MSU Extension Chestnuts

Nutrient management in chestnut trees is unique among perennial tree crops.  Routine and proper nutrition is important for tree health, vigor and optimal yield. A complete fertilization program based on soil testing, annual leaf analysis and observation of tree growth will maximize the establishment and development of chestnut trees. Many soils in Michigan provide nutrients in sufficient levels for chestnut production.  However, before planting it is recommended that growers do a soil test. A soil test provides you with valuable information on soil pH, texture and nutrient status. Chestnut trees require well drained soils and a pH of 4.5-6.5. Even though optimum nutrient levels for phosphorus, potassium, calcium and magnesium are not known for chestnuts, a soil test can provide you with information to base your nutrient and lime addition decisions.   

Nitrogen Recommendations 

Nitrogen is an essential nutrient and plays an essential role in many plant functions.  Fertilizer application is a necessary part of orchard maintenance as the nitrogen status of a tree can have a profound effect on health and vigor. When considering how much nitrogen to use, more is not necessarily better. Excessive nitrogen fertilization will over-invigorate vegetative growth on bearing trees and result in reduced flower bud formation and reduced fruit yield.   

Nitrogen recommendations, 0-5 years

Fertilizer use during the first year is not recommended and may cause damage to roots.  Fertilizer recommendations for years 2-5 are based off of better-studied systems, including apple. Using the table below, you can select the fertilizer of your choice based on availability and specific needs. Note the difference between actual nitrogen, ‘Amount of nitrogen per tree’ and product amount as indicated in the ‘Urea’, ‘Ammonium Nitrate’, and ‘Ammonium Sulfate’ columns. 

These recommendations are based on standard fruit and nut tree nutrient management from Europe. A given site may require more or less depending on soil and leaf analysis.

Visual observation of leaf color can also be a useful indicator of tree health. Leaf yellowing may be an indicator that the soil pH is too high at those locations which prohibits the tree from efficiently utilizing the macro and micronutrients you have made available. Growers should be evaluating and adjusting pH via soil testing and visual observation.

Nitrogen recommendations, bearing trees older than 5 years

Fertilizer rates for bearing chestnut trees are determined by tree size and vigor.  The diameter of the trunk is multiplied by the nitrogen rate based on the average length of last year’s terminal branch growth.

Low vigor:  If tree growth is considered low (under 8 inches per year) then a multiplier rate of 1/6 lb. (2.7 oz.) nitrogen per inch of trunk diameter is used.

Normal vigor: If tree growth is considered normal (8 to 12 inches per year) then a multiplier rate of 1/8 lb. (2 oz.) nitrogen per inch of trunk diameter is used.

Excessive vigor: If growth is more vigorous (greater than 12 inches on average) then a multiplier rate of 1/10 lb. (16 oz.) nitrogen per inch of trunk diameter

Note: Regardless of the outcome of the nitrogen calculation above, no more than 1 lb. of actual nitrogen should be applied per tree annually.   

Example of fertilizer calculation for trees older than 5 years:

5 inch trunk diameter

5 inch average terminal branch length last year

Fertilizer of choice – Ammonium sulfate (21-0-0)

How much ammonium sulfate do you have to apply to get the correct amount of nitrogen?

Your average terminal growth last year was 5 inches, indicating a low vigor which means that you would need 1/6 lb. (2.7 oz.)  nitrogen per inch of trunk diameter.

5 inch diameter X  2.7 oz. Nitrogen = 13.5 oz. actual nitrogen needed per tree

Ammonium sulfate is only 21% nitrogen, so to determine the rate of product needed, use the following formula:

Actual nitrogen (oz.) ÷ by nitrogen in product (%) = product needed (oz.)

13.5 oz. actual N ÷ 0.21 N in ammonium sulfate = 64.3 oz. of ammonium sulfate per tree

Refer to the table below for the application rates of a number of common nitrogen sources.

Fertilizer formulation, timing and application

Formulation

Fertilizers come in two types, the unmixed and mixed fertilizers. The unmixed fertilizer will have only a source of nitrogen and maybe sulfur, but there will be no phosphorus or potassium added. A mixed fertilizer will have nitrogen mixed with phosphate and potassium. Which is right for you? At an elevator you can find bags of mixed and unmixed fertilizers or order your own mixed fertilizer. There are advantages and disadvantages to each.

