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Effect of N rate on Young tree growth

report on trial by Tim Smith, WSU Extension

As there seems to be a question on the best rates and application methods for nitrogen fertilizer in the tighter plantings of modern high density systems, I placed a N rate and timing trial in a 2 acre young block of Fuji on M 26 in Okanogan County, Washington. This was a replant site, but had good weed control and water management ever since it was planted. The soil is a fine sandy loam, and leaching of N was not as likely as we might expect in very sandy soils. There was very little N present in the soil, and relatively low organic matter. The trees were planted on a modified tatura system as whips, 3 feet apart in the row with alternating trees trained to a side of the trellis. Fruit was removed in the second season to encourage tree growth.

For two years I have applied the N (by "belly grinder" spreader) carefully and evenly in a band about six or seven feet wide, down two widely separated rows for each treatment. Each treatment covers about a quarter acre. From each treated row, two sets of twenty trees were measured at planting and each year since, so 80 trees growth is averaged for each N rate and timing.

There are five different N rates, ranging from 100 to 400 pounds of nitrogen per acre. This rate per acre refers to the surface treated acre, that is the rate per surface area in the band treated. I didn't think it was important to apply (or count) the N out in the grass where the M 26 roots did not yet reach.

All of the rates were split into five parts and applied every 3rd week starting at bloom and ending in July. In other words, the 100 lb. rate was applied as 20 lbs/acre. per application, 5 times, three weeks between applications.

Two of the rate treatments, 100 and 200 lbs. N per acre were also applied in two, rather than five, applications. One half of the total product was applied at bloom, the other half six weeks later.

It was quite educational to look at the soil surface under the trees when I had finished with each application. Even the highest rate (80 pound per acre at one time) didn't look like a lot of fertilizer compared to our normal "handful". A handful works out to be about 1/10 lb. of actual N when using 34-0-0. I then calculated what N rate per surface-treated acre the tree saw from a standard smith-sized handful of ammonium nitrate scattered in a zone 2 feet out from the trunk, Whew. It worked out to 690 lbs. of N per acre! No crop can use that much N at one time. This much N at one application should set the tree up for salt stress, which leads to moisture stress until the salt level drops. This is an important concept when you are growing a small tree with a small root system. It's not the same effect when you place a handful of fertilizer over the large root system of a big tree.

If I had wanted to apply the equivalent of 100 lbs. N per acre (still quite a lot, as far as the trees' needs are concerned) in that zone of 2 feet out from the tree in all directions, I would have to cut the rate to two and 3/4 level tablespoons of amonium nitrate! (or six tablespoons of calcium nitrate.) Spread 2.75 tablespoons worth of 34-0-0, or 6 level tablespoons of calcium nitrate, in a 2 foot zone around a young tree and see what 100 lbs. N per acre looks like. You'll be surprised. This total amount of product applied over the first half of the season, split into at least two ( on fine textured soils) or four (on sandy soils) applications will grow trees very well. So, one tablespoon of 34-0-0 per young tree, applied (for instance) three times, three weeks apart is plenty for the tree. Apply another tablespoon or two if this rate seems too little, but don't expect increasing rates to lead to increasing tree growth.

When trees get into their second and third seasons, their roots develop a somewhat wider spread, so can use the N applied farther from the trunk. If you want to apply the equivalent of 100 lbs. N per acre in an area three feet in all directions from the tree trunk, multiply all of the above numbers by 2.2. So, it will take 6 level tablespoons of 34-0-0 per tree or 13 tablespoons of calcium nitrate per tree to give you that per-acre rate. Remember to split this into two to four applications, don't apply it all at one time. Also, be sure that the product is spread relatively evenly under the tree in that 3 foot zone.

Plot results:

After two seasons (involving 10 two-acre hand applications of fertilizer and umpteen tree measurments) here is the grand rolling out of the results:

100 lbs. per acre split by 5... 247 % growth

150 lbs x 5... 253 % growth

200 lbs x 5... 264 % growth

300 lbs x 5... 273 % growth

400 lbs. x 5... 264 % growth

100 lbs per acre split by 2... 269 % growth

200 lbs. x 2... 248 % growth

There is no visible or statistical difference between these treatments, either in the orchard or in the data. I apparently took care of all the young trees' N needs with the 100 lb. per acre rate, even when only applied twice during the early season. It appears that we don't need the high rates of N to get good tree growth. Other factors far out-rank the N rate when it comes to getting trees to grow in the first two seasons. N is important, but the keeping the N rate high is not. I will probably have to repeat this work at a number of sites before the data becomes valid to most folks. Stay tuned.

For those who would like to know, applying the fertilizer in a band, with the 7 foot wide weed control strip treated out of the 16 foot row width, I applied the following actual product rates per acre (using 34-0-0). Numbers indicate the total product per season, you can split that total into as many applications as you wish:

for amonium nitrate:

100 lbs. N per acre, 34-0-0 = 100 / .34 = 295 pounds product per treated acre. 7 feet out of 16 feet = .44 of the area being treated. .44 X 295 lbs = a total of130 lbs of 34-0-0 applied per acre.

If you wish to apply this split into 3 parts, apply 43 lbs. of product per acre each application.

for calcium nitrate:

100 lbs. N per acre, 15.5-0-0 = 100 / .155 = 645 pound product per treated acre. 8 feet out of 20 = .40 of the area being treated. .40 X 645 lbs. = a total of 258 pounds of product per acre. Spit this poundage into as many parts as you feel will match your soil texture.

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Wenatchee WA, 16 October 1995