K - Ca - Mg TANAH diabstraksikan oleh: Soemarno tanahfpub-2010.

KALIUM TANAH Jumlah K-tanah Mineral Kalium Lithosfer mengandung 2.6% K Tanah mengandung <0.1 - > 3%, rata-rata sekitar 1% K Tanah lapisan olah (setebal 20 cm) mengandung <3000 - >100.000 kg K/ha Sekitar 98% K dalam tanah terikat dalam bentuk mineral Mineral Kalium K-feldspar merupakan mineral utama sumber kalium, 16% K-mika sekitar 5.2%, terdiri atas Biotit sekitar 3.8% dan Muskovit 1.4% Kekuatan ikatan K dalam mineral Kation K diameternya 2.66 Å, terbesar di antara unsur hara lain; oleh karena itu ikatannya dalam struktur mineral lebih lemah dibandingkan kation lainnya yg lebih kecil dan muatannya lebih besar. Karena ukurannya besar, kation K dapat diselimuti oleh 7-12 ion oksigen, sehingga kekuatan masing-masing ikatan K-O relatif lemah

KALIUM dlm FELDSPAR KIMIA & struktur Polimorf dari feldspar Feldspar adalah aluminosilikat , formulanya KAlSi3O8, kandungan kaliumnya 14%. Di alam, sebagian kalium digantikan oleh Na dan Ca Kation pusat Si4+ sebagian digantikan oleh Al3+, satu penggantian untuk setiap empat tetrahedra, sehingga menjadi AlSi3O8- Polimorf dari feldspar Ortoklas: monoklinik - prismatik, dlm batuan plutonik Sanidin : Monoklinik, dalam batuan vulkanik Microcline : Triklinik, mengandung magmatit-pegmatit Anortoklas : Substituted feldspar, (K,Na)AlSi3O8 Nepheline : Mengandung lebih banyak Na dp K Plagioklas : (Ca, Na feldspar) mengandung sedikit kalium Pelapukan Mineral Kalium Proses pelapukan fisik menghancurkan batuan induk, sedangkan pelapukan kimia akan melepaskan ion K+ dari mineral Temperatur penting untuk pelapukan fisika, sedang hidrolisis penting untuk kimiawi Asam-asam yg penting pd hidrolisis mineral kalium adalah H2CO3 dan asam-asam organik hasil dekomposisi Bahan organik tanah

HIDROLISIS Feldspar KALIUM Abstraksi proses hidrolisis KAlSi3O8 + HOH ===== HAlSi3O8 +K+ + OH- (Fase cepat) HAlSi3O8 + 4HOH ===== Al(OH)3 + 3H2SiO3 (fase lambat) Penambahan H+ mempercepat pembebasan K+ dan merusak ikatan Al-O; Al yang dibebaskan membentuk gugusan AlOH2 koordinasi-4:  Si-O-Al  + H2O + H+ ====  Si-O + Al-OH2 + K+ | | K H Hancurnya ikatan Si-O-Si mungkin disebabkan oleh melekatnya OH- ke Si sehingga menjadi gugusan Si-OH; dengan cara ini ikatan kovalen rangkap dihancurkan. Joint reaction H2O dan H+ dlm menghancurkan ortoklas: 3 KAlSi3O8 + 12H2O + 2H+ ===== KAlSi3O6.Al2O4(OH)2 + 2K+ + 6 H4SiO4 Pelapukan ortoklas menjadi kaolinit: H2O 2KAlSi3O8 -------------- Al2Si2O5(OH) + 2K+ + 2OH- + 4H4SiO4

Ca K+, Ca++, H+ (larutan tanah) Koloid liat H KALIUM TANAH Sumber K-tanah Mineral primer yang mengandung kalium: 1. Feldspar kalium : KAlSi3O8 2. Muskovit : H2KAl3(SiO4)3 3. Biotit : (H,K)2(Mg,Fe)2Al2(SiO4)3 Mineral sekunder: 1. Illit atau hidrous mika 2. Vermikulit 3. Khlorit 4. Mineral tipe campuran Proses pelapukan mineral KAlSi3O8 + HOH KOH + HAlSi3O8 K+ + OH- K Ca K+, Ca++, H+ (larutan tanah) Koloid liat H

