PEMANASAN GLOBAL DAN PERUBAHAN IKLIM

PEMANASAN GLOBAL DAN
PERUBAHAN IKLIM

PENGERTIAN
Pemanasan Global adalah indikasi naiknya suhu

muka bumi secara global (meluas dalam radius
ribuan kilometer) terhadap normal/rata-rata
catatan pada kurun waktu standard (ukuran
Badan Meteorologi Dunia/WMO: minimal 30
tahun).
Perubahan Iklim Global adalah perubahan unsurunsur iklim (suhu, tekanan, kelembaban, hujan,
angin, dsb.nya) secara global terhadap
normalnya.
Iklim adalah rata-rata kondisi fisis udara(cuaca)
pada kurunwaktu tertentu (harian, mingguan,
bulanan, musiman dan data tahunan yang
diperlihatkan dari ukuran catatan unsur-unsurnya
(suhu, tekanan, kelembaban, hujan, angin,
dsb.nya)

Pemanasan global (global warming) pada
dasarnya merupakan fenomena
peningkatan temperatur global dari tahun
ke tahun karena terjadinya efek rumah
kaca (greenhouse effect) yang disebabkan
oleh meningkatnya emisi gas - gas seperti
karbondioksida (CO2), metana (CH4),
dinitrooksida (N2O) dan CFC sehingga
energi matahari terperangkap dalam
atmosfer bumi. Kenaikan konsentrasi gas
CO2 ini disebabkan oleh kenaikan
pembakaran bahan bakar minyak (BBM),
batu bara dan bahan bakar organik
lainnya yang melampaui kemampuan
tumbuhan-tumbuhan dan laut untuk
mengabsorbsinya.

Meningkatnya suhu global
diperkirakan akan menyebabkan
perubahan-perubahan yang lain

seperti naiknya permukaan air laut,
meningkatnya intensitas fenomena
cuaca yang ekstrim serta perubahan
jumlah dan pola presipitasi. Akibatakibat pemanasan global yang lain
adalah terpengaruhnya hasil
pertanian, hilangnya gletser, dan
punahnya berbagai jenis hewan.

Pemanasan global mengakibatkan dampak yang luas dan
serius bagi lingkungan bio-geofisik (seperti pelelehan es di
kutub, kenaikan muka air laut, perluasan gurun pasir,
peningkatan hujan dan banjir, perubahan iklim, punahnya
flora dan fauna tertentu, migrasi fauna dan hama penyakit,
dsb). Sedangkan dampak bagi aktivitas sosial-ekonomi
masyarakat meliputi :
gangguan terhadap fungsi kawasan pesisir dan
kota pantai
gangguan terhadap fungsi prasarana dan sarana
seperti jaringan jalan, pelabuhan dan bandara
gangguan terhadap permukiman penduduk

pengurangan produktivitas lahan pertanian
peningkatan resiko kanker dan wabah penyakit,
dsb).

BEBERAPA UNSUR PENYEBAB PEMANASAN
DAN PERUBAHAN IKLIM GLOBAL
Semakin tingginya Populasi
Ekploitasi lingkungan meningkat dengan
marak dan meluasnya perubahan
tataguna lahan yang berakibat pada
penciutan luasan hutan,
Kemajuan industri menimbulkan
pencemaran di darat, laut dan udara
yang berlanjut dengan perusakan gas
ozon di kutub atau lubang ozon di kutub
dan konsentrasi gas buang yang menjadi
selimut gas atau gas rumah kaca,
Dst

INDIKASI YANG TERKAIT

DENGAN PEMANASAN
DAN PERUBAHAN IKLIM
GLOBAL

TERIMA KASIH

MK. TANAH & Media Tumbuh Tanaman

AIR TANAH
&

TANAMAN
1

2

Proses fotosintesis memerlukan air

3

CO2 dari Udara

Fotosintesis:
CO2 + H2O ---- Karbohidrat
(Glukosa)

Glukosa

Pati

dan senyawa organik lain dalam buah
dan biji

Air dari tanah
4

CO2 dari Udara

Fotosintesis:
CO2 + H2O

Karbohidrat
(Glukosa)

Glukosa

Pati

dan senyawa organik lain
dalam biji

Stomata:
Pintu lalulintas CO2,
O2, dan H2O

Air dari tanah
5

Budidaya
tanaman padi sawah

memerlukan banyak air

6

KEBUTUHAN AIR
TANAMAN
A plant has different
water needs at different
stages of growth. While
a plant is young it
requires less water than
when it is in the
reproductive stage.

Kurva Penggunaan Air Musiman
oleh Tanaman

When the plant
approaches maturity, its
water need drops.

Curves have been
developed that show the
daily water needs for
most types of crops.
7

KEDALAMAN PERAKARAN TANAMAN
A plant’s root depth determines the depth to which soil water can be
extracted. A young plant has only shallow roots and soil water deeper
than rooting depth is of no use to the plant. Plants typically extract
about 40 percent of their water needs from the top quarter of their
root zone, then 30 percent from the next quarter, 20 percent from the
third quarter, taking only 10 percent from the deepest quarter.
Therefore, plants will extract about 70 percent of their water from
the top half of their total root penetration.
Deeper portions of the root zone can supply a higher percentage of
the crop’s water needs if the upper portion is depleted. However,
reliance on utilization of deeper water will reduce optimum plant
growth.
8

KUALITAS AIR & TANAH
For good plant growth, a soil must have adequate room for water and
air movement, and for root growth. A soil’s structure can be altered
by certain soil management practices. For example, excessive tillage
can break apart aggregated soil and excessive traffic can cause
compaction. Both of these practices reduce the amount of pore space
in the soil, and thus reduce the availability of water and air, and
reduce the room for root development.
Irrigation water with a high content of soluble salt is not as available
to the plant, so a higher soil water content must be maintained in
order to have water available to the plant. Increasing salt content of
the water reduces the potential to move water from the soil to the
roots. Some additional water would also be needed to leach the salt
below the crop root zone to revent build-up in the soil. Poor quality
water can affect soil structure.
9

Kebutuhan air BAWANG PUTIH (Allium cepa)
Untuk mencapai hasuil optimum tanaman onion memerlukan 350-550 mm air.

