1. Does Cassava Cultivation Degrade the Soil? Reinhardt H. Howeler July 2008

2. Almost anywhere in the world, cassava has the reputation to degrade the soil Is this a myth or a reality? Is it based on any scientific evidence, or only based on the observation that cassava is often seen growing on poor and degraded soils? Is cassava the cause or the consequence of soil degradation?

3. Table 1. Chemical properties of various horizons of Haplic Acrisols that have been under different land use for a long time in southeastern Vietnam.

4. Figure 2. Decline in fresh root yields due to continuous cultivation without fertilizers in three soil series in Thailand. Source: Sittibusaya, 1993; Howeler, 1995. Cassava root yield (t/ha) = Sattahip = Huaipong = Korat 5 10 15 20 25 30 0 1955196019651970 1975 198019851990 Crop year

5. 0 20 40 60 80 100 1234 cassava upland rice Years of continuous cropping Relative yield (%) Figure 2. Yield reduction of upland rice and cassava due to fertility decline as a result of continuous cropping without fertilizer application. 100% corresponds to 18.9 t/ha of fresh cassava roots and 2.55 t/ha of rice. Source: adapted from Nguyen Tu Siem, 1992.

6. Soil Productivity Decline due to 1)  Nutrient Depletion 2)  Erosion

7. Table 3. Average nutrient removal by cassava and various other crops, as reported in the literature.

8. Table 6. Dry matter and nutrient distribution in 12 month old cassava, cv. MVen 77, grown without and with fertilization in Carimagua, Colombia in 1983/84.

9. Table 4. Major nutrients removed in the harvested products of various crops grown during 22 months in Sri Racha, Chonburi, Thailand in 1989-1991.

10. 1.If only roots are removed and plant tops are returned, removal of N and P is lower than from most other crops; K removal is lower or similar to other crops. 2. With an average fresh root yield of 15 t/ha, only about 30 kg of N, 3.5 kg P and 20 kg K are removed with the root harvest. 3. If stems and leaves are also removed, nutrient extraction of all three nutrients can be quite high; Ca and Mg removal is also quite high. Nutrient extraction by cassava

11. Why is it that cassava can grow on very poor soils where most other crops would perish? 1. Cassava is more tolerant than most other crops of acid soils with high levels of exchangeable Al

12. Figure 4. Relative growth response of various plant species to a series of constant pH values in flowing nutrient solution. Source: Islam et al., 1980. 20 0 40 100 80 60 Relative whole plant yields (%) pH of nutrient solution 345 6 7 89 Ginger Cassava Tomato Wheat Maize

13. Figure 7. The effect of lime application on the pH and exchangeable Al in a Carimagua Oxisol. 1.0 2.0 3.0 Lime (t/ha) 0.52.0 6.0 4.0 4.5 5.0 5.5 pH Al 3+ pH Exch. Al me/100g 0 0

14. Figure 6. The response of cassava, rice, cowpea, beans and maize to the application of lime in Carimagua. The number of cultivars or lines screeneed is shown in parenthesis. Source; Howeler, 1991. 0 500 Beans, maize, rice, cowpea yield (kg/ha) 0.5 1,000 1,500 0 26 10 20 30 Cassava root yield (t/ha) Lime applied (t/ha) Maize (20) Black beans (4) Cowpea (8) Rice (96) Cassava (42) Non-black beans (8) 0

15. Why is it that cassava can grow on very poor soils where most other crops would perish? 2. Cassava is very efficient in the uptake of P and some other nutrients due to a highly effective symbiosis with naturally occuring mycorrhizal fungi in the soil

16. Cassava has a very coarse root system as compared with rice, both growing in nutrient solution

17. But, when inoculated with mycorrizal fungi (right), cassava grows much better due to a more effective uptake of P from the low-P nutrient solution

18. Fibrous root of cassava with mycorrhizal vesicles inside the root and hyphea covering the root in nutrient solution

19. Fibrous root of cassava with root hairs and mycorrhizal hyphae and vesicles

20. Masses of mycorrhizal hyphae attached to cassava roots growing in nutrient solution

21. Schematic diagram of roots with (right) and without infection by mycorrhizal fungi and the effect on the volume of soil from which P can be absorbed

