O level Notes: Agriculture - Soil and soil fertility

Soil is the medium that supports plant life and subsequently animal life. To be able to effectively sustain plant growth, the soil’s nutrient status must be very good. Nitrogen and carbon are very important elements that are recycled in different forms between plants, atmosphere and the soil. Nitrogen is a very important element whose amount and availability contributes greatly to the nutrient status of the soil. Nitrogen is required by plants to make proteins. Animals eat proteins from plants and other animals. Proteins are broken down to amino acids which animals use as building blocks to make their own proteins. The exchange of nitrogennbetween organisms, atmosphere and soil is known as the nitrogen cycle.

All agricultural activities are sustained by water. Without water, agriculture is not possible. Animals require water for body fluids, and plants require water for photosynthesis to make carbohydrates. It is not just the availability of water for agriculture that is of importance, but the quality of the water also matters. Some water sources are polluted to the extent that the water cannot be used for crops and livestock. Various human activities result in unwanted substances finding their way to water bodies thereby contaminating fresh water sources. It is very important that measures are put in place to minimize pollution of fresh water sources as the effects threaten human health, agricultural production, and the life of many beneficial organisms is also threatened. There are laws in Zimbabwe that govern how public water is used and how water as a natural resource is managed nationally.

Ground water sources that have high chances of being contaminated are the important sources for irrigation water and livestock. Irrigation makes all year-round crop production possible and is important in combating drought effects.

Irrigation infrastructure comprises of many parts and structures that farmers need to understand in order to plan, implement and manage sound efficient irrigation systems. This chapter will help you to understand the nitrogen cycle, public water legislation in Zimbabwe, issues of water pollution, irrigation infrastructure equipment and how it functions.

Soil fertility

Soil fertility refers to the nutrient and organic status of the soil. A soil’s nutrient status can be described as being poor, medium or very good. The fertility of the soil can be influenced by man through addition of inorganic fertilizers and manure. Poor farming practices can result in heavy nutrient losses and can also create conditions that make the soil poor in nutrients. Nitrogen is by far the most important nutrient for vigorous plant growth. It is very important to understand how nitrogen is recycled, understanding the processes as well as conducive physical and biological environment for efficient nitrogen recycling. Processes and activities such as leaching, burning of plant residues and waterlogged soil conditions can result in heavy losses of nitrogen from the soil. Studying the nitrogen cycle will equip you with better understanding of soil fertility management and improvement.

The nitrogen cycle The nitrogen cycle is a flow of nitrogen in different chemical forms between plants, organisms and the atmosphere. Nitrogen exists abundantly in the atmosphere and constitutes about 79 percent of the air that we breathe. However, nitrogen in gaseous form cannot be used by plants. Plants can only use nitrogen in the form of nitrates to make plant proteins. Animals get their nitrogen requirements through feeding on plant proteins. Carnivorous animals obtain their protein from feeding on other animals.

The cycling of nitrogen is done namely by particular strains of bacteria. Strains of bacteria known as nitrifying bacteria are capable of converting gaseous nitrogen into nitrates which can be absorbed by the plants through the root system. Nitrifying bacteria such as Azotobacta and Clostridium pasteurianum are symbiotically associated with legume plants. Legumes are very important in the cycling of nitrogen. Legumes are plants that produce their seeds in pods and have typical root nodules or swellings on their roots. The nitrifying bacteria penetrate legume plant roots to form nodes or nodules. As the bacteria live inside the roots, they use nitrogen in the air converting it to nitrates available to the plant roots.

For the soil bacteria to perform well in nutrient cycling, the soil must be with good air circulation, must be at field capacity moisture condition and PH range of the soil should be 5 to 7.

Lightning discharges during rainfall provide energy that oxidizes gaseous atmospheric nitrogen into nitrates that fall dissolved in rain water and are available for plant root absorption.

A farmer’s activities, such as application of artificial nitrogenous fertilizers add nitrogen to the system and become part of the nitrogen cycle. Nitrogen is converted back to gaseous nitrogenous forms through the action of decomposers that convert or decompose nitrogenous compounds from plant and animal remains to a gaseous form of nitrogen such as ammonia gas. Ammonia gas escapes from the soil back to the atmosphere.

Denitrifying bacteria in the soil remove nitrogen from the soil back to the atmosphere. The bacteria break down nitrates by obtaining oxygen from the nitrates. The chemical reduction of the nitrate by denitrifying bacteria results in the formation of nitrogen gas which freely leaves the soil into the atmosphere.

Denitrifying bacteria thrive well in anaerobic (no or limited oxygen) environment. Waterlogged soil conditions have a limited supply of oxygen, therefore promote anaerobic denitrifying bacteria which obtain their oxygen from chemical reduction of nitrates in the soil, thereby resulting in loss of nitrogen from the soil.

An example of denitrifying bacteria is the Pseudomonas aeruginosa which can survive under anaerobic or swampy conditions and can seriously reduce the amount of soil fixed nitrogen.