Pelapukan Mineral KALIUM 1. Proses fisika: Penghancuran fisik, ukuran partikel menjadi lebih halus, luas permukaannya menjadi lebih besar 2. Proses kimiawi: Hidrolisis, Protolisis (Asidolisis) Proses Hidrolisis dan Protolisis KAlSi3O8 + HOH HAlSi3O8 + K+ + OH- (cepat) HAlSi3O8 + 4 HOH Al(OH)3 + 3 H2SiO3 (lambat)  Si-O-Al  + H2O + H+  Si-O +  Al-OH2 + K+ K H Pelapukan Ortoklas: 3 KAlSi3O8 + 12H2O + 2H+ KAlSi3O6 .Al2O4(OH)2 + 2K+ +6H4SiO4 H2O 2 KAlSi3O8 Al2Si2O5(OH) + 2K+ + 2OH- + 4 H4SiO4

Sumber: aglime.com.au

Faktor Pelapukan Feldspar KALIUM 1. Faktor Internal 2. Faktor Eksternal Faktor internal: 1. Regularity of the crystal lattice. Microcline lebih stabil / sukar lapuk dibanding Ortoklas dan Sanidine 2. Na content of crystals. Anortoklas lebih mudah lapuk daripada ortoklas 3. Si content. Feldspar-substitusi lebih mudah lapuk dp Feldspar 4. Particle size. Semakin kecil ukuran partikel, maka semakin luas permukaannya untuk mengalami reaksi hidrolisis dan asidolisis. 5. …………. Faktor Eksternal: 1. Temterature. Proses pelapukan lebih cepat pd kondisi suhu yg lebih tinggi 2. Solution volume. Kondisi basah mempercepat proses pelapukan 3. Migration of weathering products. Proses pelapukan akan terhambat kalau hasil-hasil pelapukan terakumulasi di tempat 4. The formation of difficult soluble products of hydrolysis. Kalau hasil reaksi hidrolisis mengendap maka reaksi akan dipercepat 5. pH value. Semakin banyak ion H+, proses protolisis semakin intensif. 6. The presence of chelating agents.

MASALAH KALIUM TANAH Ketersediaan K-tanah Tanah mineral umumnya berkadar kalium total tinggi, kisarannya 40 - 60 ribu kg K2O setiap HLO Sebagian besar kalium ini terikat kuat dan agak sukar tersedia bagi tanaman Kehilangan akibat Pencucian Sejumlah besar kalium hilang karena pencucian : Tercuci dari tnh lempung berdebu 20 kg K2O/ha/thn Diangkut /dipanen oleh tanaman 60 -”- Konsumsi berlebihan: Luxury consumption Tanaman dpt menyerap kalium jauh lebih banyak dari jumlah yg diperlukan Pemupukan kalium harus dilakukan secara bertahap Masalah Kalium tanah: 1. Pd saat tertentu sebagian besar K-tanah tidak tersedia 2. K-tanah peka terhadap pengaruh pencucian 3. Kalium dapat diserap tanaman dlm jumlah banyak, melebihi kebutuhan optimalnya

Kadar K-tanaman (Tinggi) K diperlukan untuk Pemakaian berlebihan pertumbuhan optimum Kalium yg diperlukan (Rendah) Rendah K-tersedia dalam tanah Tinggi

BENTUK & KETERSEDIAAN Relatif tidak tersedia Feldspar, Mika, dll. (90-98% dari K-total) K segera tersedia K dpt ditukar dan K dlm larutan tanah ( 2 % dari K-total) K lambat tersedia K tidak dapat ditukar (1 - 10 % dari K-total) K tidak dapat ditukar K dalam larutan tnh K dapat ditukar

hort.wisc.edu

Sumber: ipipotash.org

LOKASI DAN JALUR KALIUM DLM TANAH K dalam mineral sekunder Pelepasan K K dalam mineral primer mis. Muskovit K dalam PUPUK Fiksasi K pd mineral primer Pelarutan pupuk Transisi mine-ral sekunder menjadi mika akibat fiksasi K K dalam larutan tnh Absorpsi K Pelepasan K mengakibatkan pembentukan min. sekunder Pelepasan Kdd atau K-terfiksasi K dalam tanaman K dalam mineral sekunder mis. Kaolinit Adsorpsi atau Fiksasi K

Pelepasan K dari mineral primer Pelepasan K dari mineral primer selama periode pertanaman intensif; media tumbuh mineral dicampur pasir kuarsa. Ukuran partikel mineral primer < 50 ; ukuran partikel illit < 20 . Pelepasan K-tukar, g / g mineral 2000 - Biotite Illite Muscovite Ortoklas 400 5 10 15 cropping periode, (0-15) days Sumber: Verma (1963)

Konsentrasi K-larutan tanah vs Kdd K dapat ditukar, mg K / 100 g tanah K-larutan tanah (me/l) 5.0 Tanah berpasir 4.0 3.0 2.0 Tanah liat 1.0 10 50 100 K dapat ditukar, mg K / 100 g tanah

