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Is it worthwhile turning the tap on?

Whoever can simply turn on the sprinkler during dry periods is looked on with envy by his colleagues. But sprinkler irrigation is also expensive and not cost-effective for every crop. Of course, it always pays dividends for potatoes and beets. But what about cereals? We asked Ekkehard Fricke.

Mr Fricke, after a year of extreme drought and two years in succession that were far drier than the average, many farmers are wondering how they should adapt to these climate changes. Do you have any tips?

The simple answer is sprinkler irrigation. But that's not as simple as it sounds, either. To be able to assess whether starting to use sprinkler irrigation is the right business decision, a few questions need to be answered beforehand:

  • Can the farm's economic situation be influenced positively in the long term by means of sprinkler irrigation?
  • Is the usable groundwater supply reserve in your groundwater body still available in sufficient quantities?
  • Are the costs for pumping up the water and routing it to your land reasonable?
  • Can you obtain a water right that is sufficient for the cultivated crops from the lower water authority within your administrative district?
  • Do you have to commission a complex, costly hydrogeological survey for this?
  • Which restrictive ancillary provisions will the administrative district impose on your farm, and are these practicable and economically feasible?

So, that's quite a lot of requirements to think about. Let's phrase the question differently: for which crops is sprinkler irrigation worthwhile? Over a number of years, we have tested different sprinkler irrigation variants at our experimental farm in Hamersdorf. While sprinkler irrigation only leads to slightly higher rapeseed yields, significant yield increases are achieved particularly with winter wheat, summer malting barley and potatoes.

So, do we 'bring on the water' for these crops?

That would be great. However, a look at the average volumes of water that are needed shows that not all crops can be irrigated 'optimally', because keeping to the quantities of water made available by the authorities (these are often around 800 m3/ha, i.e. 80 mm) is otherwise not possible. Each year, the farmer therefore has to think long and hard about the strategy used to sprinkler-irrigate his various crops depending on which crops are cultivated and the respective market prices.

The limited resource of water therefore has to be used optimally. Which crop offers the greatest effect?

An economic evaluation of these tests offers some degree of guidance. Ware potatoes are the clear leaders here. In our tests, around €2,000 per hectare remained on the bottom line for these potatoes thanks to sprinkler irrigation.Even the variant with reduced sprinkler irrigation leads to an advantage €1,800/ha. Disregarding sprinkler irrigation costs, the economic performance of the optimally irrigated variant is also better in the case of summer malting barley, silage maize and winter barley. With winter wheat, the costs of optimum sprinkler irrigation are not covered by the additional revenues, which is why reduced sprinkler irrigation was sufficient in this case.

If I have to budget my water, when should I switch on the rain gun?

You will never be able to pick exactly the right point in time, but fairly precise figures are certainly available. Naturally, irrigation should be adapted as precisely as possible to the different needs of the crops. Irrigating too little or economising at the wrong point time can have significantly negative impacts on the yield but also on the required qualities. Conversely, supplying too much water is unproductive. The precious resource is wasted and is then lacking elsewhere. Cost effectiveness suffers in both cases.

Too much water can't do any damage, can it?

Oh but it can! If excessive quantities of water are applied, nutrient losses can occur due to seepage under the root penetration zone. Apart from possible negative consequences for the groundwater, fertilisation efficiency also suffers, something that nobody can possibly want in the light of the restrictions imposed by the Fertiliser Ordinance.

So is slightly less water better than too much?

Not entirely. Applying insufficient quantities that are significantly less than the calculated amounts is not recommended either. They do not penetrate deeply enough into the soil and evaporate again very quickly. The losses caused by the evaporation of the water remaining on the leaves and the water located near to the surface of the soil are also relatively high. If too little water is applied, the fertiliser nutrients cannot be used sufficiently either due to reduced withdrawal, and partly remain in the soil. In the case of readily soluble compounds such as nitrate, this would equate to losses via the seepage water over the winter. Irrigation control according to objective criteria is therefore required.

What form does such control take?

