Millet: Soil Management

General climatic and soil requirements for pearl millet

Pearl millet is a warm season crop and is sensitive to frost. Temperatures of around 28 to 30 0C are regarded as suitable for crop growth. Pearl millet is generally sensitive to low temperatures during the seedling and flowering stages. The crop grows well in a wide range of rainfall regimes ranging from 200 to 1,500 mm per annum although 250 to 700 mm is considered as the ideal range. The rainfall is preferably evenly distributed throughout the growing season. Too much rainfall during flowering is reported to cause yield reductions. The seed matures best in a non-rainy, dry and cool climate.

Even though millets are drought tolerant, prolonged dry spell can reduce yields signifi cantly. But its rapidly developing root system, the special mechanism of its root system to prevent desiccation during periods of moisture stress, and its high tillering capacity make pearl millet tough to drought.

Millet grows well in deep to loamy sands but performs best on deep well drained fertile soils. Deep soils are ideal, as the pearl millet roots can grow to nearly 3.6 m depth. Pearl millet also performs relatively well under acidic soil conditions. But the crop does not grow well in calcareous soils.

Diversification strategies in millet production

There are many ways to diversify a millet cropping system. It can be achieved through growing different millet species or varieties, or growing millet together with other crops – in rotations or in mixtures.

Mixed cropping

In traditional cropping systems, millet is mostly grown together with other crops.

The reason lies in the numerous advantages associated with mixed cropping such as higher total and safer yields, better use of the resources, and cultural advantages such as better weed control and soil protection. In West Africa, pearl millet is often intercropped with other cereals like other millets or sorghum, or with legumes like cowpea or groundnut. Intercropping patterns vary depending on rainfall regime and other factors such as crop preferences. Intercropped legume and millet are usually sown in alternating rows. In the case of cowpea, growing two rows of millet and four rows of cowpea has proved more productive than alternating single rows of both crops. Sowing times, varieties and cropping patterns should be chosen in a way that avoids competition by the legume for water, nutrients and light. Cowpea may be sown two to four weeks after millet.

Longer growing seasons offer greater possibilities of adapting the system. Intercropping of millet with a drought-tolerant legume in general increases productivity of both crops compared to cultivation of a sole crop.

Cultivation of nitrogen fi xing trees in rows increases the diversity of the cropping system, while offering additional, highly nutritional forage for livestock, enriching and protecting the top-soil with the fallen leaves in the wet season, fi xing nitrogen on its roots and drawing nutrients from deep soil layers. All these effects contribute to improvement of yields of millet. Although millet prefers unshaded conditions, it will profi t from improved nitrogen supply and soil conditions in proximity of the trees. Drought stress, phosphorus defi ciency, and increased bird damage (harboured by the trees) may though limit advantages of diversifi cation of the farming system with trees.

Crop rotation

Pearl millet grows well after a legume or root crop. Rotation of millet with legumes allows growing the legume at higher density than it is possible in an intercropping system. Depending on the species, their nitrogen fi xation rate and their use (feed for livestock, incorporation into the soil as green manure, grains for human consumption), legume crops grown prior to millet will have a more or less positive impact on the yield of the following millet crop. Rotation (and to a smaller extent intercropping) of millet with legumes furthermore reduces infestation of the parasitic weed Striga, an economic weed for millet. Rotation of millet with fallow, which is another possibility of diversifi cation that is traditionally used, does also contribute to better growth of millet, but in general does not substantially improve soil fertility, and regeneration of soil fertility is slower under natural fallow.

For more information on crop rotation see section on soil fertility management.

Improving soil fertility

Pearl millet is highly responsive to increased soil fertility although it is reported to be a crop with low nutrient demands, requiring less nitrogen than sorghum.

Therefore, it is worthwhile for millet farmers to invest in soil fertility.

Instead of applying mineral fertilizer, which includes costs and may increase the risk of moisture stress to the crop, farmers look at avoiding loss of soil fertility and supplying fresh or decomposed biomass to the soil, and practicing planned cultivation of leguminous crops to collect nitrogen from the air.

Nutrient requirements of pearl millet are modest compared to other cereals like sorghum or maize. Nitrogen requirements of millet are around 25 to 35 kg per hectare for grain crops and common seed rates. Requirements are higher, if planting densities are higher. Excess nitrogen may produce tall crops that are prone to lodging. Phosphorus requirements of the crop are 5 to 10 kg per hectare.

In very acidic soils, application of lime 3 to 6 months prior to planting millet allows for the lime to react with the soil and improve availability of the applied phosphorus. The quantities of lime to apply depend on the amount of acid to neutralize in the soil (or the target pH to be reached after liming). Advice on this is best based on laboratory analyses.

Organic approaches to soil fertility management in millets

Basically, there are several approaches to increasing soil fertility in millet production: i) Preventing soil and organic matter loss, ii) Growing crops that feed the soil in rotation or together with millet, iii) Adding manures, compost and other organic amendments to the soil before and after planting.

