Organic Soil Amendments Are Better for Fruit Quality Than Chemical Fertilizers

• Fruit quality parameters in soil supplemented with organic soil amendments were generally better than those grown with chemical fertilizers.
• However, chemical fertilizers increased fruit size and yield and improved lycopene content.
• The type of organic manure is also crucial. Each type of manure successfully enhances one or more fruit quality parameters.

Growing food requires artificial supplementing of the nutrients removed by crops and horticultural trees in intensive agriculture. Consumer demand for organic food grown without chemicals is increasing due to awareness of inorganic fertilizers’s negative impact on food safety and the environment. In public perception, organic food is more nutritious. Find out what scientific studies reveal about the influence of organic soil amendments on fruit quality and nutrition.

Organic Soil Amendments

Improving soil fertility with mineral nutrients and organic fertilization enhances plant growth, yield, and quality of crops. Organic amendments are sourced from live material, while inorganic fertilizers are artificial or mined.

Several organic soil amendments exist, such as compost, farmyard manure (FYM), poultry manure (PM), vermicompost, and biostimulants. Organic soil amendments add macro and micronutrients, increase soil organic carbon content, and act as organic fertilizers, see Table 1.

In addition, and in contrast to chemical fertilizers, organic soil amendments increase soil biodiversity and activity by providing energy sources to microbes, mycorrhizal fungi, and earthworms. They also improve soil’s physical conditions, such as soil structure, aggregation, and permeability, which increase water holding capacity, drainage, and aeration.

The type of organic fertilization can be essential and has a varying influence on soil fertility and fruit quality. The cultivars, area, and climate influence the effect of organic fertilizers on crop growth and fruit quality.

Table 1: Organic soil amendments supply several macro and micronutrients compared to chemical fertilizers, Zahid et al. 2022. (Credits: https://doi.org/10.1155/2022/8618854)

Fruit Quality

Studies have been comparing whether organic soil amendments are better for fruit quality than chemical fertilizers. Overall, organic soil amendments result in better quality fruit than chemical addition or no soil management; however, there are exceptions, and some quality attributes perform better under conventional chemical fertilization.

Total soluble solids and titratable acidity

Fruit quality is evaluated based on sensory attributes, with the balance between sweetness and sourness being crucial.

Organically produced fruits tend to have higher total soluble solids (TSS) and lower titratable acidity (TA) compared to conventionally grown fruits, making them sweeter and less acidic than conventionally produced fruits. For example, organically-grown strawberries had higher TSS (7.1%) and lower TA (0.93%) than conventionally-grown strawberries with 6.6% TSS and 0.99% TA.

The nutritional composition of strawberries can vary based on the availability of nutrients to the plants during fruit production. Application of FYM in soils resulted in increased chlorophyll production, which, in turn, led to higher assimilate production and increased soluble solid contents in the fruit.

Total phenols

Phenolic metabolites are beneficial for human health and are associated with preventing chronic conditions like cancer and cardiovascular diseases. Phenolic compounds also play a crucial role in plant defense against various threats, including insects, fungi, and animal herbivores, and act as natural protectants.

Organically grown fruits tend to have higher levels of phenolic compounds than conventionally produced fruits. For example, the quantity of phenolic compounds in strawberries is influenced by soil fertility and the amount of fertilizer used. Adding FYM led to a remarkable 30-fold increase in the total phenols in strawberries.

More phenols in organic fruits are attributed to various causes:

• The restricted use of herbicides, pesticides, insecticides, and chemical fertilizers in organic farming may stimulate the synthesis of phenolic compounds in plants.
• Bacteria in organic manures help plants fix nitrogen from the air and produce phytohormones, including phenolic compounds.
• Supplementary phosphorus and potassium in organic manures (FYM and PM) can enhance nitrogen-fixing activity by different bacteria. This improved activity results in increased total phenolic contents in fruits.

Ascorbic acid (Vitamin C)

Vitamin C is a significant health-promoting compound found in fruits.

Organically produced fruits generally tend to have higher levels of ascorbic acid than conventionally grown fruits. For example, organic berry fruits had the highest ascorbic acid content (50.5 mg/100 g fresh weight), while conventional berry fruits had the lowest (41.25 mg/100g fresh weight). However, some studies also found no difference in Vitamin C levels between organic and conventional strawberries.

The type of manure used in farming also affects ascorbic acid levels. For instance, peppers treated with sheep manure had the highest ascorbic acid content compared to conventionally produced fruits.

The synthesis of vitamin C in plants follows the L-galactose pathway and is dependent on photosynthesis. Potassium in the soil can also contribute to increased vitamin C content in fruits.

However, intrinsic plant properties also produce variability in Vitamin C.

• Ascorbic acid content in fruits varies depending on the specific fruit cultivar. Different varieties of fruits can have varying levels of Vitamin C.
• Tissue-specific and genetic factors influence the synthesis of vitamin C. Thus, different parts of the plant may have varying levels of Vitamin C, and genetic factors play a role in determining how much Vitamin C a fruit contains.

Fruit pH

Fruit pH can affect its flavor, as well as its overall quality.

The effect of organic materials on fruit pH can vary depending on the type of fruit or plant being studied.

• Adding organic materials, specifically sheep manure, lowered the pH of strawberry fruits.
• There wasn’t any significant difference in fruit pH between organic and inorganic peppers.

Pigments

Organically grown fruits develop a more robust color than conventionally grown ones.

