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close this book Soils, Crops and Fertilizer Use
close this folder Chapter 10: Fertilizer guidelines for specific crops
View the document Cereals
View the document Pulses (grain legumes)
View the document Root crops
View the document Vegetables
View the document Tropical fruit crops
View the document Tropical pastures

Root crops


Basic Facts on Cassava

Cassava is a drought-resistant tuber crop known for its adaptation to poor soils. It's the 4th most important en;ergy source in tropics after rice, maize, and sugarcane. Though its roots are very low in protein, they are an excellent calorie source. In many areas, the leaves are also consumed (cooked) and are rich in protein (about 30% on a dry weight basis), vitamin A (as carotene), vitamin C, and folic acid. They are also a fair source of iron and calcium. Two cassava leaves provide a child with enough vitamin A for a day and cook down to a volume of only 15 cc (1 tablespoon).

The tubers and the leaves contain varying amounts of toxic hydrocyanic acid (HCN, prussic acid). Varieties are grouped into "bitter" (high HCN) and "sweet" (low HCN) types. Even the tubers of the sweet varieties must first be detoxified by peeling (most of the HCN is in the peel), followed by cooking, roasting, or sun drying. The bitter varieties are often used for commercial starch and alcohol production since they tend to be better yielders.

Cassava roots are ready for harvest from 8-1S months after planting cut sections of the stem about 25-30 cm long. For pure stands, a density of about 10,000 plants/hectare seems to be best. Although the tubers store well in the ground if harvesting is delayed, they spoil within a few days, once dug. Harvesting of the leaves will decrease tuber yields which average about 9600 kg/ha. Experimental yields of 80,000 kg/ha or more have been obtained. On poor soils, under low-moisture conditions, yields drop to about 1000-2000 kg/ha.

Cassava is unusual in that it has no critical period after establishment where drought will greatly lower yields. Surprisingly the bulk of the plant's roots are quite shallow but have the ability to proliferate in response to moisture stress. It is considered to be potentially one of the most efficient carbohydrate (energy) producers under adverse farming conditions in tropical areas and is also a relatively low-management crop.

Fertilizer Needs of Cassava

Cassava has an unusually good tolerance for very acidic soils with their high levels of soluble aluminum which would injure other crops. Although it is well adapted to lowfertility soils, it responds very well to organic and chemical fertilizers. Some agronomists feel that cassava extracts large amounts of nutrients (esp. K) from the soil, which nay lead to fertility exhaustion after several years of intensive cropping, unless nutrient additions are made. However, research has shown that, except for K, cassava actually uses fewer soil nutrients than other crops per unit of dry Batter produced. It is believed that mycorrhizae root fungi (see Chapter 1) play an especially important role in aiding the cassava plant's uptake of P on low-P soils.

NPK Needs: Excessive N will favor leaf production over tuber growth, so recommended rates fall in the range of 50-120 kg/ha. P is the nutrient most likely to be deficient in much of Latin America, but N and K shortages are more common in Africa and Asia. Recommended rates of P2O5 range from 60-130 kg/ha. Cassava has one of the highest K needs of any tropical crop, and rates range from 60-150 kg/ha of X20. Split applications of K are often recommended, especially where leaching potential is high.

Application Methods: Apply all the P and K, along with about 1/3-1/2 of the N at planting. The band or half-circle method can be used; avoid broadcasting on soils with a high P tieup capacity. The rest of the N can be sidedressed in 1-2 applications between 1 and 3 months after planting. (Under high rainfall, the K dosage should be divided into 2 applications.)


Basic Facts on Potatoes

Worldwide, potatoes rank 4th in total production after wheat, maize, and rice, although production in the tropics is often restricted by high temperatures. Potatoes are often erroneously maligned as a low-quality, "fattening" food. While not a rich protein source, they have twice the protein content of cassava or sweet potatoes, and the amino acid quality rivals that of meat. They are very low in fat (1%) and are a fair source of vitamin C. A 140 g potato (5 oz.) has about 100 calories compared with 270 for an 85 g (3 oz>) hamburger. This portion will provide about 4-5% of the daily calorie needs for an adult, along with 6% of the protein, 35% of the vitamin C, 10% of the iron, 20% of the vitamin Be, and a number of other nutrients.

In recent years, potato production has made a rapid expansion into tropical and subtropical areas for several reasons. Potatoes produce more edible energy per unit of time than almost any other crop, including maize and cassava. They are in high demand throughout the Third World and co- and a good price.

One limiting factor is the high cost of production (about $1000/hectare), largely due to the volume and cost of the required "seed" (whole or cut potatoes). Usually 1000-2400 kg/ha are needed. Recent innovative research led by the CIP (Internal. Potato Center in Lima, Peru) has devised new methods of propagation such as true potato seed (TPS), tuberlets produced from TPS or leaf bud cuttings, and tissue culture from stem cuttings.