22. Growth response to five levels of applied P in a sterilized Quilichao soil, without mycorrhizal inoculation

23. Growth response to five levels of applied P in a sterilized Quilichao soil, with mycorrhizal inoculation

24. Figure 9. Effect of mycorrhizal inoculation and several levels of applied P on the dry weight of plant tops of cassava, cultivar MMex 59, in a sterilized soil from CIAT-Quilichao in the greenhouse. Source: CIAT, 1981. P applied (kg/ha) 10 20 30 40 1,600 3,200 800 400200100 = Non-inoculated = VAM-inoculated Dry weight of plant tops (g/plant) 0 0

25. Soil sterilization with methyl bromide to eliminate all natural mycorrhiza fungi

26. Cassava growth in sterilized soil in front, in non-sterilized soil in back

27. Cassava growth in sterilized soil on left, in non-sterilized soil on right; note recuperation of border row plants due to mycorrhizal re- invasion from non-sterilized soil

28. Effect of soil sterilization and mycorrhizal inoculation on the root yield of two cassava varieties in Quilichao, Colombia

29. After several years of continuous cropping, excellent cassava growth even in P-check plot due to an efficient mycorrhizal association

30. Critical levels of soil parameters for cassava as compared to some other crops pH (1:1 in water) Cassava 4.6 Common beans 4.9 Al-saturation (%)Cassava80 Rice40 Maize30-45 Common beans10-23 P (ppm in Bray II)Cassava4-6 Common beans10-15

31. What is the long-term effect of continuous cassava cultivation on the soil’s nutrient status? Results of 19 long-term fertility trials, conducted from 4 to 31 years of continuous cropping, indicate that K had become the most limiting nutrient in 12 trials, N in five trials and P in only two trials Thus, while N and P are important nutrients in some soils, in the great majority of soils where cassava is grown, K becomes the most limiting nutrient over time due to considerable removal of K in each root harvest.

32. 0204080 80 N 80 K 2 O 04080160 80 N 40 P 2 O 5 0 5 10 15 20 0-0-080-40-80160-80-160 0 5 10 15 20 04080160 40 P 2 O 5 80 K 2 O Root yield (t/ha) = KM 60= SM 937-26 0 20 22 24 26 28 0204080 80 N 80 K 2 O 80 N 40 P 2 O 5 kg K 2 O/ha kg P 2 O 5 /ha Figure 10. Effect of annual applications of various levels of N, P and K on the root yield and starch content of two cassava varieties grown at Hung Loc Agriculture Research Center, Thong Nhat, Dongnai, Vietnam in 2006/07 (17th year). 04080160 40 P 2 O 5 80 K 2 O kg N/ha 0 20 22 24 26 28 kg N-P 2 O 5 -K 2 O/ha 0-0-080-40-80160-80-160 Starch content (%) 04080160

33. 0 20 40 60 80 100 120 Relative yield 0 20 40 60 80 100 120 0 10 20 30 40 Cassava root yield 0 10 20 30 40 0 0.05 0.10 0.15 0.20 0.25 0.30 Soil K (me/100g) 0 0.05 0.10 0.15 0.20 0.25 0.30 Critical K-level N0P0K0N0P0K0 N0P2K2N0P2K2 N2P0K2N2P0K2 N2P2K0N2P2K0 N2P2K2N2P2K2 Hung Loc 0 10 20 30 40 1234567891011121314151617 Soil P (ppm) 0 10 20 30 40 Critical P-level Crop cycle Figure 11. Effect of annual applications of N, P and K on cassava root yield, relative yield (yield without the nutrient over the highest yield with the nutrient) and the exchangeable K and available P (Bray 2) content of the soil during 17 years of continuous cropping in Hung Loc Agric. Research Center in Dong Nai, Vietnam.