Tanaman sangat peka terhadap kondisi defisit air tanah. Untuk mencapai hasil yang
tinggi, penurunan kandungan air tanah tidak boleh melebihi 25% air tanah tersedia.
Tanaman paling peka terhadap defisit air selama periode pembentukan umbi,
terutama selama periode pertumbuhan umbi yang cepat yang terjadi sekitar 60 hari
setelah transplanting. Tanaman juga sangat peka kekeringan selama masa
transplantasi. Selama periode pertumbuhan vegetatif tanaman agak kurang peka
terhadap defisit air tanah. Untuk mendapatkan hasil yang banyak dan kualitas yang
baik, tanaman memerlukan suplai air yang terkendali dan sering selama musim
pertumbuhannya; akan tetapi irigasi yang berlebihan mengakibatkan pertumbuhan
terhambat.
Untuk mendapatkan ukuran umbi yang besar dan bobot yang tinggi, defisit air
tanah terutama selama periode pembentukan hasil (Periode pembesaran umbi) tidak
boleh terjadi. Kalau supali air terbatas, maka penghematan air dapat dilakukan
selama periode pertumbuhan vegetatif dan periode pemasakan.
10

11

Komposisi tana menurut volume

Tanah subur yg ideal:
• Mineral 45%
• Organic matter 5%
• Water 25%
• Air
25%

12

Tiga komponen tanah
The soil system is composed of three major components: solid
particles (minerals and organic matter), water with various
dissolved chemicals, and air.
The percentage of these components varies greatly with soil texture
and structure.
An active root system requires a delicate balance between the three
soil components; but the balance between the liquid and gas phases
is most critical, since it regulates root activity and plant growth
process.

13

A soil profile is the sequence
of natural layers, or horizons,
in a soil. Each soil series
consists of soils having major
horizons that are similar in
color, texture, structure,
reaction, consistency, mineral
and chemical composition,
and arrangement in the soil
profile. The soil profile
extends from the surface
downward to unconsolidated
material. Most soils have three
major horizons called the
surface horizon, the subsoil,
and the substratum.
14

STRUKTUR &
CIRI
H2O

Molekul air terdiri atas atom oksigen dan dua atom
hidrogen, yang berikatan secara kovalen
Atom-atom tidak terikat secara linear (H-O-H), tetapi
atom hidrogen melekat pada atom oksigen seperti huruf
V dengan sudut 105o.

Molekul air bersifat dipolar:
Zone elektro positif

+
H

H
105o

Zone elektro negatif

-

15

Plants develop the tension, or
potential, to move soil water
from the soil into
the roots and distribute the
water through the plant by
adjusting the water potential,
or tension, within their plant
cells.
The essence of the process is
that water always moves
from higher to lower water
potential.
For water to move from the
soil, to roots, to stems, to
leaves, to air the water
potential must always be
decreasing.

Ilustrasi tentang penurunan potensial air
untuk suatu tanaman

16

Lingkaran
Tanah-AirTanaman

LTAT mrpk sistem dinamik dan terpadu dimana air mengalir
dari tempat dengan tegangan rendah menuju tempat dengan
tegangan air tinggi.

Hilang melalui stomata
daun (transpirasi)

Air kembali ke
atmosfer
(evapo-transpirasi)

Air dikembalikan ke
tanah melalui hujan
dan irigasi

Penguapan

Serapan bulu akar

17

SISTEM TANAH-TANAMAN
Structure of water transport model for the soil-leaf continuum, with
the inputs outlined in boxes.
Root and shoot components are represented by a resistance network,
each component of which varies according to the inputted K(y)
function from vulnerability curves of xylem.
Layers of roots reach to different soil depths according to an inputted
root area profile. Canopy layers reflect an inputted leaf area and Y
profile.
Soil is modeled as a rhizosphere resistance connecting roots to bulk
soil of an inputted y and K(y).
The model predicts transpiration (E) as a function of the inputs.
18

Model struktur sistem tanaman dalam konteks hubungan AirTanah-Tanaman
19

Kekuatan ikatan antara molekul air dengan partikel tanah
dinyatakan dengan TEGANGAN AIR TANAH. Ini merupakan fungsi
dari gaya-gaya adesi dan kohesi di antara molekul - molekul air dan
partikel tanah

Adesi

Kohesi

H2O
Partikel tanah

Air terikat

Air bebas

20

Air Tersedia untuk pertumbuhan tanaman
21

).

Fine textured soils with small
pores can hold the greatest
amounts of PAW.

Coarse textured sandy soils with
large pores can hold the least
amounts of PAW.
22

Status Air
Tanah

Perubahan status air dalam tanah, mulai dari
kondisi jenuh hingga titik layu

Jenuh

Kap. Lapang

Padatan

Titik layu

Pori

100g

air

40g

tanah jenuh air

100g

20g

udara

kapasitas lapang

100g

10 g

udara

koefisien layu

100g

8g

udara

koefisien higroskopis

23

TEGANGAN
&
KADAR AIR

PERHATIKANLAH proses yang terjadi kalau tanah basah
dibiarkan mengering.
Bagan berikut melukiskan hubungan antara tebal lapisan air di
sekeliling partikel tanah dengan tegangan air

Bidang singgung tanah dan air
Koef.
Koef.
padatan tanah
higroskopis layu

10.000
atm
31 atm

10.000 atm

15 atm

Kapasitas
lapang

1/3 atm

Mengalir krn gravitasi

Tegangan air

1/3 atm
tebal lapisan air

24

Representasi bola air yang menyelubungi partikel padatan tanah
25

JUMLAH AIR DALAM TANAH
The amount of soil water is usually measured in terms of water content as
percentage by volume or mass, or as soil water potential. Water content does
not necessarily describe the availability of the water to the plants, nor indicates,
how the water moves within the soil profile. The only information provided by
water content is the relative amount of water in the soil.
Soil water potential, which is defined as the energy required to remove water
from the soil, does not directly give the amount of water present in the root zone
either. Therefore, soil water content and soil water potential should both be
considered when dealing with plant growth and irrigation.
The soil water content and soil water potential are related to each other, and the
soil water characteristic curve provides a graphical representation of this

relationship.
26

TEGANGAN
vs
kadar air

Air
higroskopis

Kurva tegangan - kadar air tanah bertekstur
lempung

Air kapiler
Air tersedia
Lambat tersedia

Cepat tersedia

Air gravitasi

Zone optimum

Tegangan air, bar

31

Koefisien higroskopis
Koefisien layu

0.1

Kapasitas lapang
Kap. Lapang maksimum
27
persen air tanah

Hubungan antara kadar air tanah dan tegangan air
tanah untuk tekstur lempung

28

STRUKTUR
&
CIRI

POLARITAS
Molekul air mempunyai dua ujung, yaitu ujung oksigen yg
elektronegatif dan ujung hidrogen yang elektro-positif.
Dalam kondisi cair, molekul-molekul air saling bergandengan
membentuk kelompok-kelompok kecil tdk teratur.
Ciri polaritas ini menyebabkan plekul air tertarik pada ion-ion
elektrostatis.
Kation-kation K+, Na+, Ca++ menjadi berhidrasi kalau ada
molekul air, membentuk selimut air, ujung negatif melekat
kation.
Permukaan liat yang bermuatan negatif, menarik ujung positif
molekul air.

