The idea that you might be able to invest in tree growth in such a way that you can have your first commercial harvest and perhaps all subsequent harvests earlier is appealing to many people. Fertilizing forest stands is not a new idea and has been practiced in the Southern United States, especially on lands owned by forest industry, for many years. However, for most landowners the problem is more one of execution than application. Unless nitrogen is specifically limiting in the soil (the soil is infertile and doesn’t have enough nitrogen) then the addition of nitrogen may not actually give you the results that you want. The addition of Nitrogen forces the growth of leafy green structures (just think about what happens when you make a fertilizer application to your lawn). Essentially, if you fertilize your trees you don’t necessarily get more growth, you get more leaves. However, it is mainly a reallocation of growth; your trees will reallocates growth designated for other structures (roots, diameter increment, fruit, etc…) to invest in shoot elongation and leaf surface area. So back to execution!
Where should you start?
Have a soil test done. The University of Wisconsin – Madison Soil Testing Laboratories can perform a forest soils analysis for you. The report you receive will include information on “soil pH, organic matter and nitrogen, and the amounts of available nutrients such as phosphorus, potassium, calcium and magnesium, along with an interpretation of how to correct for deficiencies or excessive quantities of nutrients.” I cannot stress this enough. Fertilizing with nitrogen only may not give you the results that you are looking for if nitrogen is already plentiful in the soil. It may very well be that liming or some other type of amendment may give you the results that you are looking for if your soil is deficient in one of the major nutrients needed for tree growth.
Limiting nutrients and plant growth
There are 12 different macro and micronutrients that trees require for photosynthesis, growth, and maintenance of the stem, branches, roots, and leaves as well as a sufficient supply of water and carbon dioxide (CO2). When any one of these nutrients or water is not readily available then tree growth is limited by the availability of that nutrient. This doesn’t mean that no growth will occur; only that growth will only proceed according to the availability of the least available or most limiting nutrient.
Think of it this way… set up an experiment in your backyard. Mark out four ten foot by ten foot plots. Leave one plot as a control (no supplemental watering or nutrients); give one plot a one-time supplement of Nitrogen; give one plot an additional one inch of water each week; and give the last plot a one-time supplement of Nitrogen and one inch of additional water each week. Which plot will have the best growth and greenest grass? The plot you provide the additional nitrogen and water. Which has the poorest growth? The plot that did not receive any supplemental treatments. Growth proceeds according to whatever is most limiting. When water and Nitrogen are available in abundance you get a lot of leafy green plant structures (grass, which you now get to mow). Your control plot still has grass (well maybe not this summer unless you live in the Ashland area) but it isn’t as green or luxurious.
Let’s assume that your soil test comes back and everything looks good. There are no nutrient deficiencies that might limit growth if you fertilize your trees. Your next step is to consider what kind of application you would like to make and how it impacts tree growth.
When should you start making fertilizer applications?
Since growth relies on the ability of the tree to take up water and nutrients from the soil you should wait until your trees have developed a root system sufficient to support accelerated growth. I would not recommend starting a fertilization program until your trees have been in the ground at least four years. As a general rule of thumb we expect trees to experience “transplant shock” equal to their age. A seedling grown in the nursery for two years would take two years after planting to grow a root system necessary to support top growth. Since you are trying to push growth I would wait at least two more years to make sure that the root system is extensive enough to capture a supplemental application of nitrogen.
You can continue making annual applications up to the point where the crown’s “close” and sunlight becomes a limiting factor as the crowns interfere with each other and the trees compete for sunlight. As with the previous discussion on limiting nutrients, sunlight is an essential factor in photosynthesis and as the crown’s close and light becomes less abundant the impact of supplemental applications of nitrogen also become less valuable as well as more costly in terms of the reduction in additional growth you get from fertilization.
When should you make supplemental applications?