The application level for sprinkler irrigation is oriented to the amount of water that the soil can absorb. This is dependent, firstly, on the soil type and humus content and, secondly, on the root penetration depth. The light, sandy soils typically found in sprinkler irrigation areas have a usable field capacity (uFC) of 10 to 16 mm per 10 cm soil depth (corresponding to litres/m2). Pure sands lie at the lower end of this range, whereas loamy, silty or very humic sands are located at the upper end.

In the next step, the depth from which the plants can draw the water from the soil has to be clarified. Shallow-rooted crops such as potatoes, onions or spring barley can accomplish this down to a maximum depth of 60 cm. Consideration also has to be given to the fact that the root penetration depth is lower at the beginning of development in this case.

Specifically, what does this mean for maximum water application?

The soil water reservoir should only be filled to around 80 to 90% in order to avoid seepage losses. If sprinkler irrigation is commenced as of 50% of the uFC and the uFC is topped up to 80%, the maximum application level on pure sandy soils with a total uFC of 60 mm at a depth of 60 cm is 18 mm. Application of over 50 mm is therefore theoretically calculated for a soil with good water storage capacity. However, the soil is unable to absorb such high quantities in a short space of time, as a result of which surface run-off can occur. Individual applications should therefore be between 20 and 40 mm depending on the soil, crop and current soil moisture.

Does that also apply to deep-rooting crops?

Yes, it also applies to sugar beet, winter cereal or maize, which could absorb high quantities of water due to their large root area if they are accordingly dried-out. In these cases, the top-up rate of the soil as a whole remains less than 80%; in particular, the topsoil is completely soaked as a result. The plants adapt to the fluctuating soil moisture in the soil layers by drawing water primarily from this zone when a high quantity of water is available in the topsoil, even if roots have been formed at greater depths. The deep roots only begin to supply a part of the water again when conditions above become drier. Soaking the entire root area of deep-rooting crops beyond 60 cm is therefore unnecessary and, if the soil moisture is low, not even possible with a single sprinkler irrigation application.

Do you have any specific recommendations for the most important crops?

Let's start with maize. Sprinkler irrigation during the most important period of time from panicle formation to the start of milk ripeness is sufficient. During longitudinal growth, sprinkler irrigation should only take place in extremely dry conditions. Shortly before and during flowering, maize reacts very sensitively to a lack of water. Sprinkler irrigation should therefore be carried out from 40 to 45% of the uFC, but at the latest when the first leaves begin to roll.

Ware potatoes should be irrigated from the beginning of tuber formation. The soil moisture in the ridge should then be no lower than 50% of the uFC. With industrial potatoes, sprinkler irrigation can be carried out slightly later or with a lower soil moisture. As of flowering, the soil moisture should be no lower than 40% of the uFC, or preferably 50% of the uFC for ware potatoes or varieties that are sensitive to dryness. Make sure that the interior of the ridge doesn't dry out completely. Re-moisturising sandy soils is then very difficult due to the poor water conductivity of dry soil.

The advantage of sugar beets is that they directly show stress caused by dryness. From the start of development to row closure, the plants should supply themselves from the soil. Sprinkler irrigation too early on can inhibit the desired deep root penetration and foster excessive foliage growth. Conversely, a moderate lack of water during the development of the young plants fosters the crop's tolerance of dry conditions. As of row closure, sprinkler irrigation should begin when severe stress caused by dryness is recognisable, i.e. when the leaves droop during the day and have not yet fully recovered even by the morning. You can even tolerate some leaf loss until around mid-July, because the tests showed that beets can compensate for quite a lot over their long vegetation period. Sprinkler irrigation should be carried out regularly when the soil moisture has reached  25 to 35% of the uFC, as shown by extensive leaf 'drooping'.

Most years, early sprinkler irrigation up to the half-way point of shooting has little effect on cereal yields, with the exception of malting barley. Cereals generally offer the advantage that they can compensate well for temporary stress caused by dryness in individual development phases. Low crop density due to lack of water from tillering up to the half-way point of shooting can at least be compensated for in part by wheat, barley and rye thanks to good ear grain formation. Compensation is additionally possible through the grain weight. Sprinkler irrigation should therefore begin as of the emergence of the flag leaf, when the soil moisture is around 35 to 45% of the uFC. Earlier sprinkler irrigation is only indicated in very hot and dry conditions.