Potassium availability in soils

Potassium (K) is second to nitrogen in terms of the amount absorbed by higher plants. Optimum K level for plant growth ranges from 2 to 5% of plant dry weight (Marschner, 1995). Unlike P, K is present in relatively large quantities in soils coming from the weathering of primary minerals such as feldspars, mica, and others. But it is commonly deficient in highly weathered or old soils. Total K contents of soils range between 3000 and 100,000 kg/ha in the upper 20 cm of the soil profile (Sparks, 2000). The behavior of K in the soil is influenced primarily by CEC and mineral weathering and not by biological processes.

Interrelationship of various forms of soil K (modified from Sparks, 2000)

K in the soil occurs in 4 forms: solution K, exchangeable K, nonexchangeable K, and mineral K (Sparks, 2000).

a) Solution K. This is the K dissolved in the soil solution. It is the form of K that is readily available to plants and soil microorganisms and also is the form of K most subject to leaching losses. It varies in amount from 2 to 5 mg/liter K but can be dramatically changed by the addition of k fertilizers to the soil.

b) Exchangeable K. This is the form of soil K that is adsorbed on the surfaces of soil colloids. It is readily exchanged with other cations in the soil solution and is also readily available to plants. Some authors combine exchangeable K and solution K into one form called readily available form of K which comprises only 1 to 2 percent of soil K. This is also dependent on the CEC of the soil.

c) Nonexchangeable K. This is the portion of soil K that is fixed or held between adjacent layers of 2:1 clay minerals particularly vermiculite and smectite clay minerals. This is continually released to the exchangeable form when levels of exchangeable and soil solution K drops due to plant uptake and leaching losses.

d) Mineral K. This is the K that is part of the crystal structure of primary minerals such as muscovite, biotite, and feldspars. It is the most abundant and accounts for 90 to 98 percent of soil K. It is unavailable to plants and can only be released to the soil solution upon weathering of the minerals.

Leaching is the major cause of loss of K in the soil. Leaching of soil solution K is greatly dependent on the CEC of the soil and thus is influenced by the amount and type of clay and the SOM content of the soil. Soils with higher CEC like clayey soils have greater ability to hold K and thus have lower leaching losses than sandy soils with low CEC. Excess application of K-fertilizers can also enhance leaching losses especially under conditions of high rainfall.

Another form of leaching loss of K (and other nutrients) which is often overlooked is the one that occurs from the leaves of the plants. This can cause substantial nutrient loss exceeding seven times the amount in the standing crop in the case of K. Nutrients are leached from the leaves in the order K>N>P although this would also depend on the nutrient status and leaf morphology. Anything that reduces the water contact with leaves like smooth cuticle, erect leaves, etc. also reduces leaching losses (Chapin, 1980).

References
Chapin, F.S. III., 1980. The mineral nutrition of wild plants. Ann. Rev. Ecol. Syst. 11:233-260.
Marschner, H. 1995. Mineral Nutrition of Higher Plants. 2nd ed., Academic Press, London.
Sparks, D.L. 2000. Bioavailability of soil potassium. In:  In: Handbook of soil Science (M.E. Sumner, ed.). CRC Press, Boca Raton, pp: D38-D53.

Response of Abaca (Musa textilis Née) to shade, irrigation and fertilization

Abaca plants

Abaca (Musa textilis Née), a relative of the edible banana (Musa acuminata and Musa balbisiana), is a native to the Philippines. It is grown primarily for its fibers which are utilized by the pulp, cordage and fiber craft industries. Studies have shown that the specific tensile strength of abaca fiber is comparable to or even higher than that of fiberglass (Bledzki et al., 2007;Sinon, 2008).

Dr. Marlito Bande and co-workers, in a paper to be published in the international journal Industrial Crops and Products 42:70– 77, reported on the optimum light, nutrient and water requirements of abaca to attain optimum yield. They also discussed how these parameters affect fiber recovery and fiber quality under field conditions. 

