The Other Side of Weeds

An article by Sonja Burger.

Weeds have suffered from a bad reputation for centuries, and often undeservedly so. Yes, they can reduce crop yields and quality, harbour pests and diseases, taint milk, contaminate wool and poison livestock. But, they are often not given credit for colonizing bare soil and preventing erosion, for loosening up hard soil and transporting nutrients from the sub-soil. Their role in scavenging and conserving nutrients such as nitrogen and sulfur, which might otherwise leach away, is often ignored. Their role in providing shelter and food for native insects is often ignored. So is their nutritional and medicinal value. Dandelions, for example, have greater protein levels than lucerne, comparable mineral content and almost double the relative feed value.

Balancing the Soil

Graeme Sait, an Australian expert on high-production sustainable agriculture and the CEO of Nutri-Tech Solutions in Australia, recently spoke to 130 farmers in Paarl and Bloemfontein about integrated weed management. In the group were several Woolworths producers who participate in the chain’s Farming for the Future initiative.

Graeme quotes Dr Arden Anderson, a soil scientist and agricultural consultant who is also a physician and an expert on the link between soil and human health, “According to Anderson each weed species is genetically keyed to replace a specific deficiency. If the function of weeds is to correct the imbalance of minerals, the greater the imbalance, the better the weed growth. Rather than accusing weeds of being the problems, think of them as the symptom of problem soil.”

Graeme explains that many weeds will only germinate in very poor soil conditions and that flocculated, well-drained soils with a good crumb structure will generally promote the growth of crops, not weeds.

How do you know the soil is out of balance?

Nutrient balance in the soil is vital if one wants to discourage weed growth. Farmers can use the identity of the weeds that grow on their land to evaluate what is wrong with their soils. Broadleaf weeds, for example, generally present to correct an imbalanced ratio between phosphorous (P) and potassium (K). They are also a symptom of calcium deficiency. Succulents frequently point to soils deficient in the appropriate type of biologically active carbon. Weeds from the Crucifaerae family thrive in soils that have been subjected to the overuse of salty fertilizers such as potassium chloride. “Sandy, alkaline and badly drained soils all produce different kinds of weeds,” Graeme explains.

One way of determining whether the soil is imbalanced is to use the refactometer to test brix levels in weeds. If the brix of the weed is high, then the relative imbalance is extreme and should be corrected.  The aim is to have much higher brix levels in the crop than in the weeds.

Deposition of Nutrients

Weeds have the ability to improve the balance in soils because they contribute to the deposition of nutrients in the soil. “Many people find it difficult to understand that plants can deposit nutrients. Most people think of plants as extracting nutrients from the soil. What they don’t take into account is that 95% of the entire weight of the crop is determined by photosynthesis and only 5% is derived from soil nutrients. Therefore, as weeds die and decompose, there is net nutrient gain,” Sait explains.

Mineral Balance Case Study

Sait cites the example of Klaas and Mary Howell-Marten, who are large organic farmers in New York State in America. They have achieved exceptional weed control using a combination of mineral balance, crop rotation and timely cultivation. They have found, for example, that high magnesium soils encourage summer annuals that become resistant to herbicide, so they use gypsum to neutralize the magnesium and improve the soil structure. “They have had almost miraculous results,” Graeme says, “Where the fertilizer spreader stopped, it looked almost as though the spray rig had missed a row because the foxtail (setaria) grew so abundantly.”

Crop Rotation

Informed crop rotation plays an important role in integrated weed management strategies. Graeme advises that soil-building crops such as legumes or fine-rooted grasses should be alternated with soil-depleting crops such as corn.

Allelopathy can also be used to discourage weeds. There are many allelopathic plants – plants that secrete toxins to dissuade other plants from growing in their territories. All plants use biochemical tricks to establish their own control zone by producing auxin hormones 24 – 48 hours after germination. This can serve as a weed management strategy in itself if the first flush of weeds is removed from the surface immediately before germination of the crop.

