20 Mar As-salt-ing the Plains: the science behind the scenes.
Welgevonden is located on nutrient poor soils dominated by Sourveld grasses, which are less palatable than grasses in nutrient rich Sweetveld. Even the same grass species occurring in both areas often show differences in nutrient value and palatability due to differences in soil substrate.
The salt trials experiment which has been running for 5 years has kicked off for the year, with the laying of new salt. This experiment is part of the grazing lawn program to improve grazing for herbivores, with the idea emerging from a Welgevonden Scientific Advisory Committee meeting in 2015. During a discussion on the sodic soils of Sweetveld areas in the lowveld, which have a high sodium content, someone suggested spreading salt over a small area to see if increasing sodium in the soil increases the palatability of the grass.
So why would adding salt increase palatability? Well, Sourveld areas like Welgevonden, have higher rainfall which means nutrients tend to leach from the soils, but more water is more accessible to these grasses, hence their growth vigour is high with more energy put into producing stems with high bulk and low nutrient value. On the other hand, Sweetveld areas have lower rainfall and clay soils that are high in nutrients, but also higher in sodium, which makes water uptake for the grasses more difficult because of the osmotic gradient. To overcome this, these grasses produce sugars in their leaves and store some in their roots to increase the osmotic pressure above that of the salts in the soil. This facilitates water and nutrient uptake with most of the energy going towards producing leaves that are higher in sugars and therefore are more palatable. This is probably where the term Sweetveld originated and is the reason why Welgevonden began the experimental salt trials.
The results over the past five years exceeded our expectations. The grazing lawn has flourished in this area and attracted herds of generalist herbivores. A fortuitous, but unexpected, consequence of the experiment was the dying back of some shrubs that were unable to cope with the higher salt concentration. This is an additional advantage as now we also have a method that controls encroaching shrub species on the grazing lawns, while enhancing the nutritional value of these areas.
The project has been so successful that this year we added a few more sites which will be treated with salt, so if you happen to see a whitish layer over some of our plains – now you know why!