Blueberry Council of Missouri Newsletter

Health and Development

Editor: Howard Thompson
July 2009

As a grower I am constantly concerned about the health and development of my plants as we move through the year. This is centered around plant nutrition and hoofed vermin. This spring I did a small jig of delight at the same time letting out a few choice word of disgust when I got the results of the soil sample from the field that I have been prepping for planting. It finally showed that I had hit a pH in the 4's. The back story here is that this field had been prep several years ago with mushroom compost which has been heavily limed. At one time the pH was in the 6's. After years and literally TONS of sulfur, I finally got it right. This means I needed to change my regiment of sulfur and ammonium sulfate. In other words, I had to hit the books//computer to find a routine for my plants. The search was concentrated on the Universities and their extension divisions from states that are the top blueberry producers in the USA along with Missouri State University.

Plant nutrients can be divided into major nutrients, macronutrients and micronutrients. Nitrogen (N), phosphors (P) and potassium (K) are the three major nutrient, Ca, Mg and S are considered macronutrient while B, Cu, Fe, Mn and Zn are the minor nutrients. This discussion will only cover the nitrogen since it the nutrient that is most likely to be in a deficit in Missouri. Also to get optimal blueberry plant growth, nitrogen is the nutrient of greatest concern.

Nitrogen is the biggest of the big three. Plants can not survive without nitrogen. All the plant's proteins and genetic material (DNA and RNA) are centered around nitrogen in each cell. As the plant grows, each new cell requires more nitrogen to continue to grow. The interesting thing about plants is photosynthesis. It takes light and water to convert carbon dioxide into short chain carbon molecules which are used throughout the plant from everything from sugars and amino acids to proteins, nucleic acids, cellulose (used for the cell walls) and the cellular organelles. As a by-product of photosynthesis, oxygen is formed and energy is stored. To make photosynthesis work, chlorophyll is needed. As noted below (Fig 1), chlorophyll is complex carbon based molecule with four nitrogen chelating a Mg. Without sufficient nitrogen, you get smaller plants with fewer newer shoots. What new growth that does develop in a nitrogen deficient state has an anemic pasty to yellow appearance. This is different from an Fe(iron) deficiency where the vein of the leafs are green and the rest of the leaf is yellow-green.

Fig 1. Chlorophyll a molecular structure

Nitrogen in fertilizer comes in four forms, ammonia (NH3), ammonium (NH4+), nitrate (NO3-) and urea. NH3 is actually a gas or liquid which to use for blueberries is not practical. The blueberries evolved where there was minimal NO3- in the soil. Nitrates are not used well by blueberries since they have very limited nitrate reductase which converts nitrate into amino acid which is used to build proteins. Unlike NH4+, NO3- does not linger in the soil. In fact, some of the sources suggest that NO3- may be toxic to young blueberries. Figures 2 shows the effect of different forms of nitrogen on root development This leaves NH4+ and urea as a nitrogen source. Urea breaks down to NH3 and CO2 in the presence of water. It has little effect on the soil pH. It use is suggested if the soil pH less than 5.0. For soils that have pH's greater than 5.0, ammonium sulfate, (NH4)2SO4, which can lower the pH due to the sulfur. Urea is 43% nitrogen while ammoniums sulfate is only 21%, so if you have the idea soil pH, you will need to apply less urea than ammonium sulfate.

Fig 2. comparative effect of different forms of nitrogen on root development
(Spectrum Analytic, Inc)


My review found a very wide variation in the annual recommendation of nitrogen from 30-150 # nitrogen/A depending on the source, the region of the US and the number of plants per acre (Table 2) with Missouri State recommending 90-120#/A. If you look at the recommendations from a broader view point, you can make some generalizations: 1) The amount applied to each plant increases with the age of the plant. 2) Do not apply all the fertilizer to the crown of the plant. 3) Use annual leaf sample after harvest to help determine you nitrogen requirement for the following year. 4) Do a soil sample in the spring to check the pH to determine which nitrogen source to use. 5) The recommend amount of fertilizer is based on applying it to the WHOLE acre not just the plant stripe. 6) Maximal fruit production is tied with optimal plant nutrition. 7) Higher than optimal nitrogen results in larger plants and increase fruit production. 8) The age, depth and type of mulch may double to triple you nitrogen requirement. And finally 9) keep records of application and soli and tissue analysis. Below are two tables to assist you in managing your nitrogen application. Table 1 is "norms" for tissues (leaf sample) nutrient level.

