K - Ca - Mg
TANAH
diabstraksikan oleh:
Soemarno tanahfpub-2010.
KALIUM
TANAH
Jumlah K-tanah
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
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
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+ + OHK
Ca
Koloid liat H
K+, Ca++, H+ (larutan tanah)
Pelapukan
1. Proses fisika: Penghancuran fisik, ukuran partikel
menjadi lebih halus, luas permukaannya menjadi lebih
besar
2. Proses kimiawi: Hidrolisis, Protolisis (Asidolisis)
Pelapukan
Mineral
KALIUM
Proses Hidrolisis dan Protolisis
HAlSi3O8 + K+ + OH- (cepat)
KAlSi3O8 + HOH
HAlSi3O8 + 4 HOH
 Si-O-Al  + H2O + H+
K
Al(OH)3 + 3 H2SiO3
 Si-O
(lambat)
+  Al-OH + K+
2
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
Faktor Pelapukan
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)
Kadar K-tanaman
K diperlukan untuk
pertumbuhan optimum
Pemakaian berlebihan
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 dapat ditukar
K dalam
larutan tnh
hort.wisc.edu
Sumber: ipipotash.org
LOKASI DAN JALUR KALIUM DLM TANAH
K dalam mineral primer
mis. Muskovit
Pelepasan
K
K dalam PUPUK
Fiksasi K pd
mineral primer
Pelarutan pupuk
Transisi mineral sekunder
menjadi mika
akibat fiksasi
K
Pelepasan K
mengakibatka
n
pembentukan
min. sekunder
K dalam mineral sekunder
mis. Kaolinit
K dalam
larutan tnh
Pelepasan Kdd
atau K-terfiksasi
Adsorpsi atau
Fiksasi K
Absorpsi K
K dalam tanaman
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-larutan tanah (me/l)
5.0
Tanah berpasir
4.0
3.0
2.0
Tanah liat
1.0
10
50
K dapat ditukar, mg K / 100 g tanah
100
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
Pupuk
buatan
Mineral kalium
lambat tersedia
K - tersedia
Terangkut
tanaman
Hilang
pencucian
Hilang
Erosi &
Run-off
Fiksasi
Kalium
Faktor
Ketersediaan Ktanah
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
Dosis kapur, CaCO3
pH: 7.00
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
0
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 , kg /ha (Tanaman kacang buncis)
300
r2 = 0.79**
0.2
0.4
Sumber: Nemeth dan Forster (1976)
0.6
0.8
K- larutan tanah ( me K / l)
Laju
Penyerapan K
vs
Konsentrasi K+
larutan
Laju penyerapan K+ , mole/g/jam (akar tanaman barley)
10.0
0.05
Sumber: Epstein (1972)
0.10
0.15
0.20
Konsentrasi K+ larutan tanah ( mM)
Efek Ca++ thd
penyerapan K+
akar tanaman
Penyerapan K , mole/g (akar tanaman Jagung )
6
+ Ca
0
-1
0.5
1.0
Sumber: Lauchli dan Epstein (1970)
1.5
jam
2.0
KALSIUM
DALAM
TANAH
Sumber Ca-tanah
Mineral primer :
1. Dolomit
: ………..
2. Kalsit
: ………..
3. Apatit
: ………..
4. Feldspar kalsium: ………..
5. Amfibol
: …………
Bahan Pupuk:
1. Kalsium nitrat
2. Gipsum
3. Batuan fosfat
4. Superfosfat
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:
1. Dolomit
: ………..
2. Biotit
: ………..
3. Klorit
: ………..
4. Serpentin
: ………..
5. Olivin
: …………
Bahan Pupuk:
1. MgSO4.7H2O
2. MgSO4.H2O
3. K-Mg-sulfat
4. Magnesia
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
25
Sumber: Nelson (1968)
50
Daun
75
100
days after emergence
Kandungan Ktanah
vs
Respon pupuk
K
H
Tambahan hasil jagung , bu/ac
25
Kdd = 50 ppm
Kdd = 100 ppm
Kdd = 150 ppm
Kdd = 200 ppm
25
50
75
100
Dosis pupuk K ( lb / ac )
Sumber: Hanway et al. (1962)
125
Kandungan Kdaun
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
Dosis pupuk K ( lb / ac )
Sumber: Hanway et al. (1962)
125
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.
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 nonfriable.
Under these same conditions, as the ratio increases from < 2:1 to
4:1 the soil will progress through stages of friability from semifriable 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
<40
%
>20
%
<2:1
<5%
Poor structure; hard setting; nonfriable
High/Low
>65
%
<12
%
>4:1
<5%
Good structure, friable
High/High
>65
%
>20
%
<4:1
<5%
Good structure, semi-friable (very rare)
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.
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.
Toxicity
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.
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.
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
Epsom salts
Potassium-Magnesium
Sulfate
Magnesium
Oxide/Magnesia*
Mg Chelates
Formula
%Mg
MgSO4·7H2O
10
K2SO4·2MgSO4
11
MgO
55
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.
Magnesium Status
Time of
Deficie
Suffici
Sampli
Low
High
nt
ent
ng
-------------------%Mg-------------------
Crop
Plant Part
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
>1.0
oats
upper leaves
boot
stage
-
.13
.13-.40
>.4
potatoe
s
petiole of most recently
mature leaf
bloom
<.20
.20-.30
.30-.70
>.70
soybea
ns
most recently developed
trifolate
pod set
<.10
.11-.25
.26-1.0
>.10
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
ppm
Magnesium to Apply
Starter or Broadcast
----lb./acre----
0-50
low
10-20
50-100
51-150
medium
trial*
0
151+
high
0
0
Magnesium recommendations for fruit and vegetable crops.
Magnesium
Soil Test
Magnesium to Apply
Relative Level
Starter or Broadcast
ppm
----lb./acre----
0-50
low
20
100
51-100
medium
10
50
101+
high
0
0
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
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).
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