Soil Utilization Plan

Selecting the Appropriate Soil

Click here to view a video that explains how to read the soil texture triangles.

A permanent crop like citrus, which may remain in the same soil for more than fifty years, proper selection of the soil, is essential. One is, however, limited to the soil types that occur on the farm but have to select the best from what is available. Tree crops are often not planted on ideal soils and the subsequent horticultural practices need to be developed to compensate for the deficiencies of the soil.

In evaluating soils for crop production, one strives to get as close to the optimum soil characteristics as possible. This is a fairly specialised activity and should ideally be done by an experienced pedologist.

This process of soil surveys on a farm should culminate in the creation of a soil map indicating positions of the various orchards overlaid onto the map produced by the soil survey. Once this is done, soil preparation and the irrigation layout can be planned.

Slope

Orchard crops require the use of fairly heavy machinery, such as spray carts and picking trailers. During soil preparation, the tractors and implements used to prepare the soil must also be able to drive safely. The slope must therefore not be so severe that workers and equipment are put in danger. A flat surface with a slope of fewer than 2 degrees is however also not suitable, as surface drainage of rainwater will then be too slow. These recommendations also hold true for other crops.

In crop production, other than tree crops, slopes are often contoured (Fig 2.1.) to reduce the flow of water (runoff) across the land surface, thus reducing erosion.

Figure 2.1. Contour ploughing to restrict runoff in a crop field.

Apart from the influence of the slope of the site on safety and ease of vehicle movement in the fields, it is also important for row orientation, surface drainage and erosion. Planting on contours is no longer done and if the slope poses any potential problems with surface drainage and/or erosion, the orchard layout should be adapted accordingly. The orchard roads and vehicle paths between rows are usually protected by grass-cover. If the orchard layout facilitates water movement at a moderate speed, erosion can be avoided. Row orientation is more important in the southern parts than in the northern parts of Southern Africa and can be changed to facilitate surface drainage. Also remember that in South Africa, northern slopes are warmer than southern slopes. The crop selection should therefore be done in accordance, namely crops with a lower tolerance to heat on the southern slopes and those crops which growth is encouraged by more heat on the northern slopes.

Some of the benefits of contour ploughing are:

• Soil erosion can be reduced by as much as 50%
• Increased water infiltration promotes better water quality.
• Increased soil retention encourages root development, binding the soil and preventing erosion.
• Limits the release of nutrients/particulates into nearby river systems or lakes, minimizing harmful effects such as eutrophication.
• Can disrupt wind currents, therefore reducing wind erosion.

Soil Depth

In the general tree, crops will require a minimum soil depth of 30cm to 40cm if micro-jet irrigation is used, and 40cm to 50cm where drip irrigation is used. It is important that the layer below this potential rooting zone does not restrict drainage.

The ideal soil depth varies between crops. In the case of crops like onion and cabbage, they have a shallow root system that will be restricted to the top 15 cm soil layer. Such crops can therefore be planted on shallow soils. Deep-rooted crops such as tree crops require an effective soil depth of at least 60 to 90 cm, with no restriction layers present. In the case of shallow-rooted crops, an effective soil depth of 20 to 30 cm is required. In all cases, the soil requires effective drainage to prevent waterlogging.

Where the potential effective rooting depth is limited, it can be improved by ridging. This creates a thicker layer of soil where the crop will be planted.

Ridging also demarcates the path for vehicles to drive in, avoiding re-compaction of the soil after planting. Ridging lowers the preparation cost. The energy required in soil preparation increases the quadratic equation with depth; hence, costs will also increase. Where large quantities of lime are required, ridging can reduce the quantity required. For example, where 10 tonnes of lime has to be mixed in with the top 50 cm of soil, it can be more easily and cheaply done by mixing 5 tonnes into the top 25 cm and then ridge the field.

Keep in mind that ridging makes harvesting more difficult in tree crops. Due to the undulating surface, the pickers have to walk more.

Ridging improves drainage in areas with such problems and is often used in irrigation or high rainfall areas. Tomatoes, potatoes and tobacco are some of the crops which are often ridged.

