Soil & soil fertility

1. Soil & soil fertility Africa Soil Health Consortium 2014 Lecture 2: Introduction to soil and soil fertility

2. Objectives Gain knowlegde on the principles underpinning ISFM practises • Introduction to soil • Soil texture • Porosity • Mineral fraction • Organic matter • Introduction to nutrients • Understanding the function of nutrients in plant growth • Recognizing nutrient deficiencies • Soil fertility • Understanding the concept of soil fertility • Introduction to soil fertility management • Conservation agriculture & organic agriculture • Minimizing losses of added nutrients

3. Soil • Organic fraction: • Soil organic matter (SOM) • Key issue in soil fertility management Soil solids Pore space + soil fauna and flora Pore space: -space for roots and micro-organisms -air for micro-organisms -water storage • Mineral fraction: • Provides support to plant roots • Slowly releases nutrients into the soil solution

4. Pore space Porosity: volume of the soil occupied by air and the soil solution Porosity in Well-drained moist soil: sufficient moisture for plant growth and sufficient aeration for proper root function Dry soil: all pores are filled with air  drought stress Flooded soil: pores are saturated with water  roots cannot breathe and plants may die Water film 0 1.5 2.0 2.5 1.0 0.5 mm Soil particle Air space Illustration adapted from Brady 1984, The nature and properties of soils, 9th edition.

5. Mineral fraction Sand Silt Clay 4 5 3 1 2 0 mm Sand: 0.05 - 2.0 mm Silt: 0.002 - 0.05 mm Clay: < 0.002 mm Illustration adapted from: www.iconn.org

6. Mineral fraction Silt % Clay % Sand %

7. The finger test Mineral fraction

8. Mineral fraction & Porosity • Soil texture affects • Porosity • Water holding capacity • Nutrient retention and supply • Drainage • Nutrient leaching Pore Space in Sandy Soil vs. Clay Soil Sandy soil Clay soil Larger pores Smaller pores Infiltration Variations by Soil Texture Less total pore volume = Less porosity Greater total pore volume = Greater porosity Clay Silt Sand Illustrations adapted from: http://wegc203116.uni-graz.at/meted/hydro/basic/Runoff/print_version/04-soilproperties.htm

9. Mineral fraction & CEC Cations: positively charged ions (e.g. K+, NH4+) Cation exchange capacity (CEC): the maximum quantity of total cations that a soil is capable of holding. Clay fraction and SOM: Small particle size  Large negatively charged surface area  More positions to hold cations  High CEC Clay – Many positions to hold cations Sand– Few positions to hold cations H+ Ca2+ Mg2+ NH4+ Na+ K+ H+ H+ H+ K+ Sand Clay Illistration adapted from: http://www.spectrumanalytic.com/support/library/ff/CEC_BpH_and_percent_sat.htm

10. Mineral fraction & CEC CEC depends on • Clay content • Type of clay mineral • SOM content • Soil pH Clay minerals differ in structure • 1:1 clay minerals • CEC varies with soil pH • Found in most upland soils in SSA • 2:1 clay minerals • Large inherent CEC capacity • Found in fertile lowland soils Illustration adapted from Lory ‘Structure of Clays’ www.soilsurveys.org

11. Organic fraction: SOM SOM: plant and animal residues, in various stages of decompisition Picture: http://www.guiadejardineria.com/jardineria/suelos-y-abonos/page/7/

12. Organic fraction: SOM % Organic matter Litter layer • Contains essential plant nutrients • Improves the soil’s Cation Exchange Capacity • Improves the soil’s water-holding capacity (SOM can hold up to five times its own weight in water!) • Improves water infiltration • Buffers soil pH • Binds with toxic elements in the soil • Improves soil structure by stimulating activity of soil flora and fauna • Regulates the rates and amounts of nutrients released for plant uptake •  SOM is a key issue in soil fertility management! 3 1 2 4 5 Top soil Sub soil Organic matter Illustration adapted from: http://www.tekura.school.nz/departments/horticulture/ht106_p4.html

13. Soil analysis • Soil test: chemical method for estimating the nutrient-supplying power of a soil • Laboratory needs a representative composite sample of 0.5 kg • Be aware of heterogeneity within fields when sampling!

