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How to Read those Reports - 1996

By this time of year avocado growers should have finished taking their annual leaf samples for a tissue analysis.  The leaves would have been from fully-expanded, terminal spring flush, on branches without fruit.  The sampling should have occurred mid-August to mid-October.  The reason for sampling during this time period is that leaf nutrient levels change as the leaf ages, and during this period the changes are relatively stable and have been correlated with yield. 

The elemental values are expressed in percentage (part per hundred) or part per million of leaf dry weight, and the values are compared to tabulated values which show deficiency, excess or adequacy.  These leaf analysis guide values are available from the labs doing the work or by contacting your local Cooperative Extension office.  If your values fall within the adequate range, there is a sufficiency of that nutrient.  Many growers feel that if their values fall in the low part of the adequacy range they should bring the value up.  This is not the case.  The adequacy range is just that, it says your current practices are what the tree needs. 

The analysis reflects the past fertilization practices, however,  if tree yield and vigor and weather of the current year are different from the previous year, only then should a change be made in the fertilizer practice.  For example, if the crop is substantially higher this year, an increase in nitrogen or potassium might be indicated.  On the other hand if yield is lower this year, then nitrogen and potassium fertilization might be reduced.

The defoliation caused by persea mite has complicated the sampling process.  Often, it is hard finding spring flush leaves that have not been mottled by the feeding damage of the mite.  Interpretation of the analysis can be misconstrued, because the guidelines are based on healthy leaves.  For example, the accelerated aging of the leaves can often indicate a lower than normal potassium level, because older leaves tend naturally to have lower potassium.  If persea damage is significant, it is important to recognize the deviation from the standard comparison before significantly altering the current fertilizer practices.

One of the beauties of tissue analysis is that it integrates what the tree is seeing.  It reflects the weather, the yield, the irrigation practices, the disease pressure, and the soil's moisture and chemical properties.  Before embarking on a different fertility program, examine whether some change in another cultural practice may be more effective in correcting the nutritional problem.  For example, with the loss of feeder roots due to root rot, it may be more effective to correct zinc deficiency by treating the disease than applying zinc fertilizer.  Iron deficiency is more pronounced in wet soils.  Modifying the irrigation practice may be more effective at correcting the deficiency than applying expensive iron chelates.

At this time of year, growers will often take soil samples for analysis, as well.  In general, soil analysis is not well correlated to tree nutrition.  This analysis works well for annual crops, but the perennial root system of trees acts very differently.  For example, the mycorrhizal relationship of the avocado root means that it can scavenge for phosphorus much more effectively than many annual plants.  The naturally occurring beneficial fungus on the root also helps in uptake of other nutrients, such as zinc.  The degree to which a perennial root system can absorb nutrients is not adequately addressed by a soil analysis.

This doesn't mean soil analysis is without value.  Analyzing for sodium adsorption ratio (SAR), salinity, boron, sodium, chloride and especially pH, can be extremely helpful in guiding irrigation and fertility programs.  This should be done in conjunction with a water report where these same characteristics should be measured, as well as bicarbonates.  Water bicarbonates will tell if an irrigation system clogging potential exists and if long term soil pH might be a problem.  If waters exceed values of 1000 parts per million (reported often as mg/l) salinity, 100 ppm sodium or chloride, or 1 ppm boron, it should be a yellow flag that these water constituents could be at toxic levels.  If the soil report indicates two and half times these values, it means an improved leaching program needs to be addressed.  SAR is another indication of the potential for water to have a sodium problem.  A water with a SAR in excess of 3 can indicate soil permeability problems, as well as a sodium toxicity to the tree.