- Author: Craig Kallsen
University of California (UC) researchers and private industry consultants have invested much effort in correlating optimal citrus tree growth, fruit quality and yield to concentrations of necessary plant nutrients in citrus (especially orange) leaf tissue. The grower can remove much of the guesswork of fertilization by adhering to UC recommendations of critical levels of nutrients in the tissues of appropriately sampled leaves. Optimal values for elements important in plant nutrition are presented on a dry-weight basis in Table 1. Adding them in appropriate rates by broadcasting to the soil, fertigating through the irrigation system or spraying them foliarly may correct concentrations of nutrients in the deficient or low range. Compared to the cost of fertilizers, and the loss of fruit yield and quality that can occur as a result of nutrient deficiencies or excesses, leaf tissue analysis is a bargain. At a minimum, the grower should monitor the nitrogen status of the grove through tissue sampling on an annual basis.
Leaves of the spring flush are sampled during the time period from about August 15 through October 15. Pick healthy, undamaged leaves that are 4-6 months old on non-fruiting branches. Select leaves that reflect the average size leaf for the spring flush and do not pick the terminal leaf of a branch. Typically 75 to 100 leaves from a uniform 20- acre block of citrus are sufficient for testing. Generally, the sampler will walk diagonally across the area to be sampled, and randomly pick leaves, one per tree. Leaves should be taken so that the final sample includes roughly the same number of leaves from each of the four quadrants of the tree canopy. Values in Table 1 will not reflect the nutritional status of the orchard if these sampling guidelines are not followed. Typically, citrus is able to store considerable quantities of nutrients in the tree. Sampling leaves from trees more frequently than once a year in the fall is usually unnecessary. A single annual sample in the fall provides ample time for detecting and correcting developing deficiencies.
Table 1. Mineral nutrition standards for leaves from mature orange trees based on dry-weight concentration of elements in 4 to 7 month old spring flush leaves from non-fruiting branch terminals.
|
element |
unit |
deficiency |
low |
optimum |
high |
excess |
|
|
|
|
|
|
|
|
|
N |
% |
2.2 |
2.2-2.4 |
2.5-2.7 |
2.7-2.8 |
3.0 |
|
P |
% |
0.9 |
0.9-0.11 |
0.12-0.16 |
0.17-0.29 |
0.3 |
|
K (Calif.*) |
% |
0.40 |
0.40-0.69 |
0.70-1.09 |
1.1-2.0 |
2.3 |
|
K (Florida*) |
% |
0.7 |
0.7-1.1 |
1.2-1.7 |
1.8-2.3 |
2.4 |
|
Ca |
% |
1.5 |
1.6-2.9 |
3.0-5.5 |
5.6-6.9 |
7.0 |
|
Mg |
% |
0.16 |
0.16-0.25 |
0.26-0.6 |
0.7-1.1 |
1.2 |
|
S |
% |
0.14 |
0.14-0.19 |
0.2-0.3 |
0.4-0.5 |
0.6 |
|
Cl |
% |
? |
? |
<0.03 |
0.4-0.6 |
0.7 |
|
Na |
% |
? |
? |
<0.16 |
0.17-0.24 |
0.25 |
|
B |
ppm |
21 |
21-30 |
31-100 |
101.260 |
260 |
|
Fe |
ppm |
36 |
36-59 |
60-120 |
130-200 |
250? |
|
Mn |
ppm |
16 |
16-24 |
25-200 |
300-500? |
1000 |
|
Zn |
ppm |
16 |
16-24 |
25-100 |
110-200 |
300 |
|
Cu |
ppm |
3.6 |
3.6-4.9 |
5 - 16 |
17-22? |
22 |
*California and Florida recommendations for K are sufficiently different that they are presented separately. The California standards are based on production of table navels and Valencias, and those for Florida were developed primarily for juice oranges like Valencia.
