SOIL BIOLOGY on abandoned potato production circles

by Nelmarié Saayman, nelmaries@elsenburg.com

PPotato production lands are common in the Sandveld along the west coast of South Africa. These lands usually lie fallow for three to four years to break the cycle of possible soil-borne diseases before they are planted again with potatoes. Having become economically unviable, many of the lands were withdrawn from production due to high input costs and/or diseases.

Others are close to drainage lines and wetlands and pose problems for the natural environment. Many of these abandoned circles are in the endangered Leipoldtville Sand Fynbos vegetation type and rehabilitation has become a priority.

In 2017, an article was published on a planned rehabilitation project at three sites near Elands Bay, Redelinghuys and

Sandberg. This article is a follow-up with preliminary results and it focusses on soil health and high phosphorus levels in the soil, to which the local fynbos is not adapted. It also looks at which plant species’ seed are in the soil seedbank. The phosphorus (P) levels of the three abandoned lands range from 35 to 63 mg P per kg soil, compared to the adjacent veld where it is only 4 mg/kg. Phosphorus does not easily leach from the soil and will stay in the soil for two to three decades. One method to lower the P-levels in the soil is to plant a cover crop mixture including lupines, which can mobilise poorly soluble phosphorus from the soil. At the end of the growing season, the crop is harvested and removed from the land. The additional advantage of the cover crops is that it can initiate nutrient cycling in the soil.

In May 2017, soil samples were collected for a soil seedbank study and to determine some aspects of the soil biology before any rehabilitation. In June 2017, the first cover crop mixture was planted. Due to the drought of 2017, there was almost no establishment of the cover crops and it was planted again in May 2018. The cover crops were harvested and removed during September 2018 and another set of soil samples was taken to determine the impact of the cover crops on the soil health and soil P-levels.

The long-term average annual rainfall for the region is 280 mm, with only 140 mm measured during 2017 and 290 mm during 2018. Although the cover crop stand was better in 2018 than in 2017, it was still too low to have any effect on the soil P-level. The cover crops did, however, have a positive impact on the soil biology with an increase in soil microbial activity and diversity, starting the nutrient cycle in the soil. This can assist in the establishment and survival of the indigenous plants that will be planted in 2020. The plant material (seeds and cuttings) will be harvested in an area with higher soil P-levels, in order for it to be adapted to the higher soil P-levels at the study sites.

Mainly annual plant species were found in the soil seedbank, but it did include species such as rooisaadgras/veldt grass ( Ehrharta calycina ) at two of the sites in 2017. In 2018, there were fewer species and individual plants despite the better rainfall. This might be due to the low rainfall of 2017 with seeds germinating after the first rains but not surviving until seed set, resulting in fewer species and less abundance in 2018. The near absence of indigenous perennial species found in the natural veld adjacent to the abandoned lands emphasises the importance of adding seed and/or plugs of these indigenous species to the lands in an attempt to rehabilitate it to as close to a natural state as possible.

One can try to rehabilitate abandoned lands and degraded areas, but in arid regions, rainfall remains the deciding factor for the success of any project.

AP