day and heavy clay soils about three. When it rains both macro and micropores become filled with water. The increased porosity has its origin in the fact compost is significantly lighter than conventional soils.Īn ideal soil has a porosity of about 50%, equally divided between micro and macropores, which provides a good mix of drainage and retention. These peds have adequate pores to allow entry of air and water, both which are essential to plant health. Secondly, compost improves soil structure by gluing tiny particles of rock (sand, silt, or clay) together into peds (aggregates), which is the basis of all good soils. These organisms function to increase a soil’s porosity - the percentage of soil that is pore space or voids. Firstly, compost contains carbon, as well as other nutrients, that provide food for soil organisms. While compost does add nutrients to the potting mix, its major advantage is improving the water-holding capacity of the soil, which occurs through two mechanisms. Lastly, using garden soil will save you money and lower your environmental footprint. Secondly, garden soil will help improve drainage, ensuring your planter does not become waterlogged. However, there is the possibility of inducing pests and diseases, so we recommend avoiding soils that you have previously planted. These organisms will help improve the structure of your soil and break down organic matter into mineral nutrients, available for uptake by plants. Now why do I want to mix compost with garden soil? Firstly, garden soil is incredibly complex with numerous soil organisms that help boost your plant’s health. Calculating a planter’s volume (measure in litres) is relatively easy, but it is first important to work out the compost/garden soil ratio required for a particular plant. Compost is important as it will improve your soil’s structure, increasing its available water capacity (AWC), which is especially important for planters. The amount of compost needed for a planter depends both on planter size, its material and the plant you wish to grow.