How soil health affects food quality

With Africa constantly facing the dilemma of providing sufficient food for its ever-growing population, it is essential to produce crops that contain all of the inorganic and organic nutrients (minerals, carbohydrates, proteins, fats and vitamins), which are required by both humans and animals. Joyce Kinabo, a lecturer at the Department of Food Science and Technology at Tanzania’s Sokoine University, explains how good quality food depends on soil health and agricultural practices.

Kinabo says an average human body requires 42 different essential nutrients to develop fully and keep on functioning normally. “The health of top soil is critical for people’s nutrition and health. The nutrients we consume from plants are heavily dependent on the quantity and chemical forms of the elements present in the soil in which they are grown.
Soil health
To determine the health of the soil we have to evaluate the soil type, texture, structure and pH (alkalinity or acidity) levels.
1. Texture. Soil characteristics like texture will give an indication of the viability of the soil. Sandy soil is most likely to have
a low cation exchange capacity (CEC), low nutrient holding capacity, and high leaching – all resulting in below par nutrient availability. By contrast, clay has a high CEC and high nutrient holding capacity, resulting in low nutrient leaching.”
Kinabo explains that in soil science, CEC is known as the maximum quantity of total cations, of any class, that a soil is capable of holding, at a given pH value, available for exchange with the soil solution. Furthermore CEC is used as a measure of fertility, nutrient retention capacity, and the capacity to protect ground.
2. Structure. Defined as the arrangement of soil particles into aggregates, structure is another contributing factor to the soil’s viability. “Large particles possess large spaces between them, allowing high water and nutrient movement, which in turn results in leaching. Small particles lead to soil compaction and enhanced water runoff, while medium sized particles allow good space between particles for water, nutrient and root movement, so less leaching occurs,” she states.
3. pH levels. It is important to determine the pH to establish the acidity or the alkalinity of the soil. These levels facilitate adsorption or desorption, precipitation, mineralisation, or immobilisation. “For instance, when the soil is alkaline (pH >7) solubility of iron and zinc decreases and becomes unavailable to the plant. Through much research we have verified that many
nutrients are more available in slightly acid soils (pH 6-6.5),” Kinabo explains.
“In instances where the soil is more alkaline, it is less likely to pass lower nutrient contents in plants. This is due to inherently low content of such nutrients in soils and/or low availability due to low solubility caused by inherent physico-chemical properties of soils as well as moisture levels.”
Usage and storage
She says that plants require approximately 16 essential elements as well as light, heat and water to stimulate growth and development and synthesis of nutrients. While each plant has an optimum range of requirements (minimum and maximum), each essential element has a specific role to play. Plant roots normally extract elements from the soil to supply to the leaves and later to make the nutrients, which are then located in various parts of the plant.
Kinabo emphasises that the location of nutrients in the plant and eventually in the grain, fruit or tuber is, however, not the same. Various nutrients are used and stored for different functions. “Nitrogen is used for amino acid and protein formation, while enzymes and vitamins are responsible for the formation of dry matter (quantity and quality). Direct consequences of insufficiency of any of these elements are low protein content and reduction in yield. Elements like phosphorus aid with photosynthesis and
respiration as well as genetic information. A deficiency will result in stunted growth as well as poor seed/grain and fruit development.”
Other nutrients like potassium, iron, calcium and zinc are essential to the translocation of carbon, hydrogen and oxygen, protein synthesis, chlorophyll formation as well as seed production and increase in the size of grains as well as the quality of fruits and vegetables. Deficiencies of these elements can cause a range of problems including stunted growth, poor cell membrane  formation, chlorosis and reduced dry matter production and yield among other things.
• This article is based on Joyce Kinabo’s presentation From soil elements to food nutrients: does soil health affect food quality?at the South African Association for Food Science & Technology’s Biennial International Congress and Exhibition in 2013.

The impacts of food processing

According to Joyce Kinabo, a lecturer at the Department of Food Science and Technology at Tanzania’s Sokoine University, ensuring that the soil in which the plants grow is healthy and contains enough nutrients is just the first step. She also advocates good agricultural practices and the processing of fruits, vegetables, grains and seeds in such a way that the nutrients are
“How often does the industry check the nutrient content in foods prior to processing and preparation? The peeling, milling, washing and drying of food is sometimes detrimental to its nutritional content.
• Peeling. “Most phytochemicals are located in the skin of fruits and nonleafy vegetables, so peeling this outer layer reduces the amount of phytochemicals.
• Drying. “Removal of water from foods reduces the amount of water soluble vitamins. The extent of loss depends on the amount
originally present in the food item.
• Milling. “The aleurone layer and embryo contain most of the minerals, vitamins and proteins in cereals. Refining of maize
and rice reduces micronutrient and phytochemical levels. If maize or rice is grown in poor soils, it means less nutrients in the final product after processing,” says Kinabo.
She suggests creating food processes that work in such a way that they protect rather than remove the nutrients. “In this way, there will be no need for fortification or supplementation.
Agriculturalists, food scientists and nutritionists should work together to ensure the ‘wholesomeness’ of the food we consume.”
Kinabo furthermore proposes the development of the following:
1. A package of recommendations for optimal soil element uptake by crops and improved yield.
2. A database of soil elements and nutrient composition of foods.
3. An Information package of soil suitability for high nutrient crop production.
4. A package of information on conditions and practices that encourages optimal soil element uptake by plants.
5. Best practices of agricultural crop production for optimal nutrient content.