Does Cassava Cultivation Degrade the Soil?

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. 30 25 20 Cassava root yield (t/ha) 15 10 = Sattahip = Huaipong 5 = Korat 0 1955 1960 1965 1970 1980 1985 1990 1975 Crop year Figure 2. Decline in fresh root yields due to continuous cultivation without fertilizers in three soil series in Thailand. Source: Sittibusaya, 1993; Howeler, 1995.

5. 100 80 cassava upland rice 60 Relative yield (%) 40 20 0 1 2 3 4 Years of continuous cropping 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 • Nutrient Depletion • 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. Nutrient extraction by cassava • 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.

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

12. 100 80 60 Relative whole plant yields (%) Ginger 40 Cassava Tomato Wheat 20 Maize 0 3 4 5 7 6 8 9 pH of nutrient solution 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.

13. Exch. Al me/100g pH 3.0 5.5 Al 3+ 2.0 5.0 pH 1.0 4.5 4.0 0 6.0 0 0.5 2.0 Lime (t/ha) Figure 7. The effect of lime application on the pH and exchangeable Al in a Carimagua Oxisol.

14. 30 Cassava (42) 1,500 Rice (96) Cowpea (8) 20 1,000 Black beans (4) Beans, maize, rice, cowpea yield (kg/ha) Cassava root yield (t/ha) Maize (20) 10 500 Non-black beans (8) 0 0 0 0.5 2 6 Lime applied (t/ha) 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.

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. 40 30 Dry weight of plant tops (g/plant) 20 10 0 0 100 200 400 1,600 800 3,200 P applied (kg/ha) = Non-inoculated = VAM-inoculated 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.

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 (%) Cassava 80 Rice 40 Maize 30-45 Common beans 10-23 P (ppm in Bray II) Cassava 4-6 Common beans 10-15

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. 20 20 15 15 10 10 Root yield (t/ha) 80 N 40 P2O5 5 5 80 N 80 K2O 40 P2O5 80 K2O 0 0 0 40 80 160 0 20 40 80 0 40 80 160 0-0-0 80-40-80 160-80-160 28 28 26 26 24 24 22 22 20 20 0 0 0-0-0 80-40-80 160-80-160 kg N-P2O5-K2O/ha = KM 60 = SM 937-26 Starch content (%) 80 N 80 K2O 40 P2O5 80 K2O 80 N 40 P2O5 0 40 80 160 0 40 80 160 0 20 40 80 kg P2O5/ha kg N/ha kg K2O/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).

33. N0P0K0 N0P2K2 N2P0K2 N2P2K0 N2P2K2 40 40 Hung Loc 30 30 Cassava root yield 20 20 10 10 0 0 120 120 100 100 80 80 Relative yield 60 60 40 40 20 20 0 0 0.30 0.30 0.25 0.25 0.20 0.20 Soil K (me/100g) 0.15 0.15 Critical K-level 0.10 0.10 0.05 0.05 0 0 40 40 30 30 Soil P (ppm) 20 20 10 10 Critical P-level 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 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. N1P1K1; tops incorporated 50 50 N0P0K0; tops incorporated 45 45 N0P0K0; tops removed 40 40 35 35 30 30 25 25 Cassava root yield (t/ha) 20 20 15 15 10 10 5 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Cassava crop cycles N1P1K1 + compost; tops removed N1P1K1; tops removed N1P0K1; tops removed N1P1K0; tops removed N0P0K0; tops removed 50 50 45 45 40 40 35 35 30 30 25 25 20 20 15 15 10 10 5 5 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Khon Kaen 76/77 80/81 85/86 90/91 95/96 00/01 Cassava root yield (t/ha) Cassava crop cycles Figure 12. Effect of annual fertilizer application and crop residue management on cassava yields during 25 consecutive crops grown in Khon Kaen, Thailand.

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. 5 4 3 2 1 0 Bean Rice Maize Cotton Peanut Cassava Soybean Sugarcane Irish potato Castor bean Sweetpotato Maize+bean Bean Rice Maize Cotton Peanut Cassava Soybean Sugarcane Irish potato Castor bean Sweetpotato Maize+bean Soil loss (t/ha) 14 12 10 8 Runoff (% of rainfall) 6 4 2 0 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.

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

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 Sri Racha , Thailandfrom1989 to 1993. First Second No. of crop period period Total cycles (22 months) (28 months) (50 months) Cassava for root production 4 142.8 168.5 311.3 Cassava for forage production 2 68.8 138.5 207.3 Maize 5 28.5 35.5 64.0 Sorghum 5 42.9 46.1 89.0 Peanut 5 37.6 36.2 73.8 Mungbean 6 70.9 55.3 126.2 Pineapple 2 31.4 21.3 52.7 Sugarcane 2 - 94.0 - F-test ** ** cv (%) 11.4 42.7

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