Foliar fertilizer application is sometimes promoted as an effective means of supplying nutrients to avocado. On the market are various products being promoted as foliar nutrients for avocado, some proponents even suggest that their products do away with the need for soil applied nutrients. The nature of the avocado leaf severely limits its capacity to absorb foliar sprays.
The structure of plant leaves has evolved primarily to capture sunlight and exchange gases, roots have evolved to absorb nutrients and water and anchor the plant. Any absorption of nutrients by leaves is therefore likely to be more fortuitous than by design. In some crops passive nutrient absorption by leaves is occasionally sufficient to supplement the supply of nutrients taken up by the roots. Most often this involves trace elements, which as their name suggests are required in very small amounts (eg. copper and zinc). However if non-mobile elements or elements with limited mobility in the plant (eg. calcium, phosphorus, zinc, boron and iron) are absorbed when foliar sprayed they are not likely to make it down to the roots where they are also needed. Most nutrients will move freely in the water stream but the movement of many is restricted in the phloem, hence leaf applications don't meet the requirements of deficient trees. Occasionally major elements (such as nitrogen and potassium) are applied to make up for a temporary shortfall or provide a boost at a critical time. Citrus is an example of a crop where some benefits from foliar applied nutrients have been reported.
The ability of the leaf to absorb nutrients from its surface must depend to some degree on the permeability of its epidermis (outer layer) and the presence and density of stomates (pores for the exchange of gases). Scanning Electron Microscope studies of mature leaves and floral structures in avocado show the presence of a waxy layer on both the upper and lower surfaces of mature avocado leaves (Whiley et al, 1988). On the upper surface the wax appears as a continuous layer and there are no stomates. On the lower surface the wax layer is globular and stomates are present. Blanke and Lovatt (1993) describe the avocado leaf as having a dense outer wax cover in the form of rodlets on young leaves and dendritic (branching) crystals on old leaves including the guard cells (guard cells surround stomates). The flower petals and sepals in avocado have stomates on their lower surfaces and no wax layers on either surface, which might explain why floral sprays of boron might work.
Blanke, M.M. and Lovatt, C.J. 1993. Anatomy and transpiration of the avocado inflorescence. Annals of Botany, 71 (6): 543-547.
Whiley, A.W., Chapman, K.R. and Saranah, J.B. 1988. Water loss by floral structures of avocado (Persea americana cv. Fuerte) during flowering. Australian Journal of Agricultural Research, 39 (3): 457-467.
The avocado leaf, water beading up on the waxy cuticle.
An avocado leaf with its cuticle (white, paperlike surface) being exposed by underlying leaf fungi.
For centuries, the prevailing science has indicated that all of the nitrogen on Earth available to plants comes from the atmosphere. But a study from the University of California, Davis, indicates that more than a quarter comes from Earth's bedrock.
The study, to be published April 6 in the journal Science, found that up to 26 percent of the nitrogen in natural ecosystems is sourced from rocks, with the remaining fraction from the atmosphere.
Before this study, the input of this nitrogen to the global land system was unknown. The discovery could greatly improve climate change projections, which rely on understanding the carbon cycle. This newly identified source of nitrogen could also feed the carbon cycle on land, allowing ecosystems to pull more emissions out of the atmosphere, the authors said.
"Our study shows that nitrogen weathering is a globally significant source of nutrition to soils and ecosystems worldwide," said co-lead author Ben Houlton, a professor in the UC Davis Department of Land, Air and Water Resources and director of the UC Davis Muir Institute. "This runs counter the centuries-long paradigm that has laid the foundation for the environmental sciences. We think that this nitrogen may allow forests and grasslands to sequester more fossil fuel CO2 emissions than previously thought."
WEATHERING IS KEY
Ecosystems need nitrogen and other nutrients to absorb carbon dioxide pollution, and there is a limited amount of it available from plants and soils. If a large amount of nitrogen comes from rocks, it helps explain how natural ecosystems like boreal forests are capable of taking up high levels of carbon dioxide.
