Soil fertility - introduction

1

25 20 15 10 5 0

N Nutrient removal by plants

Rice Stover Grain

P K Ca

values are =/- 30%

Mg S

2

Millet 45 40 35 30 25 20 15 10 5 0 N P K values are =/- 30% Leaf Stem Grain Ca Mg S 3

Groundnuts

20 10 0 60 50 40 30 N P K

values are =/- 30% Haulm Shell Kernel

Ca Mg S

4

Yams

40 35 30 25 20 15 10 5 0 N P

values are =/- 30%

K

Tops Tuber

Ca Mg

5

Sources of nutrients

Nitrogen

Nitrogen cycle / bacteria Organic manures Inorganic fertilisers Rain

Other nutrients

Cation exchange Breakdown of rocks Inorganic fertilisers Ash – slash & burn 6

Cattle manure – as an example

1 tonne of manure from a bedded (stall) system contains 5.4 kg N 2.7 kg P 2 O 5 = 1.2 kg P 5.4 kg K 2 O = 4.5 kg K One small cow produces around 5 tonnes manure per year Rice yields are around 2 t ha -1 In theory this would require only 10 tonnes = 2 cows but there are many losses of nitrogen – dealt with in next presentation.

Many poor farmers in India hire their bullocks for ploughing.

Most cattle in developing countries are not stall fed – collection of manure would be a problem.

So would application – 10 tonnes is a lot of **** 7

Losses

Applications may be 3 to 5 times uptake levels because of losses – so for example recommended N fertiliser rate for dryland rice for which yields are unlikely to exceed 2 tonne is 100 kg ha -1 8

Influence of cultivar on uptake

Improved cultivars usually more responsive or more efficient 9

Effect of climate on uptake

     high rainfall leads to large leaching losses low rainfall means soils are dry for much of the time and nutrients cannot move easily within the soil profile higher temperatures lead to high levels of volatisation of nitrogen lower temperatures lead to lower biological activity which effects contribution of bacteria to N moisture and temperature affect soil chemistry 10

Soil factors and management

• soil nutrient status & CEC calcium and phosphate move very slowly in soils – cannot apply after planting texture & depth cultural practices ploughing in residues burning - decreases N & S 11

Importance of clay fraction

• • most chemically active because of surface area & charge consists of: • clay minerals • Fe & Al oxides • organic material 12

Clay affects : • CEC • so nutrient availability • retention of nutrients against leaching • soil physical characteristics – drainage problems 13

Cation exchange capacity 14

15

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Variable charge soils

CEC is made up of a combination of fixed electrical charges on the clay lattices and charges which vary with pH. Soils in which CEC is strongly dependent on pH are called “variable charge soils”. They are very common in the tropics.

Because CEC is pH dependent - must distinguish between CEC (using Ammonium acetate at pH 7.0) & "Effective" CEC (ECEC) using KCl solution and does not change the pH of the soil during measurement Variable charge soils are often “low activity” soils with a low capacity to retain fertilisers / soil nutrients against leaching Leaching causes variable charge soils often to be acidic with a low ECEC, low in exchangeable bases (Ca, Mg, K and Na) 17

Phosphate deficiency

Causes common in tropics because fixed to Fe & Al oxides – also forms complexes with Ca so a problem in calcareous soils, e.g in Middle East Countries such as Iran 18

Phosphorus deficient maize

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Soils of Western Kenya

• Alfisols and oxisols (Rhodudalfs & Eutrudoxs) – High P sorbing but previously fertile • P and N major constraints • 80% of farms are P deficient (< 5 mg kg Olsen) • Farmers know about fertilisers and 40% use DAP but less than optimum rates • ICRAF recommends replenishing P with 250 kg ha -1 over a 20 year period • Rock Phosphate (Minjingu) only effective if low soil pH, sandy, and OM added - otherwise low returns 20

Precip > 700 mm Potential P-def.

Lake Victoria basin

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Simulations of alternative fertilizer strategies for a typical resource-poor farm (0.2 ha).

(Inputs at 250 kg P ha -1 ; urea 60 kg N ha -1 ; 1US$=55 Kenyan shillings) ICRAF (1997) 22

Role of crop varieties

• P efficient varieties such as maize from CIMMYT

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Impact of phosphate fertilisers

Fertilisers such as superphosphate can : • increase ECEC • retard leaching of Ca, Mg, K • increase adsorption of applied Zn • precipitate Al if at toxic levels • retard nitrate leaching 24

VAM

• vesicular-arbuscular mychorrhizal (VAM) fungi on crop roots important in P uptake - trials with inoculation can increase dry weight by up to 4 to 20 X depending on crop 25

Lime, CaCO 3

• Lime may be used to increase Ca, reduce Al and Mn toxicity • also to increase ECEC - retention of fertiliser cations improved • may also increase nitrification of OM & urea

Silicon

• • silicates used to increase pH important for monocots as they often lack sufficient silicon – high silica content of straw – especially rice – effects fodder quality 26

Application methods of fertilisers

• broadcasting     most suitable for P except on calcareous soils - N fertilisers more susceptible to volatisation if broadcast but stimulates weed growth

placement

 bands or near plant manually  need to be safe distance 5 to 10 cm from seed    cost effective does not fertilise weeds top-dressing - surface placement immediately cover with soil

split applications

 especially N in 2 or more doses  reduces leaching  2 to 3 wap and 6 to 8 wap 27

  

basal

 large dose to bring low fertility up - e.g. 500 kg /ha

maintenance

 replace nutrients removed by crop & leaching

at planting

 must avoid direct contact

Forms of fertiliser

see separate handout 28

Effect of excess application of fertiliser

• • leads to yield decline - sometimes through the effect on other nutrients e.g. excess K causes Mg deficiency environmental problems waste of investments excessive application of P & N fertilisers gives high concentrations in water causing excessive growth of weeds and algae acidification of the surface in especially no-till was attributed to excessive nitrogen applications 29

Socio-economics aspects

How much fertiliser do farmers apply?

Application of fertilisers is primarily related to economic factors: • price of fertilisers   price of crop break even point  extra yield for a given fertiliser input occurs at lower rates so more economical to apply small amounts to large part of crop rather than large amounts to a small part of the crop though probably a minimum amount needed to have some improvement - perhaps 5 to 10 kg ha -1  farmers may apply fertilisers to vegetables and not to food crops because of price obtained in market 30

Reasons for not using fertiliser

• crop surpluses for sale are non-existent or very small • lack of credit • farmers are wary of credit which involves property as surety • non-availability of fertilizer • socio-economic constraints (labour, e.g shortage because of off-farm employment ) • policy uncertainties (frequent changes to subsidy rates) • lack of social capital – groups to buy fertilisers together • infrastructure - many villages have poor access roads and are remote from markets so supply of inputs and marketing of produce is a problem 31

Reasons for applying incorrect amounts

• • • • lack of information on best practices visits from extension staff often rare ineffective extension (blanket recommendations, high cost & low production farm) low level of technical training of farmers 32

Participatory soil assessment

See paper in folder as an example 33