1. Sun Light
2. Water
3. Nutrients 
4. Oxygen
5. Temperature

These elements form the building blocks for plant growth and need to be ideally balanced to ensure optimal growth. 

Plants grow mainly during daytime, when there is sunlight. Most plants grow optimally at temperatures ranging between 10°C - 30°C. When temperatures rise above 30°C, plants protect themselves by a partial or total closure of their stomata. This results in decreased photosynthesis which in turn reduces biomass production and ultimately results in decreased yields.

With an overhead system, air temperature and relative humidity can be manipulated through a process of crop cooling. Effective crop cooling will typically consist of short, frequent irrigation cycles of 5-10 minutes. Water applied in this cycle must be allowed to evaporate before the next 5-10 minute cycle is applied. Water applied to crops for cooling purposes will not be deemed as irrigation but as a means of cooling only. Normal irrigation quantities and cycles must still be applied. 

In other words through effective crop cooling plant temperatures can be kept below 30°C and transpiration can be optimized. If the transpiration rate is optimized, yields will subsequently increase.

Water is essential for the growth processes of plants. It transports nutrients and minerals that are absorbed by a plant’s root system. Water is also the principle medium for chemical processes that support plant metabolism. Evaporation between intercellular spaces provides a cooling mechanism that allows plants to maintain favourable temperatures necessary for these metabolic processes. In addition, water helps to provide physical support to plants (Turgor Pressure). Well-watered plants maintain their shape due to this internal pressure in plant cells. Insufficient water supply can result in the loss of Turgor pressure; this will manifest itself as plant wilting. This may be a temporary or in some instances a permanent condition.

Too much or too little water will have a detrimental effect on plant growth. Soil contains an average of 20% oxygen at field water capacity. Water saturated soil lacks sufficient oxygen. Insufficient oxygen reduces the uptake of nutrients and results in inhibited growth. Over-irrigation results in leaching of nutrients beyond the root zone, making them inaccessible to the plant. Large quantities of expensive fertilizer are often wasted in this manner.

Example: Vineyards

A plant’s root system may be divided into three main groups:

Anchor roots are thick, and grow deep into the soil to anchor a plant. The ability of anchor roots to absorb water and nutrients is negligible. 

Oxygen roots are thinner. They are usually found in the mid root zone and their prime function is the intake of oxygen, they do however absorb some water and nutrients.

Feeder roots are responsible for the majority of water and nutrient absorption. They are easily recognized, as they are very thin and have an abundance of root tips.  They are usually found in the upper 200mm of the soil profile. It is this portion of the soil profile which needs to be irrigated regularly.

Farmers tend to over irrigate as they invariably over estimate the depth of the roots that absorb water. All too often irrigation cycles are calculated to give anchor roots and oxygen roots far too much water. The prime objective is to concentrate on supplying the feeder roots with the correct amount of water, as they have the vital task of supplying plants with water and nutrients. Therefore irrigation cycles should be shorter and at regular intervals to be of maximum benefit to these shallow positioned roots. An irrigation cycle that is too long and/or too frequent may cause saturated conditions which result in inferior oxygen, water and nutrient absorption.

The purpose of a good irrigation system is to manage the food, water and nutrients in the root zone.

Soil is composed of solid particles, organic matter, water and air. It is important to understand the interactions between soil and water and soil-water tension. (See chapter 12 for effective irrigation scheduling practices).

WATER MANAGEMENT IN SOIL

Field capacity (FC) is the amount of water retained by soil after all excess water has drained and its lateral movement is negligible due to soil hydraulic conductivity.

Readily Available Water (RAW) is that portion of soil moisture that is easily available to plant roots and is generally between 55% and 70% of field capacity.

Permanent Wilting Point (PWP) is the point where water is no longer available to the plant.

  Fig. 1.1 Soil-water availability

Correctly scheduled irrigation is when water is applied when the soil’s refill point is reached but has not been depleted to less that 50% of plant available water. It is therefore important that a good irrigation system can apply small quantities of water at a time, with short intervals between irrigation cycles. This will prevent plant stress and promote optimal growth. The intervals between irrigation cycles can be determined by scheduling aids such as tensiometers (see chapter 12).













Table 1.1 Soil-water retention for different soil types

Example:

A soil with approximately 7% clay and silt that has a field capacity of 120mm per meter soil depth, will only have between 55% and 70% RAW (Readily available water), the remainder of the water being more difficult for the plant to absorb. Therefore this soil has only between 66mm and 84mm of easily accessible water per meter. As the majority of feeder roots are found within the top 200mm of the soil profile, an irrigation application needs to be between 13mm and 17mm at a time. Higher applications will result in water passing the feeder root zone. Depletion of the soil-water reservoir to less than 13mm in this 200mm profile will cause the plant to stress and will result in reduced growth.

A Floppy irrigation system can be easily managed by applying the correct amount of water at optimum frequencies that will result in less water and fertilizer being used to obtain increased yields and profits.