But not just any rock can leach nitrogen. Rock nitrogen availability is determined by weathering, which can be physical, such as through tectonic movement, or chemical, such as when minerals react with rainwater.
That's primarily why rock nitrogen weathering varies across regions and landscapes. The study said that large areas of Africa are devoid of nitrogen-rich bedrock while northern latitudes have some of the highest levels of rock nitrogen weathering. Mountainous regions like the Himalayas and Andes are estimated to be significant sources of rock nitrogen weathering, similar to those regions' importance to global weathering rates and climate. Grasslands, tundra, deserts and woodlands also experience sizable rates of rock nitrogen weathering.
GEOLOGY AND CARBON SEQUESTRATION
Mapping nutrient profiles in rocks to their potential for carbon uptake could help drive conservation considerations. Areas with higher levels of rock nitrogen weathering may be able to sequester more carbon.
"Geology might have a huge control over which systems can take up carbon dioxide and which ones don't," Houlton said. "When thinking about carbon sequestration, the geology of the planet can help guide our decisions about what we're conserving."
The work also elucidates the "case of the missing nitrogen." For decades, scientists have recognized that more nitrogen accumulates in soils and plants than can be explained by the atmosphere alone, but they could not pinpoint what was missing.
"We show that the paradox of nitrogen is written in stone," said co-leading author Scott Morford, a UC Davis graduate student at the time of the study. "There's enough nitrogen in the rocks, and it breaks down fast enough to explain the cases where there has been this mysterious gap."
In previous work, the research team analyzed samples of ancient rock collected from the Klamath Mountains of Northern California to find that the rocks and surrounding trees there held large amounts of nitrogen. With the current study, the authors built on that work, analyzing the planet's nitrogen balance, geochemical proxies and building a spatial nitrogen weathering model to assess rock nitrogen availability on a global scale.
The researchers say the work does not hold immediate implications for farmers and gardeners, who greatly rely on nitrogen in natural and synthetic forms to grow food. Past work has indicated that some background nitrate in groundwater can be traced back to rock sources, but further research is needed to better understand how much.
"These results are going to require rewriting the textbooks," said Kendra McLauchlan, program director in the National Science Foundation's Division of Environmental Biology, which co-funded the research. "While there were hints that plants could use rock-derived nitrogen, this discovery shatters the paradigm that the ultimate source of available nitrogen is the atmosphere. Nitrogen is both the most important limiting nutrient on Earth and a dangerous pollutant, so it is important to understand the natural controls on its supply and demand. Humanity currently depends on atmospheric nitrogen to produce enough fertilizer to maintain world food supply. A discovery of this magnitude will open up a new era of research on this essential nutrient."
UC Davis Professor Randy Dahlgren in the Department of Land, Air and Water Resources co-authored the study.
The study was funded by the National Science Foundation's Division of Earth Sciences and its Division of Environmental Biology, as well as the Andrew W. Mellon Foundation.
Photo: The stuff that makes leaves green
providing some advantage to the farmer. Frequently, these are new fertilizer mixes presented as proprietary cocktails promoted and dispensed with promises of a multitude of profitable (yet improbable) benefits to the buyer. With the large number of new products available, and the number of salespeople promoting them, it is often difficult for growers to distinguish between products likely to provide real benefit, and those that may actually reduce the profitability of the farm.
In all situations when a company approaches the University or a commodity research board with a new product or technology for sale to California growers, these institutions act as grower advocates. They are charged with sorting through the available information; asking the right questions; getting the necessary research done if the available information warrants this pursuit; disseminating accurate information on these new technologies and products, and doing all that can help maximize grower profits now and in the future. When approached with a new product or technology it is obligatory to challenge claims with the following questions:
Is there some basic established and accepted scientific foundation on which the product claims are made?
Language that invokes some proprietary ingredients or mysterious formulations, particularly in fertilizers mixes registered in the State of California, raises red flags. A wide range of completely unrelated product benefit claims (such as water savings, pesticide savings, increased earlier yield) raises more red flags. Product claims that fall well outside of any accepted scientific convention generally mean the product is truly a miracle, or these claims are borderline false to entirely fraudulent.