Anthocyanin: Anthocyanins are a group of plant pigments responsible for various fruits and vegetables’ red, purple, and blue colors.

Adding organic matter to the soil significantly increased the anthocyanin content in strawberry fruits. A study on strawberry fruits found that the highest anthocyanin content (42.88 mg per 100 g of fruit fresh weight) was achieved through a treatment involving 6 kg of organic matter per square meter compared to the control with no soil amendments, which produced the lowest anthocyanin content.

In another study involving red pepper fruits, the highest anthocyanin content (38.5 mg per 100 g) was obtained by applying a mixture of different organic matter treatments. At the same time, the conventional culture system yielded the lowest anthocyanin content.

Organic manures, which release nutrients slowly compared to chemical fertilizers, ensure better nutrition and assimilation of minerals and compounds necessary for plant development.

Total antioxidants in fruits depend on flavonoids, phenols, and anthocyanins. Therefore, an excellent fertile growing medium is associated with beneficial antioxidant activity.

Lycopene: Lycopene is a naturally occurring pigment and antioxidant found in many fruits and vegetables, particularly in tomatoes, watermelons, and red peppers.

Lycopene content is one of the few compounds whose levels are lower in organic fruits. The highest lycopene content was found in fruits grown using conventional agriculture. Additionally, there were no significant differences in lycopene content among fruits grown with various organic matter sources. PM resulted in the lowest lycopene content in pepper fruits.

Fruit size

Different farming systems can influence fruit size. Conventional agriculture involves synthetic fertilizers, which can provide readily available nutrients to plants, leading to vigorous growth and potentially more significant fruits. On the other hand, organically produced fruits rely on natural and organic inputs, which may not provide nutrients as quickly and abundantly. However, using a high amount of organic matter in the soil (6 kg O.M/m2) can improve the physical and chemical properties of the ground and produce large fruits in organic cultivation.

Fruit fresh weight

The fruit weight is primarily influenced by the type of fruit (cultivar) and the temperature conditions during growth more than the cultivation type. Most studies have found only small and non-significant differences in fruit weight between organic and conventional farming systems; however, in an experiment with strawberry plants, chemical fertilizers produced significantly heavier fruit than organic materials or no fertilizers. So, the type of fruit is also crucial in determining fruit weight in response to different soil amendments.

Fruit moisture content and dry weight

The moisture and dry matter content showed opposing trends between the two farming methods. Organic fruits had more dry matter and less water content, while conventionally produced fruits had higher water content and lower dry matter. For example, conventionally grown strawberries had the highest moisture content (93.37%) and the lowest dry matter content (6.63%). In contrast, organic strawberries had a moisture content of 92.61% and a dry matter content of 7.39%.

Dry matter content refers to the solid, non-water components of the fruit. Therefore, postharvest quality, especially of climacteric fruits, will be better in organic than conventionally produced fruits. Fruits with more dry matter store longer, taste better, and are more suitable for juice production.

Crude fiber

The higher crude fiber content in organically produced fruits is seen as potentially beneficial for nutrition and health.

The crude fiber content in fruits can vary significantly based on dry weight. Since organic fruits have more dry matter, they also have higher crude fiber content than conventionally produced fruits. The highest crude fiber content in strawberries (8.13%) was obtained by applying 4.5 kg of organic matter per square meter. This result significantly differed from the conventional and control treatments, showing that the type and amount of organic matter used in cultivation can impact fruit fiber content.

Measuring Fruit Quality

Tools such as Felix Quality Meters are suitable for studies that measure a wide range of fruit qualities. Felix F-750 Produce Quality Meter and the F-751 series can measure dry matter, SSC, titrable acidity, and color. The F-750 is ideal for several fruits, while the F-751s have been customized for specific fruits such as mango, avocado, kiwifruit, and melon. They are trusted by scientists for being accurate, non-destructive, easy to use, and for providing results in real-time. They allow scientists to collect and analyze fruit quality even in the field.

Sources

Abu-Zahra, T. R. (2016). Quality and nutrient contents of fruits produced under organic conditions. Organic Farming – A Promising Way of Food Production. https://doi.org/10.5772/61245

Davis, J.G. & Whiting, D. (2013). Choosing a Soil Amendment – 7.235. Retrieved from https://extension.colostate.edu/topic-areas/yard-garden/choosing-a-soil-amendment/

Kilic, N., Turemis, N. F., & Dasgan, H. Y. (2021). The effect of fertilizers on crop yield, fruit quality and plant nutrition of organically grown strawberry (Fragaria x Ananassa Duch.). Applied Ecology and Environmental Research, 19(3), 2201–2211. https://doi.org/10.15666/aeer/1903_22012211

Ye, S., Peng, B., & Liu, T. (2022). Effects of organic fertilizers on growth characteristics and fruit quality in Pear-jujube in the Loess Plateau. Scientific reports, 12(1), 13372. https://doi.org/10.1038/s41598-022-17342-5

Zahid, N., Maqbool, M., Tahir, M. M., Horvitz, S., Hamid, A., Khalid, M. S., Ejaz, S., Jawad, R., Awan, S. I., Rehman, A., &  Ali, A. (2022). Influence of organic and inorganic fertilizer regimes on Growth Patterns and antioxidants capacity of Strawberry (Fragaria × ananassa duch.) CV. Chandler. Journal of Food Quality, 1–10. https://doi.org/10.1155/2022/8618854