AGRONOMIC AND VEGETATIVE ASPECTS

1. AGRONOMIC AND VEGETATIVE ASPECTS Some people working in the field (such as Stocking (1985)) have stated that the only long term solution to soil erosion is vegetative protection & fertility enhancing farming systems rather than mechanised methods. - debatable for drought prone areas - water conservation extremely important - usually requires some structures. best approach = holistic one which combines various methods. Introduction

2. Reinforcement of soil • In addition to protecting the soil from raindrop impact, plant roots also help to anchor the soil in place. • Roots & rhizomes have a relatively high tensile strength and • adhesion (soil to roots) and when these are embedded in a soil matrix having low tensile strength soil, the shear strength of soil is enhanced (cf. reinforced concrete). • Fine roots most effective :- • grasses down to 1.5 m, • trees down to 3 m.

3. Organic matter mulch from residues increases infiltration rates recycles nutrients increases water holding capacity debris slows overland flow debris traps silt Effects of organic matter from plants

4. Soil preparation aspects Ploughing • aim for large clods to maximise roughness, as this improves • porosity & infiltration and temporary surface storage • deep ploughing often inadvisable as subsoil may be unstable • or infertile - best not to bring subsoil to surface so usually • adopt 15 cm maximum depth for mouldboard ploughs • - can be deeper for disk ploughs which do not invert the soil • pans can develop if soil is ploughed with heavy equipment when wet • - may decrease infiltration and increase runoff as well • as causing waterlogging • - in mechanised conditions, ripping may be necessaary /possible

7. many soils become very hard in dry season • best time to plough is at end of harvest when there is still • some soil moisture but often this is time for celebrations • and there is "no time" for ploughing; • increased risks of soil erosion after harvest ploughing • especially on steep land • if post-harvest ploughing can be carried out, moisture can • build up from occasional showers in dry season and at start • of rainy season • otherwise may have to wait until after rains • - especially if hand cultivation- wastes time • if ploughed before the rain, oxen or tractors usually necessary but even these may not have enough draught power • early ploughing may cause oxidation of organic matter • if soil inverted

8. Contour ploughing • for permeable soils, plough along "exact" contour; • for very impermeable soils, plough at angle to contour with • slope of 0.25 to 0.5 to encourage drainage • permanently mark contour or plough direction by planting • trees • can be mechanised, semi-mechanised or hand cultivation; • tractor ploughing on contour limited to 17% for 2WD • vehicles, 25% for 4WD.

10. Harrowing • best timing is just before planting to avoid erosion caused • by effect of rain on very loose soil • size of tilth is important; ideal is to achieve same average • aggregate size as seed size • when it rains, harrowed soil may experience reduction • in infiltration rate caused by crusting

12. Compaction • Compaction is usually deleterious but may have beneficial • effects on some loose textured soils. AWC and engineering properties may be increased – e.g. forested road embankments – balance needed

13. Depth of planting • plant too shallow and seed dries out - too deep and seedling cannot emerge; 25 to 50 mm typical – maize up to 100 mm if dry soil – deeper in light soils than heavier soils • some (maize?) varieties can be planted up to 15 cm deep • - seeds wait for rain to percolate down, by which time, • there is more confidence that rains have started.

14. Time of planting General principles • Early establishment of crop canopy will help to absorb • raindrop energy early in season and so reduce splash erosion • and surface sealing. • Also helps to maintain porosity and infiltration rate • increases brought about by ploughing. • Early establishment makes best use of available rain.

16. Dry planting • makes optimum use of early rain • spreads work load • agroclimatic analysis to determine best (called "trigger") • dates very important Risks and problems: • large variation in start date for rain from year to year • if rain late, seed can be lost and so replanting will be • required (but will seed be available?) • animals (e.g. ground squirrels) may eat dry seed so • guards may be necessary • delay in replanting waiting for rain to "re-start" may • lower eventual yield • increased weed problem • (as crop & weeds germinate at the same time)

17. Plant after start of rain • May be essential on hard soils especially cultivation is done • with digging stick or hand hoe • Avoids weed problem of above , weeds germinate first • then hoed • Usually wastes rain & time (i.e. reduces possible growing • period (often correlated with yield) )

18. Phased planting • Plant part of field/farm then later on (after a week?) • plant next part • leads to lack of uniformity • leads to increased risk of pest damage • (pests that only feed on the crop at certain stages can • proceed to the next "phase" as it becomes ready to eat)