They showed that abaca planted under 50% shade had significantly (p < 0.01) higher fiber yield than those planted under the other shade/light treatments (30% and 40% of full sunlight using polypropylene shade nets) since the plants pseudostem under such treatment were longer, bigger and heavier.They revealed that the combination of irrigation and fertilization further enhanced fiber yield to as much as 41% but this was not enough to offset the effects of shade on the performance of the plant which significantly (p < 0.01) increased fiber yield to as much as 165%. Shade and irrigation–fertilizer application had no significant effect on fiber fineness and tensile strength. 

They concluded that 50% shade is the optimum requirement of abaca to achieve an optimum machine stripped fiber yield of 135.04± 4.31 g/plant without affecting fiber quality for industrial purposes.

The study which was conducted in Ormoc, Leyte, Philippines was funded by the German Research Foundation (DFG).

References

Bande MM, J Grenz, VB Asio, and J Sauerborn. 2013. Fiber yield and quality of abaca (Musa textilis var. Laylay) grown under different shade conditions, water and nutrient management. Industrial Crops and Products 42:70–77.

Bledzki AK, AA Mamun, O Faruk. 2007. Abaca fibre reinforced PP composites and expansion with jute and flax fibre PP composites. eXPRESS Polymer Letters 1 (11), 755–762.

Sinon FG. 2008. Optimization of stripping technologies for the production of high quality abaca fiber. Dissertation, Universität Hohenheim, Stuttgart, Germany

The role of mycorrhiza in the mineral nutrition of plants

Mycorrhiza is the association between fungi and the roots of higher plants. The term was introduced by the German scientist A.B. Frank in 1885 (Mengel and Kirkby, 2001). Mycorrhiza is considered as the most widespread association between microorganisms and higher plants. On a global scale, between 86% and 94% of plants are mycorrhizal (Brundrett 2009). All Gymnosperms as well as 83% and 79% of dicotyledonous and monocotyledonous plants, respectively, are mychorrhizal (Marschner 1995). Nonmycorrhizal plants can be found in stressed soil environments (very dry or saline, waterlogged, severely disturbed as in mining areas, infertile) or even in very fertile soils. Mycorrhizas (or mycorrhizae) are absent under all environmental conditions in the Cruciferae and Chenopodiaceae (Marschner, 1995). Generally, in root-fungus association the fungus is strongly or wholly dependent on the higher plant, whereas the plant may or may not benefit from the association. It is not also essential for plant survival except in some plants like orchids. Mycorrhizal associations are therefore either mutualistic, neutral, or parasitic depending on the circumstances although mutualism is the dominant type.

Groups of mycorrhizas

Two mycorrhizal groups according to how the fungal mycelium relates to the root structure:
a) Endomycorrhizas. The fungi live inside the cortical cells of the roots and also grow intercellularly. The best known type is the vesicular-arbuscular mycorrhiza (VAM). This is widespread in cultivated soils.
b) Ectomycorrhizas. This group of mycorrhiza occurs mainly on roots of woody plants and only occasionally on herbaceous and graminaceous perennial plants. Some temperate tree species like beech, oak, spruce and pine cannot survive without ectomycorrhiza (Schachtschabel et al., 1998). They form a sheath or mantle of fungal mycelium over the surface of fine roots. The hyphae penetrate into the intercellular spaces of the root cortex and it extends outward into the soil.

Role of mycorrhizas in the mineral nutrition of host plants

Mycorrhizas are very important in the uptake of nutrients such as P, N, K, Cu, Zn and Ca by plants especially in soils low in these nutrients. Since P is the most limiting nutrient in tropical soils, mycorrhizas are vital for improving P nutrition particularly for cultivated plants. External hyphae can absorb and translocate P to the host from soil outside the root depletion zone. The thin mycorrhizal hyphae (2-4 μm in diameter) are able to penetrate soil pores not accessible to the root hairs which are about five times larger than the hyphae (Kirkby and Mengel, 2001). For example, studies have shown that the heavily mycorrhizal root of cassava enables it to grow well in phosphate-deficient soils where other crops fail (Wild, 1993). Also, a long-term study at the National Abaca Research Center at VSU (Armecin and Geneston-Asio, 2004) has provided the first clear evidence that abaca plant (Musa textilis) is mycorrhizal although colonization was relatively low (18-22%). In alkaline soils, mycorrhiza can prevent iron and manganese deficiencies. Mycorrhizas are also known to protect the plant from soil borne pathogens.

