**12. Soil fertility and nutrient management of sweet potato crops**

Sweet potato is a nutrient-intensive crop, and it depletes soil nutrients when cultivated continuously without application of adequate nutrients [6]. A crop yielding 35 t/ha of edible roots will remove from one hectare of land 151 kg nitrogen (N), 28 kg phosphorus or 64 kg P2O5, 263 kg potassium (K) or 328 kg K2O, 46 kg calcium (Ca) and 18 kg magnesium (Mg) in vines and roots [4]. In another case, for a tuber yield of 10 t/ha, the crop removes 50 kg N, 22 kg P2O5 and 100 kg K2O [2]. These data help us to determine the ratio of major nutrients required for sweet potato as 2.5 N−1 P2O5−5 K2O.

Potassium is essential for the synthesis and translocation of carbohydrates. For sustainable high yields, it is important to maintain soil K level at 200 parts per million (ppm) and plant tissue K concentration at 3.8% [70]. The next important nutrient is N which improves crop growth and development and root yields. However, excess N application promotes excessive vegetative growth at the expense of storage roots development [62, 63]. Sweet potato crops can tolerate low soil P levels because their roots are highly efficient in absorbing P from soil due to their association with vesicular-arbuscular mycorrhizae [4]. As Ca deficiency can lead to reduced root growth, lime or gypsum is applied to increase soil Ca levels to 1500 ppm minimum [70]. Among the micronutrients, boron (B) and manganese (Mn) are important; application B prevents blistering disorder in sweet potato [71].

Balanced nutrient use is important because deficiency of any one nutrient can depress the uptake of other nutrients. For example, crop response to N is poor when K is deficient in soils [72]. Calculated nutrient requirements will be 5.0 kg N, 2.2 kg P2O5 and 10 kg K2O for every tonne of root yield. The entire P and half of the N and K are incorporated into the soil at planting, and the second half of N and K is applied *Sweet Potato (*Ipomoea batatas *(L.) Lam): A Review of Modern Varieties and Production… DOI: http://dx.doi.org/10.5772/intechopen.106586*

and covered with soil at 35–40DAP during the second weeding and earthing-up operations. Deficiency of Ca and micronutrients, if any, must be corrected as per local recommendations. Point placement or banding of inorganic fertilizers increases nutrients uptake and reduces fertilizer requirements. The combined application of organic amendments and supplemental fertilizers is called integrated nutrient management (INM), and it helps to increase soil carbon stocks, which in turn will improve the water and nutrients retention capacity of soils and at the same time enhance nutrients uptake by crops [73, 74]. Organic inputs help neutralize soil acidity and replenish K, Ca, and Mg in soils [75].

In practice, only moderate amounts of fertilizers recommended for sweet potato in most countries: 5–15 t/ha of cattle manure plus 50–60 kg N, 30–60 kg P2O5, and 75–120 kg K2O per hectare [41]. In Africa, where fertilizers are scarce and expensive, micro-dosing of fertilizers by applying small amounts of fertilizers to individual planting holes reduces fertilizer rates and increases crop uptake of nutrients [76].

### **13. Weeds and their management**

Weeds compete with sweet potato crops for space, water, nutrients and light, thereby inhibiting crop growth and reducing crop yields. Weeds can also serve as alternate hosts for insect pests, pathogens, and vectors of diseases and provide a safe habitat for rats which feed on storage roots.

Weeds are a problem in sweet potato during the slow early growth stages (up to 40–45 DAP). Reduction in yields could be as high as 20–80% [77]. Farmers should adopt integrated weed management (IWM) strategies by using a combination of cultural, manual, mechanical and/or chemical weed control methods. Cultural weed control methods include good land preparation and levelling; selection of sweet potato varieties with vigorous early growth and long trailing vines; practising crop rotation and intercropping; surface soil mulch with crop residues; and reducing weed "seed bank" in soils. For manual weed control, two times digging the soil and earthing up around the base of plants at 15–20 DAP and 30–35 DAP are recommended. Mechanical weeding involves inter-row or inter-ridge cultivation and earthing up that uproots and buries young weeds. Finally, for chemical weed control, farmers can use herbicides [78].