19. Relay or sequence planting • Plant second crop underneath first crop before first is • harvested so that it gets off to a good start • Grows up through first crop • Symbiosis: • second crop may fix nitrogen that can be used • by first crop, • first crop provides shade to second crop, • first crop may provide physical support - not the same as • parasitism which implies feeding or using moisture, • each may help prevent spread of pests to the other)

20. Other factors affecting time of planting • Birds / pests may arrive at certain time & may be best to avoid having grain ready at that time even if late planting means yield is less than ideal (with no pests or under experimental conditions) • Air temperatures affect soil temperatures and these affect mineralisation of N - avoid high temperatures at times when crop susceptible to nitrogen supply • Radiation varies throughout year according to angle of sun. This has direct affect on potential for photosynthesis

21. Weed growth may be at a maximum at a certain time of year - avoid maximum weed growth when crop most susceptible to (e.g.) moisture supply if possible. • Crop physiology may require avoidance of rain or drought at certain stages of growth • Viruses may only attack under certain climatic conditions (e.g. groundnuts will suffer from mosaic virus if planted late) • Fungus similarly (e.g. millet more susceptible to "smut" if it flowers in heavy rain)

22. Response farming Experiments have been conducted in semi-arid East Africa (e.g. Stewart & Faught, 1984) to find best management regime for very variable rainfall conditions by estimating total rainfall on the performance “so far”. Experiments were aimed at being able to provide low risk guidelines using radio/extension workers to pass information on to farmers, based on the accumulated climatic conditions since the crops were planted. This approach is called response farming

24. Example for maize: (i) Early start to rain was correlated with higher seasonal expectation of rain. So if early start to rain advise farmers to: · use high seed rates & fertilisers · plant higher proportion of drought susceptible (usually more valuable) crops Use opposite strategy if rains start late. (ii) After 40 to 50 days, advice on thinning schedule is again based on climate so far; may need to decide on whether or not to plant catch crops such as cowpeas (iii) at 75 days - can predict harvest - alert govt. etc. re. famine if necessary

25. Optimists - aim at high production and are prepared to thin or remove alternate rows if season has low rainfall Pessimists - aim low, but add nitrogen, relay sow, plant short season -inter-crop or catch crop if rainfall is better than feared

26. Fallowing and rotations Theory is that nutrients, organic matter and water build up during year or season, then these are released and become available for following years crop. Break in disease or insect pest cycles. Return period R is defined as: This measure is also used for different cultivation systems such as shifting cultivation. NB if 1 yr in 5, R = 20%; 5 yrs in 25 = 20% also, but production and erosion may be different

27. Types Shifting cultivation R factor < 0.3 (R < 30%) Jhum cultivation system in MEGHALAYA India

28. Weed free fallow • plough field then leave for a year without planting • weed during year to reduce evaporation • yields in experimental conditions have been up to 4 times • without fallow • unpopular with farmers; difficult to convince them to weed • unplanted fields! • technique also advocated for nematode control

29. 1st season fallow • leave field unplanted in first season, plant in second when moisture has built up • in experiments, yields have been double (second season crops) • depends on having suitable (bimodal) rainfall distribution • a variation of this is to leave land fallow in hot, rainy season so that moisture can be built up for dry, cold season crop as is done on vertisols in N. Nigeria (see earlier note)

30. Partial fallow • in first season, plant with crop having low water use requirement, e.g. E. teff, proso millet • technique may not be effective in cracking soils with high available water capacities in surface horizon as subsoil may remain dry, then in second season, what little moisture there is in sub-soil evaporates directly to atmosphere

31. Effect of soil fertility and rainfall on return period R% for maintenance of nutrient levels adapted from Table 12 in Agrofor for SC

32. more fertile soils can be cultivated for a larger percentage of the time • with high inputs, land can be cultivated for a larger percentage of the time (though not 100%) • heavier rainfall areas leach nutrients more quickly so have to be rested more frequently or have more inputs

33. Minimum (conservation) tillage • Spectrum of meanings from: • machines to plant through stubble then use herbicides to kill weeds –GM soya (for example) to resist herbicide TO • burn weeds then dig holes and hand plant • rainfall is low so weed growth in dry season is small • soil may become very hard so runoff may be high – system could be improved by in situ water harvesting

