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Pre-seeding nitrate nitrogen test can save dollars

Do all fields have similar nitrate nitrogen (NO3-N)? Do all fields need similar amounts of N application for a particular crop?

April 20, 2009
By Dr. Tarlok Singh Sahota

More growers are wanting to learn about the various means of boosting yields and monitoring inputs, as evidenced by this gathering at TBARS in 2007.


Do all fields have similar nitrate nitrogen (NO3-N)? Do all fields need similar amounts of N application for a particular crop?

One might think the answer is yes, but that would be a mistake. After all, why apply fertilizers? To supplement soil nutrients to a level that is conducive to an economic optimum yield without impacting environment. It is therefore essential to know what the nutrient content of a field is before deciding upon the rates of nutrient application.

Most farmers go for a basic soil test package that gives them an estimate of phosphorus (P) and potassium (K) content in the soil. There is a need to test nitrate N as well because by and large it is the form in which crop plants take N from the soil. N content in different fields could vary with the previous crops (legumes/non-legumes), fertilizers/manures applied to previous crops, crop yields and residual N after crops’ harvest.

At the Thunder Bay Agricultural Research station (TBARS), Thunder Bay, we have observed that a field with previous crop (oats) applied with recommended rate of N (no manure application) had 8.2 ppm nitrate N (32.8 kg N/ha; ppm x 4 = kg/ha), in zero  to 30 centimetres (12 inches) soil the next spring, whereas a field with silage corn as a previous crop, applied with liquid manure and recommended amount of N had 26 ppm residual nitrate N (104 kg of N per hectare or 93 lbs of N per acre) in zero to 30 centimetres of soil. Cereals (mean over barley, wheat and oats) response to recommended rate of N (70 kg/ha or 62 lbs/ac) in the first field (8.2 ppm nitrate N) was 1,117 kg/ha (= 16 kg grains/kg N), whereas there was no response to applied N in the second field (26 ppm nitrate N). Both the fields were in the adjoining (similar) plot ranges. Is it possible that in the latter case, at current prices of ($1.30/kg) for urea N, we could have saved $91 per hectare (= 70 kg x $1.30/kg)($224 per acre)? This, compared to the direct cost of nitrate N test ($1.5/ha; assuming 25 acres unit for soil sampling), is a significant amount. If someone grows 100 acres of cereals, there is a potential of saving more than $3600. Traditionally, pre-seeding nitrate N test was not recommended, because nitrate N is generally believed not to stay in the soil, because of denitrification/or leaching losses. However, in well-drained soils, especially in tiled drained fields, with no excessive moisture, where will the nitrate N go? Nowhere; it will be taken up by crop plants or stay in the soil. In a summer/spring fallow (tile drained) plot range at TBARS, pre-seeding nitrate N was unbelievably high (70 ppm = 280 kg/ha). No wonder in this field application of N fertilizers tended to reduce the wheat grain yield.

Dr. Tarlok Sahota offers helpful advice to growers during a demonstration day at the Thunder Bay Agricultural Research Station (right). Photos courtesy of Dr. Tarlok Sahota, TBARS


Translated to spring wheat
In spring wheat, in a plot range with pre-seeding nitrate N test of 12 ppm (48 kg of N per hectare or 42.8 lbs of N per acre), response to 40 kg of N per hectare (or 36 lbs of N per acre) was 506 kg grain/ha (= 12.65 kg grain/kg N), and to 80 kg of N per hectare (or 71 lbs of N per acre) was 418.2 kg grain/ha, which equals 5.2 kg grain/kg N. This is a breakeven point for N application to spring wheat at $1.30/kg N and $260/tonne grain. Under the fluctuating/uncertain grain prices, it would be safer to apply 40 kg N/ha to spring wheat than to apply 80 kg N/ha.

Apart from nutrient content in soil, it is also important to know how many nutrients are removed by crop plants so that the nutrients are replenished adequately to maintain soil fertility. Table 1 shows N removal by cereals per tonne of grain (kg/ha) at TBARS, Thunder Bay.


