Marnus Ferreira, Head of Soil Science
Phillip Nell, Soil scientist

In search of fertile soils

Vegetable production has had a long and storied history in South Africa. Vegetables were the first crops produced in the Cape of Good Hope and served as a welcome source of refreshment to sailors during their long journeys around Africa. The original production site was chosen due to the fertility of the soils at the foot of Table Mountain and the shores of Table Bay. Today still, the size and weight of vegetable crops can be linked to a soil’s physical and nutritional make-up.

Sandy soils pose a challenge

In areas of South Africa where vegetable production takes place on predominantly sandy soils such as Vaalwater and Sandveld, challenges experienced are depth to subsurface clay layers (which are usually poorly drained), areas of intense leaching, subsurface rock and sections where heavier soil occurs within an irrigation area. These challenges, if left unmanaged, reduce/may impact the overall yield of the planted vegetables. With regards to sandy soils, the challenge is usually water and nutrient retention. Sandy soils saturate easily, but, due to the high rate of drainage, they also dry out quickly. Typically, they have low levels of active carbon, clay and organic matter, which lead to a very low nutrient and water retention ability. When these normally deep sandy soils are combined with areas that vary in depth and subsurface clay and poorly drained areas, the result is a pivot or irrigation section that is very difficult to manage.

Understand and know your soil’s physical properties

The solution is to use Agri Technovation’s soil classification service to determine where those challenging areas are, how to correct/improve them if possible, and how to successfully manage those challenges.  When managing sandy soil challenges, it is not a case of one answer fits all. For example, the addition of organic material such as compost certainly has benefits, but on leached soils – which have drainage issues – added (increased) organic material without prior installation of drainage will lead to an exacerbated wet spot. Furthermore, in areas where subsurface clay appears, which is often accompanied by saturated conditions, drainage is required to rectify the situation. However, drainage must be installed at the correct depth to remedy the problem, otherwise the drainage system will have little to no effect.

Proven results

The example shared below relates to a tendency often encountered (Figure 1). The size and weight of a cabbage crop were measured by doing a large-scale trial. The trial included a 5-hectare pivot section where the only variation was soil depth. Except for this variation, the soil was similar in every other way: nutritional status, texture, soil type, percentage coarse fragments and infiltration rate etc. The whole area received the same fertiliser, water and biostimulant applications. Total production cost per hectare came to around R40 000/ha.

The results (Figures 2 and 3) are self-explanatory, indicating the obvious difference in cabbage size achieved.

Shown in Figures 2 and 3 are three (3) randomly selected cabbages each that are representative of the specific trial site. The cabbages in Figure 2 are visibly larger (pen for scale) and weigh two-thirds more than the smaller cabbages in Figure 3. The cabbages in Figure 2 were grown on unimpeded deep Hutton soils, while the cabbages in Figure 3 were grown on 500 mm deep Hutton soils – impeded by rock.

As mentioned above, this variation in soil depth was the only difference between the two areas. The question to answer is what can be done to improve the production potential of the area indicated in Figure 1 with the presence of a root limiting layer at 500 mm (red circle).

As mentioned above, this variation in soil depth was the only difference between the two areas. The question to answer is what can be done to improve the production potential of the area indicated in Figure 1 with the presence of a root limiting layer at 500 mm (red circle).

The most important aspect is firstly, to understand the nature of the physical limitation and then, to manage it accordingly.

• Firstly, it is worth investigating the type of rock underlaying the poorer area – whether it can be mechanically broken by deep ripping or not. By mechanically breaking the subsoil, rock drainage as well as the potential rooting depth can be improved.
• The second step is to manage irrigation differently in this As the soil is shallower, it can take and store only about half the moisture of what can be taken and stored by the better area. In addition, the drainage component is much slower and therefore the amount and timing of irrigation must be adjusted to maintain optimal water-to-oxygen ratios in the soil.

Often these pivots can be segmented in order to manage water differently. This can be done either by soil moisture probes, by digging inspection pits or by drilling with an auger to ascertain moisture levels in terms of which irrigation strategies can be determined.

The above area-specific approach to management is only possible once it is known where the different areas are and the differences between them.

Soil classification is a once-off investment for your farm. The data obtained is applicable to each growing season and will allow you to successfully manage your crops, to increase yields and to avoid any unnecessary expenditure.