Biochar is a partially burned organic matter that can be used as a soil amendment. Its use was identified in the Amazon jungles where the terra preta (black earth) was first identified in areas where humans had incorporated the charcoal and improved plant growth was noted. There is a growing industry world-wide to take organic matter and create biochar because of potential environmental benefits, such as reduced greenhouse gas emissions, improved soil fertility, reduced leaching of nutrients and chemicals from plant root zones, and water retention. Biochar could potentially last for hundreds of years once incorporated in the soil. There have been many claims for biochar, but so far there is not a lot of scientific basis for these claims. One of the results recently has been the standardization and testing methods for biochar, so the material can be better characterized and studies can be standardized. The guidelines can be found at this link: http://www.biochar-international.org/sites/default/files/Guidelines_for_Biochar_That_Is_Used_in_Soil_Final.pdf
Biochar has been most notable in soils that are of poor quality which are characteristic of highly leached soils found in the Amazon. The question is can biochar perform in the relatively young soils of California where irrigation and nutrients are better controlled. We are doing a quick and dirty little trial on avocado and citrus to evaluate plant performance in rocky soils, calcareous soils and with trees that are doing “poorly”. If we see some indication that there is something working, we will expand the study with more rigor and more situations.
Photo of Terra Preta from Cornell University's College of Agriculture and Life Sciences
Travel can be enlightening. In Turkey I learned that sour orange rootstock is routinely used with lemon and mandarin scions without any fear of tristeza virus, a formidable disease of oranges. When I heard this I asked Georgios Vidalakis in charge of the UC Clonal Protection Program and a virologist. And he said that it was true and the neat thing is that the rootstock can handle heavy, calcareous soils better than other citrus rootstocks. So we are doing a trial on rootstocks and sour orange is included.
Something else I “learned” was that if you girdle citrus at flowering, the fruit has few or no seeds. Well, I talked to many growers and scientists and they all said the same thing. I went through the citrus literature and I could find no mention of this. I emailed Carol Lovatt, the plant physiologist at UCR and she said that when you alter hormone flows by girdling, who knows what might happen. So we set up a little trial in lemon that flowers pretty much all year long on the coast. Every month we girdle branches with either a hand saw or a girdling knife which make different sized cuts, flagging the branches with different colored tape to identify the girdling date. Over an 18 month period we harvested fruit and cut it to count seeds. And………………………………………there was only a slight difference in seed numbers, a few less in the girdled trees.
The goal of this trial was to see if girdling worked and if so, what was being changed in the tree and if could identify that, then maybe we could develop a nutritional program that would do the same thing. That way we wouldn’t need to girdle. But not everything you hear turns out to be true.
'Bitters', 'Carpenter' and 'Furr' trifoliate hybrids are three new citrus rootstocks released in August 2009 by the University of California Riverside. These three rootstocks are hybrids of Sunki mandarin x Swingle trifoliate orange. As rootstocks for citrus, they produce good quality fruit on small ('Bitters') or medium-large ('Carpenter' and 'Furr') trees. They all show good tolerance to Citrus tristeza virus (CTV), but vary in their tolerance of Phytophthora and nematodes. They all produce uniform seedlings due to high levels of nucellar embryony, and are graft compatible with sweet orange, Lisbon lemon and grapefruit. They also appear compatible with mandarins based on observation of 11 year-old satsuma and ‘Pixie’ trees. We are comparing their performance compared to ‘C-35’ as a standard and with ‘sour orange’. These are being grown with ‘Pixie’ and ‘Lisbon’ lemon scions. Sour orange grows well on calcareous, heavy soils and are less prone to nematodes. It turns out that mandarin and lemon are not prone to tristeza virus when grown on sour orange rootstock. Trees will be planted this coming spring. Stay tuned.
In coastal lemons, there has been a major increase of broad mite and the damage it causes on fruit and leaves this year.
Broad mites are often found in depressions on fruit where the females lay their eggs, which are dimpled, translucent, and covered in white speckles. These mites are so small you need a hand lens to see them. Broad mites are yellowish in color and adult females have a white stripe on the back.
Broad mites feed on fruit and leaves, preferring young fruit up to about 1 inch (2.5 cm) in diameter that are located on the inside of the canopy or on the inward facing side of outer fruit. Feeding results in scarred tissue that cracks as fruit grows, leaving a characteristic pattern of scars and new tissue. Although most feeding occurs on fruit, broad mites may also feed on young expanding leaves causing them to curl. This cupping and curling of leaves can appear similar to mild damage caused by glyphosate-Roundup applications.
