Crop Selection

By selecting the right crop for given conditions, one can optimise the requirement of irrigation water and added fertiliser and increase yields.

The crop water needs mainly depend on:

• The climate
• The crop type
• The growth stage

The crop water needs are high in sunny, hot and windy climates and low in cloudy, cool, humid climates with little wind speed.

The Climate

In a sunny and hot climate, crops need more water per day than in a cloudy and cool climate. One has to consider the amount of rainfall and water loss trough percolation and evapotranspiration in order to calculate the right amount of irrigation water needed. Soil moisture conservation through mulching, tillage techniques, soil cover or soil amendment.

Average daily water need of standard grass during irrigation season. Source: FAO (1986)

The Crop Type

There exist crops like rice or sugarcane, which need more water than crops like beans and wheat. The table below gives some idea about the different seasonal water needs of the most important field crops.

Approximate values of seasonal crop water needs. Source: FAO (1986)

The Growing Stage

A fully-grown crop will need more water than a crop that has just been planted. It is estimated that 50% of the crop water is needed during the mid-season stage, when the crop is fully developed. During the so-called crop development stage the crop water need gradually increases to the maximum crop water need (100%). The maximum crop water need is reached at the end of the crop development stage, which is the beginning of the mid-season stage.

Fresh harvested crops, such as lettuce, cabbage, etc. need the same amount of water during the late season stage as during the mid-season stage. The crops are harvested fresh and thus they need water up to the last moment.

Dry harvested crops, such as cotton, maize (for grain production), sunflower, etc. are allowed to dry out and sometimes even die during the late season stage these crops. Thus their water needs during the late season stage are minimal.

Water Needs

The water need of a crop is usually expressed in mm/day, mm/month or mm/season. Suppose the water need of a certain crop in a very hot, dry climate is 10 mm/day. This means that each day the crop needs a water layer of 10 mm over the whole area on which the crop is grown. It does not mean that this 10 mm has to indeed be supplied by rain or irrigation every day. For example, it is still possible to supply 50 mm of irrigation water every 5 days. The irrigation water will then be stored in the root zone and gradually be used by the plants: every day 10 mm will be used (adapted from FAO 1986). In terms of volume, with a rainfall of 10 mm, every square metre of the field receives 0.01 m, or 10 litres, of rain water. With a rainfall of 1 mm, every square metre receives 1 litre of rain water (FAO 1985).

Crop Rotation and Water Need

The success of conservation agriculture lies on a proper and good crop rotation plan. Different plant species or even varieties have different needs for nutrients, light, water, temperature and air. When one of these basic needs is not met, the plant will be stressed and will not grow properly. Selecting crop rotation varieties is therefore a very important exercise that a farmer needs to undertake (INFONET-BIOVISION 2010). The crop water need (ET crop) is defined as the depth (or amount) of water needed to meet the water loss through evapotranspiration. In other words, it is the amount of water needed by the various crops to grow optimally. The crop water need always refers to a crop grown under optimal conditions, i.e. a uniform crop, actively growing, completely shading the ground, free of diseases, and with favourable soil conditions (including fertility and water). The crop thus reaches its full production potential under the given environment. The crop water need mainly depends on (FAO 1986):

• The climate: in a sunny and hot climate crops need more water per day than in a cloudy and cool climate.
• The crop type: crops like maize or sugarcane need more water than crops like millet or sorghum.
• The growth stage of the crop; fully-grown crops need more water than crops that have just been planted.

Nutrient Requirements

Proper nutrition is essential for satisfactory crop growth and production. The use of soil tests can help to determine the status of plant-available nutrients to develop fertiliser recommendations in order to achieve optimum crop production.

There are at least 16 elements known, which are normally derived from the soil in the form of inorganic salts, to be essential for plant growth (see table below). 94 to 99.5 % of fresh plant material is made up of carbon, hydrogen and oxygen. The other nutrients make up the remaining 0.5 to 6.0 per cent.

Macronutrients refer to those elements that are used in relatively large amounts, whereas micronutrients refer to those elements that are required in relatively small amounts (adapted from MCKENZIE 1998).

Essential Plant Nutrients. Source: MCKENZIE (1998)

All 13 elements must be present in the soil for plant use, in varying degrees of availability, to ensure both the immediate and long-term needs of the crop.

Some of the commonly used terms to describe levels of nutrient elements in plants include.Source: MCKENZIE (1998)

Plant tissue analysis measures nutrient levels in the plant during their growth. The supply of available nutrients is reflected in the nutrient content of the crop. Therefore, use of plant tissue analysis allows a producer to evaluate the effectiveness of fertiliser recommendations from a soil testing service. Producers who do not soil test can still use routine plant tissue analysis to evaluate their fertiliser management programme to determine whether they used the correct kinds and amounts of nutrient (adapted from MCKENZIE 1998).

Furthermore, while an efficient plant nutrition management is essential for producing sufficient and healthy food, it is also a very complex challenge. Read more about it in the “Guide to Efficient Plant Nutrition Management” (FAO 1998). An overview about different strategies in fertilisation in different regions of the world is given by FAO (2006).

Soil Texture

Soil texture is an important soil characteristic that drives crop production and field management. The textural class of a soil is determined by the percentage of sand, silt, and clay. Soils can be classified as one of four major textural classes: (1) sands; (2) silts; (3) loams; and (4) clays. Soils can be improved with help of tillage, soil cover and reforestation, soil amendment, mulching or adding compost. There are several soil properties which are influenced by the texture (BERRY et al. 2007):

• Drainage
• Water holding capacity
• Aeration
• Susceptibility to erosion
• Organic matter content
• Cation exchange capacity (CEC)
• pH buffering capacity
• Soil tilth

Sandy soils have large particles and gaps between them. This allows water and nutrients to drain away freely, making sandy soils less fertile than heavier soils. Sandy soils also tend to dry out in the summer. But they warm up quickly in spring (allowing seedlings a good start) and they are much easier to dig than clay-based soils. If your soil is sandy, you should have no trouble growing root vegetables (such as carrots and parsnips), but you may struggle with nutrient-hungry brassicas (such as cabbages and broccoli). Also, plants and trees with shallow roots are prone to drying out as sandy soils lose moisture faster than heavier soils (adapted from GROWVEG n.y.).

Clay and silt soils – ‘heavier soils’ – have small particles. This means water is less likely to drain away but the soil is more likely to become waterlogged. Heavier soils are fertile, but take longer to warm up in the spring and are harder to dig. If you have clay soil, you should find that brassicas (such as cabbages and broccoli) grow well, but root vegetables (such as carrots and parsnips) are likely to struggle as they have to push through the heavy, often compacted soil. Shallow-rooted trees (such as pear trees) are likely to thrive in this soil as it holds moisture better than sand (adapted from GROWVEG n.y.).