Recently, Lambers et al. (2010) reported that terrestrial plants (except epiphytes, parasites and carnivorous species) acquire most mineral nutrients from the soil primarily via two pathways: 1) direct absorption through the roots, and 2) indirect absorption through symbiotic mycorrhizal fungi. The majority of plants can take up phosphorus via both pathways but depend primarily on mycorrhizal fungi to acquire phosphorus.

References

Armecin RB and LG Asio. 2004. Effects of vesicular-arbuscular mycorrhizal fungi inoculation on Abaca (Musa textilis). Unpublished research report. NARC, VSU, Baybay, Leyte.
Brundrett, M. 2009. Plant and Soil 320: 37-77.
Lambers H, MC Brundrett MC, JA Raven and SD Hopper. 2010. Plant and Soil 334:11-31.
Marschner, H. 1995. Mineral Nutrition of Higher Plants. 2nd ed., Academic Press, London.
Mengel, K. and E.A. Kirkby. 2001. Principles of Plant Nutrition (5thed.). Kluwer Academic Publishers, Dordrecht, 849pp.
Schactschabel P., H.P. Blume, G. Brümmer, K.H. Hartge and U. Schwertmann. 1998. Lehrbuch der Bodenkunde (14th ed.). Ferdinand Enke Verlag, Stuttgart, 494pp.
Wild, A 1993. Soils and the Environment. Cambridge University Press, Cambridge, 287pp.

Photo Sources:
1. G. Quinn at http://www.finegardening.com/
2. Nathan Brandt, Iowa State University Extension News at http://www.extension.iostate.edu/

Biological nitrogen fixation in corn

Corn (Zea mays L.) can establish rhizospheric or endophytic associations with various nitrogen-fixing bacteria (diazotrophs) such as Azospirillum, Klebsiella, Pantoea, Herbaspirillum, Bacillus, Rhizobium etli and Burkholderia. Most of these diazotrophs can grow in the intercellular tissue of plants without causing any disease.

Biological nitrogen fixation (BNF) is the biological process by which nitrogen (N2) in the atmosphere is converted to ammonia by an enzyme called nitrogenase. The screening of plant genotypes for their enhanced ability to acquire nitrogen by BNF can reduce the use of expensive nitrogen fertilizers in several important crops like sugarcane, rice, wheat and corn. It can greatly benefit particularly the poor farmers of developing countries.

In a recent study aimed to quantify the symbiotic biological nitrogen fixing activity of a range of commercial corn cultivars, Montanez et al. (2009) demonstrated that corn cultivars obtain significant nitrogen from BNF, the level of which varied with corn cultivar and nitrogen fertilization level. The study showed that some cultivars were more sensitive than others to nitrogen application and that 15N isotope dilution method is a useful tool to screen and select corn cultivars with any potential BNF.

Reference

Montanez A, Abreu C, Gill PR, Hardarson G, and Sicardi M. 2009. Biological nitrogen fixation in maize (Zea mays L.) by 15N isotope dilution and identification of associated culturable diazotrophs. Biology and Fertility of Soils 45: 253-263

Does Sago palm respond to nitrogen application?

Sago palm (Metroxylon sagu Rottb.) is widely found in the tropical lowland forest and freshwater swamps across Southeast Asia and New Guinea. Sago, the starch extracted from the pith of sago palm stems, is a staple food for the lowland peoples of Papua New Guinea and the Moluccas (http://en.wikipedia.org/wiki/Sago).

In recent years, the plant has received increased scientific interest as new uses for sago starch like in the manufacture of alcohol, citric acid, bio-ethanol and biodegradable plastics are being explored. One important research issue is on how to increase sago production since, like most wild plants, the mineral nutrition of sago palm is still poorly understood. Little scientific information is also available about its response to fertilizer application.