FIKSASI KALIUM TANAH Faktor yg mempengaruhi fiksasi K-tanah 1. Sifat koloid tanah 2. Pembasahan dan pengeringan tanah 3. Pembekuan dan pencairan tanah 4. Adanya kalsium yg berlebihan Koloid dan Kelembaban Kaolinit sedikit mengikat kalium Montmorilonit dan Ilit mudah dan banyak mengikat kalium, lazim disebut dengan FIKSASI KALIUM: lapisan liat 2:1 Ion kalium Ion lainnya

Sisa tanaman & Pupuk kandang Mineral kalium lambat tersedia Pupuk buatan Mineral kalium lambat tersedia K - tersedia Terangkut tanaman Hilang pencucian Hilang Erosi & Run-off Fiksasi Kalium

Faktor Ketersediaan K-tanah 1. MOBILITAS Mobilitas kalium dalam tanah ditentukan oleh bentuk K+, yaitu bentuk bebas dalam larutan tanah atau bentuk terjerap pada permukaan koloid tanah 2. Interaksi dg ion lain 3. Mass flow dan Difusi 4. Kapasitas dan Intensitas 5. Mineral Tanah: Mineral Primer dan Mineral Liat a. Kadar K mineral primer b. Kecepatan pelepasan K+ dari mineral primer c. Jumlah mineral liat d. tipe mineral liat 6. Bahan Organik Tanah 7. pH tanah 8. Aerasi 9. Lengas Tanah Difusi K+ dalam tanah terjadi melalui dua cara, yaitu: 1. Ruang pori yang berisi air, dan 2. Selaput air di sekeliling partikel tanah.

Sumber: spectrumanalytic.com

Pengaruh pH thd fiksasi K Pengaruh thd fiksasi K Pengaruh pH terhadap fiksasi K bersifat tidak langsung, yaitu melalui pengaruh pH thd jenis aktion yg dominan pada posisi inter-layer mineral liat. Pd tanah masam Al+++ menempati posisi-posisi jerapan. Pengasaman dapat mengakibatkan akumulasi ion Al-hidroksil pd inter-layer mineral liat, shg KTK lebih rendah Pada Vermikulit, ion Al+++ dapat mengusir K+ dari kompleks jerapan, sehingga menurunkan kapasitas fiksasi K+. Sehingga pengaruh pengasaman tanah thd fiksasi K tergantung pada adanya vermikulit dan adanya Al+++ yg akan mendominir kompleks jerapan Pengaruh pengapuran tanah masam thd fiksasi K tgt pada adanya Ca++ yg akan menggantikan Aldd, shg membuka peluang terjadinya fiksasi K+ Fiksasi K+ K-released pH: 3.50 Pupuk 100 kg K/ha 0.0 pH: 4.35 Tanpa pupuk K pH: 7.00 Dosis kapur, CaCO3 K-adsorbed Pencucian

Sumber: fcn.agronomy.psu.edu/2007/fcn0730.cfm

Efek Pupuk K terhadap K-tanah K-larutan tanah pH: 4.1 pH: 5.1 pH: 6.5 pH: 7.0 Dosis pupuk K

extension.umn.edu

im-dinamika.com/reduce.html

Lengas Tanah terhadap K-tanah Serapan K tanaman jagung Pupuk Kalium: 49 mg K/100 g tnh 29 9 Kadar air tanah (20-40%) Sumber: Grimme (1976)

Serapan K vs K-larutan tanah Konsentrasi K+ dlm larutan tanah merupakan indeks ketersediaan kalium, karena difusi K+ ke arah permukaan akar berlangsung dalam larutan tanah dan kecepatan difusi tgt pada gradien konsentrasi dalam larutan tanah di sekitar permukaan akar penyerap. Serapan K vs K-larutan tanah Serapan K , kg /ha (Tanaman kacang buncis) 300 r2 = 0.79** 0.2 0.4 0.6 0.8 K- larutan tanah ( me K / l) Sumber: Nemeth dan Forster (1976)

Konsentrasi K+ larutan Laju Penyerapan K vs Konsentrasi K+ larutan Laju penyerapan K+ , mole/g/jam (akar tanaman barley) 10.0 0.05 0.10 0.15 0.20 Konsentrasi K+ larutan tanah ( mM) Sumber: Epstein (1972)

Efek Ca++ thd penyerapan K+ akar tanaman Penyerapan K , mole/g (akar tanaman Jagung ) 6 + Ca -1 0.5 1.0 1.5 2.0 jam Sumber: Lauchli dan Epstein (1970)