### **14. Pruning**

Under favourable rainfall and temperature and/or high N application, sweet potato grows vigorously and produces large amounts of vines at the cost of roots formation [62, 63]. If this happens, the top parts of the vines are cut from 75 DAP and used as greens in cooking, as fodder for animals, or as planting materials for a new crop.

### **15. Management of insect pests and diseases of sweet potato**

Biotic constraints include insect pests and diseases. Sweet potato weevils are the most important insect pest, stem wilt and black rot are the two key fungal diseases, and sweet potato virus disease is the most destructive virus disease.

For control of insect pests and diseases, integrated pest management (IPM) is recommended. The three cardinal principles of IPM are (i) growing a healthy crop by adopting resistant varieties and disease-free planting materials as well as best crop management practices; (ii) field sanitation and maintaining a healthy agroecosystem; and (iii) the strategic use of external pest control inputs that are known to have a minimal impact on the agro-ecosystem and the surrounding environments and communities. IPM is thus a knowledge-intensive approach, one which requires systematic learning through continuous observation and practice (e.g. via Farmer Field Schools) [79].

Strict enforcement of phytosanitary measures at national borders is crucial to stem the spread of sweet potato virus diseases from one region to another or from one country to another.

Farmers currently can use any or any combination of the six IPM tactics: (i) genetic-host plant resistance; (ii) cultural control; (iii) mechanical methods; (iv) biological control; (v) use of safe botanicals and microbial bio-pesticides; and (vi) judicious chemical control with environmentally safe pesticides [80]. Specific IPM tactics are given below for individual insect pests and diseases.

#### **15.1 Key insect pests and their management**

*Sweet potato weevils (Cylas spp.):* The species *C. formicarius* is important in Asia, Oceania, the Caribbean and the USA, while *C. puncticollis* and *C. brunneus* are common in Africa. Adult weevils feed on epidermis of vines and leaves as well as surfaces of roots. The larvae of the weevils are the most damaging by their tunnelling of vines and roots. In response to attack by larvae, storage roots produce toxic terpenes, which render storage roots inedible even at low concentrations and low levels of physical damage.

Cultural control involves the use of weevil- and virus-free planting material; good field sanitation by removing volunteer plants and crop residues as well as alternate hosts in nearby areas; timely planting and prompt harvesting at maturity; flooding the field for 24 h soon after harvest; minimizing soil cracking by proper irrigation; and crop rotation. For biological control, fungi *Beauveria bassiana* and *Metarrhizium anisopliae* and the nematodes *Heterorhabditis* spp. and *Steinernema* spp. are used. The fungi attack and kill the adult weevils, whereas the nematodes kill the larvae. Finally, in severe cases, malathion 0.1% or carbaryl 0.2% is sprayed on the crop twice at 10-day intervals [41].

*Leaf-eating caterpillars*: The moths hover over the flowers around sunset time. The caterpillars feed gregariously on leaves. Deep ploughing after harvest and spraying Endosulfan @ 0.05% are recommended to control this pest.

#### **15.2 Important diseases and their management**

*Stem rot or wilt*: is caused by *Fusarium oxysporum* f. sp. *batatas*. This soil-borne fungus can survive in soil and in debris for several years. In affected plants, leaves turn yellow and wilt and then the vines die. It can be controlled by using disease-free vine cuttings and/or dipping vine cuttings in fungicide solutions [41].

*Black rot*: is caused by the soil-inhabiting fungus *Ceratocystis fimbriata*. Infected plants show dark to black sunken cankers in lower stems, and then the leaves turn yellow and the plants wilt and die. Transmission occurs through wounds made by sweet potato weevils, wireworms, crickets, and mice. Control measures include field *Sweet Potato (*Ipomoea batatas *(L.) Lam): A Review of Modern Varieties and Production… DOI: http://dx.doi.org/10.5772/intechopen.106586*

sanitation; use of resistant varieties and disease-free vines; and dipping the vine cuttings in 0.2% Aretan or Agallol solution before planting [41].

