Introduction

Fertilizers and fertility management serve as one of the cornerstones in potato production. A successful fertilizer program helps provide top yields of high quality potatoes while maximizing returns on fertilizer investment. As fertility programs become more specialized, one factor which cannot be overlooked is the source of nitrogen in the fertilizers applied to the crop.

In general, the nitrogen source used in potato fertilizer is not critical in the overall effectiveness of the fertilizer. There are, however, some differences in the way various nitrogen sources behave in the soil and knowledge of these differences can be useful.

Most commercial fertilizers supply nitrogen to the soil in one of two forms: Ammonium-Nitrogen and Nitrate-Nitrogen. Plants use only the nitrate form directly. However, nitrogen supplied in the ammonium form is converted to the nitrate form by soil micro-organisms during the growing season.

If only the nitrate form of nitrogen is taken up by plants, one might logically expect that fertilizers containing only nitrates would be preferable. This is not normally the case, however, because nitrates are also highly subject to leaching losses. If only nitrate-nitrogen were applied in the spring and heavy rains occurred before the nitrogen is taken up by the plant, substantial nitrogen losses could occur resulting in crop deficiencies.

An ideal nitrogen program is one that employs both ammonium and nitrate sources which, in turn, provides a steady and adequate supply of nitrate-nitrogen to the growing crop. Knowledge of the forms of nitrogen commonly used in fertilizer materials is useful,therefore, in planning a fertilizer program for potatoes.

Common Nitrogen Sources:

Diammonium Phosphate (18% N): Diammonium phosphate (DAP) is commonly used in almost all fertilizers sold on Prince Edward Island. Although it is the major source of phosphorus used in blended fertilizers, it also supplies a significant amount of nitrogen to the final product. In a 1:2:2 ratio fertilizer, such as 10-20-20, the DAP supplies approximately 80 percent of the nitrogen in the fertilizer; in a 1:1:1 ratio fertilizer, such as 15-15-15, the DAP supplies approximately 40 percent of the nitrogen in the blend.

It should also be noted that the nitrogen contained in DAP is all in the ammonium form.

Ammonium Nitrate (34% N): This has been one of the most commonly used nitrogen sources on Prince Edward Island for many years. One half of the nitrogen contained in ammonium nitrate is in the nitrate form while the other half is in the ammonium form. Ammonium nitrate is, therefore, an excellent nitrogen source in circumstances where nitrogen is required immediately for plant uptake, such as topdressing, because 50 percent of its nitrogen is readily available to the plant. This can be a disadvantage, however, in cases where fertilizer is applied early in the spring, before the crop's need for nitrogen is significant. If heavy rains occur during this period, it can result in a significant portion of the nitrate in the fertilizer being lost.

Ammonium nitrate picks up moisture very rapidly from the air and can get "sticky" very quickly under humid conditions. There is a common belief that when ammonium nitrate is used as a topdressing, it vaporizes and is lost in the air if not covered immediately upon application to the soil. This is not so. Although the fertilizer granules pick up moisture from the air and appear to "melt" into the soil, they are not in any way lost.

Ammonium Sulfate (21% N): Ammonium sulfate contains nitrogen only in the ammonium form. It will, therefore, require some time after application, particularly when soils are cold in early spring, before it is converted into nitrates. On the other hand, early season nitrate losses are minimized with this product.

Because ammonium sulfate has a relatively low nitrogen content, its relative shipping cost per unit of nitrogen is high. Therefore, its cost per unit of nitrogen is often quite high as well, and it is used only occasionally in our region. The low nitrogen content, in addition, makes it unsuitable for use in high analysis blended products such as 17-17-17 or 12-24-24.

One other important characteristic in relation to ammonium sulfate is the fact that it produces a strongly acidic reaction in the soil. This is normally a disadvantage because more limestone is required to counteract its use as compared to most other nitrogen sources. However, potato producers wishing to reduce the pH of the soil in the rooting zone to help suppress Common Scab can use a banded fertilizer containing ammonium sulfate to achieve maximum pH reduction.

Urea (46% N): Urea is another excellent source of nitrogen which has many relative advantages in manufacture and transportation. In all probability, it is and will continue to be the most economical source of nitrogen available in the region for the foreseeable future.

Urea has a somewhat unique chemistry because it contains neither nitrate-nitrogen or ammonium-nitrogen. Once placed in the soil, however, the soil micro-organisms quickly convert it into ammonium-nitrogen. It is therefore, normally considered equivalent to an ammonium source.

One aspect of the unique chemistry of urea places some restrictions on its use as a banded source of nitrogen for potatoes. During the biological conversion of urea-nitrogen to ammonium-nitrogen, free ammonia gas can be evolved in significant quantities near the fertilizer band for short periods of time. Because this also occurs near the seed-piece, this free ammonia gas can cause damage to emerging potato sprouts or root systems. This problem is more predominant under warm dry soil conditions such as with late season plantings. It is recommended, therefore, that not more than 120 Kg/ha (110 lb/A) of nitrogen be banded with potato seed if the potato fertilizer contains over 20 percent of its nitrogen in the urea form. If over 120 Kg/ha of nitrogen is needed for the crop, any nitrogen in excess of 120 Kg/ha should be broadcast, either before or after planting.

The major advantages of urea as a nitrogen source for potato production lie in its relatively low cost and its ability to resist losses in early season if excessive rainfall occurs in the first few weeks after planting.

Calcium Nitrate (15% N): This source of nitrogen is a specialty product used mainly for topdressing applications. Because it contains only nitrate-nitrogen, it is very effective as a topdressing because it is immediately available to the crop.

Calcium nitrate is not normally used in fertilizers at planting because of its high potential loss factor and its relatively high cost per unit of nitrogen.

Reccommendations:

High yields of quality potatoes can be achieved in most years, regardless of the source of nitrogen used in fertilizer blends. Rates and timing of application are usually more critical in determining final yield than the source used. The following points, however, should serve as a guide to the differences in fertilizer use as related to the source:

Because the response to different fertilizer nitrogen sources is often minimal, the relative cost of a nitrogen source must be a prime consideration. The most economical source of nitrogen is usually the best type to use unless special circumstances indicate otherwise.

The potato crop does not have a high requirement for nitrogen in the first three to four weeks after planting. Therefore, to prevent excessive loss of nitrogen due to heavy rains which may occur during that period, a fertilizer with most or all of the nitrogen in the ammonium (or urea) for is preferred at planting.

If fertilizer banded at planting contains over 20 percent of its nitrogen in the urea form, not more than 120 Kg/ha of nitrogen should be banded with the planter. Any nitrogen in excess of this amount should be broadcast prior to planting or topdressed on the growing crop.

Fertilizer placement is critical. Injury to emerging sprouts or roots can occur if the fertilizer is closer than the recommended spacing of 10 cm. below and to the side of the seed piece. This spacing is especially important if the banded fertilizer contains urea.

If the fertilizer is applied as a topdressing or sidedressing to overcome a suspected nitrogen deficiency, it should have at least a portion of its nitrogen in the nitrate form. This should ensure rapid availability to the crop.

More detailed information on soil and fertilizer nitrogen can be found in the publication Soil and Fertilizer Nitrogen in Atlantic Canada available at no charge at any District Agricultural Office.

For further information contact Brian Douglas at the Soil and Feed Testing Laboratory or your local Agricultural District Office of the P.E.I. Department of Agriculture and Forestry.