KALSIUM DALAM TANAH Sumber Ca-tanah Mineral primer : Bahan Pupuk: 1. Dolomit : ……….. 1. Kalsium nitrat 2. Kalsit : ……….. 2. Gipsum 3. Apatit : ……….. 3. Batuan fosfat 4. Feldspar kalsium: ……….. 4. Superfosfat 5. Amfibol : ………… 5. Ca-cyanamide Kation kalsium dlm larutan tanah dapat mengalami: 1. Hilang bersama air drainase: Proses pencucian 2. Diserap oleh organisme 3. Dijerap pada permukaan koloid tanah 4. Diendapkan sebagai senyawa kalsium sekunder Faktor ketersediaan Kalsium tanah: 1. Jumlah kalsium dapat ditukar (Ca++ yang dijerap oleh koloid tanah) 2. Derajat kejenuhan Kalsium dari kompleks pertukaran 3. Tipe koloid tanah 4. Sifat ion-ion komplementer yg dijerap oleh koloid tanah 5. …………….

MAGNESIUM DALAM TANAH Sumber Mg-tanah Mineral primer: Bahan Pupuk: 1. Dolomit : ……….. 1. MgSO4.7H2O 2. Biotit : ……….. 2. MgSO4.H2O 3. Klorit : ……….. 3. K-Mg-sulfat 4. Serpentin : ……….. 4. Magnesia 5. Olivin : ………… 5. Basic slag Kation magnesium dlm larutan tanah dapat mengalami: 1. Hilang bersama air drainase: Proses pencucian 2. Diserap oleh organisme 3. Dijerap pada permukaan koloid tanah 4. Diendapkan sebagai senyawa kalsium sekunder Faktor ketersediaan Magnesium tanah: 1. Jumlah kalsium dapat ditukar (Mg++ yang dijerap oleh koloid tanah) 2. Derajat kejenuhan Mg dari kompleks pertukaran 3. Tipe koloid tanah 4. Sifat ion-ion komplementer yg dijerap oleh koloid tanah 5. …………….

soils.org

Serapan K vs Dry matter production Growth & nutrient uptake, % 100 silking tasseling Biji dry matter Tongkol Kalium Batang Daun 25 50 75 100 days after emergence Sumber: Nelson (1968)

Kandungan K-tanah vs Respon pupuk K Tambahan hasil jagung , bu/ac 25 Kdd = 50 ppm Kdd = 100 ppm Kdd = 150 ppm Kdd = 200 ppm 25 50 75 100 125 Dosis pupuk K ( lb / ac ) Sumber: Hanway et al. (1962)

Kandungan K-daun vs Respon pupuk K Respon jagung thd pupuk kalium dipengaruhi oleh status K tanaman, yaitu kadar K daun pada fase silking Defisiensi akut : Kadar K daun 0.25 - 0.41 %K Defisien tanpa gejala: 0.62 - 0.91 %K Normal : 0.91 - 1.3% K Tambahan hasil jagung , bu/ac 25 Kdaun = 0.75 % Kdaun = 1.0 % Kdaun = 1.5 % Kdaun = 1.75% 25 50 75 100 125 Dosis pupuk K ( lb / ac ) Sumber: Hanway et al. (1962)

Sumber: www.maine.gov/dep/air/acidrain/

The difference between flocculated (aggregated) and dispersed soil structure. Flocculation (left) is important because water moves through large pores and plant roots grow mainly in pore space. Dispersed clays (right) plug soil pores and impede water movement and soil drainage in all but the sandiest soil.

Sumber: www.fao.org/docrep/field/003/ac1...2E05.htm Reaction of lime and fertilizers with soil. Sumber: www.fao.org/docrep/field/003/ac1...2E05.htm

Liming of the soil: Soil acidity is corrected by the application of lime material. The lime material has to be a calcium or magnesium salt of a weak acid such as limestone (CaCO3), dolomite (Ca Mg (CO3)2), quicklime (CaO), hydrated lime or slaked lime (Ca (OH)2). The reaction of lime with acidic soil is represented by the following equations

The Calcium Magnesium ratio in Soil Many people have heard of the Calcium Magnesium ratio, but few people understand what it is all about. The problem is that is has implications for both soil structure and animal health. Issues also arise in understanding both the importance and use of the Ca:Mg ratio. The desirable levels for this ratio (that are now widely accepted throughout Agriculture in Australia) were one of the first outcomes of Ted Mikhail's research in the 1960s, yet there is still an under appreciation of the significance of this simple measure. Mostly, when people discuss the calcium magnesium ratio (including many 'experts') they talk about it using plant nutrition terms and so it is often expected that extremes in the ratio will produce either Calcium or Magnesium deficiencies. This is simply incorrect. However, the problem is that the Calcium Magnesium ratio has implications for both soil structure and animal health. So, strictly speaking, there are really two ratios to talk about. Also, for soil friability, a proper assessment of the ratio cannot be made on the basis of the Ca:Mg ratio alone. It is important to also realize that the effects of the Ca:Mg ratio occur equally in all soil texture classes (sand and clay alike).

