It’s that time of year when citrus and avocado growers need to collect leaf samples for nutrient analysis to guide fertilizer applications. Leaves are collected between August 15 and October 15 and sent to the lab for analysis. For perennial crops, leaf analysis is the most important guideline for managing tree nutrient applications. Many growers think that soil analysis is as important as leaf analysis, and is for annual crops, but is much less valuable for tree crops. Because a tree stores nutrients in its various parts, such as roots, trunk, branches, stems and leaves, it does not have to get all of its immediate nutrients from the soil the way a lettuce plant does. Trees also have a root association with beneficial fungi called mycorrhizae (fungus/roots) which aid in the uptake of nutrients such as phosphorus and zinc, and this ability is not reflected in a soil analysis. A leaf analysis integrates everything the tree is "seeing" – weather, soil, in-tree storage, water, crop load, disease – which is then reflected in the leaf analysis.

Leaf analysis is done at this period, because the leaf nutrients are somewhat stabilized. Young leaves are high in such nutrients as nitrogen and potassium, but low in zinc and iron. As the leaf matures it loses nitrogen and potassium, but gains in iron and zinc. A fully expanded four-month old leaf from the spring flush taken at this time of year has been found to best reflect the tree’s nutrient status. For a discussion on leaf sampling, see our fall 2003 edition of Topics in Subtropics - http://ceventura.ucdavis.edu/newsletterfiles/Topics_in_Subtropics3707.pdf.

If leaf nutrients are low or high, it can indicate not only what nutrient is the problem, but also what sort of corrective actions should be evaluated. It may not be the lack of something like iron, but waterlogging from too long or frequent irrigations. Waterlogged soils reduce iron uptake, and this deficiency might be better addressed by correcting the irrigation practice than spending money on iron applications. Zinc deficiency might be a result of root rot killing root hairs that take up zinc and addressing the disease issue is going to have a longer term improvement on tree nutrient status than simply applying zinc fertilizer. And then of course, if leaves are showing toxicities to sodium or chloride, correcting irrigation leaching and infiltration issues is the way to solve this nutrient problem, since this the easiest way to solve the problem.

This does not mean soil and water analyses are not important, on the contrary. A pre-plant analysis for water and soil can tell you before hand what you might be dealing with and allow you to correct the problem before planting. A high pH is best corrected before trees are in the ground. Trying to correct a zinc, iron, manganese, or copper deficiency with the trees in the ground is expensive and can take years to correct. It is easier to apply sulfur or sulfuric acid to the ground before planting and can be done relatively quickly without harm to the trees. The micronutrient availability is controlled by pH and once soil pH is in the 6-7 range, it is less likely for these deficiencies to occur. Trying to lower pH when the trees show iron deficiency, must be done slowly, since adding too much acidifying material at one time can kill the tree and during the process of acidification, some sort of stop gap measure, such as foliar feeding or fertigation must be employed until the soil pH has slowly been corrected. A water analysis too can forewarn you if you will be having problems with such things as high salinity, chloride, sodium, magnesium, boron or pH, and allow you to select appropriate rootstocks tolerant of the problem or again address it with soil amendments pre-plant.

A soil analysis in conjunction with water analysis can also be used for an ongoing determination of how well irrigation is being managed. Soil from trees doing poorly can be analyzed to see if adequate leaching is being accomplished with the frequency and amounts being applied. Generally, though, a soil analysis is a poor indicator of guiding a tree nutrition program and as an ongoing practice should be used for identifying the toxicity problems of salinity, boron, sodium, chloride and pH.

UCCE Farm Advisor Gary Bender finally has  his 14 chapter book on avocado history, botany and cultural practices on the San Diego County web site.  Check it out:

http://ucanr.org/sites/alternativefruits/?story=237&showall=yes

Iron chlorosis is an off duck.  Soil is basically iron with some aluminum, silicon and oxygen added in for good measure, but plants can lack for iron because it is not available to be taken up.  This is often because the soil pH is high due to carbonates which tie up the iron, causing it to precipitate.  Another cause is asphyxiation of the roots from too much water.  Avocados are especially sensitive to asphyxiation and will show classic interveinal chlorosis and as time goes on, the leaf will turn almost white.   They will also show tip burn, like chloride damage in some cases, along with the yellowing.  The image of the tree below is of an avocado in a loamy soil with a pH near 7, but is irrigated every two weeks with about 400 gallons of water – kind of boom and bust.  The trees are drowning and showing classic iron chlorosis symptoms, but they could also be confused with root rot.

I was recently in an orchard looking at what appeared to be avocado root rot. I was checking on the symptoms and quizzing the grower on the other cultural practices.  The grower was prepared with soil, leaf and water reports.  I asked how much nitrogen was being applied and it was something in the 50 pounds nitrogen per acre.  When I saw the water report it listed nitrate in the water at the level of 84 ppm.  Rarely do you see a yield response in plants when soil nitrogen exceeds 90 ppm nitrate.  The soil nitrogen would pretty rapidly take on the nitrogen level of the water. So this grower was very close to the level at which no yield improvement would occur, and in fact would be increasing vegetative growth and hence increasing pruning problems.  I looked at the leaf nitrogen levels and they were quite high, as well.  I suggested that it would be a good idea to cut back significantly the amount of fertilizer applied, if not stopping application all together.  There are many areas along the coast where avocados are grown where there are high nitrates in the water.  Water can be a significant source of nutrients, as well as toxics, such as boron, chloride, sodium and total salts.  Learn to read all your reports – soil/leaf/water – and figure out the puzzle of fertilizing appropriately.