The sampled leaves should be placed in a paper bag, and protected from excessive heat (like in a hot trunk or cab) during the day. If possible, find a laboratory that will wash the leaves as part of their procedure instead of requiring the sampler to do this. Leaf samples can be held in the refrigerator (not the freezer) overnight. Leaves should be taken to the lab for washing and analysis as quickly as is feasible.
Often separate samples are taken within a block if areas exist that appear to have special nutrient problems. The temptation encountered in sampling areas with weak trees is to take the worst looking, most severely chlorotic or necrotic leaves on the tree. Selecting this type of leaf may be counter-productive in that the tree may have already reabsorbed most of the nutrients from these leaves before they were sampled. A leaf-tissue analysis based on leaves like this often results in a report of general starvation, and the true cause of the tree decline if the result of a single nutritional deficiency may not be obvious. Often in weak areas, it is beneficial to sample normal appearing or slightly affected leaves. If the problem is a deficiency, the nutrient will, generally, be deficient in the healthy-looking tissue as well.
Groves of early navels that are not normally treated with copper and lime as a fungicide should include an analysis for copper. Copper deficiency is a real possibility on trees growing in sandy, organic, or calcareous soils. For later harvested varieties, leaves should be sampled before fall fungicidal or nutritional sprays are applied because nutrients adhering to the exterior of leaves will give an inaccurate picture of the actual nutritional status of the tree.
Usually leaf samples taken from trees deficient in nitrogen will overestimate the true quantity of nitrogen storage in the trees. Trees deficient in nitrogen typically rob nitrogen from older leaves to use in the production of new leaves. Frequently, by the time fall leaf samples are collected in nitrogen deficient groves, these spent spring flush leaves have already fallen. Nitrogen deficient trees typically have thin-looking canopies as a result of this physiological response. Since the spring flush leaves are no longer present on the tree in the fall when leaves are sampled, younger leaves are often taken by mistake for analysis. These leaves are higher in nitrogen than the now missing spring flush leaves would have been and provide an inaccurately higher nitrogen status in the grove than actually exists.
Critical levels for leaf-nitrogen for some varieties of citrus, like the grapefruits, pummelos, pummelo x grapefruit hybrids and the mandarins, have not been investigated as well as those for oranges. However, the mineral nutrient requirements of most citrus varieties are probably similar to those for sweet oranges presented in Table 1, except for lemons, where the recommended nitrogen dry-weight percentage is in the range of 2.2- 2.4%.
A complete soil sample in conjunction with the leaf sample can provide valuable information on the native fertility of the soil with respect to some mineral nutrients and information on how best to amend the soil if necessary to improve uptake of fertilizers and improve water infiltration.
P.S. from Ben Faber
What has been said here about citrus is also generally true for avocado, although the nitrogen sufficiency levels are lower than for citrus. For a more detailed discussion see: http://www.californiaavocadogrowers.com/sites/default/files/documents/11-Final-Report-Issued-Giving-Tools-for-Fertilization-and-Salinity-Management-Winter-2016.pdf
Photo: Nitrogen deficient avocado leaf
- Author: Ben Faber
Potassium deficiency in avocado and citrus leaves often looks like salt stress and more specifically sodium toxicity. Plants will often look wilted with curled leaves, yellow areas between leaf veins and dead areas along the margins of the leaves. Salt stress refers to the excessive amount of soluble salts in the root zone which induce osmotic stress (appearance of lack of water) and ion toxicity (growing problems and often symptoms) in the growing plant. Among toxic ions, sodium (Na+) has the most adverse effects on plant growth by its detrimental influence on plant metabolism in inhibiting enzyme activities. An optimal potassium (K+) : Na+ ratio is vital to activate enzymatic reactions in the cytoplasm necessary for maintenance of plant growth and yield development These enzymes control such functions as the stomata which regulate water and photosynthesis control in the plant. Although most soils have adequate amounts of K+, uptake is exacerbated under sodic or saline-sodic soil conditions as a consequence of K+-Na+ antagonism. Here K+ uptake by plants is severely affected by the presence of Na+ in the soil. Due to its similar chemical properties, Na+ competes with K+ in plant uptake It would seem a reasonable assumption therefore that an increase in the concentration of K+ in salt-affected soils may support enhanced K+ uptake. And that has been noted in many plant species including citrus and avocado.
