Soil fertility Patma Vityakon Rambo, PhD

Transcription

1 Land Resources and Environment in Sustainable Agriculture Soil fertility Patma Vityakon Rambo, PhD

2 Soil fertility studies Soil fertility is the study of soils as a source of plant nutrients. The objectives of studying soil fertility is to learn about behaviour of plant nutrients in soils, and how soils supply nutrients to plants. To achieve the above objectives, two major concepts are employed.

3 Concepts employed for soil fertility study: Soil is a colloidal system. The availability concept: Available forms of nutrients and nutrient availability.

4 Nutrien t ions in soil solutio n Soil colloid Soil solution Nutrien t ions adsorbe d to soil colloid Nutrient absorption by root Nutrient ion being transported to root R o O O T Figure1. Profile of soil colloidal system as related to soil nutrients and

5 The availability concept The words used most commonly to describe the supply the plant nutrients in soil are available and availability. Available means susceptible to absorption by plants. Availability means effective quantity (Black, 1993)

6 Available forms of nutrients Nutrients exist in soils in different forms, such as water soluble, adsorbed, and fixed forms. These nutrients become available to plants to different degrees but with time it can be said that all will become available. However, the most readily available form is those in soil solution (water soluble form).

7 Availability factors The ability of soils to supply nutrients to plants is affected by the following factors: - Intensity - Quantity - Buffering capacity - Mobility

8 Further reading on the availability concept Black, C.A Soil fertility evaluation and control. Lewis Publishers. 746 p.

9 Nitrogen in soils

10 Table 1. Distribution of N in atmosphere, geosphere and biosphere Pools of N Total mass % total N mass (x ) Atmosphere (N gas) Hydrosphere (soluble N) Lithosphere 19,085 >98 Igneous rock 19,000 Biosphere Terrestrial ecosys (N in soil, plants) Marine ecosystem Adapted from Haynes (1986a), Stevenson (1986)

11 Global Fluxes of Nitrogen into and out of the Terrestrial Biosphere Proces Inp Wet uts and dry deposition Wet and dry (NH 3 Wet )/NH deposition and 4+ ) dry (NO x ) deposition Atmospheric (organic N fixation Biological ) (lightning ) Industrial fixation fixation s (Tg rate N yr -1 )

12 Process Out Ammonia puts volatilization Denitrification Biogenic (N 2 + N 2 O) NO x production Fossil fuel Fires burning (NO (NO x ) x ) (Tg N rate yr -1 ) Leaching and runoff (inorganic) Leaching and Source: Haynes (1986) runoff (organic)

13 Table 2. Distribution of N in a nutrient-poor Amazonian rain forest, an oak-hickory forest, a short grass Amazo prairie, Oakhickory fore grass prai owtun and a wet Short meadow Mead tundra ecosystem. nian fore Nitrogen kg st N st % kg rie N % kg dra N % Total pool vegetatio Above nground Roots Litte r Soil (sampling depth Totain cm) l ha ha % kg N (75 cm) Source: Haynes (1986) (60 cm) ha (30 cm) ha (20 cm)

14 Figure 2. The

15 Table 3. Global fluxes of nitrogen into and out of the terrestrial biosphere. Process rate (Tg N yr -1 Input ) s Wet and dry deposition (NH 3 )/NH Wet ) and dry deposition (NO x ) Wet and dry deposition (NO x ) 10- Atmospheric fixation (lightning ) Biological fixation Industrial fixation (fertilizers) 6 0 Outputs Ammonia volatilization Denitrification (N 2 + NBiogenic 2 O) NO x production Fossil fuel burning (NO x ) Fires (NO x ) Leaching and runoff (inorganic) Leaching and runoff (organic)

16 Nitrogen cycling in different ecosystems Natural ecosystems, e.g. forest Agricultural ecosystems

17 Return of plant residues Run 41 N off 16 N Plant tops and roots Soil reserves Plant uptake 126 N Product removal 85 N Gaseous losses 15 N Fertilizer 112 N Irrigatio n 10 N Leachin g Figure 3. N cycle in 15 a Ncorn crop in USA

18 Forms of N in soils Organic nitrogen: More than 90% of total N in soils is organic N. Inorganic nitrogen: Major forms: NH 4+ (soluble, exchangeable, non-exchangeable) NO - 3 (soluble, and exchangeable)

19 Nutrien t ions in soil solutio n Soil colloid Soil solution Nutrien t ions adsorbe d to soil colloid Nutrient absorption by root Nutrient ion being transported to root R o O O T Figure1. Profile of soil colloidal system as related to soil nutrients and

20 Table 4. Pools of soil organic N as chemically separated by acid hydrolysis. Soil N Quantity pools Acid-insoluble or (% soil N) non-hydrolysable Acid hydrolysa - ble -NH Amino 3 - Nacid - Amino sugar - Hydrolysable unknown-n or HUN Source: Stevenson

