Our elders told us to lead in the water, soak the soil, and have the plant feel good:
“Wet soil means bountiful yields and beautiful fruits"
That was true when water was plenty, but it seems that those times are gone by. Now, that water is the limiting resource, the allotted water must be stretched over the largest possible cropping area to maximize income. Wasting water by over irrigation is actually a waste of cropping area and income.
Irrigation can be defined in precise language as:
"Manipulation of soil water content to achieve desired crop responses by artificial water supplement to natural precipitation"
Meaning that, we add water to the soil to control crop development in order to get the highest economical returns. Remembering that irrigation is crop specific, some crops need more water in certain development stages - as for “pumping” fruit for larger size, or stressing periods in other crops to reduce growth, or to concentrate sugars in maturation.
The soil is the medium where crops grow and water is applied. It consists of granular solids with voids between them filled with water and air. In light, sandy soils the granules are coarse and the voids in between are larger, but the total void volume is smaller. In heavier soils the granules are finer and the voids much narrower, but the total void volume is larger. Water is attracted to the granules by adhesion forces to their surface. In heavy soils water is held more strongly because the fine granules have larger surface area. Coarser sandy soils hold less water, and less strongly. In light (sandy) soils, water moves faster in the large voids, and in heavy soils water moves much slower (Soil physics in a nutshell...)
The ability of the soil to hold water is measured by the pressure applied to extract the water. It is measured in atmospheric pressure units - centi-bars or kilo-Pascals. Looking from the plants perspective, pulling out the water by suction (negative pressure) we talk about soil water tension. In fully saturated soil the tension is 0 (zero). After a few days of free drainage, when the largest voids become empty, in most soils, the tension raises to 10 to 15 kPa. Plants can extract water quit easily until the tension rises to 30 to 40 kPa. In these range we will find about 10% to 15% volumetric soil water content, and that is the useful range of water available for most crops in the active growth stages. In sandy and very light soils those values are much lower. For instance, in sandy soils the useful range of water available for most crops may be as small as 5% volumetric soil water content.
Soil water tension is measured with tensiometers, tools in use for many years now. There are other, more modern tools also, however all of them, including tensiometers, need expertise in use and professional advice to learn their utilization.
Measurement of soil water content, tension and other water related properties is a very broad subject, to be discussed in another place.
Irrigation scheduling answers the two eternal questions in irrigation: WHEN to irrigate and HOW MUCH?
The link between those questions is the critical water tension (and the related water content) to which we can allow the plant to use up the soil water without hindering development. In stress tolerant crops the soil water can be depleted to a higher tension and irrigation may be in larger intervals and with more water at each application, than in a stress sensitive crop, which will be irrigated more often and with less water each time, even several times a day. This is complicated by the fact that heavier soils contain more plant available water, and so can supply water to the crop for a longer period, while sandy soils contain less plant available water and so can supply water to a given crop for a relatively shorter time before irrigation is needed.
The active root zone is the soil water volume to be refilled at each irrigation. After winter rains, the whole soil profile is wetted in and between the crop rows. When micro irrigation (drip and micro sprinklers) is started, the active soil volume shrinks to the effective diameter of the applicator, and to the depth of the wetting in irrigation. In high frequency irrigation, amounts applied each time will be low, so the active volume may shrink much more than in low frequency irrigation, where larger single amounts are given. Calculation of the water amount in a single application must take into account the ability of the soil to absorb water in the vacant voids of the active soil volume. Over irrigation will lead to loss of water to deep percolation.
So the answers will be:
When to irrigate? Apply water when the water tension (and the related water content) in the active soil volume rises (or, the water content is drawn down) to the critical point where the plant will suffer. This point is called the “Irrigation Threshold”. This point is crop dependent, but as a rule of thumb for most crops and soils in the active growth period, will be about 20 to 25 kPa.
How do we know that we have reached the irrigation threshold?
Water balance method: If the usable soil water content to the threshold in the active root volume is known, just sum up the evapotranspiration (ET) for the crop (using the MERIMIS data) until the depleted water reaches the threshold.
Stress monitoring method: It is possible to follow soil dry-down with tensiometers (Caution, that needs experience!) and irrigate when the threshold is reached. Experienced growers can read visual signs of stress on the crops. There are many other crop stress monitoring devices, like pressure chambers, dendrometers, fruit size growth rate, main stem elongation, leaf temperature and others to use, each crop with its specific tools. Consult your crop advisors for the best tools in your situation.
In a fixed irrigation interval situation, “when” is not a question, only “how much”.
How much to irrigate: The simple answer is to return the water lost from the soil since the last irrigation, according to the MERIMIS ET data.
The basic ET amounts should be adjusted for application efficiency, application failures, surface evaporation, and for desired crop responses if necessary.
Thinking about the active soil volume as a pail of water, and the evapotranspiration since the last irrigation as the water emptied from the pail, it is easy to think about the irrigation as returning the lost water to refill the pail. If we will refill too much, the pail will overflow – in our case the water will be lost to deep drainage. If we will refill less, at the end of irrigation the pail is not yet full, and if we will continue to under irrigate more times, the pail will go empty. That is why proper estimation of the water used up between irrigations is so important, and that is what we do in the MERIMIS
