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SUB-SURFACE DRIP IRRIGATION ON LUCERNE

14 May 2013

AGRO-TECHNICAL PLANNING - REPLANTING

In South Africa the oldest SDI systems on lucerne are now more than 10 years old and the crop will have to be replanted. There are mainly two considerations involved; the crop and the irrigation system.

Questions often asked regarding replanting of lucerne that needs to be answered:
1.    Can I directly re-establish lucerne on the same field?
2.    Which rotational crop can be planted?
3.    Can I use the old drip laterals or should I install new ones?
4.    How should I remove the old drip lines?
5.    What cultivation is necessary before replanting?
6.    What should I do regarding chemical imbalances in the soil before replanting

THE CROP
The life expectancy of lucerne is in general less than ten years. A decline in vigour and weed infestation are the biggest reasons for the reduction in yield.  Once the lucerne stand declines to less than 65% it is advisable to replant.  Sometimes when the vigour declines but the stand is still fine a phosphate application seems to have a positive response. This expected reaction can only be ascertained through soil analysis. It is advised to replant in phases as not to lose all production during replanting. The best time to establish lucerne is during autumn. Plan in advance to spray lucerne and weeds with herbicide and have enough time for all replant actions.
It is not desirable to try and replant lucerne on the same soil directly after the previous lucerne crop. Lucerne is subject to auto toxicity or allelopathy which means that the plant excretes toxins into the soil which inhibits the germination and growth of its own seedlings.
The duration of the inhibiting period is correlated to the amount of organic matter present before replanting. When lucerne is killed with an herbicide and proper soil tillage done the problem seems to be less. Climate and soil type also seems to play a role; drier areas and heavier soils with more organic matter take longer to get rid of inhibiting agents. According to Dr Johann van Heerden the cultivar also plays a role with SA Standard germinating better in replant situations.

Rotational Crop 
It is advised to plant a rotational crop with the purpose of ridding the soil of allelopathy and continued use of the land for fodder production. With this irrigation method the aim is to wet the entire soil volume.  Thus, any other crop with a relative deep root system can, theoretically, be cultivated.  For instance, maize and wheat are already successfully cultivated with this type of irrigation.  Emergence could be problematic due to the fact that water must reach the surface for seed to germinate.  The possibility for these practices would depend on the soil type and the likelihood of rainfall during establishment. It is mostly recommended to plant a small grain crop like oats for one season before replanting Lucerne. Teff is also a possibility. These crops have aggressive deep root system which will improve soil structure and help control weeds.  

Soil physics
The success of this type of irrigation system is dependent on the root system of the cultivated crop to extract soil water from deeper soil layers. It is thus of the utmost importance not to have any compact soil layers that could restrict deep root development. The same applies towards the follow-up lucerne crop. It is therefore advised to ascertain yourself regarding root development and possible soil compaction or restricting layers by soil profile inspection. This would primarily determine the need for a deep tillage soil preparation to be carried out. In the absence of any root restrictive layers at depth the soil can be cultivated quite shallow – primarily to incorporate the soil chemical amendments or do seedbed preparation.

Soil and water chemical composition
After ten years or more of the soil being irrigated the chemical suitability or deterioration of the soil towards crop production will primarily be a function of the nutrient (phosphates, potassium, etc.) extraction of the previous crop and quality of the water being used for irrigation. The balance between sodium and other cations like calcium and magnesium influences water movement through the capillary pores in the soil.  This type of irrigation system will over time lead to varying concentrations of chemicals in the soil related to the mobility of the chemical, depth and position from the drip line.  No two situations will be the same so it is of the utmost importance to take representative soil samples and have it analyzed for suitability for lucerne production and possible soil amendments to be incorporated with soil tillage operations.
      
THE IRRIGATION SYSTEM
Although it was originally anticipated that the drip system should last ten years many people ask the question if they can replant lucerne on the same system. The original thoughts were to install the drippers at 20 to 30 cm depth to offer us the opportunity to replant the lucerne on the same system. Some producers report that their system is still fine to last another ten years. For potential hazards like water quality, flushing, root intrusion and anti vacuum valves related to clogging you are referred to the SDI Lucerne guidelines. Under irrigation proves to be the main culprit when it comes to root intrusion. In some situations the drip lines were installed too shallow and this automatically means that it will have to be replaced to enable adequate soil tillage. 

Hydraulic accuracy
The potential to reuse the same system can only be determined by doing a few field tests to calculate the hydraulic accuracy. Total block flow rate can be determined and compared to the original designed flow rate. This should be within 10% of the original to consider reusing the old system.

Removing the old drip lines
Kill the lucerne and weeds with an herbicide. All soil amendments like phosphates, lime, gypsum, etc. should be broadcast before any soil tillage operation as this will ensure proper incorporation. Removal of the drip lines are done by ripping on both sides of the lateral 10cm deeper than the installation depth.
By cross ripping at a 30 degree angle afterwards will pull the drip lines from the soil.  Install new system and start with the rotational crop.

If you are fortunate and be able to reuse the old system start by killing the lucerne and weeds, broadcast soil amendments and do soil tillage not deeper than 20cm. Plant the rotational crop where after lucerne can be re-established.

 
REFERENCES:
Die Landbou Burger 2006; Dr Johann van Heerden, LNR Vee en Weidingsinstituut.
Oupa Vermeulen 2013; Kleinkaroo Saad. Personal communication.
Netafim Lucerne Guidelines.
Netafim SDI Guidelines.


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