Soil Friability For an assessment of how the proportions of Calcium and Magnesium influence soil friability, the Calcium Magnesium ratio should be calculated from the exchangeable cation figures in me/100g. If the Calcium percentage is close to or within its desirable range (60% to 70% of the adjusted CEC) and the Ca:Mg ratio is less than 2:1, then the soil will have poor structure and be classified as non-friable. Under these same conditions, as the ratio increases from < 2:1 to 4:1 the soil will progress through stages of friability from semi-friable to friable and very friable, but above a ratio of 4:1 there will be no further improvement in friability.

It should be noted that the Ca:Mg ratios given here represent general conditions found in soils with the kinds of Ca% and Mg% shown. The ratio is not and should not be calculated from these percentages. Ca% Mg% Ca:Mg* Na% Soil condition Low/Low <40% <12% >2:1 <5% Poor structure; semi-friability Low/High >20% <2:1 Poor structure; hard setting; non-friable High/Low >65% >4:1 Good structure, friable High/High <4:1 Good structure, semi-friable (very rare)

The ionic form is considered to be available to crops. Magnesium in the Soil Magnesium is a component of several primary and secondary minerals in the soil, which are essentially insoluble, for agricultural considerations. These materials are the original sources of the soluble or available forms of Mg. Magnesium is also present in relatively soluble forms, and is found in ionic form (Mg++) adhered to the soil colloidal complex. The ionic form is considered to be available to crops.

Balances and Ratios Ca:Mg For many years, there have been a few people who claim that there is an "Ideal" ratio of the three principal soil cation nutrients (K, Ca, and Mg). This concept probably originated from New Jersey work by Bear in 1945 that projected an ideal soil as one that had the following saturations of exchangeable cations 65% Ca, 10% Mg, 5% K, and 20% H. The cation ratios resulting from these idealizes concentrations are a Ca:Mg of 6.5:1, Ca:K of 13:1, and Mg:K of 2:1. It is generally accepted that there are some preferred general relationships and balances between soil nutrients. There is also a significant amount of work indicating that excesses and shortages of some nutrients will affect the uptake of other nutrients (see later sections of this paper). However, no reliable research has indicated that there is any particular soil ratio of nutrients.

Some of the claims are that the correct soil Ca:Mg ratio will Over the years, a significant amount of conversation and salesmanship has revolved around the concept of the ideal soil Ca:Mg ratio. Most of the claims for the ideal ratio range between 5:1 and 8:1. Some of the claims are that the correct soil Ca:Mg ratio will Wisconsin research found that yields of corn and alfalfa were not significantly affected by Ca:Mg ratios ranging from 2.28:1 to 8.44:1in all cases, when neither nutrient was deficient, the crops internal Ca:Mg ratio was maintained within a relatively narrow range consistent with the needs of the plant. These findings are supported by most other authorities. A soil with the previously listed ratios would most likely be fertile. However, this does not mean that a fertile soil requires these specific values (or any other). Adequate crop nutrition is dependent on many factors other than a specific ratio of nutrients. It will rarely be profitable to spend significant amounts of fertilizer dollars to achieve a specific soil nutrient ratio.

High Response Crops to Mg While this is an essential element for all plants, these crops have been found to be especially responsive: Alfalfa, blueberry, beet, broccoli, cabbage, cauliflower, celery, clover, conifers, corn, cotton, cucumber, eggplant, lettuce, onion, pepper, potatoes, pumpkin, spinach, squash, tobacco, tomato, and watermelon.

Mg Deficiency Symptoms The classic deficiency symptom is interveinal chlorosis of the lower/older leaves. However, the first symptom is generally a more pale green color that may be more pronounced in the lower/older leaves. In some plants, the leaf margins will curve upward or turn a red-brown to purple in color. Full season symptoms include preharvest leaf drop, weakened stalks, and long branched roots. Conifers will exhibit yellowing of the older needles, and in the new growth the lower needles will go yellow before the tip needles.

Magnesium toxicity's are rare. Crops grown on heavy Montmorillonite clay soils that have been poorly fertilized with potassium may exhibit excesses of Magnesium in their tissue. But, before the tissue level approaches toxicity, Potassium deficiency will occur. Higher tissue levels of Magnesium are usually found in the older leaves on the plant and may be associated with diseased or damaged leaves. Grass Tetany This is a magnesium deficiency in ruminants. It occurs when livestock are fed a diet of forages low in Mg.