21 Fig Hypothetical structure of humic acid. Nitrogen is incorporating into the humic acid in three ways: (a) as a bridge unit, (b) in the form of N phenylamino acid, (c) in the

22 Soil processes which transform organic N to inorganic N N mineralization : organic N is transformed to inorganic N. N immobilization: inorganic N is transformed to organic N. Both are microbially mediated. Mineralization Immobilization

23 Nitrogen mineralization and immobilization occurs simultaneously in soils but in opposite directions. Higher N mineralization relative to N immobilization results in net N mineralization and vice versa. This is affected by the relationships between energy and nutrient nitrogen in decomposing organic materials.

24 Carbon to nitrogen ratio The C/N ratio is used as an indicator of the likelihood of net N mineralization or net N immobilization taking place upon decomposition of organic materials or soil organic matter in question. The C/N ratio estimates the energy/n ratio of organic materials.

25 Figure 4. Schematic diagram of the different forms of cations associated with a 2:1 clay mineral.

26 Loss of nitrogen from soils This can occur through different processes: Ammonia volatilization (Sherlock, 1986) Denitrification Leaching (especially of nitrate) (Cameron and Haynes, 1986)

27 Adsorbed NH 4 + (1 ) NH 4+ (aq) in soil solution (2 ) NH 3+ (aq) in soil solution (3 ) NH 3 (g) gas in soil (4 ) NH 3 (g) gas in Fig. 5. The various equilibria that atmosphere govern ammonia loss from soils

28 Fig 2. Hierachical class of soil pore space accoding to Elliott and Coleman(1988). 1= macropores, 2= pore between

29 Nitrogen requirement of plants

30 Figure 6. Accumulation and distribution of abovegroud biomass and N in different parts of corn: A. seed, B. cobs, C. tassels, D. leaf sheath, and E. leaves (Goh and Haynes, 1986)

31 Evaluation of nitrogen availability 2 types of soil analysis for this purpose: Evaluation of residual nitrogen in soils, Evaluation of nitrogen mineralization potential (Goh and Haynes, 1986)

32 Table2.10 Chemical extraction methods to determine soil available N Extrac Te (h N tion Mild extraction methods Water 0.01 M 0.01 CaCl 2 M CaCl M 1 NaHCO M KCl 3 2 M KCl 0.01 M CaCl 0.1 M 2 Ba(OH) Source: 2 Goh and mp. 100 C100 C 121 CRoo m100 C80 C 121 C Roo m r) Total NTotal or NH NH + 4-N 4-N Total N or UV absorbance NH + 4 and NH + NO 4and 3 Soluble NO 3 -N Soluble carbohydrate carbohydrate

33 Extrac Te (h N tion Intermediate intensity Alkaline extraction KMnO Na 2 CrO 4 4 +H 3 P ONeutral N Na 1 M 4 PNa 2 O 7 1 OHM Na 1 OHM H Cl 1 M H 2 SO 4 Source: Goh and mp. 100 C100 C C Roo C Roo m mroo m r) NH + NH 4-N + 4-N NH + NH 4-N + NH 4-N + NH 4-N + NH 4-N + 4-N

34 Extrac Te (hr N tion mp ) Intensive extraction NH N Ro H 2 SO M 4 Na Na omoh 8 -N NH + 4- OH K 2 Cr 2 O 7 + H 2 SO 4 distillation Walkleyblack oxid n NH + 4 -N Source: Goh and

35 Further reading on soil nitrogen Haynes, R.J. (ed.) Mineral nitrogen in plant-soil system. Academic Press. New York, London. Stevenson, F.J. (ed.) Nitrogen in agricultural soils. American Society of Agronomy, Madison, Wisconsin, USA. Stevenson, F.J Cycles of soil: Carbon, nitrogen, phosphorus, sulfur, micronutrients. Wiley, New York, 380 p.

36 Soil Organic Matter (SOM)

37 able 3.1 Classification of SOM into livin on-living components. SOM Compon ents of SOM livi ng Compon ents of SOM Living/non -living Quantit % y of livi ng compon C ent % total nonliving Ro ots Macroorganism Micro- Particula sorganisms Hum te OM us

38 Table1. Pools of SOM and nutrients, generalized turnover rates, and hypothesized Po Turn primary controls Pool of pool ol size tim control over size e s Unprotec ted BI (microbial O biomass) LAB (labile) Protecte dco M(colloidal protection) POM (structural protection) 2. yr+, yr+, temperate humid tropics yr+, temperate yr+, humid 100 tropics yr 0 + Depends on Physical disturbance Substrate availability Residue inputs, climate Soil mineralogy, Tillage and texture aggregate disruptio so particle distributi n, il size