*Foot rot*: is a minor fungal disease caused by *Plenodomus destruens*. Brown lesions form on the stem at or below the soil surface. Plants wilt and die in severe cases. It spreads through infected roots and stems.

*Charcoal rot*: is the fungal disease of the fleshy roots in storage, and it is caused by *Macrophomina phaseolina*. Good aeration and sanitation of the warehouse will prevent this disease.

*Sweet potato virus disease (SPVD)*: Plants attacked by this virus are stunted with small and narrow chlorotic leaves with mottled pale spots. Using resistant varieties and SPVD-free planting materials will help control this disease [35] and also increase yields by 40% [81].

### **16. Harvesting and postharvest processing**

#### **16.1 Harvesting**

Sweet potato is ready for harvest when leaves turn yellow and drop. After maturity, storage roots can be kept in the ground for 1–2 months and harvested progressively as and when required. Its wide harvesting window allows it to act as a famine reserve food crop and is invaluable in managing labour schedules and in improving family's cash flow by selling the roots in local market over an extended period. Although root yields can increase with delayed or progressive harvesting, root quality declines and attack of roots by weevils and fungi increases [78]. The field is wet by a flush irrigation 2–3 days before harvest to facilitate easy lifting of the roots; the vines are cut and the storage roots are lifted carefully without causing any injury to the roots [45].

#### **16.2 Postharvest management**

Huge losses (40–80%) of storage roots occur during and after harvest due to high temperatures of 32–35°C and high RH of 80–95% prevailing at harvest time in the tropics [25] and inefficient handling, storage and transport of the harvested roots [23–25]. Being bulky and tender, losses in storage roots occur in both quality and quantity due to physical injury, sprouting, attack by weevils and fungal diseases, and loss of weight [25]. Therefore, sweet potato storage roots must be handled carefully at all stages.

*Curing*: It is intended to heal injuries, if any, caused during harvesting and to keep the roots in good condition for marketing or to preserve 'seed' roots for the next crop. Soon after harvest, the roots must be cured in well-ventilated curing rooms which provide adequate circulation of air for breathing of roots and to prevent excessive condensation of moisture [41]. Curing efficiency varied with dry matter content of storage roots and curing periods; roots with low dry matter content have high curing efficiency [25]. Curing methods such as fungicide treatment, bio-control, gamma irradiation, hydro-warming, and storage in sand and saw dust helped limit spoilage of roots and enhanced shelf life of sweet potato roots [25].

*Storage*: Red-skinned varieties store better than white-skinned varieties. An ideal storage room must be maintained at 15 °C and 85% relative humidity (RH). Two commonly used, inexpensive storage methods are pit storage and indoor storage. In pit storage, a pit covered with straw is used to store roots for 2–3 months [41]. Storage in

sand and saw dust helped limit spoilage and enhanced shelf life of sweet potato roots [25]. Some farmers store the roots inside their houses, while others keep the roots in soil and harvest as and when required. In developed countries, refrigerated storage and transport are used to keep the roots fresh for long periods of time [41].

*Processing and value addition*: In addition to simple boiling and eating, sweet potato roots can be used to prepare several convenient foods: flour, chips, crisps, breads, biscuits, pies, candies, and juices. Coloured sweet potato flour is used in various bakery and noodles preparations.

Sweet potato roots are used to produce starch, glucose, sugar syrup, and industrial alcohol [82]. Sweet potato starch is used in textiles, paper and food manufacturing industries, preparation of liquid glucose and adhesives. Yellow- and violet-fleshed sweet potatoes are used to extract beta carotene and anthocyanins that are used as natural food colours and antioxidants [82]. Enzymes such as sporamin and betaamylase are also produced from storage roots.