Using Magnesium in a Fertility Program Soil testing is the first step in determining a need. If the analysis shows a need and a supplemental application is indicated, you can be confident the application will be economically sound. As always Plant Analyses are also useful in uncovering "hidden" Magnesium shortages and when a need is determined, treatment should follow. Magnesium is a constituent of most agricultural lime, as well as specific Mg fertilizers. Magnesium containing materials applied to the soil may serve two functions.

As MgCO3, to neutralize soil acidity A nutrient As MgCO3, to neutralize soil acidity Proper liming with dolomitic limestone is almost always the most practical solution to low Mg, even if the dolomite is more expensive. Supplemental broadcast and row applications will most likely need to be repeated over a period of several years. If row applied fertilizers are used where magnesium shortage is a problem, it is desirable to minimize in-row K applications to avoid K-Mg competition. However, materials such as Sul-Po-Mag and K-Mag that contain both nutrients have been used to partially satisfy Mg needs on soils where the crops had significant Mg stress caused by extremely high K levels. Likewise, broadcast recommendations of K20 equal to, or in excess of 400 lb/A should be split into two or more applications. Good responses have been obtained from foliar applications of both Epsom salts (MgSO4) and Magnesium chelates. The basic consideration with these materials is total cost per acre. Also remember, foliar applications are only supplements to a sound soil fertility plan. They are rarely successful in replacing a sound soil fertility program where soil Mg levels are weak.

per Manufacturer Recommendation Recommended rates of Mg Method Rate Broadcast: 22 to 66 lb./A In-row: 11 to 33 lb./A Foliar. 0.5 To 2 lb./A (from MgSO4) Foliar: per Manufacturer Recommendation

Some Mg fertilizer sources Name Formula %Mg Epsom salts MgSO4·7H2O 10 Potassium-Magnesium Sulfate K2SO4·2MgSO4 11 Magnesium Oxide/Magnesia* MgO 55 Mg Chelates Various 3 to 5.5

The Role of Magnesium in the Plant Magnesium is the central core of the chlorophyll molecule in plant tissue. Thus, if Mg is deficient, the shortage of chlorophyll results in poor and stunted plant growth. Magnesium also helps to activate specific enzyme systems. Enzymes are complex substances that build, modify, or break down compounds as part of a plant's normal metabolism.

Magnesium in the Soil Magnesium is abundant in the earth's crust. It is found in a wide variety of minerals. Magnesium becomes available for plant use as these minerals weather or break down. The majority of the soils in western Minnesota have naturally high levels of Mg. For the acid soils of the eastern counties, the addition of dolomitic limestone in the crop rotation, when needed, should supply adequate Mg for crop growth. Magnesium is held on the surface of clay and organic matter particles. Although this exchangeable form of Mg is available to plants, this nutrient will not readily leach from soils. In Minnesota, Mg deficiency has only been observed on very acid soils. These soils usually have a sandy loam, loamy sand or sand texture. A Mg deficiency is not likely to occur until the soil pH drops below 5.5. In Minnesota, the acid sandy soils occur in the central and east-central part of the state. The low levels of Mg in soils can occur where potatoes are grown on acid sandy soils or where corn follows a potato crop. Sometimes, grass tetany, a livestock disorder caused by low levels of Mg in the diet, is reported where high rates of potash have been applied to grass pastures. Research trials, however, have shown that the use of Mg in a fertilizer program for these pastures has not increased forage yields. For these situations, it is less expensive to supplement the animal diet with a salt that contains Mg.

Relationship of Magnesium to Calcium in Soils There are some who believe that there is an "ideal" ratio of calcium to magnesium in soils and one of these two nutrients should be added in a fertilizer program if this "ideal" ratio does not exist. The need for this "ideal" ratio has never been verified by various research efforts throughout the Corn Belt which have focused on the importance of ratios. In Wisconsin, for example, the ratio of calcium to magnesium in soils was adjusted in a range of two to eight by adding different amounts of calcium and magnesium in a fertilizer program. This variation had no significant effect on alfalfa and corn yields. Therefore, as fertilizer recommendations are developed, emphasis should be placed on providing adequate amounts of magnesium in soils rather than the maintenance of a certain ratio of one nutrient to another.

Predicting the Need for Magnesium The critical plant tissue concentrations of Mg in selected crops are listed . Since Mg is a mobile element in the plant, the concentration of Mg usually decreases from the top to the bottom of the plant. Also, the Mg concentration usually decreases as the plant approaches maturity. It is, therefore, important to indicate the age of the plant and the part of the plant that was sampled when samples are submitted for a measurement of Mg in plant tissue.