There are several types of unmixed fertilizers where the nitrogen, although unmixed with phosphate and potassium, can be found in various molecular forms and amounts within the fertilizer. Below, we have listed the most commercially available and commonly used unmixed fertilizers.

Ammonium Sulfate (NH4)2SO4 (21-0-0)

This means that 21% of the weight of the bag will be nitrogen and there will be no phosphorus (P) or potassium (K) in this fertilizer. This formulation contains both ammonic nitrogen and sulfur. Ammonium sulfate moves easily into the soil with rainfall or irrigation. Ammonium sulfate is the most acid-forming of the common available nitrogen fertilizers (one pound of (NH4)2SO4 will generate enough acidity to neutralize 5.3 lb. of free lime. We know that chestnut trees require low pH values so this fertilizer is useful in supplying nitrogen and keep the pH of the soil low. Do not use it in soils that are already low in pH (if your soils are pH 5.5 or lower, you don’t need it and don’t use it on a continuous basis). Problems can arise with the availability of other macro and micro nutrients if the soil pH drops too low.
This fertilizer is an excellent sulfur fertilizer, containing more sulfur than nitrogen (24% vs. 21%).


Urea CO(NH2)2 (46-0-0)

This means that 46% of the weight of the bag will be nitrogen, and again, there will be no phosphorus (P) or potassium (K) in this fertilizer. This formulation contains both ammonic nitrogen and carbon (which is useless to the plant as it obtains carbon from photosynthesis. A carbon-containing, factory synthesized nitrogen that mimics the urine of animals. Water-soluble. When this fertilizer is dissolved by rain or irrigation water, it moves rapidly into the soil. It is the cheapest form of nitrogen. Does not absorb humidity from air, thereby allowing relatively long-term storage. The ammonic nitrogen form is trap by clay particles and organic matter reducing leaching. Recommended used in soil with pH lower than 6.5 (because the free ammonia). 



Different manufactures process various formulations of mixed fertilizers. Those used for ornamentals, grass, fruit trees will all have different mixes of nitrogen, phosphorus and potassium. Then the different companies that make the material may have slight differences even if used for the same purposes. Elevators will develop mixed fertilizers for their specific area, so an elevator in Mason may not, on a routine basis; produce a mixed fertilizer that an elevator in Hemlock routinely mixes. However, they may be similar. For example, one elevator may mix a mixed fertilizer called Triple 12 (12-12-12) and another may manufacture Triple 16 (16-16-16). These premixed fertilizers are cheaper than others at the elevator since they process and sell so much of it. By now you know that these fertilizers mentioned above would have 8 pounds of nitrogen, 8 pounds of phosphorus and 8 pounds of potassium in a 50 pound bag of fertilizer.

Fertilizer timing and placement

There are several standard ways available to apply nitrogen and other nutrients to your trees in your orchard and probably dozens of less than standard ways that work. The guidelines below are based on soil application of the nitrogen. While some people may apply it to the leaves, there is no precedent for foliar applications on chestnut. 



Timing of nitrogen fertilizer applications to the soil surface influences the type of response that trees are likely to exhibit. With most tree crops, early season growth potential and strength of flower buds are largely determined by the nitrogen reserves that the buds contain when growth begins that season. This is a standard statement used for most fruit trees. However, most fruit trees flower in the spring. Chestnut flowers in the very late spring or early summer. We may be able to have some influence with our spring nitrogen application on the strength of the flower bud with s[ring application of nitrogen.



With most tree crops, nitrogen fertilizers applied during the dormant season as soon as the snow clears will stimulate vegetative growth and generally do not influence the nitrogen status or strength of current season flower buds or fruit set. This may be true for chestnut, too.



Applications during the summer, particularly after current season shoot growth has been completed, are more likely to result in improved nitrogen status of the buds for the next season. However, applications of nitrogen late in the summer may delay or reduce fruit development, increase the pre-harvest fruit drop, delay maturation of buds and woody tissues and/or stimulate late season growth, thus increasing susceptibility of woody tissues and buds to cold injury. In regions where cold injury is of concern, summer applications of nitrogen must be carefully managed to ensure the tree properly shuts down in preparation for winter.  Fall applications of nitrogen may delay hardening of buds and woody tissues and increase the potential for desiccation during the winter, particularly if made before trees have become completely dormant.

