Water management beats precision fertilizing on irrigated land

Higher organic matter gives crops a big nitrogen boost and so it’s better to focus on water management, study finds

Crop being irrigated with an overhead irrigation system.
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It seems fertilizer prescriptions matched to the needs of various areas of a field doesn’t give much of a return under irrigation.

That’s the conclusion of an Alberta Agriculture and Rural Development study of responses to applying fertilizer according to the needs of the field’s soil zones.

The project included dryland and irrigated fields with varying slopes, knolls and depressions that were soil tested separately. Prescription maps were developed and used to apply fertilizer at rates tailored to each zone.

Under irrigation, yields from the soil zones didn’t change when the fertilizer was adjusted to precisely match the perceived needs of each area.

“Water drives yields with irrigation as much as on dryland,” says Doon Pauly, a provincial soil fertility research scientist. “One site caught several big dumps of rain that left water pooled in the low spots. And even though those areas stayed very wet, perhaps as much as 60 per cent of the field did not yield as well as it should have, due to underirrigation.

“We generally assume applied nutrients are the main drivers of yield, but from our work, I think you’d be hard pressed to pay a consultant’s fee for a prescription map for variable-rate fertilization. Management of irrigation seems to be a much more important yield driver.”

Estimating the amount of nutrients an area needs for maximum yield is difficult because of the high levels of organic matter that have built up in irrigated soils, said Pauly.

At last winter’s Irrigated Crop Production Update, scientists reported nitrogen releases from organic material are about 100 pounds per acre a year on irrigated land. A similar amount is stored in new organic matter and can be used by following crops. Heavy tillage to bury crop residues, a common practice in the past, exposed organic matter and so it more quickly broke down into nitrous oxide and carbon dioxide that were lost into the atmosphere. Probably only 40 to 60 pounds of organic nitrogen remained in the soil with those practices. It’s also less with low-residue crops in the rotation.

“With such a high level of nutrient cycling in the soil, modifying nutrient levels has only very small incremental effect on crop yields in most fields,” said Pauly. “I’d say apply a blanket rate of major nutrients across the whole field and pay more attention to water management unless you’re already doing a fantastic job of irrigation management.”

Low-pressure pivots

Low-pressure pivots have made a world of difference to irrigation management, especially compared to wheel-move systems. One farmer weighing the value of investment in a low-pressure system compared his irrigation costs to those of his neighbour and found his pumping costs were double and his yield was about half as much. That was bad enough, but then add on the system’s limitations, the time involved in moving his system and he was very glad to see the current Growing Forward program included funding for more efficient irrigation systems.

Last year, Alberta Agriculture published a new irrigation management manual that focuses on running low-pressure systems for optimum yield of any crop and efficient use of water.

The manual is based on the Alberta Irrigation Management model (AIMM), an online system that accounts for actual weather — along with the soil type of each field and the crop and its development stage — to show how much of the soil moisture is being used by the crop.

“Irrigation application timing and amounts, fertilizers, disease risk, fungicide application, (and) harvest decisions are a complex process,” says Alan Efetha, author of the manual. “You have to get everything right to produce a high-quality crop.”

Watching crops for signs of moisture stress and hand checking soil moisture levels is time consuming and can result in irrigating after plants have lost yield potential. The AIMM model gives an irrigator real-time information on the moisture available to a crop and the rate it’s being depleted.

During early stages of crop growth, running the pivot fast keeps water application rate below the infiltration rate, which prevents run-off — which both wastes water and causes soil erosion. But having it run over the field a few times builds up water reserves for later in the season when the crop’s peak water demands exceed the capacity of the system.

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