This was according to Dr Michael Southwood, independent agricultural consultant, during a panel discussion hosted by Farmer’s Weekly at this year’s Nampo Cape in Bredasdorp. The topic was El Niño and managing drought and heat stress.
Southwood said lessons from previous droughts in South Africa, Namibia and Zimbabwe showed the value of being proactive rather than waiting for water stress to become a problem.
“We need to partner with nature, we need to see water as a strategic asset, and when we apply it, we need to apply it slowly and deep into the soil profile,” he said.
Build a reservoir early
For irrigated crops, Southwood said some of the most important water-management decisions are made early in the season.
Good establishment and a uniform stand are important, while sufficiently deep irrigation early on can help build a reservoir of moisture in the soil profile and create a favourable microclimate in the orchard.
“If you can fill up the soil early, nice and deep, and create that reservoir, it will take you very far into the season,” Southwood explained.
Fertiliser applications also need to correspond to the amount of water available.
“As the soil dries out, fertiliser salts become more concentrated around the roots, making it harder for plants to take up water, and increasing the risk of salt stress. So, don’t apply too much fertiliser when water is limited,” he said.
Precision irrigation can help farmers take this even further.
Southwood explained that instead of treating an entire orchard block in the same way, growers were increasingly placing soil-moisture probes according to different soil textures and irrigating these areas accordingly.
This allowed irrigation to be better matched to what the soil could hold and what the trees needed.
Start from the roots
Southwood said his approach when dealing with stressed orchard and berry crops was to work “from the bottom up”.
Healthy roots are central to a plant’s ability to take up water and nutrients, which makes rootstock and soil condition particularly important. In orchards, this can include choosing rootstocks with good disease resistance to help maintain a healthy and efficient root system for longer.
High-density planting is another option being used in some crops. Southwood explained that smaller trees generally have less vegetative growth to support than large trees and therefore place less demand on water and nutrients. This allowed growers to direct more of the plant’s resources towards production.
However, even the right rootstock or tree architecture can only do so much if soil conditions restrict root development or the soil cannot hold water effectively.
Southwood said farmers, therefore, need to pay close attention to what is happening below the surface of the soil, including soil structure, compaction, and water-holding capacity.
Cover crops must serve a purpose
Cover crops can help improve soil condition, but Southwood cautioned against simply planting what works for a neighbour.
“We can’t just do what the neighbour does,” he said.
The choice of cover crop should depend on the problem the farmer is trying to address.
For example, where soil compaction is a concern, the solution may differ from a situation where the main problem is nematodes or poor water-holding capacity.
Southwood said he was increasingly looking at annual cover crops that could produce biomass relatively quickly, as well as species with deep roots that could improve conditions deeper in the soil profile.
For nematode and disease suppression, for instance, he was looking at mixtures including Saia oats (black oats), Scala mustard, and Sunhemp; while for biomass and soil health, he was looking at Saia oats, Haymaker vetch, and Sunday fodder radish.
Cover crops use water and nutrients themselves, particularly while becoming established, but Southwood said their longer-term contribution to the soil also needs to be considered.
Carbon helps hold on to water
Building soil carbon is another important part of improving water-holding capacity.
Southwood said changes in soil carbon happen slowly, but even relatively small improvements can make a difference to the amount of water stored in the soil.
In the sandy soils he was referring to, he estimated that every 1% increase in soil carbon could hold roughly 5mm to 7,5mm more water over a soil depth of 30cm.
He added that an increase of about 0,5% in soil carbon could potentially represent enough additional stored moisture to save the equivalent of an irrigation day during a summer week under those conditions.
“Carbon is the currency,” Southwood said.
He added that work conducted in the Western and Southern Cape indicated that a soil-carbon level above roughly 1,75% could be regarded as a useful target.
However, farmers should not expect to change soil carbon quickly. Even with regular applications of the best organic amendments, Southwood said it could take at least three years to bring about meaningful changes in the soil.
Know what is happening in your soil
Southwood encouraged farmers to get a better feel for their soils themselves, rather than relying on laboratory results alone.
An auger can show where moisture is sitting in the profile, while a penetrometer can help identify compaction. Farmers can also dig soil profiles to see how roots are developing and observe how quickly water moves into the soil.
“Get your hands dirty,” was his advice.
Laboratory testing still plays an important role, particularly when it comes to soil chemistry and biology, but Southwood stressed that the physical condition of the soil should not be overlooked.
Pre-plant testing can also help farmers identify problems before a crop is established. He referred to the Cornell Soil Health Test as one example that assesses physical, chemical, and biological soil properties.
The results can help identify the main constraints in a soil and show which problems need immediate attention and which can be addressed over the longer term.
Southwood suggested that a general soil-quality assessment can be done about every three years, depending on the crop and production area. Testing for soil-borne diseases or nematodes might need to be done more frequently.
He added that he provides the same test as the Cornell Soil Health Test, and invited farmers to contact him.
Adapt the crop to the conditions
Farmers can also make changes to the crop or production system when water, salinity, or other constraints become more serious.
Southwood gave several examples.
In strawberries, growers struggling with saline or degraded soils can move production into substrate, although this increases production costs.
For lucerne, salt-tolerant cultivars, such as BAR ST, offer another option where salinity is becoming a problem, although Southwood noted that these varieties generally have a shorter productive lifespan than conventional varieties like BAR 7.
Tomato growers can also rethink the habitat in which their plants grow. Some growers are moving away from tall, indeterminate plants of around 2,5m to shorter, semi-determinate plants of about 1,2m to 1,5 m.
The shorter plants require fewer inputs, particularly water, and produce a more concentrated crop. This allows growers to remain in tomato production while adapting to reduced water availability.
Southwood’s point was not that every farmer should make the same changes, but that production choices can be adjusted according to the constraint facing the farm.
For him, drought resilience begins with understanding what is happening beneath the surface of the soil.
Farmers need to know where their water is going, how much their soils can store, whether roots can reach it, and what they can change in order to make better use of the available water.
The less water there is to work with, the more important these decisions become.
For more information, email Dr Michael Southwood at [email protected].