For most efficient use, nitrogen fertilizers should be spread over the area where the herbicide treatment eliminated the weeds (weed-free zone) or along the cultivated tree-row strips where the majority of the active tree roots are located. Application to weeds or grasses will act to fertilize the weeds and the tree roots will get the leftovers. For this reason broadcasting over the entire orchard floor is less efficient, requires considerably greater rates of application, and is more likely to benefit ground covers than the trees.



Fertilizer should be focused near the tree’s feeding roots. The feeding roots can extend beyond the spread of the outermost branches by as much as 40 to 50%. Shallow-rooted species such as elm or maple typically have roots that extend well beyond the spread of the branches. In such cases, extend the area fertilized to match the estimated root spread.  Applying nitrogen to organic mulches, such as shredded bark or leaves, will act to generate microbial degradation of the mulch; the nitrogen will be used primarily by the fungi and bacteria active in the mulch. If you are using organic mulches, apply the nitrogen to the soil and then put down the mulch.

General discussion of nutrient management and plant physiology

Fertilizer is generally thought of as plant food. Actually, most of the food or sugars of the plant is made by the process known as photosynthesis. Through photosynthesis, the plant removes carbon dioxide from the atmosphere, combines it with water and captures the carbon, hydrogen and oxygen atoms. It is through photosynthesis that the plant manufactures glucose, which is then used as an energy supply as well as providing the essential building blocks of the plant cells. Other atoms and molecules are also needed and some may be in short supply because photosynthesis occurs every day (unless dormant) and the other molecules must be scavenged through the root system. These other nutrients such as nitrogen, phosphorus, sulfur, potassium and a host of others must be found in the soil and then removed from the soil by the plant’s root system in combination with a fungal symbiont known as mycorrhizae. It is ironic that one of the most limited of these nutrients is nitrogen (N); the most abundant atom in the atmosphere.

Nature supplies N through the metabolism of various bacteria found in the soil, but this is a slow and study method and too often, humans want the plants to grow fast and produce abundant fruit—so we have learned to add N to the roots. Supplying nitrogen to plant systems usually induces a flush of growth whether it is redwoods or algae. This is one of the reasons why algae begin to grow in lakes when excessive nitrogen is released to the water through fertilizer use at home as well as agricultural runoff. For this reason, nitrogen is the focus of our story on fertilization. Too much, and it not only hurts the growth of the plant, it can cause large-scale environmental problems in watersheds. Too little, and your plants may be harmed directly through a lack of growth or indirectly through stresses that may be applied now or years later. Most of this information is taken from many sources, few of which deal with chestnut trees. We provide it for your interests and consideration.

A statement about genetics. You can only enhance the growth and yield of a plant with fertilizers to the level of genetic superiority of the particular plant or cultivar. During the Green Revolution, researchers found they could increase the productivity of certain genetic grain varieties by providing higher levels of N. If the same amount of N was added to the older, inferior lines, they would only have responded to a limit and peaked. The new lines responded much better and produced incredible yields compared to the older lines. This was accomplished by improving the genetics of the grains. If you use inferior plant material to start your chestnut farms, all the fertilizer in the world will not get you out of this situation. You need to start your farms with the best material possible that has demonstrated its ability in nearby plantings. Fertilizing inferior trees will only waste the fertilizer and result in water contamination.

Nitrogen is one of the macronutrients required for proper plant nutrition and growth. Nitrogen becomes part of many organic compounds within the plant cell. Somewhere between 1 and 5% of the total dry weight of a leaf can be accounted for by nitrogen. The plant takes up nitrogen from the soil in two forms, nitrate (NO3-) and ammonium (NH4+). Nitrogen plays an important role in many of the basic molecules of the plant cell such as amino acids used as the building blocks of proteins, and the nucleotides used in making DNA. Nitrogen is also an important component in molecules that make up various pigments and in generating cellular energy. Nitrogen is present in so many compounds that one begins to understand why it is that a lack of nitrogen will lead to growth deficiencies and the symptoms associated with these deficiencies. To make up for a lack of N the plant has devised various methods for its efficient use of N. For example, older leaves will exhibit a general chlorosis and may turn yellow and fall off since plants will commonly transfer nitrogen from older leaves to younger leaves to meet the nitrogen requirements of the new growth.