Supplemental applications of nitrogen should be made in the spring of the year only (there are some issues around slow release formulations that I will cover later) prior to when the tree is actively budding out. Think about how your tree grows. The shoots elongate and the leaves are put on in the spring when moisture is good. Overall, tree growth declines in the summer as temperatures increase and water loss becomes a problem for the tree. Summer wood or diameter increment occurs from late spring into early fall. Once the shoots elongate the tree puts on diameter growth to stabilize the new growth to make it sturdy (makes sense right?!?!). Finally, late summer into fall the tree grows its root system. All three of these things are usually occurring simultaneously. However, there are times during the year where one takes precedence over the other. This is why a fast release application made in the fall can give you an overabundance of shoot growth without the stem diameter growth or root system to support it. It will create stress on the tree which can lead to top dieback, breakage (wind or snow / ice loading) or slowed growth as the tree puts on diameter and root growth at the start of the next year trying to play catch-up.
Take a look at the figure below describing seasonal differences in height growth, diameter growth, and root elongation of an eastern white pine tree (taken from UW – Extension publication G3277 “How Forest Trees Grow”). This graphic provides a nice description of the timing of tree growth. Looking at this graph a spring application of a fast release fertilizer should occur around mid-April (weather dependent on tree green-up and when the frost comes out of the soil). Notice shoot elongation peaks around Memorial Day. So any applications made after that point will stretch out height growth and leaf growth at the expense of diameter growth and root growth.
You can make a late fall application with a slow release formulation of nitrogen but it should be part of a maintenance program that is designed to knockdown competition and destroy any sod-forming grasses in your plantation. The root systems of sod forming grasses will capture the nitrogen from any treatment whether spring or fall, slow release or fast release; but they are especially problematic for slow release applications since the actual amount of nitrogen released at any given time will be capture entirely by the root system of sod forming grasses.
How much should you apply?
Your soil analysis will provide you with a recommendation for how much you should apply. However, as a general rule of thumb, around 10 lbs. per acre is all you really need. You may want to experiment with this but remember the additional growth you see from a supplemental application of nitrogen will still be limited by something. 10 lbs. per acre is a reasonable place to start. You may find that you can go as high as 35 lbs. per acre as the trees get older. I would recommend being strategic in your treatments. If your goal is to push growth to get a quicker financial return then throwing away money by applying more nitrogen than the trees can use just defeats your purpose. Experiment with your applications throughout your stand and make applications where you think you may get your best returns, especially in areas where the soil may be (more?) deficient in nitrogen.
Please remember that too much nitrogen can actually cause damage and lead to damaged foliage, lost foliage, and reduced growth. This isn’t a case of “if a little is good, then more is better, and too much is just right.” Over-application of nitrogen can cause serious and long-term damage to your trees and your property.
How do you make a supplemental application?
The type of application and how it is incorporated are also important. You want to place the nitrogen within easy reach of the roots system. Liquid applications are most effective when they are applied just outside the “dripline” of the tree where the highest proportion of feeder roots often found. Dry applications are often made in “bands” on either side of the tree. Both work well but the way you make the application is usually proscribed by the type of equipment you use.
Remember, sod forming grasses are the absolute death of these types of applications. You need to introduce the fertilizer to the soil. Liquid applications that are sprayed on the surface will be captured by the roots of sod forming grasses before it can ever reach the root zone of the tree. So maintenance and fertilization often go hand-in-hand. If you want to soil incorporate a dry fertilizer then a fall application of a slow release fertilizer would be ideal.
If you can manage it, provide your trees with supplemental water as well as fertilizer. Remember, if you give a plant fertilizer it produces foliage and if it produces foliage then it loses even more water through photosynthesis and transpiration. Since there is no way to tell whether we will have a good rain year or drought you are always going to be a hostage to precipitation (which you are anyway when it comes to tree growth) so any supplemental water you can provide will make your nitrogen application more effective.
Also, don’t forget good cultural practices. You may actually find that you will need to increase your efforts to control competition as these plants will also surely benefit from these applications. You may find that you need to prune your trees as well. Good growth means that your trees will be putting on additional foliage and the branches to support that foliage. You are going to have to prune your trees to insure that the branches are strong enough to support the additional growth as well as removing weakly attached branches to prevent breakage or damage to main stem if they tear away from the tree.
At the end of the day you can get really impressive results from fertilizing trees at a young age (4 year-old minimum all the way up to annual treatments until canopy closure of the plantation). However, it has to include good cultural practices, like weeding and pruning, and the addition of all factors that contribute to growth (water, nitrogen, phosphorous, etc…) to insure that you get the result that you want.