You know that you are leaving the plant ‘thirsty’ when during the day you see leaves becoming fluffy.
PART 3: A simplified science behind irrigation
By Eng. Kant Ateenyi Kanyarusoke
In the last article in the New Vision of December 30, 2016, I suggested that for someone to gainfully use lessons of Museveni's message, it was necessary to understand a bit of irrigation science. The peasants he was demonstrating to may - out of respect - simply do what he did. Without further guidance, they are more likely to be disappointed by crop failure. In this article, I try to give some details these people definitely need in order to reduce chances of failure.
Why irrigate? You irrigate primarily to supply sufficient water to the soil so that the roots of plants can take it up along with other dissolved nutrients to keep the plant healthy. The tips of the roots are the ‘mouths' of the plant. Water and dissolved salts are the ‘drink and food' of the plant.
This ‘drink and food' need to be at the right position in the soil to enter the ‘mouth' of the plant. If too high above the root tips in the soil, or if lower, the plant starves because it has no ‘hands' to pick them or ‘muscles' to move the ‘mouth' to them. Some plants - like millet have many roots - some of which spread sideways in the soil. If the applied water is only high up in the soil, some of these will get it while those lower down will miss it.
In that case, the plant will be ‘underfed' or even ‘malnourished' if some of the nutrients are lower down. The purpose of irrigation, therefore, is to ensure all root tips of the plant are able to be surrounded by wetted soil.
What is the problem, therefore? There are several issues you need to be aware of: One - the plant loses water mainly through the leaves to the atmosphere on daily basis. Therefore, you need to ensure its daily ‘drink' is more than it can lose during the growth period. While some causes of this loss are under its control (e.g. respiration), many others are dependent on weather conditions of sunshine, temperature, wind and humidity (or how dry the air is). In Uganda's dry seasons, all these tend to be adverse.
You know that you are leaving the plant ‘thirsty' when during the day you see leaves becoming fluffy in spite of a daily irrigation routine. The temptation in these conditions is for peasants to attempt watering the plant during the hot day, probably pouring water on the leaves. This will waste water and the latter action could destroy the plant because some weak leaves may fall off.
Alternatively, the water on the leaves may nourish pests on them. The peasants must be advised against this temptation. To circumvent some of these uncontrollable conditions, seedlings - or very young plants - should be kept under shades or even shielded against the wind but without total occlusion of sunshine. Considering that the President appeared ‘incensed' by loss of seedlings provided under his wealth creation programme due to the dry season, these measures should be made clear to all peasants and enforceable on all those receiving government seedlings.
The second problem is that as the plant grows its needs for water increase; its roots spread out and also grow deeper. This means it is not possible to keep a constant irrigation regime without either being ‘extravagant' with water (inefficient) or being a ‘miser' to the plant. Therefore, advice needs to be given to peasants that they will later need more than a single 20 litre container, if this is what is necessary at the beginning of the irrigation season.
Arising out of these two issues is the critical question of how much water to supply to the plant at a given growth stage. There is no straight forward answer because apart from weather conditions mentioned above, it depends on the type of plant and more importantly, on the nature of soil. Barren, loose sandy soil, such as shown in Figure 3 below, requires a lot more water than almost all Uganda peasants' cultivated soil. This is because most of the water simply percolates through the soil and sinks to lower levels where the roots of some plants do not reach it.
This explains why the flowered plant in the picture looks healthier than the spinach in the same area while irrigated spinach plants in a different sandy ‘garden' that received proportionate compost exhibit better health. To overcome such a problem and in the interest of conserving water, peasants need to be advised against their normal practice of mixing plants of different root systems in one garden, if they are to have successful dry season growths.
Some soils are a lot more compact because of presence of too much clay. These do not easily allow water to pass through to reach the root tips in good time before either running off the surface or evaporating into the air. The answer to this is to plough the land properly, breaking up the clay. Then block water runoff (a kind of terracing) and irrigate only in cool and calm (not windy) conditions. This means irrigating early morning before sunrise and late evening towards or just after sunset.
All that said, peasants can only know they are supplying enough water, if in the hot afternoons the leaves do not appear to wilt (i.e. kuhotoka in Runyakitara). They will know they are not being extravagant with water, if they do not see it running out from the irrigated part of the garden or when they pick wet soil from below the surface and squeeze it in their palms, water does not ooze out. Too much water kills the roots just like you can drown under water.
The fourth problem is how to supply the water. This brings us to the types of irrigation available. I discuss them in the next article.
The writer is a pan Africanist Engineering Don and CEO of Progressive Africa Solar Engineering Pty Ltd. in Cape Town
ALSO RELATED TO THIS ARTICLE
PART 2: A brief Irrigation History behind the President's act