Conservation Agriculture Training Course, Zimbabwe 2010

1. Conservation Agriculture Training Course, Zimbabwe 2010 The Role and Importance of Residues in Conservation Agriculture by Christian Thierfelder

2. Basic principles of Conservation Agriculture • Minimum soil movement • Residue retention • Crop rotations and GMCCs

3. Residues – what are we talking about? Maize stover covering a field before planting

4. What are residues? continued…. Maize residues in maize-cotton rotation

5. Sunnhemp residues (left) Cotton residues (right)

6. How to manage residues - rolled maize stalks or sunnhemp

7. The challenge – burning of stover CIMMYT Research Station

8. Competing uses for stover residues…!

9. The Role and Importance of Residues in CA • Water erosion • Water infiltration • Evaporation • Soil temperature • Weed control • Soil organic matter • Soil biological activity

11. Impact of raindrop on different soil surfaces

12. Crusting and sealing leads to rill erosion

13. Amount of soil being eroded after 7 years of bare fallow (plot size 224 m²)

14. - 1 - 2 Treatment Erosion in t ha Runoff l m Conventional ploughing (CP) 11.99 a 545.1 a Direct seeding (DS) 8.01 b 383.9 b Magoye ripper + intercropping (MR) 6.91 b 314.7 b M . pruriens LSD (p ≤ 0.05) 3.50 75.7

15. About 50% of rainfall is lost on conventional systems

16. 100 80 Relative Erosion % 60 40 20 Cover % 80 20 40 60 2 4 Residue t/ha 6 8 Relationshipbetween residue amount, ground cover and water erosion Erenstein, 1997.Based on data of Shaxon et al., 1989, Tripp and Barreto, 1993, and Kok and Thien, 1994.

17. Why soil erosion? Because water is not infiltrating! • The soil needs to be permeable (no crusts) • The soil has to have pores (earthworms) • CA plots covered with residues provide for soil micoorganisms, build stable aggregates (natural swelling and shrinking) and increase the organic matter which all improve infiltration

18. Pore system has to be unsealed and easy to moisten to enable infiltration

19. Shamva Zimuto Conventional 6.26 b 6.60 b Rip-line seed 11.50 a 11.50 a Direct Seed 11.99 a 10.88 a CV% 22.7 23.9 SE Diff 1.30 1.24 Time to Pond – On-farm Results

20. Infiltration is reduced on conventional systems

21. - 1 - 1 Mulch rate, t ha Precipitation storage , mm Sorghum yield, t ha 0 72 c 1. 8 c 1 99 b 2.4 b 2 100 b 2.6 b 4 116 b 3.0 b 8 139 a 3.7 a 12 147 a 4.0 a Effects of mulch rate on soil water conservation during fallow and on subsequent sorghum grain yield, Texas Source: Unger 1978

22. Mulch protects the soil from drying out! • Residues reduce radiation energy of the sun - less radiation => less evaporation • Drying forces of the wind are lower • Vapour flux from the soil into the air is reduced • Capillary uptake in the soil maintains moisture transportation from the subsoil to the soil surface

23. Comparison between capillar uptake in conventional and residue covered soils • Conventional: Radiation is high at the soil surface (less cover), capillary uptake generally is not sufficient from the subsoil. The consequence is drying out of the upper layers • Residue covered: Radiation is lower, capillary uptake can be maintained. Soils supply more moisture for germination and plant growth.

24. Bare soil Temperature (°C ) Days in August 1973 Effect of Crop Residues on Soil Temperature (10 cm Depth) Source : Lal, 1978

25. Effects of different residues on weed soil cover (in %) and weed population (plants/m2) 7 and 58 day after roling Source: Almeida and Rodríguez, 1985

26. Residues build up the humus in the soil „Theorganic matter in tropical soils is, besides moisture, more important than any other soil characteristic“ Young, 1976

27. Effects of residues on nutrient availability • Residues increase organic carbon content over time • Plant available phosphorus, potassium, calcium and magnesium increases have been reported • Residues may lead to nitrogen lock-up initially especially when soils are very limited in nitrogen (microbes use the nitrogen for their biomass) • After some years the system is in equilibrium, N immobilization will no longer negatively effect nutrient availability

28. Why is surface residue retention important? • Decomposition in tropical climate • Surface effects (protection, temperature, moisture retention, fodder for soil fauna and flora) • Slow nutrient release • Build-up of organic matter pool

29. No Burn CO2 80% Crop Residue Microbial Decomposition Soil OM 20% Crop Residue CO2 60% CO2 92% Burn CO2 Ash Microbial Decomposition Soil OM 8% 32% 40% Source: Albrecht et al., 1994 USDA-ARS Pendleton, OR

30. Residues encourage earthworm activity

32. Effective destruction by ants and termites, fungi and bacteria

33. Factors influencing decomposition • Lignin content • C:N ratio • Microbial preferences (maize/sunflower) • Microbial variability (clay soils) • Soil moisture content • Temperature • pH

34. Effect of residue addition on biological activity (% increase in biological activity over control) Day Wheat strawburned stubbles 1 80 29 4 61 9 7 47 0 10 28 0 Source: Albrecht et al., 1994 USDA-ARS Pendleton, OR

35. Problems and challenges with residues (pests and diseases carry over) Perithecia of Pyrenophora trichostoma (Helminthosporium tritici-repentis) on wheat straw

36. Residues increase micro-, meso,- and macro-fauna thus facilitating a biological equilibrium If a system is in equilibrium, insects and pests are controlled by their predators

37. Some people just prefer „clean“ fields The mindset is very important

39. Thank you very much!