Kation hidrasi

Selubung air

Tebalnya selubung air tgt
pd rapat muatan pd permukaan kation.
Rapat muatan =
muatan kation / luas permukaan

29

STRUKTUR
&
CIRI

IKATAN HIDROGEN
Atom hidrogen berfungsi sebagai titik penyambung (jembatan)
antar molekul air.
Ikatan hidrogen inilah yg menyebabkan titik didih dan viskositas
air relatif tinggi

KOHESI vs. ADHESI
Kohesi: ikatan hidrogen antar molekul air
Adhesi: ikatan antara molekul air dengan permukaan padatan
lainnya
Melalui kedua gaya-gaya ini partikel tanah mampu menahan air dan
mengendalikan gerakannya dalam tanah
TEGANGAN PERMUKAAN
Terjadinya pada bidang persentuhan air dan udara, gaya kohesi antar
molekul air lebih besra daripada adhesi antara air dan udara.
Udara
Permukaan air-udara

air
30

ENERGI AIR
TANAH

Retensi dan pergerakan air tanah melibatkan energi, yaitu:
Energi Potensial, Energi Kinetik dan Energi Elektrik.
Selanjutnya status energi dari air disebut ENERGI BEBAS,
yang merupakan PENJUMLAHAN dari SEMUA BENTUK
ENERGI yang ada.
Air bergerak dari zone air berenergi bebas tinggi (tanah basah)
menuju zone air berenergi bebas rendah (tanah kering).

Gaya-gaya yg berpengaruh
Gaya matrik: tarikan padatan tanah (matrik) thd molekul air;
Gaya osmotik: tarikan kation-kation terlarut thd molekul air
Gaya gravitasi: tarikan bumi terhadap molekul air tanah.
Potensial air tanah
Ketiga gaya tersebut di atas bekerja bersama mempengaruhi energi bebas air tanah,
dan selanjutnya menentukan perilaku air tanah, ….. POTENSIAL TOTAL AIR
TANAH (PTAT)
PTAT adalah jumlah kerja yg harus dilakukan untuk memindahkan secara
berlawanan arah sejumlah air murni bebas dari ketinggian tertentu secara isotermik
ke posisi tertentu air tanah.
PTAT = Pt = perbedaan antara status energi air tanah dan air murni bebas
Pt = Pg + Pm + Po + …………………………
31

( t = total; g = gravitasi; m = matrik; o = osmotik)

Hubungan potensial air tanah dengan energi bebas

Potensial
positif

+

Energi bebas naik bila air tanah berada pada
letak ketinggian yg lebih tinggi dari titik
baku pengenal (referensi)

Energi bebas dari air murni

Potensial tarikan bumi

0
Menurun karena pengaruh osmotik

Potensial
negatif

-

Menurun karena pengaruh matrik

Potensial osmotik
(hisapan)
Potensial matrik
(hisapan)

Energi bebas dari air tanah

32

POTENSIAL
AIR TANAH

POTENSIAL TARIKAN BUMI = Potensial gravitasi
Pg = G.h
dimana G = percepatan gravitasi, h = tinggi air tanah di atas posisi
ketinggian referensi.
Potensial gravitasi berperanan penting dalam menghilangkan kelebihan
air dari bagian atas zone perakaran setelah hujan lebat atau irigasi

Potensial matrik dan Osmotik
Potensial matrik merupakan hasil dari gaya-gaya jerapan dan kapilaritas.
Gaya jerapan ditentukan oleh tarikan air oleh padatan tanah dan kation jerapan
Gaya kapilaritas disebabkan oleh adanya tegangan permukaan air.
Potensial matriks selalu negatif
Potensial osmotik terdapat pd larutan tanah, disebabkan oleh adanya bahan-bahan terlarut
(ionik dan non-ionik).
Pengaruh utama potensial osmotik adalah pada serapan air oleh tanaman

Hisapan dan Tegangan
Potensial matrik dan osmotik adalah negatif, keduanya bersifat menurunkan energi bebas air tanah. Oleh
karena itu seringkali potensial negatif itu disebut HISAPAN atau TEGANGAN.
Hisapan atau Tegangan dapat dinyatakan dengan satuan-satuan positif.
Jadi padatan-tanah bertanggung jawab atas munculnya HISAPAN atau TEGANGAN.
33

Cara
Menyatakan
Tegangan
Energi

Tinggi unit
kolom air (cm)
10
100
346
1000
10000
15849
31623
100.000

Tegangan: dinyatakan dengan “tinggi (cm) dari
satuan kolom air yang bobotnya sama dengan
tegangan tsb”.
Tinggi kolom air (cm) tersebut lazimnya
dikonversi menjadi logaritma dari sentimeter
tinggi kolom air, selanjutnya disebut pF.
Logaritma
tinggi kolom air (pF)
1
2
2.53
3
4
4.18
4.5
5

Bar

Atmosfer

0.01
0.1
0.346
1
10
15.8
31.6
100

0.0097
0.0967
1.3
9.6749
15
31
96.7492

34

KURVA ENERGI - LENGAS TANAH
Tegangan air menurun secara gradual dengan meningkatnya kadar air
tanah.
Tanah liat menahan air lebih banyak dibanding tanah pasir pada nilai
tegangan air yang sama
Tanah yang Strukturnya baik mempunyai total pori lebih banyak, shg
mampu menahan air lebih banyak
Pori medium dan mikro lebih kuat menahan air dp pori makro

KANDUNGAN
AIR DAN
TEGANGAN

Tegangan air tanah, Bar
10.000

Liat

Lempung

Pasir

0.01
10

Kadar air tanah, %

70
35

Tekstur tanah dan air tersedia

36

Hubungan antara kadar air tanah dengan tegangan air tanah

37

Jelaskan bagaimana tektur tanah mempengaruhi jumlah air tersedia bagi
38
tanaman? Sebanyak 250 kata

Jelaskan tanah-tanah yang tekturnya halus mampu menahan lebih banyak air
dibandingkan dgn tanah-tanah yang teksturnya kasar? Sebanyak 250 kata
39

Kapasitas air tersedia dalam tanah yang teksturnya berbeda-beda
40

Gerakan
Air Tanah
Tidak Jenuh

Gerakan tidak jenuh = gejala kapilaritas = air bergerak dari
muka air tanah ke atas melalui pori mikro.
Gaya adhesi dan kohesi bekerja aktif pada kolom air (dalam pri
mikro), ujung kolom air berbentuk cekung.
Perbedaan tegangan air tanah akan menentukan arah gerakan
air tanah secara tidak jenuh.

Air bergerak dari daerah dengan tegangan rendah (kadar air tinggi)
ke daerah yang tegangannya tinggi (kadar air rendah, kering).
Gerakan air ini dapat terjadi ke segala arah dan berlangsung secara
terus-menerus.