Adaptation: Potatoes prefer cooler temperatures and will withstand light frosts. The best yields are usually obtained in areas where the mean daily mean temperature (average of high and low) doesn't exceed 20-21 C (68-70F) during the tuber formation period. Higher temperatures depress tuber production, since the plants tend to respire (burn up) much of the starch they produce instead of storing it. (Higher temperatures are OK during early growth). The yield-depressing effects of high daytime temperatures can be partially offset by cool nighttime temperatures. Potato varieties vary a lot in their heat tolerance. Recent breeding work spearheaded by the CIP has led to the development of more heat-tolerant varieties.

Growth Stages: Most varieties mature in about 100-125 days after planting the seed pieces. Emergence occurs about 2-4 weeks after sowing, and tuber formation begins about 3 weeks later (it has nothing to do with flowering). Full maturity is often not attained due to defoliation by fungal leafspot diseases like early and late blight. Potatoes require more skill and care to grow than most other field crops and are prone to many leaf and tuber diseases.

Fertilizer Needs of Potatoes

Potatoes are heavy feeders and respond very well to organic and chemical fertilizers, especially since their root system is small and tubers develop over a relatively short time period. They prefer a soil pH of 5.0-6.5 and are fairly tolerant of acidity. One way of controlling potato "cab disease (a soil fungus; Streptomyces scabies) is to maintain the pH below 5.5.

Nitrogen: Overfertilization with N favors top growth over tuber growth, but most improved varieties will show a good response up to 110 kg/ha of N or ave. Recommended rates range from about 50-80 kg/ha for Third World small farmers.

Phosphorus: P increases the number rather than the size of tubers, shortens maturity, and improves quality. Rates as high as 100-200 kg/ha of P2O5 are recommended for low-P soils and should be banded, not broadcast.

Potassium: Potatoes have especially heavy K needs, and even high K soils may become depleted after a few years of potato growing. Rates for medium K soils run about 50-100 kg/ha of K2O. with even higher amounts for low-K soils. At K,0 rates much above 50-60 kg/ha, potassium sulfate should be used instead of potassium chloride (muriate of potash), because excess chloride lowers the starch content ~nd quality of the tubers.

Magnesium deficiencies are sometimes a problem in acid soils below pH 5.5. When liming, use dolomitic limestone. Epsom salts (magnesium sulfate) can be applied to the soil at 200250 kg/ha, or plants can be sprayed with a solution of 2.0-2.5 kg of epsom salts in 100 liters of water.

Hunger Signs in Potatoes: See Appendix B.

Application Methods for Chemical Fertilizer: Apply 1/3-l/2 of the N and all of the P and K at planting in a band about 5-7.5 cm (3-4 fingers-width) to the side of the seed row and 6 cm below its depth. Sidedress the regaining N about 40 days later in a band about 25-30 cm out from the row. The N can be worked into the ground a bit by combining the sidedressing with a weeding or hilling-up operation.


Sweet potatoes are an excellent energy source and are also low in fat like other root crops. The orange-fleshed varieties are very high in vitamin A (as carotene]. One average sweetpotato (5 cm x 12.5 cm) provides about half the daily adult requirement of vitamin C, along with twice the vitamin A needed (if orange-fleshed). In many areas, the leaves are also consumed either fresh or cooked and are good sources of vitamin A, vitamin C, folic acid, iron, calcium, and potassium; they also contain a fair amount of protein.

Unlike Irish potatoes, sweet potatoes are a ware-weather crop; the roots are ready for harvest in about 4-5 months. ID the tropics, planting is usually done with vine tip cuttings about 30-40 cm long. About 2/3 of the cutting's length (at least 4 nodes) should be covered with soil and the remaining third left exposed. (Tubers originate from the buried nodes.) Cuttings are hardy and begin rooting in just 2-3 days. A recent study in Puerto Rico showed that pre-rooting the cuttings by holding them for 2 days before planting increased the number of tubers and the yield; however, removing the the leaves from cuttings to reduce transpiration decreased yields. Plantings can also be started by planting "slips" (young plants produced by planting tubers densely in a nursery bed). Vine tip cuttings have the advantage of not spreading soil-borne sweet potato diseases.

Fertilizer Needs of Sweet Potatoes

Both organic and chemical fertilizers give good responses. Excessive amounts of N will favor top growth over root growth, so rates of 50-80 kg/ha are recommended. Phosphorus rates range from about 40-70 kg/ha of P2O5 5. Sweet potatoes are heavy K feeders; on low-K soils, rates of 80-130 kg/ha of K2O are recommended. Boron deficiency sometimes occurs and can be treated by mixing 5-6 kg/ha of borax (11% boron) with the NPK fertilizer; this equals only 0.5-0.6 grams of borax per sq. meter. Higher rates may cause plant injury.

Apply 1/3-1/2 of the N and K at planting tine, along with all of the P. Sidedress the remaining N and K in 1-2 applications about 1-2 months after planting. Use the band method at planting. If planting is done on high ridges, the NPK fertilizer can be applied in a band running right below the future plant row; the ridge can then be built right over it and will separate the fertilizer with a enough soil HO that burning won't occur.

Hunger Signs in Sweet Potatoes: N deficiency causes the leaves to turn light green to yellow, and the vines become deep red. P hunger causes dark green leaves that have a purpling over the veins on the backside of the leaves. K hunger begins with a yellowing and bronzing of the leaf tips and margins which gradually moves toward the center. Bee also Appendix E.