Water available to plants: understand the soil and rooting.

Spread the knowledge
The droughts are looming, it is time to talk seriously about water in the garden . For gardeners, the soil is often quite mysterious. Any gardener knows if his soil is “clay, acid, limestone, rich, humus or sandy”, but few understand finely the influence it has on the availability of water for plants.

Today, we are going to tell you how plants are rooted , and what influence soil has on access to water stocks.

1. The depth of the soil.

We can compare the soil to a reserve of elements in which the plant can draw. The deeper the soil , the more water storage capacity increases linearly – ie it stores twice as much in 20 cm as in 10 cm. A soil twice as deep can be enough to provide half as much rainfall – provided that it is twice as intense, and that the roots actually go to the bottom of the ground – 
This is important to understand for any pot holder or bins, that explains why we need to water less often in a large pot. It’s obvious , but we wanted to say it.

2. The behavior of each type of soil.

Draining soil or soil water retention?

It’s super important! Each type of soil behaves differently with respect to water.

    • Sandy soil : the water goes deep down very quickly, and in general it rises little by capillarity, so the plants need very deep roots to get moisture. This type of soil is poorly adapted to the cultivation of annuals that can not form roots deep enough, and it requires watering often in small quantities . Note, however, that perennial plants installed in drained soil have an advantage in case of drought: forced to make very deep roots, they are able to draw several meters deep where water remains for months.
  • Slightly clayey soil : clay stores water between its microscopic layers in large quantities. If these soils lack organic matter they form large clods that trap water, and re-deliver it with difficulty. Hence the importance of the clay-humic complex which we will discuss below. If your soil is of this type, water normally, but straw a lot to avoid the formation of weather-related crusts . Attention, in case of drought the clay releases very difficult its water
  • Very clay soil : if you have a doubt, it is often dark brown and sticky soils. The clay stores a lot of water, but in general it keeps it for her! It takes a very high humus rate to successfully improve the composition of this type of soil. On the other hand, the exchanges of water and nutrients between the surface and the depth are done quite easily. If your soil is of this type, water normally, but be careful in case of drought: if the soil is very compacted, the water may get stuck in depth, and in any case the clay when dry needs to a lot of water to rehydrate. .
  • Silty soil with little clay : they store a lot of water that is easily available for plants because it goes back by capillarity without problem. The risk is that these soils easily form a compact and watertight crust if they are worked in wet weather, which can block upwellings of deep layers. Water normally, but straw and break surface crust if it forms .
  • Heathland and loams : the purely organic substrates have a behavior that varies a lot depending on the fineness of the elements that compose them. The fine elements prevent the water from infiltrating , and they keep it on the spot sometimes at the risk of drowning the roots. The coarse elements allow to infiltrate the water in depth, and do not form capillarity by which the water can go up. These soils do not fear settlement, and re-deliver water quickly to the roots, which requires watering very regularly .

3. The clay-humic complex.

Humus associated with clays makes water stored in the soil more readily available to plants – not to mention nutrients that are not the subject of this article. Which means that by depositing your compost for years in the garden, you can limit watering . It is magic ! Attention, important detail: humus and clays need the action of soil fauna – like earthworms – to form the clay-humic complex , the reaction is not spontaneous! It only works with living soil .

Does the soil’s richness and acidity have an influence?

The suction force of a root must be greater than that of the soil! Simply put, it means that all plants do not attract water with the same force, and that some soils hold water less well. 
Of course, the more dry the soil, the more sucking power it has – and the more the roots have to work hardto suck up the water. 
On the other hand, the osmotic pressure – related to the concentration of solutes in the water – has a negligible influence. So that the soil is rich, poor, acidic or basic does not fundamentally change things in need of watering.

4. How to encourage your plants to produce quality rooting?

That’s the question!

Plants react to stimulations to develop: if they are thirsty, they will develop roots . Conversely a lack of light or frequent cuts will cause the lengthening of the stems and leaves to the detriment of the roots. 
So when you welcome a new plant to your home, once the recovery period is over put it on a diet  by watering little for several weeks! Better to do it in the spring, so that your plants are ready to spend the summer.

4. Automatic watering on large areas, the worst of the worst.

The consequence of excessive “pecking” of the plants – as can be a daily watering – is that the plants develop a superficial rooting . In this case they do not explore deeply and do not facilitate exchanges between the soil layers. In the long run, it causes soil degradation!

You will tell me that it is not so bad for individuals. Maybe not, but before installing an automatic watering on the grass, better know that it will encourage the roots to go shallower, they will recover less nutrients, and it will also bring fertilizer.

Dr. Kimberly Seltzer

Postdoctoral Scholar, UC Berkeley Research Assistant, MIT

Leave a Reply

Your email address will not be published. Required fields are marked *