Pelapisan tanah berpengaruh terhadap gerakan air tanah.
Lapisan keras atau lapisan kedap air memperlambat gerakan air
Lapisan berpasir menjadi penghalang bagi gerakan air dari lapisan
yg bertekstur halus.
Gerakan air dlm lapisan berpasir sgt lambat pd tegangan
41

Air hujan dan irigasi memasuki tanah, menggantikan udara
dalam pori makro - medium - mikro. Selanjutnya air bergerak
ke bawah melalui proses gerakan jenuh dibawah pengaruh gaya
gravitasi dan kapiler.
Gerakan air jenuh ke arah bawah ini berlangsung terus selama
cukup air dan tidak ada lapisan penghalang

Gerakan Jenuh
(Perkolasi)

Lempung berpasir
cm

Lempung berliat

0
15 mnt

4 jam

30
60
90

1 jam

24 jam

120
24 jam

48 jam

150
30 cm

60 cm
Jarak dari tengah-tengah saluran, cm

42

Pola Penetrasi dan Pergerakan Air pada tanah Berpasir dan
tanah Lempung-liat

43

Pola pergerakan air gravitasi dalam tanah

44

Pengaruh struktur tanah terhadap pergerakan air tanah ke arah
bawah

45

PERKOLASI

Jumlah air perkolasi
Faktor yg berpengaruh:
1. Jumlah air yang ditambahkan
2. Kemampuan infiltrasi permukaan tanah
3. Daya hantar air horison tanah
4. Jumlah air yg ditahan profil tanah pd kondisi
kapasitas lapang

Keempat faktor di atas ditentukan oleh struktur dan tekstur tanah
Tanah berpasir punya kapasitas ilfiltrasi dan daya hantar air sangat
tinggi, kemampuan menahan air rendah, shg perkolasinya mudah
dan cepat

Tanah tekstur halus, umumnya perkolasinya rendah dan sangat
beragam; faktor lain yg berpengaruh:
1. Bahan liat koloidal dpt menyumbat pori mikro & medium
2. Liat tipe 2:1 yang mengembang-mengkerut sangat berperan
46

LAJU
GERAKAN
AIR TANAH

Kecepatan gerakan air dlm tanah dipengaruhi oleh dua faktor:
1. Daya dari air yang bergerak
2. Hantaran hidraulik = Hantaran kapiler = daya hantar
i = k.f
dimana i = volume air yang bergerak; f = daya air yg bergerak dan k =
konstante.

Daya air yg bergerak = daya penggerak, ditentukan oleh dua faktor:
1. Gaya gravitasi, berpengaruh thd gerak ke bawah
2. Selisih tegangan air tanah, ke semua arah
Gerakan air semakin cepat kalau perbedaan tegangan semakin tinggi.

Hantaran hidraulik ditentukan oleh bbrp faktor:
1. Ukuran pori tanah
2. Besarnya tegangan untuk menahan air
Pada gerakan jenuh, tegangan airnya rendah, shg hantaran hidraulik berbanding
lurus dengan ukuran pori
Pd tanah pasir, penurunan daya hantar lebih jelas kalau terjadi penurunan kandungan
air tanah
Lapisan pasir dlm profil tanah akan menjadi penghalang gerakan air tidak jenuh
47

Gerakan air
tanah

Gerakan air tanah dipengaruhi oleh kandungan
air tanah

Penetrasi air dari tnh basah ke tnh kering
(cm)
18
Tanah lembab, kadar air awal 29%

Tanah lembab, kadar air awal 20.2%

Tanah lembab, kadar air awal 15.9%
0
26

156
Jumlah hari kontak, hari

Sumber: Gardner & Widtsoe, 1921.

48

GERAKAN
UAP AIR

Penguapan air tanah terjadi internal (dalam pori tanah) dan eksternal (di
permukaan tanah)
Udara tanah selalu jenus uap air, selama kadar air tanah tidak lebih
rendah dari koefisien higroskopis (tegangan 31 atm).

Mekanisme Gerakan uap air
Difusi uap air terjadi dlm udara tanah, penggeraknya adalah perbedaan tekanan uap
air.
Arah gerapan menuju ke daerah dg tekanan uap rendah

Pengaruh suhu dan lengas tanah terhadap gerapan uap air dalam tanah

Lembab Dingin

Kering

Dingin

Kering Panas

Lembab Panas
49

RETENSI AIR
TANAH

KAPASITAS RETENSI MAKSIMUM adalah:
Kondisi tanah pada saat semua pori terisi penuh air, tanah jenuh
air, dan tegangan matrik adalah nol.
KAPASITAS LAPANG: air telah meninggalkan pori makro, mori
makro berisi udara, pori mikro masih berisi air; tegangan matrik
0.1 - 0.2 bar; pergerakan air terjadi pd pori mikro/ kapiler

KOEFISIEN LAYU: siang hari tanaman layu dan malam hari segar kembali,
lama-lama tanaman layu siang dan malam; tegangan matrik 15 bar.
Air tanah hanya mengisi pori mikro yang terkecil saja, sebagian besar air
tidak tersedia bagi tanaman.
Titik layu permanen, bila tanaman tidak dapat segar kembali

KOEFISIEN HIGROSKOPIS
Molekul air terikat pada permukaan partikel koloid tanah, terikat kuat
sehingga tidak berupa cairan, dan hanya dapat bergerak dlm bentuk uap air,
tegangan matrik-nya sekitar 31 bar.
Tanah yg kaya bahan koloid akan mampu menahan air higroskopis lebih
banyak dp tanah yg miskin bahan koloidal.
50

Klasifikasi Air
Tanah

Klasifikasi Fisik:
1. Air Bebas (drainase)
2. Air Kapiler
3. Air Higroskopis

Air Bebas (Drainase):
a. Air yang berada di atas kapasitas lapang
b. Air yang ditahan tanah dg tegangan kurang dari 0.1-0.5 atm
c. Tidak diinginkan, hilang dengan drainase
d. Bergerak sebagai respon thd tegangan dan tarika gravitasi bumi
e. Hara tercuci bersamanya
AIR KAPILER:
a. Air antara kapasitas lapang dan koefisien higroskopis
b. Tegangan lapisan air berkisar 0.1 - 31 atm
c. Tidak semuanya tersedia bagi tanaman
d. Bergerak dari lapisan tebal ke lapisan tipis
e. Berfungsi sebagai larutan tanah
AIR HIGROSKOPIS :
a. Air diikat pd koefisien higroskopis
b. Tegangan berkisar antara 31 - 10.000 atm
c. Diikat oleh koloid tanah
d. Sebagian besar bersifat non-cairan
e. Bergerak sebagai uap air

51

Agihan air
dalam tanah

Berdasarkan tegangan air tanah dapat dibedakan menjadi tiga bagian:
Air bebas, kapiler dan higroskopis

Koef. Higroskopis
kurang lebih 31 atm

Kap. Lapang
kurang lebih 1/3 atm

Jml ruang pori
Lapisan olah

Air higroskopik
Hampir tdk
menunjukkan
sifat cairan

Air Kapiler
Peka thd gerakan
kapiler, laju penyesuaian meningkat dg meningkatnya kelembaban tanah

