By Charles Wortmann

Crediting some of the nitrate-N applied in irrigation water for determination of fertilizer-N rates has long been advised in Nebraska. However, nutrients applied through irrigation have not often been directly credited in fertilizer use decisions, although some may have been credited through the interpretation of soil test results.

The essential soil-derived nutrients for crop growth are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), zinc (Zn), iron (Fe), copper (Cu), manganese (Mn), boron (B), chorine (Cl), molybdenum (Mo) and nickel (Ni). Each of the essential nutrients has its special functions, and any deficiencies can be limiting to growth. The sodium adsorption ratio and electrical conductivity also are indicative of irrigation water quality.

In a University of Nebraska-Lincoln survey, 1,200 irrigation water samples from 85 of the 93 counties in Nebraska were analyzed for some nutrients (see Table 1 below). This survey was done in the 1960s, largely forgotten, and apparently not systematically repeated.

It is likely that some concentrations have increased since the 1960s because of leaching of mobile nutrients such as N, S and B. However, the results are indicative of the value of nutrients currently applied through irrigation.

The nutrients applied in irrigation water are expected to be as or more effective compared with those applied in fertilizer. The exceptions may be that the timing of irrigation is too late for starter fertilizer effects, where starter applications occur, and for enhanced greenness that often is achieved with an early S application. However, applications contribute to enhanced nutrient availability in future years.

Table 1. Statistics for nutrient concentration and amount applied per acre-foot of irrigation water, and estimated nutrient removal in 250 bushels-per-acre corn grain harvest, based on a UNL survey and analysis of 1,200 samples of irrigation water in Nebraska.

Results across Nebraska

The results were calculated from county averages (Table 1). In general, the average and median amounts of nutrients applied in 1 acre-foot of groundwater, relative to removal in the harvest of 250 bushels of corn, is small for P and about 65% for K, but in excess for Ca, Mg, S and B.

A typical recommended application rate for Cl in Kansas, where a soil test indicates deficiency, is 20 pounds per acre, which is less than that typically supplied with an acre-foot of irrigation water.

The minimum and maximum groundwater concentrations indicate the wide variation across counties, but there also was much variation within counties. 

Irrigation water sources will have much higher concentrations of one or more nutrients than indicated, but other nutrients may be at very low levels, and each water source should be tested. Some differences with neighboring counties may result from too few samples or a single sample with a very high concentration, and interpretation by groups of counties is more meaningful.

There was a small amount of elemental P supplied from irrigation with groundwater with generally less than 1 pound per acre-feet P (pound per acre-feet = 2.72 x ppm and pound per acre-inch = 0.227 x ppm). Sandhills counties tended to have relatively low concentrations of K, Ca, Mg, S and Cl.

South-central and west-central counties tended to have relatively high nutrient concentrations, with exceptions. Eastern Nebraska counties tended to have intermediate or high concentrations, except for P, K and Cl.

The very high average concentrations for some counties were sometimes due to an outlier value, such as single samples with 3.1 pounds per acre-feet P in Stanton County, 180 pounds per acre-feet K in Grant County, 455 pounds per acre-feet S in Dundy County, 2.45 pounds per acre-feet B in Brown County, and 139 pounds per acre-feet Cl in Dixon County.

While this information is too generalized and too old to be used in fertilizer use decisions, it demonstrates the value of testing each irrigation water source, including surface water sources, for concentrations of essential nutrients.

In addition to the seven nutrients addressed here, analysis also can include zinc (Zn), iron (Fe), copper (Cu), manganese (Mn) and molybdenum (Mo), as well as nitrate-N, sodium adsorption ratio, electrical conductivity, pH, and bicarbonate concentration. Concentrations are not expected to change much over the years, and the results of one test can be used for several years. However, each water source should be tested for greatest confidence in the results.

Testing tips

• About 10 ounces of water are needed for the full analysis, but confirm this with the laboratory. The laboratory may be able to provide sample containers.

• Any contamination can greatly affect results, and containers should be rinsed several times before collecting samples.

• Wells should be pumped for at least 24 hours before sample collection, preferably at the peak of the irrigation season.

• Samples of surface water should be collected from below the surface.

Wortmann is a Nebraska Extension soil nutrient management specialist.

Source: UNL CropWatch, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.