Has the product undergone thorough scientific testing in orchards?
Frequently, products are promoted based on testimonials of other growers. While testimonials may be given in good faith, they are most often not backed up by any real scientific testing where a good control was used to compare orchard returns with and without the product.
A “test” where a whole block was treated with a product and which has no reliable untreated control does not meet accepted standards for conducting agricultural experiments. Also, a treated orchard cannot reliably be compared to a neighboring untreated orchard; and a treated orchard cannot be compared to the same orchard that was untreated the previous crop year. Even a test with half a block of treated trees and half untreated is not considered dependable by any known scientific standard of testing.
Only a well designed, statistically replicated, multi-year trial allows for direct comparison of untreated versus treated trees with statistical confidence. Verifiable data from tests that meet acceptable standards of scientific design, along with access to raw baseline (before treatment) yield data from the same trees (preferably for the two years prior) should be used to determine the validity of test results provided.
Are the test results from a reliable source?
If the testing were not done by a neutral party, such as university scientists, agency, or a reputable contract research company using standard scientific protocols, this raises red flags. If the persons overseeing the tests have a financial interest in seeing positive results from the product, it raises red flags.
Does the product have beneficial effects on several unrelated farm practices?
A product that increases production of trees, makes fruit bigger, reduces pests, reduces water use, and reduces fertilizer costs, is more than a little suspicious. In reality, if such a product really existed, it would not need any testing at all because its benefits would be so obviously realized by the grower community that it would spread rapidly by word of mouth and embraced by the entire grower community.
Are other standard and proven farm products put down in the new product sales delivery?
If a new product vendor claims that their product is taken up 15 times faster than the one growers are currently using, or is 30 times more efficient, it probably costs 15 to 30 times more per unit of active ingredient than the standard market price. Growers should always examine the chemical product label to see what active ingredient they are buying. There has to be a very good reason to pay more for an ingredient where previously there had been no problem supplying the same ingredient at a cheaper price to trees in the past.
There are impartial sources of such information available to farmers to help corroborate information provided by product vendors. Perhaps the most reliable and accessible impartial research and education resources for growers are their local Cooperative Extension Farm Advisors and commodity research boards.
When promising products emerge, local university Farm Advisors can advise growers on how to evaluate these products and may help design a small trial to test a particular product on a few trees under local orchard conditions. If in these pursuits a truly promising new product or technology emerges, research board funding may follow but only on the recommendation of that board's Research Committee.
At a recent Fresh Index-sponsored meeting, David Crowley recently of UC Riverside talked of a five year-long study that assessed nutritional status and yield. This has been a study area that has long been confused by the problems of alternate bearing, weather-dependency of the avocado, soil variability, root rot, etc. etc. etc. that we all know about. There are nutrient interactions that confound results, as well. High phosphorus affects micronutrient uptake of zinc, copper and others. Zinc impedes copper uptake. Loss of roots from Phytophthora especially affects micronutrients. Irrigation and aeration again affects nutrient uptake, and especially micronutrients.
The elements coming from the soil are divided into primary nutrients, secondary nutrients and micronutrients. This grouping is based on the relative amounts required by plants, but all are essential. Crowley describes the relative need for each element being based on the “Law of the Minimum”; if only one element is deficient it eventually affects growth and yield of the entire plant in a negative manner. It doesn't matter how much the other nutrients are raised, if one is limiting, growth is limited by that one. The primary nutrients required by avocados are nitrogen, phosphorus and potassium. The secondary nutrients required are calcium, magnesium and sulfur. The micronutrients are zinc, iron, manganese, copper, boron, molybdenum, nickel and chlorine.
The Law applies not just to nutrients but to light, temperature, water, disease, pests – anything that affects growth. The limiting input needs to be fixed before the others can boost growth to whatever the biological maximum might be in that environment. In irrigated agriculture, water is the most common limiting input.
