Nutrient availability from organic sources has been considered “slow release” by many growers and advisers. This may be true in environments are colder and especially soils are cooler. Organic nutrients are dependent on microbes to break down materials and release those nutrients, and when soils are cold, microbes can't do their thing. Soils in much of agricultural California tend to be warm and lack the freezing conditions that occur in many soils in the continental US. Imagine how much microbial activity occurs in the Mid-West when soils cool down to 32 deg F at a four inch depth and deeper. The top layers of soil are where organic matter accumulates and where most microbial activity occurs. When soils cool below 50 deg F, nitrogen leaching becomes less common, because less activity is occurring which also coincides with much less plant growth.
Soils in coastal California rarely fall below 50 deg F in the surface layers, so microbial activity is ongoing, all year long. So the question is, how “slow acting” are organic fertilizers? A recent study by Tim Hartz, Richard Smith and Mark Gaskell looked at release rates of injectable organic fertilizer and found that much of the nutrient release occurs within about a week after application depending on the formulation and temperature during the study. The results conform to another study that they did where they evaluated the nitrogen release rates of dry formulations of organic fertilizers – compost, manures, feather meal, etc.
Aside from the issues of the higher costs of these materials and their potential clogging, there is the issue of application timing. In the case of avocados and citrus, adequate levels of nitrogen are needed in the trees going into to fruit set in order to optimize set. And then after fruit set, in order to maintain growth into the fast growth period, again nitrogen needs to be adequate. Using organic fertilizers with a rapid conversion to useable forms of nitrogen, means that application timing should coincide with these critical periods in tree phenology or growth cycle.
Using information on organic nutrient management based on work from cold soil climates needs to be carefully evaluated before applying it to California soils. One of the most common problems in organic production is nitrogen management. Part of the problem is the cost of supplemental nitrogen amendments, but also learning to anticipate when that applied nutrient becomes available to the plant. Developing better estimates for local release rates and patterns will better help manage organic nutrient sources.
Summary: Limited soil nitrogen (N) availability is a common problem in organic vegetable production that often necessitates additional N fertilization. The increasing use of drip irrigation has created a demand for liquid organic fertilizers that can be applied with irrigation. The N availability of three liquid organic fertilizers was evaluated in an incubation study and a greenhouse bioassay. Phytamin 801 contained fishery wastes and seabird guano, while Phytamin 421 and Biolyzer were formulated from plant materials. The fertilizers ranged from 26 to 60 g·kg−1 N, 8% to 21% of which was associated with particulate matter large enough to potentially be removed by drip irrigation system filtration. The fertilizers were incubated aerobically in two organically managed soils at constant moisture at 15 and 25 °C, and sampled for mineral N concentration after 1, 2, and 4 weeks. In the greenhouse study, these fertilizers and an inorganic fertilizer (ammonium sulfate) were applied to pots of the two organically managed soils with established fescue (Festuca arundinacea) turf; the N content of clippings was compared with that from unfertilized pots after 2 and 4 weeks of growth. Across soils and incubation temperatures, the N availability from Phytamin 801 ranged from 79% to 93% of the initial N content after 1 week, and 83% to 99% after 4 weeks. The plant-based fertilizers had significantly lower N availability, but after 4 weeks, had 48% to 92% of initial N in mineral form. Soil and incubation temperature had modest but significant effects on fertilizer N availability. Nitrification was rapid, with >90% of mineral N in nitrate form after 1 week of incubation at 25 °C, or 2 weeks at 15 °C. N recovery in fescue clippings 4 weeks after application averaged 60%, 38%, and 36% of initial N content for Phytamin 801, Phytamin 421, and Biolyzer, respectively, equivalent to or better than the N recovery from ammonium sulfate.
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
Silicon is currently under consideration for elevation to the status of a "plant beneficial substance by the Association of American Plant Food Control Officials (AAPFCO). Silicon has been shown in university and field studies to improve cell wall strength and structural integrity, improve drought and frost resistance, decrease lodging potential and boost the plant's natural pest and disease fighting systems. Silicon has also been shown to improve plant vigor and physiology by improving root mass and density, and increasing above ground plant biomass and crop yields.
Silicon (Si) is the most abundant element (27.2%) present in the earth's crust following oxygen (45.5%). Silicon is known for a number of important chemical and physical properties, i.e. semiconductor property that are used in various scientific and technical applications. In most soils near a neutral pH, the composition is a complex of iron, aluminum, oxygen and silicon. Silicon is one of the most important constituents of dust, which is carried by winds all over the world. Geologists know silicon as the rock quartz and the many silicate materials, such as opal. Formally, silica (SiO2) is a silicic acid (H4SiO4), which is water soluble and stable in highly dilute aqueous solutions. There are many forms that silicon can take in the natural environment, often complexed with water. Plants take up a form of silicic acid and in highly leached, low pH environments, much of the silicon may have been removed over time.
It appears that grains, such as wheat and especially rice have an absolute need of supplemental silicon to improve plant growth. Few non-grass plants have shown this need other than cucurbits apparently. Much of the improvement typically is for improved disease control and improved stature (prevention of lodging).