However, allelopathy, can be more specific and potent than this initial discouragement. The absence of plants beneath pine and gum trees is one example. In fact, the only effective natural herbicide is based upon pine oil. Allelopathy involves the release of secondary metabolites by one plant that either inhibits or stimulates the growth of another. The concept of companion planting is based upon allelopathy. There is exciting research involving the isolation of the biochemical’s involved and this may provide a viable alternative to chemical herbicides in the future. Rye is a well known example of a crop that provides excellent weed suppression but other allelopathic crops include barley, oats, wheat, corn, sorghum, soybeans, alfalfa, red clover, peas, field beans and sun flowers.

Howell-Martens Case Study

Rotations play an important role in the weed management of the Howell-Martens.  A typical crop rotation schedule on their organic farm might look like this: They plant a heavy cover crop of red clover in winter. They rotavate the clover in spring and then plant corn. Soy beans follow corn and the soy plant residues are then worked into the soil prior to planting winter wheat or spelt, which is interplanted with red clover. Red clover is always interplanted with cereals. After the second red clover crop a vegetable crop like snap beans or cabbages is planted for diversity.


Tillage plays a major role in weed determination. Working wet soil, for example, can lead to compaction as well as major losses of humus as CO2. Humus oxidises, clods accumulate CO2 and this process triggers the hormonal reactions that awaken specific weed seeds such as foxtail. Light-sensitive weeds can also be triggered by tillage.

However, certain minimal-interference techniques such as blind cultivation can play an important role in effective weed management. Blind cultivation is an early form of cultivation that is done immediately after planting and before emergence. The entire surface of the soil is stirred lightly introducing air, which causes germinating weed seeds to dry out. “The goal of this type of control is to get as much as possible differential size between crops and weeds, “Graeme explains, “You are taking advantage of the difference in seed size between weeds and crop and also the difference in depth of emergence to take out the first flush of weeds.”

The Howell-Martens use this type of cultivation very successfully, followed by two more sessions of cultivation, the timing of which is crucial. The second flush of weeds is attacked with a finger weeder after the crop is up. This finger process can cause minor crop losses, but the removal of the second flush of weeds gives the crop a substantial head start against the third flush of weeds. The third flush is eradicated very aggressively and rapidly with mechanical cultivation at full throttle. The Howell-Martens have always contended that their weed management strategy was as cost effective as chemical management and last year they won a National competition called “the tightwad of weeds”, which proved their point.

Overcoming Weed Management Barriers

“Often the greatest barrier preventing organic growers from switching to organics is weed management,” says Graeme. Farmers often feel they are shackled to the spray rig, because they think mechanical or manual weed control is not economically viable. “The reality is there is a great number of organic growers who have mastered the art of non-toxic weed management so there is no doubt it is possible. It could be argued these growers are masters of their trade because they have jumped this final barrier by working with nature instead of against her,” Graeme concludes.

Getting Rid of Herbicide Residues

A removal of herbicide residues can often sponsor substantial yield increases,” says Graeme, “The best technique is to dilute and apply 2.5 kg – 3 kg of soluble fulvic acid powder per hectare.” Fulvic acid is the most powerful stimulant of bacteria – the organisms that are chiefly responsible for the biodegradation of glyphosate, the world’s most widely used herbicide. Glyphosate has a very negative effect on the soil organisms that are responsible for making manganese and iron available to plants. This technique can remove accumulated residues but there is another strategy that can minimise ongoing damage from glyphosate. This involves the inclusion of an NTS product called Herbi-Safe™ with contact herbicides. This input stimulates rapid biodegradation of herbicide residues and increases the sustainability of weed management.

Sonja Burger is an award-winning freelance writer whose first love is education. She writes about topics that fire her passion or spark her imagination and advocates for issues that are close to her heart. She has written for a wide variety of magazines and newspapers in South Africa about topics as diverse as sustainable farming, biodynamic winemaking, the carbon market, French culinary traditions and the role of mitochondrial DNA in tracing maternal lines. Sonja is also a co-author of two of South Africa’s most popular English textbook series – English in Context and Spot On.

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