To protect plants successfully from frost, the water should be applied continuously,
especially during the time when thawing occurs. However the total amount of water
used should always be kept to a minimum, both to minimise the ice load and to avoid
waterlogging the soil. A balance is required ... not too much, not too little!
Filtration
Problems with Clogging
Irrigation water that is a bit “soupy” (more than about 1000ppm TDS) should be filtered
to some extent, unless surface or subsoil irrigation is used. Nozzles, water channels and
pipes can and do become clogged with a range of foreign bodies, including organic
matter, iron oxide, calcium carbonate, algae, fungi and microbial slimes.
Clogged nozzles reduce emission rates and cause uneven water distribution, as well
as increasing the amount of water usage and wastage. The problem can become very
serious and can only be solved by either cleaning/filtering the water or, sometimes, by
using bigger nozzles, which in turn can create its own problems.
Filtration methods are many and varied: sand media, wire mesh screens, strainers,
centrifugal separators and settlement tanks. In order of importance, it is increasingly
desirable to clean or filter water for:
misting, fogging, drip, spray, sprinkler, bubbler and mobile sprays.
Saline Soils and Irrigation Water
A) Problems with Saline Soils
Saline soils are also discussed in 1Ca.
If the irrigation water, or the soil, is saline, plants can only use some of the water
because the osmotic pressure exerted by the salts in solution reduces the intake of water.
As a result, salts accumulate within the plant, eventually reaching a level that becomes
harmful to plant growth.
As a general “rule of thumb”:-
“very saline” soils have more than 3 mmhos/cm^1 (or about 1920 ppm TDS);
“moderately saline” soils: between about 1 and 3 mmhos/cm^1 (640–1920 ppm TDS).
{1 mmho/cm^1 = 1 mg/litre = approx. 640 ppm TDS}
Solving the problem of saline soils generally involves leaching the soil, to wash away
the accumulation of salts to below the bottom of the root zone. Obviously, the amount of
water required to do the job depends on the quality of the water used to leach the soil,
the salt content of the soil before it is watered and the soil type.
The land must be well drained, and many soils need to have a drainage system
installed before the salts can be properly leached out.