Sustainable Nutrient Management: a Review.

Almond harvest looks to be progressing well (knock, knock).  Orchard fertility/nutrition planning for the 2014 is beginning.  Now is a good opportunity to review the basics of almond orchard nutrient management.  While some of what follows will be review for many readers, a quick review may be of value.

Sustainable nutrient management comes down to the four R’s – the Right Rate, the Right Time, the Right Place, and the Right Material.  A sustainable approach to orchard nutrition is intended to optimize crop production and maximizing net profit while reducing the risk of environmental contamination.

Rate:  Sustainable nutrient management is focuses on replacing tree nutrient use on an annual basis.  Cropload drives nutrient demand in a mature orchard.  The bigger the crop, the more nutrients/acre required.  The amount of key nutrients needed to produce 1000 pounds of kernels are presented in Table 1 (below).  Not all of the nutrients are contained in the kernels, but the numbers in Table 1 represent the amount of nutrients leaving the orchard in hulls, shells, and kernels for every 1000 lbs of kernels produced.  I have included phosphorus in Table 1 as I hear reports that growers regularly use this nutrient.  It should be noted that almond orchards use a lot more potassium (K) and nitrogen (N) than phosphorus (P).  Phosphorus is both an essential plant nutrient and a surface water contaminant, causing algae bloom in rivers, lakes, and streams that can harm fish and other aquatic life.  Surface applications of P fertilizer or manure without prompt incorporation risk losing P from the field and harming surface water quality.

Time:  The key time for adequate nutrition in almonds is during nut growth – bloom to hull split, with B and Zn most needed at bloom – early shoot growth, N most needed in March-June,  and K and P use steady during  nut growth.  Fall is a time when certain nutrients can be front loaded into the tree or the soil – depending on the nutrient and soil chemistry — before the next crop.  Knowing how much of a certain nutrient the trees or soil can hold is vital to maximizing return on your fertilizer dollar (sustainable nutrient management).   Click HERE to see an earlier post on fall fertilization and HERE for an earlier post on N fertilization in almonds.  Potassium is held on the surface of clay and soil organic matter, so any fall applications should be done with an eye to matching rates to soil type.  Higher rates work on heavier (loam/clay loam) soils, while lower rates should be applied to sandier soils that hold less K.  [Want an easy way to estimate soil texture in the field?  Click HERE to check out the UC California Soil Resources Lab’s Soil Web App for smart phones, which tells you the soil information for the spot you are standing.]  In-season fertigation via drip or smaller micro-sprinklers is a good option to fall K application.   Fall is also an excellent time for foliar applications of boron (B) and/or zinc (Zn).  Soil applications of these nutrients may not reach the buds before bloom.  Of all the recommended timings for N fertilizer use, fall soil N fertilization is the timing with the greatest risk of N loss (inefficiency).  Why?  All mineral N fertilizer sources are rapidly transformed to nitrate and nitrate moves readily in the soil with water.  In winter, in high rainfall areas or under freak high rainfall in normally low rainfall areas, the net direct of water movement is down through the rootzone towards groundwater.  Fall N fertilization decisions should be a balance between orchard need and potential for loss.  Trees don’t absorb N once leafs are off, so canopy health is key to effective fertilizer N use in the fall.

Place:  Soil applied nutrients enter a plant through its roots.  Fertilizer should be placed in or very near the active root zone.  Roots are most active where there is water during the growing season.  Fertigation via drip and small microjets efficiently delivers nutrients to the active root zone.  Nutrients with moderate to low soil mobility (K, P, and Zn) should be banded on the soil surface if not directly fertigated.  Broadcast application of these moderately to highly mobile nutrients is inefficient (expensive).

Material:  In my experience, basic commodity-type fertilizer materials from reputable producers/manufacturers are the most cost-effective tools for sustainable nutrient management.  “More efficient” fertilizer products may be just that, but at the end of the day, you still need to get into mature trees the amounts of nutrients removed in a crop (see Table 1) to sustain production and not deplete/exhaust soil reserves.        

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Table 1.  Almond crop (hull, shell & kernel) nutrient use per 1000 lbs of kernels (nut meats), and the relative soil mobility of each nutrient.  The crop nutrient values may vary slightly with crop load and location, but are accurate for the purpose of fertility planning.

*equal to 90 lbs K₂O

** equal to 18.2 lbs P₂O₅

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