Because orchard performance is rooted in the soil. By Anna Mouton.
Most growers would agree that soil conditions can determine the success of an orchard. This is why so much effort goes into soil preparation when planting. But what about the next twenty or thirty years? Should growers be thinking about long-term soil health? And which interventions will have the most impact?
Fresh Quarterly spoke with two soil scientists about their views on soil carbon, soil health, and where growers should focus their attention.
The carbon connection
Organic compounds by definition contain carbon — because carbon is the basic building block of all life on earth. Soil analysis measures the amount of carbon that is available to soil microbes. The result is multiplied by 1.7 to estimate the percentage of organic matter in the soil.
“Soil carbon and organic matter are linked,” says soil scientist Dr Pieter Raath, coordinator of the nutrition and water management programme at Citrus Research International, “and people use the terms interchangeably.”
Anything that is or was alive can contribute to soil organic matter — from active microbes to expired weeds. Soil can also contain inert carbon — think coal — but that is not reported in soil analysis because it’s not available to soil microbes.
Ultimately, all organic compounds trace their origin to carbon dioxide in the atmosphere. “Remember that plants connect the above-ground with the below-ground,” says Luan le Roux, soil scientist at agricultural consultancy Agrimotion. “All the carbon in the soil was laid down through photosynthesis — even if it went through an animal.”
Good things about organic matter
Soil has physical, chemical, and biological characteristics, and organic matter plays a role in all three, according to Le Roux. “The ideal physical medium is one which allows water to penetrate the surface, and allows excess water to move through, but which also retains enough water for the plant to use. Soil with more organic matter will have better water infiltration and retention than soil with less.”
Organic matter improves the physical structure of soil by promoting the formation and stability of soil aggregates — clusters of soil particles held together by organic material. Aggregate formation opens up more space for water movement, while aggregate stability keeps those spaces open by resisting compaction. Increased aggregate stability also helps prevent erosion.
When it comes to soil chemistry, Le Roux explains that organic matter increases fertility by improving the cation exchange capacity. “Cation exchange capacity is responsible for holding positive molecules — cations — in the soil, so that the plant can extract these cations. Organic matter has an incredibly high cation exchange capacity, so even a small amount can make a big difference.”
Another effect of organic matter is improved buffering — this means that the soil is more resistant to pH change. Le Roux points out that buffering makes it harder to correct acidification by liming. Soils that are high in organic matter will require the addition of large amounts of lime compared to soils that are low in organic matter.
Lastly, organic material feeds all the microbes and mini-beasts in the soil. “Microbes are responsible for all the biochemical processes in the soil,” says Le Roux, “nutrient cycling — specifically of nitrogen, phosphate, and sulphur.”
The underground movement
“The biggest source of organic matter in the soil is plant roots,” states Le Roux, and he isn’t only talking about the decomposition of dead roots. Trees release sugars from their roots to feed the microbes on which the tree depends. The carbon in these sugars becomes part of the microbes — and living and dead microbes are ingredients of soil organic matter.
“A plant wants a healthy microbial community around its roots,” explains Raath, “and a healthy tree creates this for itself.” Trees have a root microbiome just like humans have a gut microbiome. Both microbiomes improve nutrition and prevent disease. The narrow layer of soil occupied by the root microbiome is called the rhizosphere.
Soil bacteria make nitrogen available to plants through mineralisation — the conversion of organic nitrogen to soluble inorganic forms. This is how plants get most of their nitrogen, and fertilisation is only supplementary. But herein lies a potential downside to increased soil microbial activity.
“High organic-matter levels in soil can lead to uncontrollable nitrogen release, often in the warm season,” cautions Raath. “And the peak release is often when you would be giving little or no nitrogen as part of your normal fertilisation programme.” Raath believes that over-zealous addition of organic matter can lead to issues with fruit quality and growth vigour.
Le Roux agrees, saying that different situations call for different practices. “For example, if you start putting compost on a vigorous Packham orchard, you can expect problems. Then you should perhaps try a compost that contains less nitrogen, or rather go for a mulch.”
The ins and outs of soil carbon
The amount of organic matter in soil changes all the time. “It’s not that you can build up your soil carbon, and then walk away. It’s always going to be a continuous process,” says Le Roux. Microbes in the soil consume organic matter. They will reduce soil-carbon levels unless more is added.
Le Roux points out that microbial activity drives many of the benefits of organic matter – losing organic matter to microbes is a positive. But there are also advantages to retaining organic matter. The answer is to keep feeding the soil.
The main options on the organic-matter menu are compost, mulch, and cover crops. All of these have advantages and disadvantages, and the best option will depend on the goal in each situation. We hear about how certain growers are using the various options in our article on The New Agricultural Revolution.
The rate at which soil carbon is lost differs according to soil type — faster in sandy than clay soils — and climate — faster in warm and wet weather. Soil disturbance turbo-charges carbon loss by increasing the access of microbes to organic matter, and by providing more oxygen. Tillage should therefore only be done when absolutely necessary, for example when establishing a new orchard.
“Producers should ensure that their practices don’t have a negative impact on soil-carbon levels,” says Raath. “But it’s not necessary to chase after a specific carbon level, or to feel that you have to achieve a certain percentage of soil carbon.”
Raath is concerned that South African growers sometimes have unrealistic expectations about soil carbon based on figures from soils in other parts of the world. “In our environment, and our climate, the inputs you’ll need to achieve those levels are not only not financially justified, but also not environmentally sustainable.”
Le Roux agrees that growers shouldn’t focus on an arbitrary metric. “Soil carbon is the means to an end. But, as a grower, someone who works with soil, you need to realise that you need certain processes in your soil. And you either have to make them happen yourself, or have organic matter driving them for you.”
Bonus: A quick guide to organic matter
Compost is decomposed organic material. Common ingredients include plant material and animal manure. Compost can also be made from food waste, garden refuse, animal carcasses, and seaweed. As a result, the nutrient levels in compost are highly variable, and growers would do well to request an analysis before use.
Compost can be worked into the soil or spread as a mulch. The plant-boosting power of compost is great for young or struggling orchards, but not so good for vigorous trees. Cost and availability limit the large-scale application of compost.
Mulch refers to either organic or inorganic material spread on the soil surface. All mulches suppress weeds and slow evaporation. Some mulches also improve infiltration and stabilise soil temperature. Organic mulches are incorporated into the soil over time — the rate varies according to the material. Compost and straw break down much faster than wood chips.
Growers tend to use the term mulch for wood chips or straw. These materials have a high ratio of carbon to nitrogen. As for compost, cost and availability can restrict the widespread application of mulch. Buying in mulch may have a high carbon footprint, due to transport, and may deprive other regions of organic inputs to their soils.
Cover crops are plants which are grown specifically to cover the soil. Cover crops can suppress weeds, slow evaporation, improve infiltration, stabilise soil temperature, and increase soil organic matter — thereby basically offering the same benefits as organic mulches. But cover crops can also compete with trees for water and nutrients at certain times of the year.
The addition of cover crops diversifies the plant population in the orchard, which in turn promotes diversity of other organisms, both above and below ground. Certain cover crops are associated with reduced pressure from certain pests — red spider mite comes to mind. Unfortunately, cover crops can also worsen some pest problems.
A huge advantage of cover crops is their low cost relative to compost and mulch. On the other hand, cover crops must be sown, and kept fed and watered like any other crop. They require careful management, usually combined with trial and error, if they are to reach their full potential. Whereas the benefits of cover crops in work rows are widely acknowledged, their effect in tree rows is still under investigation.