34. Figure 12. Effect of annual fertilizer application and crop residue management on cassava yields during 25 consecutive crops grown in Khon Kaen, Thailand. 0 5 10 15 20 25 30 35 40 45 50 12345678910111213141516171819202122232425 0 5 10 15 20 25 30 35 40 45 50 Cassava root yield (t/ha) Cassava crop cycles Khon Kaen 76/7780/8185/8690/9195/96 00/01 N 1 P 1 K 1 ; tops incorporated N 0 P 0 K 0 ; tops incorporated N 0 P 0 K 0 ; tops removed Cassava crop cycles Cassava root yield (t/ha) N 1 P 1 K 1 + compost; tops removed N 1 P 1 K 1 ; tops removed N 1 P 0 K 1 ; tops removed N 1 P 1 K 0 ; tops removed N 0 P 0 K 0 ; tops removed 0 5 10 15 20 25 30 35 40 45 50 12345678910111213141516171819202122232425 0 5 10 15 20 25 30 35 40 45 50

35. Severe K deficiency in K check plot in Carimagua, Colombia

36. Growth response to K application in Carimagua, Colombia; zero K in front, zero NPK and high NPK in back

37. Strong fertilizer response in Tamanbogo, Lampung, Indonesia; without fertilizers in front, with high fertilizers in back

38. Table 10. Average nutrient content of one tonne of various types of wet manure as compared to 50 kg of 15-15-15 chemical fertilizers.

39. Nutrient Depletion It was found that cassava extracts less nutrients from the soil than most other crops, except when yields are very high or stems and leaves are also removed from the field Cassava grows better than most crops on infertile soil, but does respond well to fertilizer applications Cassava requires relatively high applications of N and K, but is very tolerant of low P or high Al in the soil

40. Effect of Erosion

41. Even on very gentle slopes a lot of run-off water accumulates in natural drainage channels

42. ……which can break the contour ridges and cause serious gully erosion

43. ………or worse……..

44. Figure 13. Effect of crops on annual soil loss by erosion (top) and on runoff (bottom). Data are average values (corrected for a standard annual rainfall of 1,300 mm) from about 48 experiments conducted from 1943 to 1959 on sandy, clayey and Terra roxa soils in Sao Paulo state of Brazil with slopes of 8.5-12.8%. Source: Quintiliano et al., 1961. 0 1 2 3 4 5 Castor bean Bean Cassava Peanut Rice Cotton Soybean Irish potato Sugarcane Maize Maize+bean Sweetpotato Soil loss (t/ha) 0 2 4 6 8 10 12 14 Runoff (% of rainfall) Castor bean Bean Cassava Peanut Rice Cotton Soybean Irish potato Sugarcane Maize Maize+bean Sweetpotato

45. Figure 14. Percent soil cover of various crops and cropping systems. Source: Aina et al., 1979. 0 60 100 80 Soil cover (%) Cassava monoculture 204060 80 40 Days after planting 20 Cassava+maiz Soybean Pigeon pea 0

46. When grown on slopes, production of cassava was found to result in greater soil losses by erosion than other crops

47. Table 11. Total dry soil loss by erosion (t/ha) due to the cultivation of eight crops during four years on 7% slope with sandy loam soil in SriRacha, Thailand from 1989 to 1993. FirstSecond No. of cropperiod Total cycles(22 months)(28 months)(50 months) Cassava for root production4142.8168.5311.3 Cassava for forage production268.8138.5207.3 Maize528.535.564.0 Sorghum542.946.189.0 Peanut537.636.273.8 Mungbean670.955.3126.2 Pineapple231.421.352.7 Sugarcane2-94.0- F-test** cv (%)11.442.7

48. Fertilizer application improves canopy development and markedly reduces runoff and erosion

49. Contour ridging (foreground) can markedly reduce erosion as compared to up-and-down ridging (back)

50. Erosion control experiments indicate that soil losses can be markedly reduced by various agronomic practices

51. Contour hedgerows of vetiver grass (in back) were the most effective in reducing erosion in north Vietnam…..

52. …….as well as in south Vietnam

53. Research has shown that various agronomic and soil conservation practices are very effective in reducing erosion and may also increase cassava yields 1.Maintaining adequate soil fertility using fertilizers, animal manures, green manures or alley cropping 2.Planting contour hedgerows of grasses or legumes 3.Contour plowing and ridging 4.Intercropping 5.Mulch application 6.Zero or minimum tillage 7.Planting cassava on plastic mulch

54. When poorly managed, cassava can indeed cause soil degradation, mainly by: 1. Slow depletion of soil nutrients, especially K 2. Soil compaction (in case of mechanical land preparation) 3. Serious soil erosion Conclusions

55. With good management, including adequate fertilization, minimum tillage and erosion control, cassava cultivation does NOT degrade the soil, or its effect is similar to that of other food crops Conclusions