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Rehydrating Soils – turn your soil into a sponge for your plants


When it comes to growing plants, and especially when growing your own food, having the right balance of air and water in your soil is one of the keys to maintaining good, healthy growth. Water not only keeps your plants hydrated and upright, it is also the vehicle by which nutrients are taken up by plant roots and transported to where they are needed. So, a lack of water also means no nutrients as well, a double whammy that holds back growth. The best possible garden soil we can have for the majority of plants is one that retains significant amounts of moisture, whilst still having good drainage. At first this sounds contradictory but let us explain how it works, and how to create this desirable state in your soil.
The great news is that building both water and nutrient holding capacity in our soil is something that each of us can do to not only help our gardens, but also to help reduce our carbon footprints. We can recycle all of our organic materials (that some refer to as waste but we refer to as natural resource) to nurture and nourish our soils in ways that are also reversing the processes that are making climate change worse. The organic end point of composting, called humus, acts like a sponge for both water and nutrients in your soil. Recycle, Rehydrate (our soil) and Restore (nutrients and humus). In turn this enables us to grow abundant organic produce to feed back into our bodies.
Let’s go back to soil basics to show you how we do all of this. When water enters the soil, either as rainfall or irrigation it moves downwards under the force of gravity and fills up all the pore spaces it reaches. The soil has pore spaces (gaps) of all different sizes, with the smaller pores mainly holding water, while the largest pore spaces usually are filled with air. The smallest pore spaces that predominate where there is lots of clay or humus (well-rotted compost) that will attract water molecules and hold them to their surface against the force of gravity. The water in these pore spaces is what the soil is able to store during the periods between water entering the soil. On the other hand, larger pore spaces do not hold water against the force of gravity so they tend to be filled mostly with air. Therefore, to create our ideal soil that has both good drainage AND holds a good amount of water against the force of gravity we need to ensure our soil has the right balance of finer and coarser pore spaces.
The mineral part of a soil is made up of sand, silt and clay with the main difference being in their particle sizes. The greater the percentage of sand in your soil, the greater percentage of larger pore spaces you will have and hence much better drainage (but poor water holding capacity) with the opposite being true for soils dominated by clay. Humus is the end product of composting and this very fine form of organic matter acts in much the same way as clay in holding a lot of water against gravity. It also acts like a glue between soil particles and helps to create aggregates of soil particles that hold water around the finer particles inside them, but also provide drainage in the larger pore spaces around the outside.
Whatever your soil type in mineral terms, whether it is sandy or clayey, the use of organic soil conditioners (in the form of composts, worm castings and charcoal/biochar) is the best way to improve your soil’s water holding capacity. The use of biochar (charcoal form a camp fire is one form of biochar) goes back thousands of years in agriculture to the Amazon jungles where the original farmers are thought to have improved their soils by adding the charcoal from their fires. Hundreds of years later these soils still have a very high carbon content and are particularly fertile and productive. They are known as terra preta soils. At an industrial scale biochar is made by combusting carbon-based materials such as sawdust and wood chips until they form red hot coals which are then doused with water. We are left with a carbon-rich porous black substance that acts as both a sponge for water and nutrients, but also as a ‘hotel’ for beneficial microbes that live on the nutrients. The extra benefit of biochar is that it is actually storing lots of carbon in the soil while it is also doing so much good for plant growth at the same time. This is a way we can all do something positive about our carbon footprints!
Compost and worm castings are other powerful ways to rehydrate our soils. These carbon-rich forms of organic matter eventually break down into humus that not only clings on to water and nutrients, but also acts as a glue between mineral particles such as sand and clay. This in turn leads to aggregation of soil particles as mentioned above. Whether you have sand or clay-based soils the addition of these organic materials will be beneficial to water storage.
Whatever composting system you use, whether it be worm farm or compost bin, you need to make sure that the compost is fully mature before you use it. This ensures that the compost or castings are as fully broken down as possible and do not require any more nutrients to supply the composting microbes that break them down. When you think your compost is ready to use sow some fast-germinating seeds such as radishes and if they produce happy, healthy seedlings within a couple of weeks you know the compost is ready to use.
Raising carbon levels in our soils is a great idea in so many regards, with water holding being one of the very best. Whether it is compost or biochar we can help reduce carbon emissions into the atmosphere while turbocharging plant growth in our gardens and on our farms.

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