Preventing soil and organic matter loss - soil and moisture conservation

A useful approach to solving the land shortage problem due to degradation is to build the productivity of the existing land. Special attention is necessary to prevent loss in soil fertility, as the soils favourable to millet production are generally sandy and are extremely prone to degradation. Farmers therefore need to conserve the soil by preventing the loss of top soil through erosion and conserving soil organic matter.

Soil protection measures include (these are discussed in more detail in Module 2: Soil Fertility Management) i) growing a soil cover (intercropped with the millet crop, or grown after the millet), ii) mulching (with straw, crop residues etc.) iii) relay intercropping to increase soil cover, and iv) construction of barriers and terraces to hold the soil in place.

When growing millet on slopes, the fi rst measure to reduce soil erosion by water run-off is to plant millet across the slope and dig trenches and build bunds along the contour lines. Farmers are discouraged from burning plant residues or burning fallow fields. Instead, they are encouraged to retain residues in the field to protect the soil and also to provide livestock feed. If not collected for use as thatch or reserve fodder, any remaining residues at the end of the dry season can be gathered and placed in trash lines along the contour ridges or at the periphery of the field where they will help to stabilize the soil and reduce erosion.

Integrating leguminous crops into millet production

Planned rotation or intercropping of millet with leguminous food crops or green manures improves soil fertility, hinders the build-up of pest populations, diseases and weeds, and reduces the risk of total crop loss in cases of drought.

Intercropping

Leguminous crops like cowpea, pigeon pea, green gram, chick pea or soybean fi x atmospheric nitrogen during their growth. Part of this nitrogen becomes available to the following crop such as millet. Other crops commonly intercropped with millet include pumpkins, melons, okra, cassava, indigenous cucumbers, indigenous vegetables and others. They help to suppress weeds and provide cover to the soil while providing a good source of nutrition to the household. When short duration varieties of millet are grown, in a good rainfall season a second crop of a short duration legume can be planted after the millet has been harvested (or in relay). Besides improving the soil, this helps to provide the household with a good source of protein (if a grain legume has been planted), or livestock feed.

Rotations with food and non-food legumes

When food legumes such as cowpeas are rotated with a millet/cowpea intercrop or sole millet crop, productivity of the millet and soil fertility can be increased signifi cantly. When grown in between millet seasons or during fallow periods and incorporated into the soil green-manure crops like jack beans, perennial peanut or mucuna, will add substantial amounts of organic material to the soil. This will help to feed soil organisms and enhance their activity, and as a result improve nutrient supply to the millet crop. Pearl millet is sometimes rotated with non-legume food crops or cash crops. The types of crops used in the rotation however will vary among farms and among different geographic and agroecological regions.

Farmers with livestock are more likely to grow green manure legumes in rotation with millet as this would increase their options for dry season feed. In areas where markets for legume crops exist, the farmers are also more likely to grow legumes in rotation with millet so as to generate income to the households. In such a case, while income generation will be the main objective of growing the legume, the rotations would help to break cycles of pests and diseases and to improve the soil.

Different legumes grown in rotation or mixtures with millet have different capacities to fix nitrogen. Their ability to fix nitrogen is further affected by the growing environmental conditions such as soils. In the sandy soils, where millets are commonly grown, nitrogen fi xation by legumes is affected by low soil fertility, as most legumes have lower capacity to fi x nitrogen under conditions of low phosphorus. Amendments which help to improve the phosphorus content of soils, like applying rock phosphate, can help to improve the performance of legumes. The nitrogen fi xation capacity of legumes is also compromised in acidic soils. Lime can be applied to increase soil pH (and reduce acidity) and create a more conducive environment for the legumes and subsequent crops. For other legumes such as pigeon pea, which are deep rooted, breaking the hard pans in the fields through practices such as ripping will also help them grow better.

Addition of organic materials

In many areas, nitrogen and phosphorus are the main limiting nutrients in millet production. Typical symptoms of phosphorus defi ciency are stunted plants, reduced tillering and discolouration of leaves, while indiscriminate yellowing of leaves indicates nitrogen defi ciency. It is reported that the nitrogen demands for pearl millet can be met from organic sources since modest quantities are required compared to other major cereals such as sorghum and maize.

Application of farmyard manure

Regular addition of organic materials to the soil from farmyard manure or compost improves the availability of nutrients to the millet crop. The use of farmyard manure is often constrained by the limited availability of suffi cient quantities.

In general, most of the manure available is of low quality prompting the need for higher application rates. The ideal application rates reported for farmyard manure range from 2 tons per hectare to 7.5 tons per hectare for a rain fed crop.

Higher application rates of up to 15 tons per hectare are recommended for hybrids and high yielding varieties cultivated under irrigated conditions. In Niger in zai pits targeted application of manure at 300 g per plant was very successful.

The optimal application rate was reported at 3 tons per hectare.

