The moisture of the soil is an essential concept either in intensive, extensive, orchards organic crops, plants a house and everything that has to do with plant development. We all imagine what it is, although today we are going to delve into something more. We get into the ground to see how the water behaves inside.
WE KNOW THAT THERE IS WATER IN THE GROUND, BUT HOW DOES IT BEHAVE?
We start from the fundamental law of all life on the planet whose need for water is unconditional. We are linked to it in one way or another and plants are no exception. We could think of cacti but they need water like any other (in less quantity) only that they store it and barely perspire so that it does not escape. In the article published a long time ago by Eduardo, he told us in detail how water influences plant growth and how it should be watered .
Once the importance has been commented, the usual means of absorption of water by the plant kingdom is the soil. The water contained in it. This brings us to the term soil moisture . What soil moisture should I have for the plants? This is too broad a question and the answer as always is: it depends. It depends on the species, the conditions, the type of soil, its structure, its composition, but above all on the first: the species. The plant is what determines its tolerance to a greater or lesser amount of water in the substrate. Its roots are those that rot or not, depending on which plant we are talking about.
Mentioning two extremes: the root of the cactus is very little tolerant to an excess of humidity and it rots quickly. It will have happened to more than one that he has watered a cactus more than necessary and has finally died. And on the other we have the water lily or related swamp plants, which literally live in the water without flinching. Its plant structure is designed not to “drown”. Therefore, the term adequate soil moisture is highly variable in the plant world.
HOW DO WE DEFINE SOIL MOISTURE?
It is easy to think that soil moisture will simply be the water that the soil contains, without more. This statement is true, but somewhat biased from the agronomic point of view. We have to define when the soil moisture is measured. In very sandy soil, right after a rain, the humidity will be high, but how long does that last? The water will drain just as quickly as it has fallen and will be unavailable to plants in a matter of hours or even minutes. The interesting thing is to see what capacity the soil has to retain that water and that it is available to the plants and also that the latter tolerate the retained water. This leads to thinking, how does the water get trapped in the ground, what phenomena take place.
HOW IS WATER FOUND IN THE SOIL?
First and foremost:
Free water: It is the fraction of water that is housed in the largest pores of the soil without any molecular force that prevents it from moving freely (hence the free). The force of gravity is what makes this water move towards deeper layers of the soil relatively quickly. The most extreme example is thinking of beach sand. The gaps between sandy particles are very large. The water will percolate rapidly due to the effect of gravity. This water is quickly lost through the drainage of the soil. The direct deduction is that this water will not be available to the plants.
Capillarity phenomena. More than once we have heard of the surface tension of water. When we see a Gerris lacustris (commonly called a shoemaker) walking on water, we are facing a phenomenon of surface tension. That is, the force exerted by the insect on the water does not exceed the binding forces of its surface tension. This same surface tension exists in soil water and causes capillary movements in the soil. For this, the soil structure must be very different from that of a sandy soil. The pores or spaces between particles must be much smaller (micropores) than in a sandy soil (macropores) and this allows surface tension forces to retain that water. From a nutritional point of view,this is the fraction of soil water that interests us. Most of the capillary water is what the plants can absorb without difficulty. These bonding forces are not very strong and the roots can overcome them to absorb this water.
On the other hand we have the water that is part of the organic matter housed in the soil. Organic compounds have water in their molecular constitution, except that it is not available either. The same happens with water adhering to soil particles . Their union is so strong that it will not be available either. These two fractions of soil water are practically insignificant. First, because they are very small fractions, and second, because they are not available to the plant. But they exist and must be mentioned.
From this we can deduce that of all the forms of water present in the soil, the one that interests us is the capillary:
Soil moisture content =
Free water+ capillary water + molecular and adherent (hygroscopic) water
Taking into account that the last two fractions are not very significant and that free water drains relatively quickly, we can establish that the water-holding capacity of a soil corresponds for the most part to capillary water. And of this, a good part will be available for plants. We already have our definition of soil moisture, from the point of view of agronomic interest. And from here, two terms widely used in the agronomic world are deduced that indicate the interval for which the plant is with adequate levels of humidity or available water. This interval is the one that will determine to a large extent, the intervals and frequencies of irrigation.
- Field capacity: It is the maximum amount of water that a soil can retain after free water drainage. If we take into account the above, the fraction of water corresponding to the field capacity will be practically equivalent to capillary water.
- Wilting point: It is the moment when there is no water available for the plants and they begin – as the term to be defined indicates – to wither.
AND HOW DO WE MEASURE SOIL MOISTURE?
This is a difficult point. The amount varies a lot on the ground. As we well know, the homogeneity of a soil is almost always conspicuous by its absence. The most accurate methods are usually laboratory methods, pressure chambers, gravimetry etc. that determine the wilting points, field capacity, etc., of a homogenized soil sample. These methods give us the most accurate and reference values, but many times we need to know the approximate values almost in real time, in order to act accordingly with our crops.
Measurement “in situ” is mainly carried out by tensiometers or currently by electrical methods. Current moisture meters are capable of determining Field capacity and Wilting point values in seconds. We put them in English because the instrumentation usually comes in this language.