Ruang diisi udara
Biasanya jenuh uap air
Setelah hujan lebat
sebagian diisi air,
tetapi air cepat hilang krn gravitasi
bumi
Lapisan bawah tanah
Karena pemadatan ruang
pori berkurang

Strata bawah (jenuh air)

Kolom tanah

Jumlah ruang pori
52

Klasifikasi berdasarkan ketersediaannya bagi tanaman:
1. AIR BERLEBIHAN: air bebas yg kurang tersedia bagi tanaman.
Kalau jumlahnya banyak berdampak buruk bagi tanaman, aerasi
buruk, akar kekurangan oksigen, anaerobik, pencucian air

Klasifikasi
Biologi
Air tanah

2. AIR TERSEDIA: air yg terdapat antara kap. Lapang dan koef. Layu.
Air perlu ditambahkan untuk mencapai pertumbuhan tanaman yang
optimum apabila 50 - 85% air yg tersedia telah habis terpakai.
Kalau air tanah mendekati koefisien layu, penyerapan air oleh akar tanaman
tdk begitu cepat dan tidak mampu mengimbangi pertumbuhan tanaman

3. AIR TIDAK TERSEDIA: AIR yg diikat oleh tanah pd TITIK LAYU permanen,
yaitu air higroskopis dan sebagian kecil air kapiler.

KH
31 atm

KL

KP

15 atm

1/3 atm

Air
Higroskopis

Air
Kapiler

Tdk tersedia

Tersedia

100 % pori

Ruang udara dan
air drainase

Berlebihan
Daerah Optimum

53

Faktor yg berpengaruh:
1. Hubungan tegangan dengan kelengasan
2. Kedalaman tanah
3. Pelapisan Tanah

Faktor yg
mempengaruhi
Air Tersedia

TEGANGAN MATRIK : tekstur, struktur dan kandungan bahan organik
mempengaruhi jumlah air yg dapat disediakan tanah bagi tanaman
TEGANGAN OSMOTIK: adanya garam dalam tanah meningkatkan tegangan
osmotik dan menurunkan jumlah air tersedia, yaitu menaikkan koefisien layu.

Persen air

Sentimeter air setiap 30 cm tanah
10

18

Kap. Lapang
Air tersedia
Koef. Layu

6

5
Air tidak tersedia

Pasir Sandy loam

Loam

Silty-loam Clay-loam

Liat

Tekstur semakin halus
54

SUPLAI AIR
ke TANAMAN

Dua proses yg memungkinkan akar tanaman mampu menyerap air dlm
jumlah banyak, yaitu:
1. Gerakan kapiler air tanah mendekati permukaan akar penyerap
2. Pertumbuhan akar ke arah zone tanah yang mengandung air

LAJU GERAKAN KAPILER
Bulu akar
menyerap
air

Gerakan
kapiler
2.5 cm
sagt penting

Jumlah
air tanah
berkurang

Laju gerakan
tgt perbedaan
tegangan dan daya
hantar pori tanah

Tegangan
air tanah
meningkat
Terjadi
gerakan kapiler
air menuju
bulu akar

Terjadi
perbedaan
Tegangan
dg air tanah di
sekitarnya

LAJU PERPANJANGAN AKAR
Selama masa pertumbuhan tanaman, akar tanaman tumbuh memanjang dengan
cepat, sehingga luas permukaan akar juga tumbuh terus.
Jumlah luas permukaan akar penyerap yang bersentuhan langsung dengan
sebagian kecil air tanah (yaitu sekitar 1-2%)
55

HADANGAN HUJAN OLEH TUMBUHAN
Tajuk tumbuhan mampu menangkap sejumlah air hujan, sebagian air ini
diuapkan kembali ke atmosfer.
Vegetasi hutan di daerah iklim basah mampu menguapkan kembali air
hujan yg ditangkapnya hingga 25%, dan hanya 5% yg mencapai tanah
melalui cabang dan batangnya.

KEHILANGAN
UAP AIR
DARI TANAH

Awan hujan

presipitasi

Pembentukan Awan

transpirasi
evaporasi

Run off

infiltrasi
Tanah permukaan
perkolasi
Batuan

Groundwater

Sungai - laut
56

Hadangan hujan
oleh tanaman
semusim

Sekitar 5 - 25% dari curah hujan dihadang tanaman dan dikembalikan
ke atmosfer.
Besarnya tergantung pada kesuburan tanaman dan stadia pertumbuhan
tanaman .
Dari curah hujan 375 mm, hanya sekitar 300-350 mm yang mencapai
tanah.

Hadangan curah hujan oleh jagung dan kedelai
Keadaan hujan

Persen dari curah hujan total untuk:
Jagung
Kedelai

Langsung ke tanah
Melalui batang

70.3
22.8

65.0
20.4

Jumlah di tanah
Yang tinggal di atmosfer

93.1
6.9

85.4
14.6

Sumber: J.L.Haynes, 1940.

57

HUBUNGAN ENERGI LTTA:
Perubahan tegangan air pd saat bergerak dari tanah melalui akar, batang, daun , ke atmosfer

Atmosfer
Daun

Batang

Akar

Tanah berkadar air rendah

500

300

100

25

20

Tanah berkadar air tinggi

15

10

5

Tanah

0

Potensial negatif air (Tegangan air)
58

EVAPOTRANSPIRASI

Kehilangan uap air dari tanah:
1. EVAPORASI: penguapan air dari permukaan tanah
2. TRANSPIRASI: Penguapan air dari permukaan tanaman
3. EVAPOTRANSPIRASI = Evaporasi + Transpirasi
Laju penguapan air tgt pd perbedaan potensial air = selisih tekanan uap
air = perbedaan antara tekanan uap air pd permukaan daun (atau
permukaan tanah) dengan atmosfer

Faktor Iklim dan Tanah:
1. Energi Penyinaran
2. Tekanan uap air di atmosfer
3. Suhu
4. Angin
5. Persediaan air tanah
Air tanah
Jagung
Tinggi
Sedang

17.7
12.7

Evapotranspirasi (cm:
Medicago sativa
24.4
20.5

Sumber: Kelly, 1957.

59

Ketersediaan Air
Tanah vs
Evapotranspirasi

Ketersediaan air di daerah perakaran sangat menentukan besarnya
evapotranspirasi.
Kedalaman daerah perakaran tanaman 50 - 60 cm.
Air tanah pada lapisan olah mengalami pengurangan karena evaporasi
permukaan
Air tanah pd lapisan bawah mengalami pengurangan karena diserap
akar tanaman

Kedalaman tanah (cm)

0 - 17.5
17.5 - 180.0

Evapotranspirasi (cm):
Jagung Padang Rumput

Hutan

24.25
20.75

23.27
22.25

23.45
21.17

Sumber: Dreibelbis dan Amerman, 1965.