To be more effective, the manure needs to be applied before ploughing and then properly incorporated during ploughing. Effectiveness of the manure also depends on other factors such as its state and composition at time of application, timing of incorporation. In the double cropping system (wheat-rice and maize-millet) of Nepal it was demonstrated that yields of maize, millet and rice were greater when manure rather than mineral fertilizer was applied. In Niger, an intercrop of millet and cowpea yielded between 11 and 18 % more grain compared to yields from a pure millet field. Application of manure was also found to signifi cantly increase the combined millet-cowpea biomass.

Corralling the farm animals during the nights on the fields during the dry season or allowing them rotationally to feed on plots destined to millet production, simplifi es manure application by reducing the labour required in gathering, transporting and spreading the manure. However, the limitation with this approach is that the livestock droppings will not be well decomposed and the crop may not maximize benefi ts from it.

Where manure or compost are in short supply, the benefi ts to the crops can be increased by banding the manure or compost in the furrows or zai pits where the millet will be planted. Targeted application is only possible with row planting, when seeds are sown in furrows or in lines on a fl at surface.

Application of ‘organic’ mineral fertilizers

Some mineral fertilizers, naturally occurring and used in that form, are permitted in organic production. Before use in certifi ed organic production, farmers are encouraged to consult with their extension experts or certifying agents about the use of different fertilizers. Based on the East Africa Organic Standards which are also in compliance with the IFOAM Standards, lime and rock phosphate are some of the mineral nutrient sources permitted in certifi ed organic farming. In their research, ICRISAT demonstrated that millet yields can be signifi cantly increased by applying phosphorus in millet/cowpea intercrop systems.

Use of biofertilizers

Azospirillum, a biofertilizer, can be used for organic production at 2 kg per hectare.

To facilitate its application, the Azospirillum can be mixed with manure or soil and applied at the fi nal ploughing or at sowing. Use of this biofertilizer is reported to enhance utilization of applied nitrogen sources by the plants. Alternatively, the biofertilizer can be used to inoculate millet seedlings before transplanting.

To prepare the Azospirillum (biofertilizer) solution 1 kg of Azospirillum is added to 40 litres of water. Then, before transplanting, the roots of the seedlings are dipped in this solution for 15 to 30 minutes.

Practicing suitable agroforestry techniques

When clearing land for cultivation, farmers in different parts of Africa retain certain tree species within the fields to provide fruits, fi rewood, medicine and other products, and services such as shade. They are aware of the positive interactions between some trees and crops. Acacia species, baobab (Adansonoa digitata),

Faidherbia albida, African Locust Bean Tree or Néré (Parkia biglobosa), marula tree (Sclerocarya birrea), Strychnos species, and Ziziphus species are some of the trees often found growing in millet fields. The trees help to provide shade, supply nutrients to the growing millet crop and protect it from strong winds. The crop also benefi ts from nutrients released during decomposition of animal droppings left when animals rest under the trees during the non-cropping season. Products from some of the trees, e.g. pulp from fruits of Acacia digitata can be mixed with millet porridge to enhance its taste and improve the vitamin content of the meal.

One major setback though about retaining or planting trees in the millet fields is that bird damage to the millet can increase, as the trees will provide a good habitat for the birds.

Moisture and nutrient management using the Zai pits

The zai pits is a planting techniques used in dry parts of West and East Africa to harvest water and to help concentrate nutrients where the crops will grow. This system can help farmers to conserve moisture and to target application of the often scarce organic soil inputs. The little available water and the little organic soil inputs are used more efficiently resulting in better grain and biomass yields.

A number of case studies report improved yields of millet when grown using the zai system in West Africa. Using the zai system versus the normal planting on the fl at increased millet yields in Niger by 3 to 4 times.

Millet biomass is a good source of fodder in dry regions. Techniques, which help to increase biomass yields therefore help to make more fodder available to the livestock. Although the results from various researches point to increased grain and biomass production, the responses are likely to vary from site to site depending on many other factors such as overall management, timing if planting, pest and disease control, weeding practices etc.

Proper water management

In Africa, millet is commonly grown as a rain-fed crop in dry climates. It is very drought resistant and under conditions of low soil-water the crop is generally more productive than sorghum. The soils in which millet is grown are generally sandy with low moisture holding capacity, and are generally defi cient of organic matter. Although it tolerates low soil moisture conditions, millet responds very well to additional water supply through better water harvesting and conservation practices or irrigation thereby producing higher yields.

Evaporation from the soil surface constitutes a large proportion of evapotranspiration.

Practical methods of reducing evaporation from soils to conserve water are very important in millet production. The use of crop residues as mulch or soil cover is one solution except where they are fed to livestock during the dry season, or consumed by termites. Early/timely planting is another important strategy to ensure that the growth pattern of the crop coincides with the rainfall pattern.

Millet needs little water after germination, a small amount as the leaves appear, and light rain during the growing period. Moisture stress during flowering through to grain formation reduces yields, as does heavy rainfall. During grain development hot and dry weather is needed.

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