Ridging reduces the soil temperature around seed tubers. The deeper one goes into the soil, the cooler it becomes. If seed tubers are to be planted in the summer in very hot soils (> 30°C), they may rot. To prevent high soil temperatures, the soil is ridged to a height of 15 – 20 cm which effectively cools the soil temperature to 20°C around the seed tubers.

Soil Clay Content

The optimum clay content varies between crops. In the case of citrus optimum clay content are between 5% to 20% for micro-jet irrigation and 5% to 35% for drip irrigation.

In general, the ideal soil type for most crops is sandy loam soils, with a clay content of 10 to 20%. Most crops will adapt to soils with higher clay content provided that no other growth factors are limiting. The use of non-ideal soils may require additional production practices, which could lead to additional costs. One should therefore ideally use a recommended soil type as far as possible.

Some crops, such as potatoes and peanuts that bear their produce under the soil surface, may be more sensitive to soil clay content. With these crops, clay soils may affect the crop by reducing the value of the crop by discolouring the crop or making it difficult to harvest. These crops are also more prone to diseases as the soil tends to remain wet for longer periods.

Under dryland production crops soils with higher clay content (>25%) are often preferred, as the heavier clay soils have a better water holding capacity than sandy soils.

The clay content of soil influences its water holding capacity, (volume of easily available water), its cation exchange capacity and its aeration. These are all factors that will influence the crop.

The water holding capacity influences the irrigation scheduling, yield and fruit size.

The cation exchange capacity influences the frequency of fertilizer application, leaching of nutrient cations and utilisation of potassium.

Poor aeration affects the root system negatively. Under anaerobic conditions, roots cannot function properly and diseases like Phytophthora proliferate.

As one cannot increase or decrease the clay percentage in the soil in the field, one has to adapt to the conditions or try to improve the soil conditions. Adapting implies the use of adapted crops and applicable irrigation systems and good irrigation scheduling. Improvement may include the incorporation of organic matter to improve aeration or installing a drainage system to improve drainage.

Soil Stratification

During soil preparation, all forms of stratification must be removed, i.e. layers in the soil are broken up and mixed. The type and depth of these layers will determine the implements that may be required. The fewer layers present in a soil, the lower the preparation cost, as soil with little or no stratification can merely be loosened.

Stratification, or layering, restricts water movement and root development, resulting in soil volumes with few roots. These volumes become waterlogged and create pockets in the soil where root-rot starts. These unoccupied volumes also contain water and nutrients that are unavailable to the plant due to a lack of roots. During soil preparation, all forms of layering must be eliminated. This can be done by deep ripping the soil. Special implements (Fig 1.2.) are needed but are worthwhile in using as they promote root growth and development.

Soil Salts Content

Salts may accumulate in soil for various reasons. If salt accumulation is due to poor drainage and high levels concentrations of sodium are also encountered, such sites should be avoided. It is difficult and expensive to remove salts from soil because the causes of accumulation also require removal.

If the high salt content is caused by the lack of leaching, the site may be reclaimed. Consult a specialist to determine the reclamation process, cost and impact. Calcium carbonates accumulate in the subsoil due to limited leaching. Depending on the depth of accumulation, the soil can still be utilised successfully.

Soils with accumulated salt anywhere in the top 60cm to 100cm should however be dealt with caution. The measurement of resistance or EC of the soil may aid in identifying accumulated salts. Clay soils that provide resistance readings below 250 ohms should be investigated intensively before preparation. The same applies to lighter soils where resistance readings are below 500 ohms. If such soils are identified, it is best to consult an expert.

While accumulated salts also include nutrients, it usually contains high concentrations of sodium, calcium, chlorides, sulphates and carbonates. The chemical conditions in the layers where the salts accumulated restrict root development and function.

Where sodium is the dominant cation, the conditions damage the structure of the soil and hence the physical properties required for proper root functions.

Where calcium and carbonate are dominant, the pH will reduce the availability of many nutrients such as Fe, Mn, Cu, Zn, P and K.

Where chloride and sodium are the dominant ions, the osmotic pressure of the water in that zone will restrict the utilisation of water by the crop plants.

Managing soils with high salt concentrations are extremely difficult and expert advice should be called upon to prevent further degradation of the soil. Managing tools can vary from planting salt-tolerant crops (for example Oldman Saltbush), to adapting the irrigation scheduling to the application of soil amendments.