A few questions come up every year in regards to fertilizing first, second, and third leaf trees. Since these trees are rapid growing, and in some cases, producing crop, adequate fertilization is crucial for growth.
First leaf trees: As a guideline, I generally recommend no more than one ounce of elemental nitrogen per tree per application. Three to four (or more) applications using a general blend (i.e. 12-12-12 NPK) fertilizer per year will produce a nice result. Using a triple 12, this totals about 8 ounces of actual fertilizer applied per tree. Applications should begin upon leaf out and continue about every 4-6 weeks. To prevent any nitrogen burn, the first applications of the year should be less than one ounce while later applications should not be greater than one ounce.
Although I know some growers are successful, I have observed enough tree damage to caution against using liquid based fertigation products for first leaf trees. It is easy to overdose the trees with nitrogen, especially in hot weather, causing tree die-back. I agree that using granular fertilizers is a conservative approach, but one that has been tested and used extensively over the years.
Second leaf trees: The rootzone of 2nd leaf trees can be quite extensive, but is still limited in comparison to mature trees. Even if the grower is able to fertigate, I usually still like to see the first application to be granular. Why? In many cases adequate potassium and phosphate have not been applied in the previous dormant season, thus applying a 12-12-12 fertilizer will ensure at least some level of these nutrients as the tree begins the rapid growth period of April and May. Later applications can be made through the fertigation system. Again, follow the rule of one ounce per tree per year of growth. So, no more than 2 ozs of elemental nitrogen at any one application. This is about 16 ozs of Triple 12, 11 ozs of CAN-17, or 6 ozs of UAN-32.
Third leaf trees: As the trees enter their “adolescent years,” caution must be used again when fertilizing. Follow the one ounce per tree per year of growth and in most cases burn should be avoided.
Slow orchard tree growth: If the trees are slightly stunted in size, it is better to reduce the rate of fertilization, apply more frequently, and apply nitrate sourced nitrogen to encourage growth. Usually, a stunted tree means a compromised root system. Using these strategies can help “spoon feed” the struggling tree.
Increasing Fertilizer Efficiency: With trees, it is better to fertilize smaller doses more frequently. This increases percentage of fertilizer uptake while reducing the risk of nutrient leaching. Following this principle, the rates mentioned above can be reduced and applied more frequently to ensure greater use efficiency amongst smaller trees.
Flood Irrigation: Be careful when applying a large dose of fertilizer which is followed quickly by an irrigation. Although this practice is recommended to increase fertilizer use efficiency, it can damage trees in hot weather. If the application is going to be close to the maximal rate as described above, and the weather conditions are greater than 85 degrees, making a slight reduction of applied rates (~10%) will reduce the risk of damage. This commonly occurs with hot weather because the trees are pulling large amounts of water which carries a high amount of nutrients into the trees. I have observed tree damage from this scenario about a handful of times with rates that initially appeared safe.
Fertilizing in cool, spring weather: Research suggests that using nitrate sourced nitrogen earlier in the growing season (Potassium nitrate, Calcium nitrate, etc) is more efficient in the earlier part of the season than urea or ammonium based fertilizers. Since urea and ammonium based fertilizers must be converted to nitrate by the soil-borne bacterial community, nitrate sources are more readily available for trees – especially in cooler soils. After soil temperatures increase, switching back to urea/ammonium based fertilizers is possible and can be done to reduce costs.