60

PEMAKAIAN
KONSUMTIF
(PK)

Pemakaian Konsumtif merupakan jumlah kehilangan air melalui
evaporasi dan transpirasi.
Lazim digunakan sebagai ukuran dari seluruh air yg hilang dari tanaman
melalui evapotranspirasi
Ini merupakan angka-praktis untuk keperluan pengairan

Dua faktor penting yg menentukan PK adalah:
1. KEDALAMAN PERAKARAN TANAMAN
2. FASE PERTUMBUHAN TANAMAN
PK dapat berkisar 30 - 215 cm atau lebih:
1. Daerah basah - semi arid dg irigasi: 37.5 - 75 cm.
2. Daerah panas dan kering dg irigasi: 50 - 125 cm.
EVAPORASI vs TRANSPIRASI
Faktor yg berpengaruh adalah:
1. Perbandingan luas tutupan tanaman thd luas tanah
2. Efisiensi pemakaian air berbagai tanaman
3. Perbandingan waktu tanaman berada di lapangan
4. Keadaan iklim
Di daerah basah : EVAPORASI  TRANSPIRASI
Di daerah kering:
1. EVAPORASI  70 - 75 % dari seluruh hujan yg jatuh
2. TRANSPIRASI  20 - 25%
3. RUN OFF  5%

61

WUE : Water Use
Efficiency

WUE  Produksi tanaman yg dapat dicapai dari pemakaian sejumlah air
tersedia
WUE dapat dinyatakan sbg:
1. Pemakaian konsumtif (dalam kg) setiap kg jaringan tanaman yg
dihasilkan
2. Transpirasi (dalam kg) setiap kg jaringan tanaman yg dihasilkan
……… NISBAH TRANSPIRASI

Jumlah air yg diperlukan untuk menghasilkan 1 kg
bahan kering tanaman
NISBAH TRANSPIRASI
Untuk tanaman di daerah humid: 200 - 500, di daerah arid duakalinya
Tanaman

Nisbah Transpirasi

Beans
Jagung
Peas
Kentang

209 - 282 - 736
233 - 271 - 368
259 - 416 - 788
385 - 636

Sumber: Lyon, Buckman dan Brady, 1952.
62

Faktor yang mempengaruhi WUE: Iklim, Tanah, dan Hara
WUE tertinggi lazimnya terjadi pd tanaman yg berproduksi
optimum;
Adanya faktor pembatas pertumbuhan akan menurunkan WUE

FAKTOR
WUE

Nisbah evapo-transpirasi tanaman di lokasi yg mempunyai defisit kejenuhan dari
atmosfer
800
Kentang
Kacang polong
400
Jagung
0
0

Defisit kejenuhan dari atmosfer (mm Hg)

12

14

Jumlah air unt menghasilkan 1 ton bahan kering
30
Kadar air tanah rendah

15
Kadar air tanah tinggi

0

63

0

Pupuk P, kg/ha

600

Pengendalian
Penguapan

MULSA & PENGELOLAAN
Mulsa adalah bahan yg dipakai pd permukaan tanah untuk mengurangi
penguapan air atau untuk menekan pertumbuhan gulma.
Lazimnya mulsa spt itu digunakan untuk tanaman yang tidak
memerlukan pengolahan tanah tambahan

MULSA KERTAS & PLASTIK
Bahan mulsa dihamparkan di permukaan tanah, diikat spy tdk terbang, dan tanaman
tumbuh melalui lubang-lubang yg telah disiapkan
Selama tanah tertutup mulsa, air tanah dapat diawetkan dan pertumbuhan gulma
dikendalikan

MULSA SISA TANAMAN
Bahan mulsa berasal dari sisa tanaman yg ditanam sebelumnya, misalnya jerami padi,
jagung, dan lainnya
Bahan mulsa dipotong-potong dan disebarkan di permukaan tanah
Cara WALIK DAMI sebelum penanaman kedelai gadu setelah padi sawah
MULSA TANAH  Pengolahan tanah
Efektivitas mulsa tanah dalam konservasi air-tanah (mengendalikan evaporasi) masih
diperdebatkan, hasil-hasil penelitian masih snagat beragam
64

Olah Tanah vs
Penguapan Air
Tanah

Alasan pengolahan tanah:
1. Mempertahankan kondisi fisika tanah yg memuaskan
2. Membunuh gulma
3. Mengawetkan air tanah.

Pengendalian Penguapan vs Pemberantasan Gulma
Perlakuan

Hasil jagung (t/ha)

Tanah dibajak dg persiapan yg baik
1. Dibebaskan dari gulma
2. Gulma dibiarkan tumbuh
3. Tiga kali pengolahan dangkal
Persiapan Buruk
4. Dibebaskan dari gulma

Kadar air tanah (%)
hingga kedalaman 1 m

2.9
0.4
2.5

22.3
21.8
21.9

2.0

23.1

Sumber: Mosier dan Gutafson, 1915.

Pengolahan tanah yg dapat mengendalikan gulma dan memperbaiki kondisi fisik tanah akan
berdampak positif thd produksi tanaman
Pengolahan tanah yg berlebihan dapat merusak akar tanaman dan merangsang evaporasi,
shg merugikan tanaman

65

Beberapa proses penting dalam siklus air:

Precipitation is condensed water vapor that falls to the
Earth's surface.
Most precipitation occurs as rain, but also includes snow,
hail, fog drip, graupel, and sleet.
Approximately 505,000 km³ of water fall as precipitation
each year, 398,000 km³ of it over the oceans.

66

Canopy interception
is the precipitation that is
intercepted by plant
foliage and eventually
evaporates back to the
atmosphere rather than
falling to the ground.

67

LIMPASAN = Runoff

includes the variety of ways by
which water moves across the land. This includes both surface
runoff and channel runoff.
As it flows, the water may infiltrate into the ground, evaporate
into the air, become stored in lakes or reservoirs, or be extracted
for agricultural or other human uses.

Infiltration is the flow of water from the ground surface into
the ground.
Once infiltrated, the water becomes soil moisture or groundwater.

68

Subsurface Flow is the flow of water underground, in
the vadose zone and aquifers. Subsurface water may return
to the surface (eg. as a spring or by being pumped) or
eventually seep into the oceans.
Water returns to the land surface at lower elevation than
where it infiltrated, under the force of gravity or gravity
induced pressures.
Groundwater tends to move slowly, and is replenished
slowly, so it can remain in aquifers for thousands of years.

69

Evaporation

is the transformation of water from liquid to gas
phases as it moves from the ground or bodies of water into the
overlying atmosphere.
The source of energy for evaporation is primarily solar radiation.
Evaporation often implicitly includes transpiration from plants,
though together they are specifically referred to as
evapotranspiration.

Approximately 90% of atmospheric water comes from evaporation,
while the remaining 10% is from transpiration. Total annual
evapotranspiration amounts to approximately 505,000 km³ of water,
434,000 km³ of which evaporates from the oceans.
70

SUBLIMASI is the state change directly from solid
water (snow or ice) to water vapor.