Relative magnesium levels in selected tissue of several crops. Plant Part Time of Sampling Magnesium Status Deficient Low Sufficient High -------------------%Mg------------------- alfalfa upper 1/3 of plant 1/10 bloom <.20 .20-.30 .31-1.0 >1.0 corn ear leaf silking <.10 .11-.25 .26-1.0 oats upper leaves boot stage - .13 .13-.40 >.4 potatoes petiole of most recently mature leaf bloom .30-.70 >.70 soybeans most recently developed trifolate pod set >.10

Magnesium Soil Test Relative Level Magnesium to Apply A soil test to measure exchangeable Mg is offered by most soil testing laboratories. In Minnesota, the potential need for Mg in a fertilizer program is highest where sandy soils are very acid. If dolomitic lime has been used in the crop rotation, soils usually have a relatively high level of Mg and it is not necessary to test the soil for this nutrient. Magnesium Soil Test Relative Level Magnesium to Apply Starter or Broadcast ppm ----lb./acre---- 0-50 low 10-20 50-100 51-150 medium trial* 151+ high

Magnesium recommendations for fruit and vegetable crops. Magnesium Soil Test Relative Level Magnesium to Apply Starter or Broadcast ppm ----lb./acre---- 0-50 low 20 100 51-100 medium 10 50 101+ high

Sources of Magnesium The application of dolomitic limestone is the most cost effective method for applying the Mg that is needed. The Mg content of dolomitic limestone varies from 8-10%. To be effective, this Mg source should be broadcast and incorporated before planting. There are fertilizers that are a combination of potassium sulfate and magnesium sulfate. The Mg content is 11%. The sulfur (S) concentration is 22% and the K2O percentage is 22%. This fertilizer is easily used in a starter fertilizer for corn or as a Mg source when there is no desire to increase soil pH. Although the need for the addition of Mg to a fertilizer program is not widespread in Minnesota, this nutrient can increase crop production when needed. The potential for need should not be ignored. If there is doubt about the need, analyze the soil to be sure.

Soil magnesium level, corn (Zea mays L.) yield, and magnesium uptake Corn grain yields were unaffected by a wide range of exchangeable Mg levels in the experimental soils. Since there was no reduction in yield at the highest (28.8% Mg saturation, exchangeable Ca/Mg = 1.8) or lowest (1.8% Mg saturation, Ca/Mg = 36.9) soil Mg level, it was not possible to identify critical limits. It was apparent from these and results in the literature, though, that a lower limit of 5% Mg saturation should be adequate for corn grain production and that Mg toxicity in corn will not occur in soils that have an exchangeable Ca/Mg equivalent of 1.0 or higher. As 1.0 is the ratio found in pure dolomite, it is unlikely that Mg toxicity will occur in normal agricultural soils. Four of the tested Mg availability indexes (exchangeable Mg, Mg saturation, exchangeable Mg/K ratio and Baker test pMg) were well correlated (|r| = 0.84 to 0.93) with corn ear leaf and silage Mg concentrations and total Mg uptake. At least 10% Mg saturation was required to obtain 0.2% Mg in the corn silage grown on those soils. It was also found that the Mg concentration in ear leaves at silking could be as low as 0.1% with no decrease in grain yield.

South African Avocado Growers’ Association Yearbook 1993. 16:33-36 EFFECT OF POTASSIUM, MAGNESIUM AND NITROGEN SOIL APPLICATIONS ON FUERTE AVOCADO FRUIT QUALITY SYLVIE KREMER-KÖHNE, J.S. KÖHNE AND J.M. SCHUTTE Merensky Technological Services, P O Box 14, Duiwelskloof 0835, RSA

South African Avocado Growers’ Association Yearbook 1993. 16:33-36 EFFECT OF POTASSIUM, MAGNESIUM AND NITROGEN SOIL APPLICATIONS ON FUERTE AVOCADO FRUIT QUALITY SYLVIE KREMER-KÖHNE, J.S. KÖHNE AND J.M. SCHUTTE Merensky Technological Services, P O Box 14, Duiwelskloof 0835, RSA

South African Avocado Growers’ Association Yearbook 1993. 16:33-36 EFFECT OF POTASSIUM, MAGNESIUM AND NITROGEN SOIL APPLICATIONS ON FUERTE AVOCADO FRUIT QUALITY SYLVIE KREMER-KÖHNE, J.S. KÖHNE AND J.M. SCHUTTE Merensky Technological Services, P O Box 14, Duiwelskloof 0835, RSA

Soil and Fertilizer Magnesium Magnesium is a common constituent in many minerals, comprising 2% of the Earth’s crust. It is also a common component in seawater (1,300 ppm). Magnesium is present in the divalent Mg2+ form in nature, but can be processed into a pure metal. Since metal Mg is one-third lighter than aluminum (Al), it is commonly used in lightweight alloys for aircraft and automobiles. In the powder or ribbon form, metallic Mg burns when exposed to air.