14. Guidelines for soil sampling Take a representative sample!!! • Check the area to be sampled for notable features (e.g. slope, soil types, vegetation, drainage). • Draw a sketch map, and identify and mark the location of sampling sites. • Take soil samples with a soil auger at the sampling depth (0-20 cm or 20-40 cm). • Take 10-35 sub-samples per site, the number depending on the size and heterogeneity of the field. • Combine the sub-samples to one composite per site and mix thoroughly. • If necessary, reduce sample weight by sub-dividing • Label the sample of soil properly. • Air-dry the sample and when dry, store it, properly labelled, in a plastic bag or a glass bottle for further analyses.

15. Nutrients Macronutrients: at least 0.1% of plant dry matter per macronutrient • Nitrogen (N): • Amino acid/Protein formation • Photosynthesis • Phosphorus (P): • Energy storage/transfer • Root growth • Crop maturity • Straw strength • Disease resistance • Needed in large amounts during plant growth • Required for N2-fixation by legumes • Potassium (K): • Plant turgor pressure maintenance • Accumulation and transport of the products of plant metabolism • Disease resistance • Required for N2-fixation by legumes • Sulphur (S): • Part of amino acids (protein formation) • Synthesis of chlorophyll and some vitamins • Required for N2-fixation by legumes • Magnesium (Mg): • Photosynthesis • Activates enzymes • Carbohydrate transport • Calcium (Ca): • Cell growth and walls • Activates enzymes (protein formation and carbohydrate transfer) • Essential in ‘calcicole’ plants (e.g. Groundnut) for seed production. • Influences water movement, cell growth and division • Required for uptake of N and other minerals Very mobile Very mobile Very mobile Quite poor mobility Very mobile Very mobile Poor mobility Quite poor mobility Medium mobility Poor mobility Quite mobile Very mobile

16. Nutrients Micronutrients: less than 0.1% of plant dry matter • Iron (Fe): • Photosyntheiss • Respiration • Manganese (Mn): • Photosynthesis • Enzyme function • Boron (B): • Development/growth of new cells • Zinc (Zn): • Nucleic acid synthesis and enzyme activation • Copper (Cu): • Chlorophyll formation • Seed formation • Protein synthesis • Molybdenum (Mo): • Protein synthesis and N uptake • N2-fixation by legumes • Chlorine (Cl): • Movement of water and solutes • Nutrient uptake • Photosynthesis • Early crop maturity • Disease control • Cobalt (Co): • N2-fixation by legumes • Nickel (Ni): • Required for enzyme urease • Sodium (Na): • Water movement and balance of minerals • Silicon (Si) • Cell walls • Protection against piercing by sucking insects • Leaf presentation • Heat and drought tolerance

17. Nutrient deficiency Healthy N-deficient P-deficient K-deficient Diseased

18. Nutrient deficiencies

19. Nutrient deficiency: exercise

20. Nutrient deficiency: exercise • P-deficient • Stunted growth • Purplish colouring • K-deficient • Browning of leaf edges

21. Nutrient uptake

22. Nutrient availability Readily available - Nutrients from soluble fertilizers (e.g. KCL), readily mineralized SOM, nutrients held on the edges of soil particles, and in the soil solution Slowly available - Nutrients in organic form, such as plant residues and organic manures (particularly with a high C/N ratio), slowly soluble mineral fertilizers (e.g. Phosphate rock) and the SOM fraction resistant to mineralization Not available - Nutrients contained in rocks, or adsorbed on soil particles

23. Soil fertility The capacity of soil to supply sufficient quantities and proportions of essential chemical elements (nutrients) and water required for optimal growth of specified plants as governed by the soil’s chemical, physical and biological attributes. • Chemical elements for plant nutrition • Adequate soil volume for plant root development • Water and air for root development and growth • Anchorage for the plant structure

24. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Nutrient deficiencies prevent a good harvest • Nutrient deficiencies can be expressed during plant growth • Use mineral (fertilizer) or organic (manure, crop residues) to supply nutrients • Use special fertilizer blends containing micronutrients or manure in case of micronutrient deficiencies Healthy N-deficient P-deficient K-deficient

25. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Acidity is caused by • inherent soil properties • acidity inducing management (e.g. long-term use of ammonium based fertilizer) • Acid soils have high exchangeable Al (Al toxicity) Lime • Increases pH • Prevents Al and Mn toxicity in acidic soils (pH <5.5) • Supplies Ca • Increases P and Mo availability • Can increase microbiological activity • Apply lime to reduce exchangeable Al to +/- 15%

26. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Compaction  sub-surface soil barrier to root growth • Break hardpans by ploughing or chisel ploughing to 30 cm depth Porous soil allows good root development Surface crust Sub-surface barrier to roots Illustration adapted from: http://locallygerminated.wordpress.com/

27. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Capture more rainfall in areas that are prone to drought • Harvesting additional water (e.g. Zaï) • Promoting infiltration by coversing the soil surface with mulch • Labour intensive Zaï pits in Niger Mulching of bananas, western Uganda Pictures: fao.org

28. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Prone to erosion: fields on steep slopes, or on gentle slopes with course-textured top soil • Measures: live barriers (e.g. grass strips), teracces, surface mulch Bunds on sloping land in Burundi

29. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Good seedbed preparation improves germination and reduces the chance for diseases • A delay in planting date often affects yield negatively • Planting time is important especially when the growing season is short

30. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Crops compete for nutrients, water and light • Use a correct planting density, adjusted to crop type and the environment. Consider the distance between rows, between plants within rows and the number of plants per planting hole.

31. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Use viable seed (at least 80% germination) • Plant seeds at the correct depth and insert cuttings at correct angle • Plant more seeds than required for optimal plant density. • Weeds compete with crops for nutrients, water and light. • Timely removal of weeds is essential • Weed before top dressing crop with fertilizer • Control pests and diseases at specific growth stages Delayed weeding reduces the crop response to fertilizer

32. Soil fertility management practices Correcting nutrient deficiencies Soil acidity correction Breaking hardpans Water harvesting Erosion control Land preparation Planting date Spacing Planting practices Weeding Pest and disease management Intercropping • Intercropping arrangements: take into account specific growth features and needs of individual crops to minimize intercrop competition. • Examples: delayed planting of one intercrop, adjusting spacing, strip intercropping Maize-cassava Maize-pigeonpea Cassava-soybean

33. Conservation agriculture (CA) Basic principles • Soil disturbance is minimized by reduced or zero-tillage • Use of at least 30% soil cover (mulch or cover crops) • Use of crop rotations/associations • Pitfalls • Competing uses of crop residues needed for mulch • Yields may decrease on the short-term (the increase often comes on the longer-term) • Increased weed pressure caused by reduced tillage • Full CA requires a fundamental change in the farming system. This may not be practical or enomic for the farmer • Possible decrease in agronomic efficiency of fertilizer use • Advantages • Rapid planting of large areas • Reduction of soil erosion

34. Organic agriculture Reliance on organic resources to provide nutrients to sustain soil fertility and produce economic crop yields However, mineral fertilizers are an essential component in sustainable agriculture in SSA • Soil nutrients stocks in large parts of SSA have already become depleted and require replenishment • Organic resources are not available in large enough quantities to replenish and sustain nutrient stocks in the soil • Large and economic responses to mineral fertilizer are obtained in many parts of SSA • Organic resources are bulky and their management is labour intensive ISFM: use of mineral fertilizer in combination with organic resources. The combination provides the greatest benefits!

35. Minimizing losses of added nutrients Losses of nutrients into the environment • Depletion of nutrients in farming systems • Eutrophication in case of excessive mineral fertilizer use (not common in SSA) Losses through • Harvesting crops recycling • Water and wind erosion • Leaching • Volatilization Nitrogen is the most susceptible to losses • Very mobile, can be lost through different ways • NO3- is susceptible to leaching.

36. Losses: Water and wind erosion 10 kg N/ha, 2 kg P/ha and 6 kg K/ha lost in low-input production systems in SSA Measures: grass strips, stone rows, mulch layer, soil preparation methods (e.g. Zaï), improving SOM Tied rigdes Bunds on sloping land in Burundi

37. Losses: Leaching • Problematic in high rainfall areas and coarse-textured sandy soils (>35% sand) • Mainly NO3- and exchangeable bases (K and Mg) percolate beyond the reach of crop roots Measures: • Improving soil structure to promote good root development for increased accessibility of nutrients • Growing annual crops in association with trees, which can ‘pump’ water and nutrients from deeper layers

38. Losses: Volatilization Denitrification of NO3- • NO3- N2O and N2 (gasses) • Occurs under anaerobic conditions Measures: improved soil drainage and maintain a good soil structure to avoid anaerobic growing conditions Volatilization of NH3 in alkaline soils (high pH) Measures: deep placement of N-fertilizers Volatilization of NH3 during storage and handling of manure Measures: use anaerobic storage pits

39. Summary Soil fertility management options Soil organic matter CEC Porosity • Nutrients • Functions • Availability • Mobility • Deficiencies Conservation agriculture Texture Organic agriculture • Mimimizing losses of added nutrients • Erosion • Leaching • volatilization