ADVEKSI is the movement of water — in solid, liquid,
or vapour states — through the atmosphere. Without
advection, water that evaporated over the oceans could not
precipitate over land.

KONDENSASI is the transformation of water vapour
to liquid water droplets in the air, producing clouds and
fog.
71

Aktivitas manusia yang dapat mempengaruhi siklus air :
Pertanian
Alteration of the chemical composition of the atmosphere
Construction of dams
Deforestation and afforestation
Removal of groundwater from wells
Water abstraction from rivers
Urbanization .

72

KAPASITAS PENYIMPANAN AIR:
WATER HOLDING CAPACITY
Soil "holds" water available for crop use, retaining it against the pull
of gravity.
This is one of the most important physical facts for agriculture.
If the soil did not hold water, if water was free to flow downward with
the pull of gravity as in a river or canal, we would have to constantly
irrigate, or hope that it rained every two or three days.
There would be no reason to pre-irrigate. And there would be no such
thing as dryland farming.

73

Soil Moisture Level (Depletion, %) vs. Soil Moisture Tension (Bars).

74

Hubungan antara Potensial Air
Tanah dnegan Air Tersedia pada
tiga macam tekstur tanah

75

The soil's ability to hold water depends on both the soil texture
and structure.
Texture describes the relative percentages of sand, silt, and clay
particles.
The finer the soil texture (higher percentage of silt and clay), the
more water soil can hold.
Gravity is always working to pull water downwards below the
plant's root zone.
To counteract the pull of gravity, soil is able to generate its own
forces, commonly called "matric forces" ("matric" because of
the soil "matrix" structure that forms the basis for the forces).

76

An important fact about the soil's water-holding forces is that as
the level of soil moisture goes down, the soil generates more force.
This is the reason that some water will move up into the root zone
from a shallow ground water table. As the plant extracts water in
the root zone, the soil pulls water up from the area with more water
to the area with less.
As you would expect, the rate at which the water-holding forces go
up with decreasing soil moisture is different for different soils. In a
coarse soil, they will go up slowly.
This means that plants can extract a great amount of water from
coarse soils before they stress. In contrast, these forces rise quickly
in finer soils.
77

Graphically, the relationship can be described by the Figure SWP-1.
Looking at the lowest line for a coarse soil.
You can see that at A, the soil moisture level is very high and the
water-holding forces are low.
This means that the plant can extract water easily from the soil.
At B, the soil moisture level is lower but the water-holding forces
haven't gone up that much.
The plant can still extract water easily.
However at C, the soil moisture level is very low and the waterholding forces have increased greatly.
The plant cannot extract water easily and will be stressed.
78

Looking at the top line for a finer soil.
At A, as with the coarse soil, the water-holding forces are low
when the soil moisture level is high.
However, at B, the soil moisture level has dropped somewhat but
the water-holding forces have gone up greatly.
And at C, where the soil moisture level is low, the water-holding
forces have gone up very high.
We will be coming back to this idea of increasing soil waterholding forces with decreasing soil moisture many times
79

HUBUNGAN TANAH-AIR
The role of soil in the soil-plant-atmosphere continuum is unique.
It has been demonstrated that soil is not essential for plant growth
and indeed plants can be grown hydroponically (in a liquid culture).
However, usually plants are grown in the soil and soil properties
directly affect the availability of water and nutrients to plants.
Soil water affects plant growth directly through its controlling effect
on plant water status and indirectly through its effect on aeration,
temperature, and nutrient transport, uptake and transformation.
The understanding of these properties is helpful in good irrigation
design and management.
80

The soil system is composed of
three major components: solid
particles (minerals and organic
matter), water with various
dissolved chemicals, and air.
The percentage of these
components varies greatly with
soil texture and structure.
An active root system requires a
delicate balance between the
three soil components; but the
balance between the liquid and
gas phases is most critical, since
it regulates root activity and
plant growth process.
81

The amount of soil water is
usually measured in terms of
water content as percentage
by volume or mass, or as soil
water potential.

Jumlah air tersedia dipengaruhi
tekstur tanah

Water content does not
necessarily describe the
availability of the water to
the plants, nor indicates, how
the water moves within the
soil profile.
The only information
provided by water content is
the relative amount of water
in the soil.
82

Soil water potential, which is
defined as the energy required
to remove water from the soil,
does not directly give the
amount of water present in the
root zone either.
Therefore, soil water content
and soil water potential should
both be considered when
dealing with plant growth and
irrigation.
The soil water content and soil
water potential are related to
each other, and the soil water
characteristic curve provides a
graphical representation of this
relationship.
83

The nature of the soil characteristic curve depends on the physical
properties of the soil namely, texture and structure. Soil texture refers
to the distribution of the soil particle sizes.
The mineral particles of soil have a wide range of sizes classified as
sand, silt, and clay.
The proportion of each of these particles in the soil determines its
texture.
All mineral soils are classified depending on their texture. Every soil
can be placed in a particular soil group using a soil textural triangle .
For example a soil with 60% sand and 10% clay separates is
classified as a Sandy loam
84

Kapasitas Lapangan
Field Capacity
There are limits on the amount of water that soil holds for crop use.
The upper limit is termed "field capacity".
During an irrigation, or whenever excess water is added to soil, water
drains down through the soil due to the pull of gravity.
At first, this internal drainage is relatively rapid.
However, it soon slows to almost nothing.
(The increasing soil water-holding forces finally start to counteract
gravity.) At this point we would say the soil is at field capacity.
85

You can demonstrate field capacity using a visualization of a sponge
(like soil, a porous material that will hold water).
Using a pan of water, hold a sponge under water until it is saturated.
Now, pull the sponge out of the water.
It will immediately start to drip water, quickly at first, then slower
and slower.
At some point it will essentially stop dripping.
The internal drainage has stopped and the sponge is at field capacity.
It is very important to note that you can soak more water into soil
that is already at field capacity.
There will be open soil pores that will take the water. However, the
excess water will not be held.
It will just drain down until the soil moisture returns to field capacity.
86

You can use the sponge again to demonstrate this important fact.
With the sponge at "field capacity", use a cup to pour water on it.
The water will soak in, there will be open pores in the sponge that will
take in water. But you will see that the sponge starts dripping again
as the excess water starts to drain off the bottom.
Because of this ability to hold water against the pull of gravity, soil
does not act like a bathtub during irrigations.
That is, irrigation water does not have to go to some "bottom" and
then fill back up to the top. Rather soil fills to field capacity from the
top down.

