If properly applied, urea and fertilizers containing urea are excellent sources of nitrogen for crop production.

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Chemical reactions

After application to the soil, urea undergoes chemical changes and ammonium (NH4 +) ions form. Soil moisture determines how rapidly this conversion takes place.

When an urea particle dissolves, the area around it becomes a zone of high pH and ammonia concentration. This zone can be quite toxic for a few hours. The free ammonia that has formed can kill the seed and seedling roots within this zone.

Fortunately, this toxic zone becomes neutralized in most soils as the ammonia converts to ammonium. Usually it's just a few days before plants can effectively use the nitrogen.

Although urea imparts an alkaline reaction when first applied to the soil, the net effect is to produce an acid reaction.

How and how much to apply

Urea or materials containing urea should, in general, be broadcast and immediately incorporated into the soil.

If applying urea-based fertilizer in a band, separate it from the seed by at least 2 inches of soil. Under no circumstances should urea or urea-based fertilizer be seed-placed with corn.

With small grains, you can generally apply 10 pounds of nitrogen as urea with the grain drill at seeding time, even under dry conditions. Under good moisture conditions, you can apply 20 pounds of nitrogen as urea with the grain drill.

Research findings

Research from North Dakota State University indicates that, under dry conditions, urea can reduce wheat stands more than 50 percent (Table 5). This was for urea applied with a grain drill in a 6-inch spacing, at the rate of more than 20 pounds of nitrogen per acre.

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University of Wisconsin research indicates that seed-placed urea with corn, even at low rates of nitrogen, is very toxic to the seed and greatly reduces yields (Table 6). However, when urea was side-placed as a 2-by-2-inch starter, researchers noted little, if any, damage (Table 7).

In Minnesota, good crop production usually requires an application of more than 20 pounds of nitrogen per acre. Farmers can avoid damage from urea by broadcasting most of the urea nitrogen fertilizer ahead of seeding. Data in Table 8 indicate that urea broadcast prior to seeding is equal to or more effective than similar ammonium nitrate treatments.

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Soil Fertility Guide

Calculating Fertilizer Rates from Nutrient Recommendations

Soil test recommendations are given in lb/ac or kg/ha of nutrients. To determine the fertilizer rate for a particular nutrient, multiply the rate of the desired nutrient by 100 and divide by the percentage of the nutrient in the fertilizer.

Example 1

• Recommended rate of N is 80 lb/ac Using 46-0-0, the rate of fertilizer required is:  (80 x 100) / 46 = 174 lb/ac

Example 2

• Recommended rate of P 2O 5 is 40 lb/ac Using 11-52-0, the rate of fertilizer required is:  (40 x 100) / 52 = 77 lb/ac 77 lb/ac of 11-52-0 would also supply (11/100) x 77 = 8.5 lb/ac of N.

Example 3

• Recommended rate of K 2O is 15 lb/ac. Using 0-0-60, the rate of fertilizer required is:  (15 x 100) / 60 = 25 lb/ac

Converting Fertilizer Prices into Price per unit of Nutrient

The cost of a fertilizer is related to its plant nutrient content. If a nitrogen fertilizer such as 34-0-0 is being purchased, the cost should be about three-quarters that of 46-0-0. When buying fertilizer, one should compare prices on the basis of cost per pound of "actual" nutrient, not the price per tonne of fertilizer material.

Example 1

If urea (46-0-0) costs $367/tonne, the cost per pound of nitrogen (N) is calculated as follows:

• Nitrogen in one tonne (1,000 kg or 2,204 lb) of 46-0-0 (containing 46% N):  (46/100) x 2,204 = 1,014 lb Cost per lb of N is:  $367/1,014 = $0.362

Example 2

(Based on 11-52-0 at $391/tonne)  In order to calculate the cost of phosphate in 11-52-0, the value of nitrogen must first be subtracted.

• Nitrogen in one tonne (1,000 kg or 2,204 lb) of 11-52-0 is (11/100) x 2,204 = 242 lbs The value of nitrogen is 242 x $0.362 = $88 (from example 1, which calculated the value of N to be $0.362/lb) Cost of phosphate per tonne is $391 - $88 = $303 Phosphate in one tonne (1,000 kg or 2,204 lb) of 11-52-0 is: (52/100) x 2,204 = 1,146 lb Cost per lb. of P 2O 5 is :  $303/1,146 = $0.264

For further information, contact your MAFRD GO Representative.

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