While planning for N application rates, it will be reasonable to assume at least five tonnes grain yield per hectare (or 139 bu/ac) for barley. N removal by barley at this yield will be 116.5 kg/ha (104 lbs/ac), whereas only 70 kg of N per hectare (62 lbs of N per acre) is recommended for barley (in northwestern Ontario). Where does the rest of the N come from? It could come from the soil or the air. N from the soil could include N left over from previous crops, plant residues, dead earthworms/microorganisms, wildlife excreta, fertilizers and manures; N held by clay and organic matter; and N in lower soil layers from 30 to 60 centimetres (12 to 24 inches) deep and 60 to 90 centimetres (24 to 36 inches) deep, for which no nitrate test is done. N from the air could include N that is fixed by legumes or leguminous weeds such as clover, trefoil or even volunteer alfalfa; free living soil bacteria such as azotobacter; and N dissolved in rain. How much N could there be in deeper soil layers (30 to 60 and 60 to 90 centimetres)? In our research plots (four to five experiments over two years), nitrate N content in 30 to 60 centimetres (12 to 24 inches) of soil varied from 26 to 43 percent (average of about 35 percent; it may be safe to assume 30 percent) in the top zero to 30 centimetres (zero to 12 inches) of soil. Nitrate N in 60 to 90 centimetres was either 50 percent of or the same as that in 30 to 60 centimetres of soil. As the crops grow, they can draw nutrients from deeper soil layers. Based on these assumptions, let us see how much nitrate N could be in the soil in a spring barley field with a pre-seeding nitrate N test of 8.2 ppm:

Soil – zero to 30 centimetres: 32.8 kg of N per hectare (8.2 ppm x 4)

Soil – 30 to 60 centimetres: 9.8 kg of N per hectare (30 percent of N in zero to 30

Soil – 60 to 90 centimetres: 4.9 kg of N per hectare (50 percent of N in 30 to 60 centimetres)

Fertilizer N: 70.0 kg of N per hectare (62 lbs of N per acre)

Total (Soil + Fertilizer)
: 117.5 kg/ha (or 105 lbs of N per acre).

This more or less equals N removal by barley (116.5 kg/ha; see paragraph 4). It may therefore be logical to assume that fields having more than 8 ppm pre-seeding nitrate N would require less than 70 kg of fertilizer N per hectare. Our experience also shows that if the pre-seeding nitrate N is 25 ppm or more, spring cereals would not respond to N application. In fact going by these calculations, N application to spring barley may not be required even if pre-seeding nitrate N test is 20 ppm. Why waste N and dollars on N then? It is quite probable that silage corn applied with manure and fertilizers and crops such as alfalfa and soybean could have residual nitrate N equal to 25 ppm or more (or about two to three times more than the residual N after cereals, especially those which didn’t get a manure application).

Additional points to consider
Limited time for field operations and seeding in spring is a constraint in taking pre-seeding soil samples. Though ideally, soil samples should be taken before seeding, other options could be explored to overcome time constrains. These could be seeding without application of N or using MAP (11-52-0 at recommended rates of P application) at seeding and taking soil samples after seeding. Cereals take three to four weeks after seeding to initiate crown roots (which are close to soil surface and can absorb top dressed N) and tillering during which time nitrate soil test results could be available to decide if N needs to be applied to spring cereals or not and if needed, how much? The principle/practice can be extended to other crops (and areas based on location-specific research) as well, and certified crop advisors or researchers/specialists in your area could be of help.

The bottom line is that pre-seeding nitrate N tests could not only save a lot of dollars, but also minimize potential impact of agricultural practices on the environment.

 *Dr. Tarlok Sahota, CCA, is the director of research and business for the Thunder Bay Agricultural Research Station (TBARS) in Ontario. He has more than 20 years of experience in research and extension worldwide and has worked extensively in nutrient management and integrated pest management. He has been with TBARS since 2004.

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