   Element       normal range           Plant age                Nitrogen
       N          1.76-2.00%            Years in field    Per plant / pounds per Acre
       P            .1- 0.4%            0                  0              0
       K         .   41-.70%            1                  0.6 oz
       Ca            41- .8             2                  0.7 oz        15
       Mg            13-.25             3                  0.9
       S             11-.16%            4                  2             40
       B           31-80 ppm            5                  3                    100
       Cu           5-15 ppm            6                  4             45,100,125
       Fe         61-200 ppm            7                                   125,145
       Mn         30-350 ppm            8                                65,    165
       Zn              8-30

     Table 1: Leaf sample means            Table 2: Nitrogen recommendations

The Oregon State Extension publication presented an equation that converts pounds per acre into ounce per plant which assumes nitrogen has to be incorporated into the mulch or sawdust when the field was prepped. One also has to divide the final numbers by 0.21 for Ammonium Sulfate or 0.43 to obtain the actual amount of fertilizer that needs to purchased and supplied

(Oz/plant ) X (plants/A)/16 = pounds of Nitrogen per A

I found a 1/4 cup of urea was sufficient to 2, 1-3 year old plants. If my calculation are correct, 1 cup of urea was sufficient for 2 mature plants. If you are using ammonium sulfate the amount has to be double. This measurement of urea was arrived at assuming that only one half the annual amount was applied. It was then doubled due the use of sawdust and woodchip mulch. Since my planted row is 4-5' wide and the between row spacing is 5-6' and I was NOT fertilizing the grass, the amount had been decreased by half. It is interesting the effect of the mulch and not fertilizing between the rows cancel each other out. This basically leaves the original formula unaltered but the amount is divided by the number of application per year. The only alternation to the amount that is applicable is the increased amount of nitrogen needed by Earliblue, Duke, Chandler and Nelson cultivate as reported by Byers, et al and Missouri State to result in comparable cane production of other cultivars.

Timing of applying nitrogen is important for the plant to make the best use of the fertilizer and to keep cost down. Rutgers University has shown than applying fertilizer before bud break is just wasting money. Blueberries absorb nitrogen best between May and early July. By late July the nitrogen absorption diminishes. Depending of the source and the type of fertilizer, my review found the 4th of JULY to the 1st of AUG as the last day to apply fertilizer.

Since blueberries have shallow root, frequent application of small amounts become more effective. The later statement is one of the leading reasons for fertigation. If you are using solid fertilization, you need water (rain) to dissolve the crystals and drive the nutrient to the plants roots. Since fertigation is part of the irrigation system, water is already part of the delivery system. Unfortunately feritgation supplies that same about of fertilizer to the space between the plants. If you have young plants, there is more space between plants than plant space, this wastes fertilizer. If you have plants in the same irrigation zone that have different needs (age, plant size, cultivar), applying the crystals by hand allow individualization of needs.

Finally, too much nitrogen can be bad. Besides wasting money, if applied too late in the season there is not enough time to allow the new plant growth to harden off before winter. This increases winter damage which can set the plant up for diseases. Also excessive nitrogen delay can fruit ripening and can increase fungal infections.

Fig.3 Obtaining a leaf sample(Oregon State extension)

Leaf samples should be taken after harvest and when the plant is still actively growing but has slowed. This time is late July to August. To collect leaves, take mature leafs near the top of the cane but not part of the opening whorl as noted in Fig 3. The leaf should include the stem. A sample needs to be at least 10 leaves but 50 would be better. Since each cultivar absorbs nutrients differently, a sample needs to be collect from each cultivars. If there are plants of different maturity, they should be sampled separately. If you have plants that appeared to have problem, they should be sampled separately.

After collection, the leaves should be briefly washed in water with a touch of detergent before bring rinsed in clear water. The leaves need to blotted dry and then allowed to air dry for a few days before submitting for analysis to prevent them rotting before analysis (MichSU). Once you get the results back, then you can plan for the following year. This is were records are important. If the tissue results are too low, the recommendation is to increase the nitrogen by 10 per cent and pray for the best.


Fertilizing Blueberries, Spectrum Analytic, Inc

Growing Blueberries in Missouri, Bulletin 44, Fuqua,, Missouri State University

Nutrient Management for Blueberries in Oregon, J.Hart, et,al, Oregon State University Extension Service, 2006.

Managing the Nutrition of Highbush Blueberries, Ext Bul E-2011, E Henderson & J Hancock, Michigan State University Extension, 1996