Calcium Nitrate Benefits in Watermelon Late-Season Growth and Development

Balanced crop nutrition is critical for achieving high watermelon yields and ensuring that nutrients don’t limit fruit quality. Important quality parameters for watermelon include uniform fruits that are free of defects, healthy skin and flesh color, good fruit firmness and fruit sweetness.

During the later stages of watermelon growth, it is particularly important to ensure that environmental and growth constraints do not limit fruit quality. Many common disorders in watermelon occur as a result of environmental and plant growth stress, including nutrient deficiencies. Hollow heart (fig. 1), blossom-end rot (fig. 2) and white heart (fig. 3) are three common physiological disorders influenced by nutritional factors.

watermelon fruit defects.jpg

Figure1- Hollow heart. Figure 2- Blossom end rot. Figure 3- White heart.

Reduce Nitrogen Fertilizer Applications in the Late Growth Stages to Maintain Fruit Fill and Sugar while Avoiding Rot and Skin Blemishes

Nitrogen is a very important nutrient for achieving high yields and quality. However, too much nitrogen during later stages of growth can spoil fruit quality by increasing the level of rind blemish and encouraging fruit rot. Too much nitrogen and excessive moisture during fruiting stages of watermelon can also encourage hollow heart and lead to poor tasting fruit.

watermelon blemishes and nitrogen rate.jpg

Nitrogen and Yield. REF: Coelho et al ,2001 – Nitrogen and Quality. REF; CTIFL


It is important to adjust both the nitrogen application rate and the timing following local trials and recommendations that take into account the need for high yield, but also good quality fruit. The form of nitrogen fertilizer is also particularly important, and like most crops, watermelons require a balanced supply between ammonium (NH4+) and nitrate (NO3-) nitrogen to maintain fast growth, high productivity, and good fruit quality.

Watermelon trials confirm higher yields when NO3nitrogen is the major nitrogen source used in the fertilizer program. In the example below watermelon plants were grown under three NO3-:NH4+ ratios; 3:1, 1:1, and 1:3. The plants receiving the high NH4+ treatment (NO3-:NH4+ = 1:3) expressed symptoms of toxicity and declined rapidly after bloom. As a result, these plants showed reduction in growth and very little fruit yield.

NO3-N : NH4+N 3:1 1:1 1:3
Fruit Yield (lb/plant) 3.75 2.56 0.13

Cultivar Sugar Baby – Watermelon, USA.


In the same trial, where NH4+ was the main nitrogen source, the NH4+ was preferentially taken up by the plants compared to other cation nutrients such as potassium, magnesium, and calcium.

N form and cation uptake - watermelon.png

Total Soluble Solids (TSS) content were also higher in fruits treated with a higher proportion of NO3- . TSS is used to assess sweetness in watermelon and, generally, the sweeter the watermelon, the better the end quality. These results also support the theory that detoxification of excessive NH4+ reduces the availability of carbon skeletons, and subsequently inhibits sugar production in the fruits.

Calcium Maintains Good Fruit Firmness and Quality

Calcium has a major influence on watermelon quality, especially rind strength and prevention of blossom end rot. From the end-user perspective, watermelon consumers are looking for a fruit with firm flesh. A lack of firmness can be a result of poor calcium nutrition. Calcium builds strong cell walls and robust cell membranes that help increase watermelon firmness and shelf life. Strong cell walls and membranes also increase resistance to bacteria and fungi because the pathogens are not able to ‘digest’ the cell wall.

As mentioned in the articles on early and mid-season nutrition of watermelons, the vast majority of calcium is taken up during early plant growth and is held within the leaves and stems. Thus, high rates of calcium application may be needed to provide the relatively low concentrations of calcium found in the fruit, which are critical levels in maintaining good fruit firmness with minimal cracking. Unfortunately, calcium deficiency problems may not be visible until after harvest, during transportation, or when the fruit is on the shelves. For this reason, providing a continuous supply of soluble calcium throughout the growing season is a proactive approach to ensure calcium is available all the way through fruit fill in order to prevent post-harvest decay of watermelons.

YaraLiva® Calcium Nitrate provides 100% water-soluble calcium. Field trials have shown that regular applications of calcium nitrate throughout the growing season to early fruit maturation are particularly effective to ensure that the fruit gets the calcium required to maximize quality potential and storability.

melon quality rating and calcium - firmness and cracks.png

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Brett Dixon
Brett Dixon
Senior Sales Agronomist

Texas, Oklahoma, Louisiana, Arkansas, Mississippi & Tennessee

Dan Clarke
Dan Clarke
Regional Sales Manager

Texas, Oklahoma, Louisiana, Arkansas, Mississippi, Tennessee, Kansas, Missouri, Nebraska, Iowa, Minnesotta, South Dakota, North Dakota, Montana