Nitrogen can be found in both inorganic and organic forms in the plant. It is usually found in combination with carbon (C), hydrogen (H), oxygen (O), which are brought into the cell through photosynthesis. Sometimes N is found in combination with sulfur (S) in specific amino acids used to build enzymes and pigments such as chlorophyll. Although inorganic N can accumulate in the nitrate form (NO3-), organic nitrogen, that is, nitrogen in combination with hydrogen (NH4+), predominates in the cell due to the high concentration of the super large molecules known as proteins.


Nitrogen consists of 1.5 % to 6 % of the dry weight of many crops. Usually the amount required in most plants for optimal metabolism, also called the sufficiency value, is from 2.5 to 3.5 percent when measured in leaf the tissue. A lower range of 1.8 to 2.2 percent is commonly found in most fruit crops. The highest concentration of nitrogen is found in the new leaves, and the nitrogen values generally decrease with the increasing age of the tissue or tree. 



Many of the cellular molecules that we have already mentioned also require phosphorus (P). Phosphorus, like N, needs to be taken up from the soil by most plants. Generally, more nitrogen is needed in the cell than phosphorus because nitrogen is used in more molecules than phosphorus. Plants have an enormous nitrogen craving because nitrogen is in such demand. However, the molecules that use phosphorus are some of the most important molecules in the cell. When nitrogen is added, plant growth is stimulated, and the more a plant grows due to this nitrogen addition, the more phosphorus will be needed. Therefore, when nitrogen is added, you may notice that phosphorus is sometimes added for good growth. Whether or not phosphorus is added is dependent on the amount of phosphorus already available in the soil. Michigan soils, as a rule of thumb, are not phosphate poor.

Potassium (K) is generally added because the more a plant grows, the faster the metabolism and potassium is involved in the metabolic functions of the cells, such as moving compounds into and out of cells. Therefore, once nitrogen is added, it is important to provide the other macronutrients needed if the plant is to fully utilize the nitrogen.



Available soil forms. Nitrogen exists in the soil as either nitrate (NO3-) or ammonium (NH4+). However, soil characteristics such as soil pH (for chestnut, this is very important), type of soil, temperature, and the presence of other chemicals in the soil can influence the uptake of either form of nitrogen.

Plants deficient in nitrogen are slow growing, weak, and stunted. Typically, the plant foliage is light green to yellow in color. The initial and more severe symptoms of yellow-leaf deficiency are seen in the older leaves, since nitrogen is diverted from the older tissue for transport to the actively growing portions of the plant. Nitrogen deficient plants will mature early with yield and quality of fruit significantly reduced.
Plants with an excess of nitrogen are dark green in color with succulent foliage, which may be more susceptible to disease and insect invasion than plants with optimal amounts of nitrogen. The plants may easily bend or break (lodge), and are susceptible to drought. If ammonium (NH4+) is the only or major form of nitrogen available for plants to take up from the soil, a toxic condition may develop which results in a breakdown of vascular tissue, thereby restricting water uptake.

An understanding of the general relationships between nitrogen and tree response is fundamental in managing nitrogen. Tree growth, flowering, fruit set, fruit growth, and fruit quality are all influenced by the nitrogen status of the trees. Numerous factors must be considered in developing appropriate nitrogen management programs. Methods and rates of nitrogen application, nitrogen sources, and economic factors must be included in the decision process.



Tree vigor is generally measured by amount of shoot growth or gain in trunk circumference per unit of time, usually one year or growing season. This relationship is important in developing the potential fruit-bearing canopy of newly planted and young nonbearing trees. Cropping potential of young trees is directly related to canopy development, especially in chestnut, which in turn is directly related to the nitrogen status of the trees.


Inadequate nitrogen supply when the tree is young may hinder the future cropping potential of the tree because a poor canopy will develop. Limited nitrogen availability may also encourage premature defoliation in the fall, and increases the susceptibility of the trees to injury when exposed to low temperature stress.
Excessive nitrogen supply: Increases vegetative growth, extends late season growth, delays leaf fall and the maturation of woody tissues, which in turn increases the susceptibility of trees to cold injury during the late fall and early winter period. Flowering and fruit set. Fruit set has been shown to increase with increasing nitrogen status of the flower buds. The period of receptivity of the ovules for fertilization is relatively short when flower buds are weak. Improving the strength of flower buds by increasing their nitrogen content increases the length of time that the ovules remain receptive for fertilization.
Inadequate nitrogen: Limits flower development and increases the tendency toward biennial cropping.



Excessive nitrogen can delay flowering of young trees. In trees of bearing age, excessive nitrogen may also stimulate development of excessive shoot growth leading to reduced flowering and fruit set.