Broad mites are occasional pests of coastal lemons from late July through early October; infestations are enhanced by the presence of Argentine ants. This mite often occurs in conjunction with Citrus Rust Mite, with the rust mite usually predominating in number. Populations of broad mite tend to be most severe in warm, humid conditions such as found in greenhouses. No treatment thresholds have been developed for broad mite in citrus. If high and increasing populations warrant treatment, use miticides with the least toxicity to predaceous mites. The predatory mite Neoseiulus californicus appears to be a good biocontrol agent.
Check out the IPM website for a greater discussion of pesticides available for use on broad mite.
Broad mite on fruit and the damage it causes to leaves.
- Author: Elizabeth Fichtner and Rachel Elkins
Lime-induced Iron Chlorosis: a nutritional challenge in the culture of several subtropical perennial crops in California
Elizabeth Fichtner, UCCE Tulare County and Rachel Elkins, UCCE Lake and Mendocino Counties
Spring, and new leaves are coming out, but this could, but yellow could be a sign of iron chlorosis, as well.
Although iron (Fe) is the 4th most abundant element in the lithosphere, Fe deficiency is among the most common plant micronutrient deficiencies. Fe deficiency in plants is common in calcareous soils, waterlogged soils, sandy soils low in total Fe, and in peat and muck soils where organic matter chelates Fe, rendering the element unavailable for plant uptake. In California, lime-induced Fe deficiency is often observed in soils and irrigation water containing free lime, and is exacerbated by conditions that impede soil drainage (ie. compaction, high clay content), resulting in reductive conditions. Given that over 30% of the world's soils are calcareous, lime-induced Fe deficiency is a challenge in numerous perennial cropping systems including: grapes, pears, apple, citrus, avocado, pecans, and stone fruit (prune, almond, apricot, peach, nectarine, cherry).
In most soils, Fe oxides are the common source of Fe for plant nutrition. Solubility of Fe oxides is pH dependant; as pH increases, the free ionic forms of the micronutrient are changed to the hydroxy ions, and finally to the insoluble hydroxides or oxides. In calcareous soils, the bicarbonate ion inhibits mobilization of accumulated Fe from roots to foliage and directly affects availability of Fe in soil by buffering soil pH. When irrigation water is also high in bicarbonate, probability of Fe deficiency is enhanced because bicarbonate is continuously supplied to the soil, and more importantly, the roots may become crusted with lime as water evaporates, thus inhibiting root growth and function. Inside the plant, bicarbonate inhibits nutrient translocation from roots to aboveground plant parts. The adverse effects of high bicarbonate levels are exacerbated in very saturated, very dry, or compact soils, where bicarbonate levels increase concurrent with diminished root growth and nutrient uptake.
Symptoms of Fe deficiency in plants
Fe is immobile in plants; therefore, symptoms appear in young leaves. Interveinal chlorosis (Figure 1) is the main symptom associated with Fe deficiency, followed by reduced shoot and root growth, complete foliar chlorosis, defoliation, shoot dieback, and under severe conditions may result in tree mortality. Overall productivity (yield) is reduced, mainly from a reduced number of fruiting points.
Plant species and cultivars vary in their sensitivity to Fe deficiency, and are categorized as either "Fe-efficient" or "Fe-inefficient". Fe-efficient plants have Fe uptake systems that are switched on under conditions of Fe deficiency. Fe-inefficient plants are unable to respond to Fe deficient conditions. All Fe-efficient plants, except grasses, utilize a Fe-uptake mechanism known as Strategy 1. Strategy 1 plants decrease rhizosphere pH by release of protons, thus increasing Fe solubility. Some plants may excrete organic compounds in the rhizosphere that reduce ferric iron (Fe3+) to the more soluble ferrous (Fe2+) forms or form soluble complexes that maintain Fe in solution. Additionally, roots of Strategy 1 plants have specialized mechanisms for reduction, uptake, and transfer of Fe within the plant. Strategy 2 plants (grasses) produce low molecular weight compounds called phytosiderophores which chelate Fe and take up the chelated Fe with a specific transport system.