87

Field capacity
is a soil-based concept.
That is, it depends on the
texture and structure of the
soil as well as the physical
conditions in the field.
Coarse soils have lower field
capacities than fine soils.
If there is a high water table
or severe stratification that
would restrict drainage, the
field capacity would be higher
than normal.
88

AIR TERSEDIA & ZONE AKAR EFEKTIF
The water held by the soil between field capacity and permanent
wilting point is termed the "available water holding capacity" of the
soil.
It is water that is "available" for the plant to use. Water added to the
soil in excess of field capacity will drain down, below the active root
system.
Water held by the soil that is below the permanent wilting point is of
no use, the plant has died.
As a crop manager you are concerned with the soil moisture
throughout the depth of the plant's active root system, the "effective
root zone".
89

The effective root zone is that depth of soil where you want to control
soil moisture (just as you control fertility and weed/pest pressures).
The effective root zone may or may not be the actual depth of all
active roots. It may be shallower because of concerns for crop quality
or development (as with many vegetable crops).
For a pre-irrigation though, you may want to consider the maximum
potential root zone as the effective root zone for that irrigation.
For example, with cotton you may estimate the effective root zone as
6 feet for a preirrigation, 2 feet for the first seasonal irrigation, 4 feet
for the second seasonal, and 6 feet thereafter. For an almond orchard,
you may estimate the effective root zone as four feet for the entire
season. With onions, the major concern is with the top 2 feet.
90

Hubungan Air – Tanah
The soil is composed of three major parts: air, water, and solids .
The solid component forms the framework of the soil and consists
of mineral and organic matter.
The mineral fraction is made up of sand, silt, and clay particles.
The proportion of the soil occupied by water and air is referred to
as the pore volume.
The pore volume is generally constant for a given soil layer but
may be altered by tillage and compaction. The ratio of air to water
stored in the pores changes as water is added to or lost from the
soil. Water is added by rainfall or irrigation, as shown in Figure 2.
Water is lost through surface runoff, evaporation (direct loss from
the soil to the atmosphere), transpiration (losses from plant tissue),
and either percolation (seepage into lower layers) or drainage.
91

The pore volume is actually a reservoir for holding water. Not all of
the water in the reservoir is available for plant use.
Figure 3 represents a "wet" (saturated) soil immediately after a large
rainfall.
Note that all of the pores are filled with water. Gravity will pull some
of this water down through the soil below the crop's root zone.
The water that is redistributed below the root zone due to the force of
gravity is gravitational water. In general, gravitational water is not
available to plants, especially in sandy soils, because the
redistribution process occurs quickly (in two days or less).

92

Kapan tanah perlu ditambah air agar tanaman tidak terganggu pertumbuhannya?
Jelaskan pendapat Saudara dnegan 250 kata?
93

Sumber dan perilaku air yang ditambahkan ke tanah
94

Saturated (wet) soil. All pores (light areas) are filled with water. The dark areas
represent soil solids.
95

Water distribution in a soil at field capacity. Capillary water (lightly shaded
areas ) in soil pores is available to plants. Field capacity represents the upper
96
limit of plant-available water.

Water distribution in a soil at thw wilting point. This water is held tightly in thin
films around soil particles and is unavailable to plants. The wilting point
represents the lower limit of plant-available water.
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Plant-available water, PAW, adalah volume air
yang disimpan dalam tanah yang dapat
digunakan oleh tanaman .
It is the difference between the volume of water
stored when the soil is at field capacity and the
volume still remaining when the soil reaches
the permanent wilting point (the lower limit), as
shown in Figure 6.

98

Figure 6. HUBUNGAN ANTARA AIR-TERSEDIA DAN DISTRIBUSI AIR
DALAM TANAH .

99

Kapasitas tanah menyimpan air

100

Jumlah air tanah pada tiga macam tekstur tanah

101

Tabel 1. Jumlah air tersedia dalam tanah yang teksturnya
berbeda-beda

102

AIR-TANAH dan CEKAMAN (stres) TANAMAN
Kalau tanaman menyerap air dari tanah , jumlah air tersedia yang tersisa
dalam tanah menjadi berkurang.
The amount of PAW removed since the last irrigation or rainfall is the
depletion volume.
Irrigation scheduling decisions are often based on the assumption that
crop yield or quality will not be reduced as long as the amount of water
used by the crop does not exceed the allowable depletion volume.
The allowable depletion of PAW depends on the soil and the crop. For
example, consider corn growing in a sandy loam soil three days after a
soaking rain.
Even though enough PAW may be avai1able for good plant growth, the
plant may wilt during the day when potential evapotranspiration (PET) is
high.
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AIR-TANAH dan CEKAMAN (stres) TANAMAN
Evapotranspiration merupakan proses hilangnya air tanah ke atmosfer,
melalui evaporasi dari permukaan tanah dan proses transpirasi dari
tanaman yang tumbuh di tanah .
Potential evapotranspiration is the maximum amount of water that could
be lost through this process under a given set of atmospheric conditions,
assuming that the crop covers the entire soil sur- face and that the
amount of water present in the soil does not limit the process.
Potential evapotranspiration is controlled by atmospheric conditions and
is higher during the day. Plants must extract water from the soil that is
next to the roots.
As the zone around the root begins to dry, water must move through the
soil toward the root (Figure 7). Daytime wilting occurs because PET is
high and the plant takes up water faster than the water can be replaced.
104

Gambar.
Kalau tanaman
menyerap air, tanah di
sekitar perakaran
menjadi mengering .
If the rate of water
movement from moist
zones is less than the
PET, the plant
temporarily wilts.

105

Pada malam hari, pada saat PET menurun hingga
mendekati nol , air tanah bergerak dari tanah yang lebih
basah memasuki zone tanah yang lebih kering di sekitar
akar tanaman.
The plant recovers turgor and wilting ceases (Figure 8).
This process of wilting during the day and recovering at
night is referred to as temporary wilting.

Proper irrigation scheduling reduces the length of time a
crop is temporarily wilted.
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Gambar .
At night when the
PET is low, the plant
recovers from
wilting as water
moves from moist
zones (dark areas)
to eliminate the dry
zones around the
roots.

107

Hubungan antara distribusi air dalam tanah dan konsep jadwal irigasi
ketika 50 percent air tersedia telah habis
108

FAKTOR TANAMAN
Three plant factors must be considered in developing a
sound irrigation schedule: the crop's effective root depth, its
moisture use rate, and its sensitivity to drought stress (that
is, the amount that crop yield or quality is reduced by drought
stress).

KEDALAMAN EFEKTIF AKAR
Rooting depth is the depth of the soil reservoir that the plant
can reach to get PAW. Crop roots do not extract water
uniformly from the entire root zone. Thus,the effective root
depth is that portion of the root zone where the crop extracts
the majority of its water. Effective root depth is determined by
both crop and soil properties.
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PENGARUH TANAMAN thd KEDALAMAN EFEKTIF AKAR
Different species of plants have different potential rooting depths.
The potential rooting depth is the maximum rooting depth of a crop when grown
in a moist soil with no barriers or restrictions that inhibit root elongation.
Potential rooting depths of most agricultural crops important in North Carolina
range from about 2 to 5 feet. For example, the potential rooting depth of corn is
about 4 feet.
Water uptake by a specific crop is closely related to its root distribution in the soil.
About 70 pe