Thoughts on watering potatoes

Is this happening to you? We see this on a range of different soil types where the application rate of the irrigator is too high for the infiltration of the soil. You will only know if you measure. In extreme cases when all the water runs off the ridge and there are no roots in the valley it can happen that the irrigation does no good at all. But, in most cases this happens only to a degree - you need to know and then see what can be done about it. Speak to a good adviser if you need some help.

Remember:For every millimetre of water use lost from potential, you could be losing .24 t\ha! We have many examples, even in a 'wet' year like 2000, of crops using 50mm less water than they could. This equates to 12 t\ha. Does this explain why some growers are getting 70-100 t\hacompared to the national average of below 40 t\ha?

It is suggested you read this next section as the whole success of watering potatoes is down to understanding and knowing when your soil is really full.

Filling your soils after planting.

With most water monitoring systems it is important to know when a soil is full. We see many fields of potatoes that have poor soil structure due to the de-clodding and planting activities. This, we believe causes scab in soils that in some years have no problem, as well as causing poor growth early in the season. The normal practice is for growers to go over their crop with 15mms of irrigation to overcome these difficulties. Using continuous monitoring equipment, we can see how much water it takes to refill a soil.

However, many growers who do not have the luxury of such equipment, do not know how much water is needed. (In 1996, after a dry winter many soils needed 70mm to refill the soil to 50cm. Not many growers were able to achieve this and hence had disappointing crops. If you guesstimated a 30mm deficit, you would have been 40mm out for the season!).

Potatoes are usually grown in ridges and some years the ridges are formed better than others.

The tubers are planted and without doubt there is a deficit.

What is it and how do we find it?
By this stage most growers know how they will irrigate and most hope that nature will fill the soils before tuber initiation. Depending on the market supplied, skin finish and scab are not important to some growers, but for the packing trade, good scab control is vital.Some soils never get scab while others get scab in most seasons. It is complicated, but generally maintaining a high soil moisture content in the ridge will solve most of the problems. The majority of problems are caused by water not behaving as expected.

Is this happening to you?

Recently, an experiment was conducted in filling a heavy soil after planting potatoes. Moisture readings were taken at multiple depths. Then the tubes were watered with a watering can and more readings taken. The agronomist who did the work was 'amazed' at how long it took and how much water was needed.

The conclusions are that it took 1.5 hours to refill the soil by 51mm! No wonder the 'average' grower does not refill his soil and hence has poor crops. (Probably much more water was applied to achieve this.)

You do need good equipment that is agronomically accurate to be able to see this. See Monitoring section.

Watering crops correctly and accurately is becoming more and more important as we have to be aware of not only increasing yields, but improving quality, reducing wastage of water and hence fertilisers and all the 'green' issues of the day.

Farming is under considerable pressure, not only in the UK, but throughout the world. With increased understanding we are able to help achieve all of these issues, because it is only by increasing 'profit' that farmers will survive. Therefore, if a grower can increase yield and quality by 10%, he could double his profit per acre.

Once the crop starts to grow the roots grow deeper and more water becomes available to the crop. For example, if a soil has 25% moisture at all depths down to 50cm, then there is 75mm of water when full in the top 30cm. Of this about half - 37mm is easily available to the crop. So watering with 25mm every 5/6/7 days can do a very good job, if the water goes into the ridge as we would like. However, in many soils much of the water runs into the valley as shown in the picture at the top of this page, not where the plant's roots would like it for maximum water use and growth!

However, with rooting down to 50cm the total water content increases to 125mm, of which the plant can use 65mm easily. If you delay watering until the plant reaches a stress level and then water with 25mm (this may be the maximum you feel that your soil can cope with, but do you know this?) then even after watering you will have a 40mm deficit. With strategically placed soil moisture sensors, we can see that the water applied only wets the top 20cm and that because the roots have run out below this, the plant can still be under serious stress. It may be that your crop would like to be using 5 to 6mm per day, but it can only get 3mm per day. This may give a reduction in yield, from potential, of .5 to.75 tons per hectare per day.

See graph illustrating this point in Agronomy 2

So, should you water earlier and keep the deficit smaller, or water later, but accept potential reduced yield? The longer you delay watering means you have more chance of rain doing the job for you. Also, there is the problem of wet soil round the tubers, which can cause some serious lenticel problems. Depending on your market, this may or may not be a problem.

Ultimately, drip solves a lot of these problems. The soil can be allowed to supply the water needed until it reaches the 'refill' point and then it is brought up to 'full' or thereabouts.By leaving well alone for as long as possible it means the tubers are only sitting wet for small periods at a time.

But, always try different things and monitor them so that you know which is best and repeatable.

Water Management experiences in potatoes

• It is imperative to know when soil is really full.
  In practice actual measurement is very different from theory.
  
  
• There is a need to know what is the real rooting depth at any time during the season, and hence the amount of water available.
  Compaction can have a big effect on the growth of the crop.
  
  
• There is a need to know what are the important water use depths.
  
  
• It is critical to apply the 'correct' amount of water.
  
  
• Is the water applied, going where you think?
  Water-use efficiency- can vary from 20% - 95% - you should know.
• How is quality affected by water and watering?
  Quite considerably.
  
  When measuring moisture make sure you use agronomically 'correct' equipment.
  

Drip at the World Potato Congress 2000

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