But aside from the role of potassium in drought tolerance there are many functions of potassium in plants:
• Increases root growth and improves drought resistance
• Activates many enzyme systems
• Maintains turgor; reduces water loss and wilting
• Aids in photosynthesis and food
• Reduces respiration, preventing energy losses
• Enhances translocation of sugars and starch
• Produces grain rich in starch
• Increases protein content of plants
• Builds cellulose
• Helps retard crop diseases
In the case of avocado and citrus there is about twice the amount of potassium as nitrogen harvested in the crop, yet many growers do not consider potassium in their normal practices, much less when drought has increased salt stress on the trees. The end of August through September is when leaf analysis is best used to adjust a fertilizer program.
Sodium toxicity and Potassium deficiency in avocado
- Author: Ben Faber
At a recent avocado meeting, Carol Lovatt of the Botany Department at UC Riverside pointed out that avocado fruit take up more potassium than nitrogen, almost twice as much, and that much of that uptake occurs later when the fruit is expanding. She reminds growers that all to often, the potassium needs of the tree are overlooked.
Click on "attached files, potassium nitrogen uptake" to view graphs.
potassium nitrogen uptake
- Author: Ben Faber
Citrus and avocado are of subtropical and tropical origin are cold-tender plants that have not developed the effective cold hardening process of deciduous trees that drop their leaves and go dormant. Even within deciduous, temperate tree species there are ranges of frost tolerance. Subtropicals do have the capacity to develop some cold tolerance and this is by going through quiescence, a sort of resting condition of no or slow growth when cooler temperatures arrive. Quiescence is induced several weeks after 40-50 degree F temperatures arrive. Cold tolerance develops most when trees are not flushing. The healthier and less stressed the tree, the more responsive it is to the cooler temperatures that induce quiescence. A tree that has been recently pruned or nitrogen fertilized is more likely to continue flushing through the cold-induction of early winter and is more subject to cold damage. Unfortunately along the coast, there can be warm temperatures that occur during the winter. This can break quiescence and the trees start flowering or flushing and when cold weather arrives again, the trees are now more susceptible to cold.
Because an evergreen canopy like avocado continues to transpire, the roots of subtropicals continue to operate to deliver water to the leaves. The leaves though are not doing much other than losing water. Therefore, there is not much demand nutritionally on the part of the tree. In fact, fertilizing with nitrogen can break the quiescence and make the tree more susceptible to cold. The tree already stores the bulk of its nutritional needs in the roots, branches, stems and leaves and can call on nutrients if it needs them. Supplemental fertilizer at the time of quiescence, though, can result in an excess of the tree’s needs and induce flushing. The goal is to go into winter with adequate storage, so that when spring comes, the nutrients are there for the demanding flowering period. Adequacy is based on previous fertilizer applications, tree condition, leaf analysis and crop load. Nitrogen applications applied in winter are also susceptible to high losses from leaching and volatilization, resulting in environmental problems.
So fertilizer timing starts with when the first frosts might come. Along the coast in Southern California, the last nitrogen application should be no later than October 1, in preparation for frosts than can occur in December. Nitrogen can resume March 1 in most cases. Then the next issue is how frequently to apply nitrogen after that. The more frequent/small applications that are made, the more efficiently it is taken up, so the less that is required. Continuous injection is ideal, but most operations are not equipped to do it this way. Setting a monthly time for injection, such as the first of the month, is the next most efficient. Whatever timing you use, though, nitrogen applications should be confined to the spring and summer months with some possible in early fall.
As for potassium, it is not so liable to cause flushing and is not so susceptible to leaching and has no volatilization. It can be applied most any time and will not go anywhere other than through erosion, waiting for the tree’s roots to absorb it. Microelements such as iron and zinc though, need an actively growing root system for uptake. Their soil application should be limited to the summer time.