Magnesium in Primary and Secondary Minerals Several ferromagnesian minerals (such as olivine, pyroxene, amphibole, and mica) are major Mg sources in basic igneous rocks. Secondary minerals, including carbonates... For example, dolomite [MgCO3.CaCO3], magnesite [MgCO3], talc [Mg3Si4O10(OH)2], and the serpentine group [Mg3Si2O5(OH)4] ...are derived from these primary minerals. When serpentine is present in large amounts, it gives rise to the term “serpentine soil.” In these ultramafic serpentine soils, high Mg concentrations lead to poor plant growth and poor soil physical conditions. Undesirably high concentrations of nickel may also occur in these soils. These primary and secondary minerals are important sources of Mg for plant nutrition, especially in unfertilized soil. But plant-available Mg concentrations cannot be accurately predicted based only on the parent material composition due to differences in mineral weathering rates and leaching. In some cases, the contribution of minerals to meeting the entire crop demand for Mg during a single growing season is insufficient to prevent plant and animal deficiencies.

Non-Exchangeable and Exchangeable Magnesium Magnesium is located both in clay minerals and associated with cation exchange sites on clay surfaces. Clays such as chlorite, vermiculite, and montmorillonite have undergone intermediate weathering and still contain some Mg as part of their internal crystal structure. The Mg release rate from these clays is generally slow. Illite clays may also contain Mg, but their release rate is even slower. The details of clay weathering and mineralogy are available elsewhere.

Semi-Soluble Mg Sources Dolomite – MgCO3.CaCO3; 6 to 20% Mg – Depending on the geologic source, the concentration of Mg will vary considerably. Pure dolomite contains 40 to 45% MgCO3 and 54 to 58% CaCO3. However a concentration of 15 to 20% MgCO3 (4 to 6% Mg) is common for material called “dolomitic limestone”. Dolomite is often the least expensive common source of Mg, but may be slow to dissolve, especially where soil acidity is lacking. Hydrated dolomite – MgO.CaO/MgO.Ca(OH)2;18 to 20% Mg – This product is made by heating dolomitic lime (calcined) to form MgO and CaO. It is then hydrated to form dolomitic hydrated lime, which may contain only hydrated calcium oxide or it may also contain hydrated magnesium oxide. These compounds dissolve faster than untreated dolomite. Magnesium oxide – MgO; 56% Mg – Composed of only magnesium and oxygen, it is formed by heating MgCO3 to drive off carbon dioxide. It contains the highest concentration of Mg of common fertilizers, but is rather insoluble. Applying in advance of plant demand and using a fine particle size will help make this nutrient source useful for plant growth.

Soluble Mg Sources (with approximate solubility at 25°C) Kieserite – MgSO4.H2O; 17% Mg – Kieserite is the monohydrate of magnesium sulfate, produced primarily from mines located in Germany. As a carrier of both Mg and S, kieserite finds multiple applications in agriculture and industry (360 g/L) Kainite – MgSO4.KCl.3H2O; 9% Mg – Kainite is the mixed salt of magnesium sulfate and potassium chloride. It is most commonly used as a K source, but is useful where both Mg and K are required (variable solubility). Langbeinite – 2MgSO4.K2SO4; 11% Mg – A widely used source of Mg, as well as K and S, this mineral is an excellent multi-nutrient source. While totally soluble, langbeinite is slower to dissolve than some Mg sources and not typically delivered through irrigation systems (240 g/L). Magnesium Chloride – MgCl2; 25% Mg – Generally sold as a liquid due to its high solubility, this material is frequently used as a component in fluid fertilizers (560 g/L). Magnesium Nitrate – Mg(NO3)2.6H2O; 9% Mg – Widely used in the horticultural industry to supply Mg in a form that also provides a soluble N source (1,250 g/L). Magnesium Sulfate (Epsom salt) – MgSO4.7H2O, 9% Mg – Epsom salt derives its name from naturally occurring geologic deposits in Epsom, England. It is a common mineral and a byproduct from various brines that makes an excellent Mg source. It is similar to Kieserite, except it contains seven water molecules associated with the MgSO4 (357 g/L).

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