So, it is complex. Really complex. But with computers and different techniques of analysis and just looking at nutrients, Crowley has been able to get a better handle on what could be limiting growth in an individual grove. This applies not to what is lacking, but what might be in excess – too much chloride, too much nitrogen, too much…………….
So, in the case of all this data collection the Crowley team has done, something unusual has popped up. Copper deficiency.
Copper deficiency is not commonly recognized as a problem in California avocado groves, but occasionally a grower will report a leaf analysis showing less copper than the 5ppm recommended by Embleton (http://ucavo.ucr.edu/General/LeafAnalysis.html). Typical copper deficiency was reported by Barnard and others (http://www.avocadosource.com/Journals/SAAGA/SAAGA_1991/SAAGA_1991_PG_67-71.pdf). They reported the symptoms of copper deficiency as follows: • Dull appearance of older leaves • Prominent leaf veination • Reddish-brown leaf color • Premature defoliation and twig. This is an extreme case, and Crowley is suggesting there may be some low, chronic level that limits avocado. His final report can be found at:
Of course, why copper might be limiting is another question. Is it due to root rot? Interaction with other applied materials like phosphorus (not phosphite, phosphorous, phosphonate) fertilizers? With irrigation management? Something(s) to think about.
And citrus in California is a different beast. It can commonly show copper deficiency and be a limiting nutrient. We apply copper as a frost/brown rot/septoria spray and as a result don't often see deficiency in citrus.
Liebig's Barrel. Optimum production occurs when all the barrel staves are as high as they can be. When one element is low, that becomes the limiting factor for production. Increased production doesn't occur until that uptake is improved and then the next limiting input restricts production. When that next one is corrected, then some other input then limits production. Correction keeps improving production until the biological limit is reached.
Have any readers actually seen a wooden barrel?
At a recent meeting the question came up about the fate of nitrogen fertilizer applied through the irrigation system. If it is applied as urea, how long does it take to convert it to nitrate? If applied as ammonium, how long does it take to convert to nitrate? Urea and nitrate pretty much move wherever water moves and is very susceptible to leaching. Because of the positive charge on ammonium, it is not as mobile as nitrate, but once bacteria transform it to nitrate, it moves with water.
This is an important question, since if more water is applied than is needed by the plant, the nitrate is going to move out of the root system and no longer be available to the plant and ends up heading to ground water. Reading the literature, growers get the sense that all this transformation takes time, maybe a long time.
It turns out that soils in coastal California have a pretty rapid conversion of nitrogen. Francis Broadbent at UC Davis did a bunch of studies back in the 1950's and 60's and found enzyme hydrolysis of urea to ammonium occurring within hours. Other researchers have looked at nitrification, the conversion of ammonium to nitrate by soil bacteria, occurring within days and much of the conversion occurring within a week depending on soil temperature (see chart below).
So there is all this nitrate present and the key is what happens to it. It turns out that most plants when actively growing absorb nitrate at about 5 pounds of nitrogen per day. So with a 100% efficiency, applying 20 pounds of nitrogen, all of it would be taken up in four days. Of course, nothing in nature is that efficient. But the point is a big slug of nitrogen applied is not going to be taken up immediately and if more water is applied after that than is needed by the crop, it likely is pushed out of the avocado root zone.
Of course all the nitrogen a plant uses does not come from applied fertilizer. The bulk is coming from soil organic matter that is slowly decomposing. This nitrogen is being released at a rate that is probably in balance with the growth of the tree.
The applied fertilizer, however, is much more unstable and needs to be handled accordingly. The rule of thumb is to break the irrigation application into thirds. In the first third, run the irrigation to fill the lines and wet the soil. In the second third, run the fertilizer. This spreads it through the system and onto the ground. The last third is clear the irrigation system of the material and to move the fertilizer into the root zone. Then given time, the tree will take up the applied nitrogen. At the next irrigation then the bulk of that nitrogen will have been taken up and little will be pushed through the root system.
Low and High Nitrogen Avocado Leaves
Chart showing rapid conversion to nitrate with soil temperature