Misshapen Fruit or bitter fruit (deformed fruit)
Misshapen fruit or bitter fruit is caused by drought (dry soil or high temperatures), inadequate pollination, poor soil fertility or other developmental problems. Watermelon vines need to be inspected weekly to remove diseased or misshapen fruits. Bad fruit should be thrown in the row middles to avoid inspecting them again. Depending on variety, between two to five good fruits will usually mature on a single plant. Keep the fruit number to that recommended by the seed supplier.

Hollow Heart
Seedless watermelons are more susceptible to this problem. Good pollination is important to prevent this disorder.

Poor flavour or lack of sweetness
Poor flavour or lack of sweetness probably caused by poor soil fertility, low potassium, magnesium or boron .

Nutrient deficiency is not a common problem for watermelon growers in Malaysia. Application of organic fertiliser in the form of chicken dung or bio-organic compost as commonly practiced by watermelon growers prevents nutrient deficiency occurring. Another reason for good nutrition is probably due to usage of foliar fertilisers that contain trace elements such as Grofas Yellow and Grofas Red.

Application of  Ground Magnesium Limestone (GML) at 4 - 5 tonnes/ha during land preparation is also a standard practice adopted by most growers. This is probably the reason for the absent of Ca deficiency in watermelon grown in Malaysia.

However, any form of nutrient deficiency that occurs in the middle of growing season will reduce fruit yield. Application of enough fertilisers is very important to avoid nutrient deficiencies. Table 1 describes symptom of nutrient deficiencies in watermelon and how to avoid it.

Table 1: Problem diagnosis for some of nutrient deficiencies in watermelon

Fertiliser losses via leaching are a major problem in watermelon grown without plastic mulch, especially on sandy soil. Losses due to weeds are also inevitable due to the difficulty of weeding once the plant has started to creep. These problems are addressed in fertilisation programmes by increasing the amount of fertiliser applied and splitting the application. The amount of fertiliser to be applied per hectare of watermelon is 5 – 10 tonnes organic manure, 400 kg NPK 15:15:15 and 1,000 kg NPK 12:12:17:2. These fertilisers are applied in split applications as shown in Table 1.

The amount of fertiliser losses due to leaching in tropical climates is quiet substantial especially in sandy soil. Therefore, using `silver-coated' plastic mulch is highly recommended. Research on sandy soil by the Malaysian Agricultural Research and Development Institute (MARDI) indicated that the amount of fertiliser required is about half that required for non-mulched watermelon. The fertiliser application schedule for watermelon grown using plastic mulch is shown in Table 2. All organic manures and NPK 12:12:17:2 are applied before installation of the plastic mulch. NPK 15:15:15 should be incorporated into the planting holes after installation of the plastic mulch. Additional fertilisers are given as a liquid feed, applied to leaves together with any insecticides or fungicides required.

Table 1: Fertiliser Application Schedule for Watermelon Grown Without Plastic Mulch.

* The direct sowing method is normally used for Hybrid F1 watermelon, whilst transplanting method is used for seedless triploid watermelon.

Table 2: Fertiliser Application Schedule For Watermelon Grown Using Plastic Mulch*

*Recommended width of plastic mulch is 1.8m

The only way to accurately manage soil fertility and pH is to have the soil tested. Soil sampling must be conducted in such a manner that it is representative of the field being sampled. This is essential to ensure accurate results and recommendations.

A good fertilizer management programme for watermelon production answers four basic questions:

•  What fertiliser materials (including lime) are to be applied?

•  In what quantities will they be applied?

•  How frequently will they be applied?

•  By which method(s) (broadcasted, banded, etc.) will they be applied?

In addition, the most successful management programmes include frequent evaluations and modifications, if needed, to deal with unanticipated problems such as floods, droughts and other factors that affect the plants' ability to utilise nutrients.

Soil pH measures the acidity or alkalinity of the soil. A pH of 7 is considered neutral, with values above 7 being alkaline and values below 7 acid. Soil pH will have a profound effect on plant growth, development and, ultimately, yield. Soil pH affects the availability of nutrients for plant growth. A slightly acid soil with a pH of 6 to 6.5 is ideal for watermelons.

The only accurate way to determine the soil pH is to have the soil tested. This analysis can determine if lime is required to raise the pH. Lime is relatively slow acting in raising soil pH and is relatively immobile in soils. For this reason lime should be added two to three months before planting and completely incorporated into the top 15 to 20 cm. Soils that are also deficient in magnesium should receive dolomitic lime instead of calcitic lime.

For watermelon production, the maximum recommended amount of nitrogen (N), phosphorus (P2O5) and potassium (K2O) is 136kg per hectare. Watermelons are a relatively long-season crop; therefore, applying fertiliser in small amounts several times throughout the season will maximise production. Rain and overhead irrigation can leach nutrients from the soil, particularly N and K if plastic mulch is not used. All required phosphorus can be applied pre-planting and should remain available throughout the growing season because it is relatively immobile in the soil.

Many different methods exist for applying the recommended fertiliser. A simple method would be to broadcast and incorporate all of the P and K and apply half the N pre-planting and half the N 4 to 6 weeks after seeding.

More complex application methods generally result in maintaining optimum nutrient levels throughout the growing season. In one such method, a modified broadcast concentrates the fertiliser in the area of the roots compared with broadcasting. With the modified broadcast method; apply the fertiliser in bands 60 to 100 cm wide in the row prior to planting. This method will also eliminate the potential for burning emerging plants if fertiliser were banded near the emerging seedlings. In this method, all the p is applied pre-planting with any micronutrients. One-third to one-half of the recommended N and K is applied in this modified broadcast also. At approximately three weeks after seedling emergence, apply one quarter of the remaining N and K on the sides of the beds just past vine tips. At approximately six weeks after emergence, apply the remaining N and K.

For crop under plastic mulch, lost of nutrients through leaching are greatly reduced. As such, all N, P and K that required by the crop can be applied before the planting bed is covered by the plastic. Further more, subsequent application of fertiliser through soil incorcoporation is not possible without breaking and damaging the plastic mulch. Any additional fertilisers can only be applied through foliar or drip line.

Watermelon growers have occasionally experienced unsatisfactory fruit set even with sufficient bee activity. Two to three foliar applications of water-soluble boron (approximately 25g by weight of actual boron per application) at weekly intervals coinciding with opening of the first female flowers can enhance pollination and improve fruit set. A good fertiliser management program includes frequent observations of plants for any nutrient deficiency symptoms.

Nutrient content of commercially available fertilisers in Malaysia

*Organic fertilisers are generally slow acting, providing plants with a steady supply of food over a long period
**Inorganic, compound and foliar fertilisers are generally quick acting, providing plants with a boost when used as top dressing
***TE = Trace elements