Many of the studies showing benefits of silicon amendment have occurred in low soil pH environments or in solution culture where it has been possible to create low silicon growing media. Several years ago, potassium silicate was being promoted as a fungistat for controlling Phytophthora root rot in avocado. A number of field and greenhouse trials were tried in California during the early 2000s to assay its effect. For whatever reason, the material showed no effect on the disease. Potassium and calcium silicates are liming materials, raising soil pH. The effect that was noticed in its use in other countries may simply have been a soil pH effect on either the avocado tree, the Phytophthora or both.
Especially when there are no winter rains to leach accumulated salts from the root zone of trees, there is major concern about increasing the levels of salts going into the root zone. Chlorides, boron, sodium and total salts all should be minimized as much as possible in order to optimize tree production and health. Evaluating the fertilizer and irrigation management programs is important and in doing so, finding out how much is being put into the orchard.
A wonderful way to evaluate what is being applied through the irrigation system is to go online to AvocadoSource (avocadosource.com) and go to the ‘Tools' section and click on the ‘Irrigation Water Mineral Content Calculator'. Once there click on ‘Retrieve District Water Analysis Data' and there are several water qualities that can be downloaded onto the calculator.
I chose one of the Metropolitan Water District sources – Castaic Lake – which is representative of water delivered to the south from northern California. It shows a chloride level of 81 mg/L (81 ppm) which translates to 220 pounds of chloride for every acre-foot of water. Which means about 440 pounds of chloride per acre (about 2 ac-ft/ac) to grow avocado and citrus in Fillmore. And the same water coming out of Lake Skinner further south but nearly the same quality as Castaic, would be 880 pounds of chloride per acre in Fallbrook (4 ac-ft/ac).
So the question comes up about the use of potassium fertilizers. Citrus and avocado haul off about twice the potassium in their fruit as nitrogen. A typical harvest for either crop is about 50 pounds of K per acre – more fruit, more K. So to apply potassium, a grower can use several different materials – KMag, potassium thiosulfate, potassium sulfate, potassium nitrate, potassium chloride. A 100 pounds of either potassium sulfate or chloride put on about the same amount of potassium, 50 pounds. With the potassium chloride or course, there is 50 pounds of chloride.
The cheapest source of K is potassium chloride, but growers are concerned about the added chloride. The material is highly soluble and is easily injectable. It also is rapidly moved through the soil, so when it is injected through the irrigation in small amounts, the chloride tends not to accumulate in the root zone. So looking at the total amount of chloride that is applied in our normal irrigation waters, the chloride in the fertilizer doesn't represent a large proportion of the total chloride the tree sees. It could be considered in a fertilizer program, or at least a supplement to other sources of potassium.
Potassium is relatively immobile in soil, more so with more clay. Chloride on the other hand is quite mobile. It goes wherever the water goes. Applying it any time of the year basically results in its staying there until it is taken up or the soil is washed away. So applying potassium chloride in a wetter time of year, could be a cheap way to get potassium on with the least effect of chloride. Or potassium chloride could be applied in rotation with more expensive forms of potassium, such as potassium thiosulfate (KTS).
By the way, that Castaic water would contain 87 ppm sulfate and 74 ppm sodium which would mean over 200 pounds per ac-ft in the water and 110 ppm bicarbonates. The pH would be around 7.8. And this is good water by southern California standards. Many of the well water in southern California have much lower qualities than these waters from norther California and we get good yields from them. We have learned to use some pretty awful waters to grow crops here.
It is more than just the confusion about the effects of phosphonates, but also how to spell the words associated with the P atom. Phosphorus with an ending in “us” is the element we know as P, while Phosphorous with a “ous” ending is the adjective of P. So an acid containing Phosphorous acid is written H3PO3 while phosphoric acid is H3PO4. These are both strong acids and can hurt and cause damage if splashed on the skin. When either is reacted with calcium or potassium hydroxide, a salt is formed which is less dangerous to users, but as with any chemical can be misused.
The salt formed from Phosphorous acid is called calcium phosphite or calcium phosphonate depending on what naming system is used to describe it. Whereas when these bases are reacted with phosphoric acid, the result is calcium or potassium phosphate. These salts are relatively benign in contact with skin. Labels on containers often call phosphorous acid, “soil applied” whereas the phosphite forms are called “leaf applied”. The “soil applied” when applied to a leaf can cause damage, whereas, the leaf applied is much less likely to cause damage to both plant and applicator. It can be applied to the soil, as well. It's much safer to use the leaf applied in either application technique.
The phosphites are often registered as fertilizers, but they have little nutrient effect. Most of their effect is to boost the plant's immunity to Phytophthoras and pythiums. This is called fungistasis and the material is called a fungistat. They don't act as a fungicide when normally applied to kill these organisms.
So you can see there is a lot of confusion in the phosphorous world. Knowing the proper spelling, pronunciation and use is note only good grammar, it makes good farming.
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There are few documented cases of phosphorus (P) deficiency in tree crops in California.