Evaluating nitrogen status

It is important that the nitrogen in the plant be monitored in order to determine if the nitrogen levels are limited, optimum or excessive. To do this, a standard protocol needs to be developed and is generally worth the time and trouble. Below, we have outlined some methods you can use to determine the nitrogen levels in your trees.



Soil testing is an important diagnostic tool in evaluating nutrient imbalances and in understanding plant growth problems. Soil test results help growers adjust fertilizer application to provide nutrients that are lacking in the trees. Also, soil testing helps growers maintain soil pH within an optimum range (5.5-6.5 for chestnut), which keeps nutrients available for plant uptake. The soil test section is usually placed with the fertilizer section of a report like this, but we place it here to inform you that it should be used before you even plant your orchard. The soil test report includes soil pH, lime index, available phosphorus, potassium, calcium, and magnesium, liming and /or fertilizer recommendations based on the crop to be grown and soil test results. Michigan State University recommendations are given in “pound of nutrients needed,” not pounds of commercial fertilizer to be applied. You can pick up soil testing kits at your local county Extension office or buy a soil test online at the Bookstore.


Nitrogen content of leaves from the current season’s new growth is generally used as the basis for evaluating the nitrogen status of fruit trees. It is important to recognize that the procedure used in collecting these samples needs to be used in all collections if the information is to be compared and used in establishing standards. Most standards are based on mid-shoot leaves collected from 60 to 70 days after petal fall. Obviously, as chestnut growers we would be collecting leaves in September if we used that standard. So we must make up our own. Probably sometime in late July would be best. Older leaves collected at the same time, or mid-shoot leaves collected at a later time will have lower nitrogen contents. Likewise, younger leaves collected at this time, or mid-shoot leaves collected earlier than this will have higher nitrogen content. If it is necessary to collect samples later than the specified time, the last fully developed mature leaf toward the shoot tip should be sampled.
 Standards used in interpreting results of nitrogen analysis must account for differences among fruit tree types, stages of tree development, and the purpose for which the fruit is intended. Nitrogen levels of 2.4 to 2.6 percent, or higher, may be necessary to support adequate growth of the young nonbearing tree. As trees begin to produce fruit, levels of 2.2 to 2.4 percent may be appropriate to support continued growth while supporting fruit set. 
Several factors may influence the level of nitrogen in leaf samples:

• Shortage of soil moisture, whether as a result of inadequate rainfall or irrigation, or from competition by sod covers or weeds, reduces availability of soil nitrogen to the trees.


• Injuries to the roots or trunks restrict nitrogen uptake and result in lower leaf nitrogen contents.


• Leaf nitrogen contents are also influenced by the level of cropping, being lower in trees that are carrying a light crop and higher in trees that are carrying heavy crops.


• The lower concentration of leaf nitrogen in lightly cropping trees is the result of dilution of the nitrogen in a greater amount of vegetative growth than that produced by trees carrying full crops.


• Tree age and vigor are important factors and with newly planted trees and young nonbearing trees, the primary objective is to encourage rapid development of the potential fruiting canopy and early fruit production. 

• 
When the trees have reached maximum allowable size, greatest emphasis must be placed on fruit quality while still maintaining adequate tree vigor to support production of consistently high yields of top quality fruit.


• During the first stage of tree development, nitrogen fertilization rates are primarily based on maximizing growth, to the limits imposed by climatic conditions in the area. During the second stage of tree development, somewhat lower rates of nitrogen application become necessary in order to limit fruit size while still encouraging canopy development.


• Finally, with mature trees, careful control of nitrogen fertilization is necessary to avoid undesirable effects on the fruit quality.



Soil management practices

Practice that reduces competition by sod cover
or weeds generally results in increased growth and productivity.  With young trees, practices such as cultivation or maintenance of weed-free bands around trees or in the tree rows may increase rates of tree growth by 40 percent or more by reducing competition for water and availability of nitrogen.



In mature orchards, effective weed control has allowed reductions of 60 percent or more in amounts of nitrogen fertilizer application. However, the adverse effects of sod or weed competition have not been overcome solely by increasing rates of nitrogen application.
 Mulches of either organic or inorganic materials provide similar increases in availability of moisture and nitrogen. Mulching or sod increases soil organic matter levels compared to clean cultivation, and orchard soil management greatly influences nitrogen fertilizer requirements.