Amelioration of Fe chlorosis
Planting sites in calcareous soils should be well drained to provide optimal conditions for root growth and nutrient uptake. Waterlogged and compact soils contain
more carbon dioxide, which reacts with lime to form even more bicarbonate. These conditions, as well as very dry soils, also inhibit microbial activity which aids in
solubilization and chelation of Fe. Prior to planting, soils and water should be tested to determine the pH, lime equivalent, and bicarbonate concentration. Bicarbonate concentrations greater than 3 meq/L in irrigation water increase the hazard of lime accumulation on and around roots. If high bicarbonate water must be used, the pH must be adjusted to 6.0-6.5 to dissolve the bicarbonate and prevent it from negating the effects of soil-based treatments. In microsprinker and drip systems, acidification of irrigation water will also reduce the risk of emitter clogging, a common problem at bicarbonate levels over 2 meq/L. The cost of reducing the pH of irrigation water will more than compensate for the savings incurred from avoiding wasted investment in failed soil- and plant-based remedies. Systems can be set up to continuously and safely inject water with acids such as sulfuric, urea-sulfuric, or phosphoric during irrigations. Specific choice and rate will depend on crop, soil type, other nutrient needs, availability, and cost. Downstream pH meters are available to continuously adjust rate of acid use. Acetic and citric acid can be utilized by organic growers.
Soil based pre-plant treatments to reduce pH include elemental sulfur (S) and acids as mentioned above. It is only necessary to treat a limited area near the root zone to ameliorate symptoms because the tree only needs to take up a small amount of Fe. Material can be shanked in or banded and incorporated in the prospective tree row. One ton of elemental sulfur per treated acre is needed to mitigate three tons of lime, and may need to be re-applied every 3 to 5 years after planting. The addition of organic matter such as well-composted manures will benefit poorly drained or compact soils by increasing aeration for better root growth, fostering chelation of nutrient cations, and reducing pH (depending on source material).
If possible, choose a Fe efficient species or cultivar. In perennial systems, lime-tolerant rootstocks may be the first line of defense in combating Fe deficiency. Some rootstocksmentioned are peach-almond and Krymsk-86 for stone fruit, Gisela 5 for cherry, and Pyrus communis for pear. Ongoing research studies in Europe focus on screening rootstocks of grape and olive for lime tolerance.
Once soil and water quality improvements are made, post-plant management strategies may also be implemented to ameliorate lime-induced Fe chlorosis in the short term. Soil can be acidified as described above. Individual trees can be treated by digging four to six 12-24 inch
holes around the drip line and burying a mixture of sulfur and Fe fertilizer. Historically, two principal methods have been utilized: 1) foliar application of inorganic Fe salts (ie. ferrous sulfate), and 2) soil or foliar application of synthetic chelates. Application of Fe salts to foliage may have mixed results due to limited penetration of Fe into leaves and inadequate mobilization within the plant. Use of Fe chelates may be of benefit; however, they are expensive and pose an environmental concern due to their mobility within the soil profile. Because soil lime interferes with Fe mobility with the plant, repeat application of inorganic Fe salts or Fe chelates may be necessary throughout the growing season.
Choice of nitrogen (N) fertilizer may also influence solubility of rhizosphere Fe. When N is applied in the ammonium form (NH4+), the root releases a proton (H+) to maintain a charge balance, thus reducing rhizosphere pH. Alternately, fertilization with nitrate (NO3-) results in root release of hydroxyl ions (OH-), resulting in an increase in rhizosphere pH. Solubility of Fe3+ increases 1000 fold with each one unit decrease in pH; therefore, fertility-induced rhizosphere pH changes may significantly influence Fe availability.
New methods for amelioration of Fe chlorosis are under investigation. For example, container studies have demonstrated that inter-planting sheep's fescue, a Strategy 2 plant, with a Fe-inefficient grape rootstock may ameliorate Fe chlorosis in grape. In this system, the grass produces a phytosiderophore that enhances Fe availability to the grape. Additionally, soil amendment with Fe3(PO4)2• 8H2O), a synthetic iron(II)-phosphate analogous to the mineral vivianite, has been effective at preventing Fe chlorosis in lemon, pear, olive, kiwi, and peach. Vivianite has a high Fe content (~30%) and serves as a slow release source of Fe in calcareous soils.
Figures below: 1) Shoot dieback in citrus, 2) Interveinal chlorosis in citrus and 3) Various stages of iron chlorosis in avocado.