Water use and salinity
Potatoes are moderately tollerant to salinity. It may be a major constraints in irrigated potato cultivation.
Tollerance and yield
Salinity problems
The exposure to saline conditions can have different causes:
- The crop is grown on soil that is saline by nature.
- The stress can be caused by the use of saline irrigation water
- The salinity problem may arise as a consequence of salt accumulation.
Water of mediocre quality may not have a salt level high enough to cause direct stress.
But due to evapo-transpiration the salt level may gradually increase in rooted soil layers.
The salt level of the soil water then becomes so high that salinity stress is unavoidable.
Regular irrigation or frequent flushing will solve this problem.
In some cases flushing with relatively brackish water is already a solution
- Salinity problems may arise as a result of improper fertiliser application (too close to the plant). This is especially the case if weather conditions prevent a slow release and regular uptake.
Description of salinity
Saline conditions can have three different negative effects:
- it can affect soil structure
- it can directly impede crop growth and development
- it may have indirect effects. An example of the latter is the possible interaction between salinity stress and biotic stress.
Salinity is usually assessed in terms of content of soluble salt.
This content is measured by the electrical conductivity (EC) of water.
This parameter is expressed in dS/m (deci Siemens per meter) or mS/cm.
Effects of salinity on soil
A high salinity of the soil can deteriorate the soil structure.
This happens when calcium ions are replaced by sodium ions in the exchangeable ion complex.
As a consequence, the infiltration rate of water and the soil aeration are reduced.
This will only occur in extreme cases of salinity.
Direct effects on crop growth and development
Direct effects of salinity on crop behaviour can be immediate or brought about in the long run.
An example of an immediate effect is disruption of chloroplasts, resulting in reduced photosynthesis.
Toxic effects happen when high quantities of ions are taken up.
This may occur through the roots (in the case of furrow or drip irrigation).
It may also occur through absorption by leaves, in the case of sprinkler irrigation.
An example of a long-term effect is the reduction of leaf size, resulting in less light interception.
The most common effects of salinity are suppressed canopy growth and photosynthesis.
Cultivars may differ slightly in their response to salinity.
Direct effects of salinity
Indirect effects on crop growth and development
Salinity affects the crop behaviour to such an extent that its susceptibility to other stresses may change.
A well-known example is the interaction between salinity and Verticillium wilt.
Under salt stress, the yield decline caused by Verticillium wilt is much greater.
Also for other fungal diseases, interaction with salinity has been reported.
Examples are early blight, stem canker and black scurf.
Quantification of effects on yield
There is a certain threshold level below which crop growth is not affected by salinity.
In the example this threshold is about 1.2 (sprinkler irrigation) or 2.7 mS/cm (drip irrigation).
Above this threshold, crop growth is reduced linearly with an increase in salinity
Salinity and yield
The figure shows the relative yield as affected by salinity.
Yield reductions of 50% are expected when the ECe is almost 6 mS/cm (sprinkler irrigation).
For drip irrigation this 50% point is at much higher salinity (about 10 mS/cm).
Potato is ranked as a moderately salinity-sensitive crop.
It is more sensitive than crops like sugar beet or wheat, but less so than onion.
Effect of soil salinity as a result of irrigating (through sprinkler irrigation or drip irrigation) with saline water on tuber yield.
Practical implications for irrigation
As indicated above, the effects of a high EC depend on the method of irrigation.
With sprinkler irrigation the salt water immediately gets in touch with the foliage.
The indiscriminate absorption of ions by leaves may affect photosynthesis or cause structural damage.
With drip irrigation the water enters the plant after selective uptake of ions by the roots.
Solution
- Effects of salinity may be minimised by increasing the quantity of irrigation water.
- An alternative is to increase irrigation frequency.
- Leaching salts downwards before planting and during the cropping season will prove beneficial.