Organic matter the key to holding water: book extract

Summer comes and everything in the garden goes, or rather grows, into double time.

You need to be everywhere, picking and planting, weeding and watering. And watering and watering. I know it seems basic, but you’d be amazed how many problems that under-watering, over-watering and even irregular watering causes, especially over summer.

Forgive me for stating the obvious but plants need water. Water is as vital to plants as it is to you and me. And gardeners think watering successfully is all about the action of watering, as in how and when you apply water. But actually, it all comes back to the soil (spoiler alert: in gardening, it always comes back to the soil).

Because when you water a plant, you are applying water to the soil in the fond hope that your vegetable crops will get the benefit. But good soil is both a sieve and a sponge. A sieve in that water needs to be able to move through it. If soil stays saturated, then all the pores — those spaces between the mineral particles and organic matter that make up soil — fill up with water and there is no oxygen available to diffuse into the roots. Your plants will effectively drown.

But you also need soil to be a sponge, as in to hold water around the roots so the plants can take it up as they need it. With large pores (or macropores, which have a diameter of more than 0.08mm), water moves easily through them and drains away. But smaller pores (or micropores, which have a diameter of less than 0.08mm) are small enough that surface tension holds the water in place. But with very small pores, like you find in very fine, silty soil, the water can be held so tightly to the particle surfaces that the force of osmosis in the plants’ roots is not strong enough to pull it up. So even though the soil is holding on to any water you apply, it is still not available to your growing crops.

So, watering is not only about how much water you apply but about how much water your soil is able to hold around a plant’s roots. That water is called plant available water (PAW). The PAW in a loamy soil can be three times as high as a sandy soil after both soils have been wet to the same degree. And the easiest and quickest way to increase the PAW in your soil is by adding organic matter.

Organic matter in soil — whether that’s compost you have applied, winter green crops you have chopped and dropped, or any other soil amendment you have applied that was once alive and is now in a process of decomposition — acts like a sponge. It absorbs and holds water. Some types of soil organic matter can absorb up to 20 times their own weight in water — and that water is released slowly as plants need it.