34. Need to note the following points: • Soils with low flora & fauna activity such as in dry areas may be unstable and have poor structure. Such soils probably benefit from tillage for improvement of infiltration & root penetration • Herbicides are too expensive for lower income countries • Results have been disappointing, e.g. in N. Ghana, it was found that minimum tillage was not as effective as ridging, mulching, or even traditional mixed cropping. More research required. • Analyse each situation independently

35. Intercropping & mixed cropping • Difference between intercropping and mixed cropping • Advantages • Depresses weed growth • Increased return / unit area (but probably not if stressed due to water shortage) • Reduced lodging • Reduced soil erosion because of better ground cover • Legume intercrop increases soil fertility (N producing bacteria) and improves neighbouring crop

36. More efficient use of available water (e.g. Ikeorgu, 1987) • Insurance (pests, diseases & drought may affect one crop but not the other) • Shading effect often beneficial (e.g. coffee shaded by bananas) • More difficult for disease to spread especially in case of intercropping • Sometimes an ad hoc mixed cropping system is adopted in which farmers fill in gaps in crops planted earlier which have failed to establish well

37. Disadvantages • System is difficult to manage, i.e. to decide on relative mix and spacing • There may be negative (i.e. competition) as well as positive interactions. The situation is complex. • There is some doubt about the benefits of its adoption in lower rainfall semi-arid areas

38. Examples of complex intercropping practices in eastern Gujarat, India

39. Growing crops in dry season on residual moisture • It is sometimes possible to plant a crop at the end of the rainy season that is capable of growing on residual moisture possibly combined with light rain in a second (usually shorter) rainy season [if there is a bimodal distribution] • Examples from India are shown in the diagram.

40. Mulching • Stubble mulch • needs a lot, e.g. 10 tons/acre is typical; 5 t/ha for straw/crop residue mulches is a common recommendation • termites may eat it but if that encourages termite activity in soil also they will benefit the soil by inducing increased infiltration • stubble absorbs water rather than allowing it to infiltrate; later evaporates directly • may increase disease risk, there is a need for frequent inspection

41. crop residue is often needed for feeding to animals especially if used for ploughing • incorporation of residue may depress N at critical time • sometimes removed weeds are used; left on surface between crop rows • can reduce weed growth by preventing sunlight reaching surface • greatly reduces runoff so mulch is good for soil conservation • can reduce splash erosion e.g. Sesbania sesban prunings used @ 5t/ha combined with 50 kg P/ha 1 to 4 weeks before planting, or after planting (but not at planting) • for e.g. from IITA, see following table

43. Grass mulch • 1 to 4 t/ha a typical recommendation • may be difficult for farmers to obtain - conflict with fodder requirements

45. Dust mulch • works on basis that unsaturated hydraulic conductivity of course pores is lower than that of finer pores • dust produced by tillage • increases erosion risk • there is some evidence of negative effects on water availability & reduction of evaporation

46. Gravel mulch • very expensive, only used in horticulture • occasionally, there are naturally occurring gravely soils which local farmers recognise as reducing soil evaporation. • Surface stones are a problem for cultivation, but there may be advantages too. Possibly use large stones only for stone bunds and leave smaller stones on surface but many workers recommend that stone bunds should have a grade of sizes from large to small to improve filtering capacity

47. Others e.g. Vermiculite, ash, sand, sawdust, bark

48. Soil nutrition • Erosion has a great effect on soil nutrition - and vice versa. Hudson said that a bag of fertiliser may control erosion better than a physical structure • The concentration of nutrients in eroded soil is usually higher than the soil from which it was eroded. • The ratio is known as the “Enrichment factor” which is of course a misnomer, “impoverishment factor” may be better. For Kenyan soils, the ratios are in the following ranges: • 1.1 to 2.1 for P • 1.1 to 2.8 for Ca • 1.1 to 3.0 for Mg • 1 to 2.3 for K

49. 10 t/ha/yr of eroded soil • may be equivalent to • 40 kg N/ha/yr (2 bags). • In Zimbabwe, • 30 t/ha/yr of eroded soil • = 50 kg N/ha/yr plus 5 kg P2O5 • [with a replacement cost of $20 to $50]

50. Effect of soil erosion is hidden in many temperate region countries. Option of soil mining, with massive replacement of nutrients may not be open to many farmers in LDCs. • Rapid early growth and good cover is desirable for Soil Conservation, so irrespective of yield, adding fertiliser may be beneficial • Drought will usually suppress benefits of application of N (e.g. Chinene & Pauwelyn, 1987)…..