Saturday 12 April 2014

Nutrients

lime
 normally dolomitic lime, supplies the soil with the important nutrients, calcium and magnesium, and reduces soil acidity and potentially toxic elements such as aluminium as well as manganese. The quantity of lime required will depend on the pH, aluminium (extractable acidity) and the clay content of a soil. Because lime moves slowly in the soil, it must be broadcast and incorporated into the soil to a depth of at least 500 mm before planting. After planting annual maintenance lime applications of about 1 to 2 t/ha are necessary to avoid reacidification of the soil caused by acidifying fertilisers, rainfall and irrigation. Apply early in the season but not within 3 weeks of nitrogen applications

phoSphoruS

Phosphorus (P) is an important nutrient for papayas because moderate applications at the vegetative stage of the plant will encourage an active root system, stimulate vegetative growth and enhance flower initiation as well as fruit set. However, P fertilisation remains in the soil for a long time and because the P requirements 
of papaya plants are relatively small, growers must avoid overapplication of this element. Overapplication can induce an iron and a zinc deficiency and has negative consequences on yield and fruit quality. Because P fertiliser moves relatively slowly in the soil, a preplant phosphorus application is essential. A single superphosphate which also contains sulphur, is normally recommended as a broadcast application and 
incorporated into the soil about 1 to 3 months before planting. Other frequent sources of P, such as MAP (mono ammonium phosphate) and DAP (double ammonium phosphate) are recommended on alkaline soils (pH >7,5) where uptake of P can be a problem. An annual maintenance application of 450 g uperphosphate 
during spring from the second year after planting is recommended. If, however, soil and petiole sampling indicate that these levels are high, P fertilisation should be reduced or stopped.

nitrogen

Nitrogen (N) is most important for increasing yields. However, overapplication will cause the plants to produce a large number of smaller sized fruit. Overapplication is also associated with soft fruit, which has serious implications in the shipment of papayas from the field to the packing plant and eventually to the market and the consumer
.The most frequent source of nitrogen fertiliser is LAN (28 % N) (limestone ammonium nitrate), urea (46 % N) and ASN (27 % N) (ammonium sulphate nitrate). LAN is recommended in most cases, except on alkaline soils (pH >7–7,5) where ASN is more suitable. Urea is also used frequently and is suitable for soils that are not too acid or alkaline, or too sandy.
Once papaya plants are established and growing actively, apply 60 g LAN every 6 weeks during the first year. Thereafter apply 120 g LAN every 2 months during the active growing period (between about September and April). During cold winters, when root activity is inhibited, foliar application of urea at 5 g/l can also promote growth and fruit set.

organic FertiliSer

• Organic fertiliser such as kraal manure improves the physical and biological properties of the soil.
• Farm manure should, however, be analysed before it is used on papaya because manure, such as poultry manure, has a high P content, which can be detrimental to plants and induces micronutrient deficiencies if excessive quantities are applied. However, poultry manure can be useful on very sandy soils if P uptake is very poor and the plants lack vigour.
.If well-decomposed kraal manure is available, a preplant application of about 5 to 10 t/ha can be incorporated into the soil.
• Keep in mind that overapplication of manure/organic fertiliser can result in soft fruit, especially if applied during spring.
• Do not apply manure within 300 mm of the trunk

.FertiliSer mixeS

 Fertiliser mixes are described by their ratio of nitrogen:phosphorus:potassium (N:P:K), for example a 3:1:5 (30) mixture will contain 10 g N/100 g of fertiliser. No single mix can be recommended for papaya because the N:P:K ratios will depend on soil and petiole analyses. However, on a soil with a high P content a 1:0:1 (47) mix at 70 g/application/plant during the first year can be used if potassium is required.

potaSSium

An adequate supply of potassium (K) is necessary for the development of roots, stem, leaves and fruit of papaya plants as well as for fruit quality and size. Excessive supply 18of potassium will reduce the uptake of calcium, magnesium and boron, which can have a negative effect on fruit quality.
Potassium is recommended as a preplant soil application if the K levels in the soil are low. As a guideline, maintenance K applications of 150 g potassium chloride (KCl) or 180 g potassium sulphate (K2SO4) per tree per year during the first year and 200 g KCl or 240 g K2SO4 during the second year are ecommended. Apply about 40 % of the fertiliser during the vegetative phase and 60 % during the fruit growth and development phase. Two to 3 applications should be sufficient on most soils. Further adjustments must be made according to petiole K levels. On certain heavy soils where the Ca + Mg/K ratios are very high, higher K-fertiliser applications will be required to maintain optimum levels.

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