In a new study published in the international journal Soil Science and Plant Nutrition, Lina and co-workers (Lina et al. 2009) found that N uptake of sago palm increased significantly but inconsistently with increasing N application. The few significant increases in N uptake that were observed did not translate into significant improvements in the growth parameters of sago plant, except for the number of leaflets in the pot experiment. No significant difference was likewise observed between the fertilizer use efficiency at the two fertilization rates (50 and 100 N kg ha-1) for either sago seedling or 2-year-old sago plants.

The study demonstrated that sago palm did take up N from the added fertilizer at low rates. Moreover, it showed that the growth parameters of sago plant are not sensitive to N application suggesting that the form of N and the timing of N fertilization are important factors for sago production.

Reference
Lina Suzette B., Okazaki M, Kimura DS, Yonebayachi K, Igura M, Quevedo MA, and Loreto AB. 2009. Nitrogen uptake by sago palm (Metroxylon sagu Rottb.) in the early growth stages. Soil Science and Plant Nutrition 55: 114-123.

Concentration affects plant uptake of inorganic and organic forms of N

It is now recognized that plants take up N from the soil in three forms: nitrate, ammonium, and amino acids (dissolved organic N). Although scientific evidence on plant uptake of amino acids has existed in the last few decades, it is only recently that the contribution of amino acids to plant nutrition has been recognized (see Warren 2009 and literatures cited). So the traditional view that organic N has to be mineralized first into nitrate and ammonium in order to be available to the plant is not anymore valid.

Different plant species vary in their preference for N forms. For instance, early successional plant species are known to have a higher capacity for nitrate uptake than late successional species. Uptake of N in the form of ammonium and amino acids is thus more important for the latter species. In a recent study to test the hypothesis that substrate concentration affects plant preference for N forms, Warren (2009) used the herb Ocimum basilicum and the evergreen tree Eucalyptus regnans. He placed roots of intact seedlings in equimolar mixtures of nitrate, ammonium and glycine (amino acid). His results revealed that substrate concentration influenced the preference of both plants for N forms. This means that whether the plant prefers one N form over another (e.g. nitrate over ammonium and amino acid or vice versa) depends on their concentrations in the growth medium or soil.

Reference

Warren CR. 2009. Does nitrogen concentration affect relative uptake rates of nitrate, ammonium, and glycine? J. Plant Nutr. Soil Sci. 172: 224-229.

Relation between nutrient status of rainforest trees and environmental factors

The mineral nutrition of native plant species is still poorly understood. This is particularly true for the various tree species in rain forest ecosystems. In order to evaluate the mineral nutrient status of the dominant tree species and its relation to environmental factors such as elevation, slope, landscape position, and soil nutrient status, Z.S. Chen and co-workers (Wu et al., 2007) collected leaf, stem, and wood samples for nutrient analysis from a total of 636 trees belonging to 20 dominant species from 27 contiguous 20m x 20m quadrants along an altitudinal transect in a subtropical rain forest in southern Taiwan. They also collected composite soil samples from the 0-5 and 5-15 cm depths in each quadrant for chemical analysis.

Their results revealed that leaf concentration was better correlated with the environmental factors than stem and wood nutrient concentrations. This means that leaf analysis is more appropriate than stem and wood analyses to evaluate the nutrient status of native tree species. They also found wide concentration ranges for most mineral nutrients except P and Cu and most tree species were clustered at the lower end of the concentration ranges indicating they have low nutrient status. Among the macronutrients, P had the lowest and narrowest foliar concentration (0.25-2.8 g kg^-1) confirming the results of other studies from other tropical areas that P is the most limiting nutrient in tropical ecosystems. For the micronutrients, the lowest concentration was shown by Cu (3.88-17 mg kg^-1). A few tree species were found to accumulate (called “accumulator species”) nutrients like N, P, K, Ca, Mg, Cu and Zn indicating high absorption capacity for these nutrients. Foliar mineral nutrient concentration of the trees was generally correlated with the environmental factors such as elevation, topographic position, slope, vegetative type and soil nutrient status.

Reference

Wu CC, Tsui CC, Hseih CF, Asio VB and Chen ZS. 2007. Mineral nutrient status of tree species in relation to environmental factors in the subtropical rain forest of Taiwan. Forest Ecology and Management 239: 81-91.