Green manure crops are rising in popularity once again, finding favour as a sustainable management tool to improve various soil conditions and yield. Unlike fertiliser, green manure crops are not a quick-fix solution and the initial capital investment might raise scepticism among farmers, but Robyn Joubert finds they’ll save money in the long run.
With farmers looking at biological and sustainable farming with renewed interest, the use of green manure crops is picking up once again. Green manure and cover crops have come full circle. First documented in 1134 BC by the ancient Chinese, traditional agriculture made extensive use of them until they were displaced by the faster results delivered by modern farming methods.
“There’s a need to recognise the dynamic and complex nature of our soil,” says Simon Hodgson, MD of KwaZulu Hybrid Seeds in Cato Ridge, outside Durban. “The soil needs an interdependent web of physical and chemical properties to sustain the living components. Using cover crops, we can start a long-term rehabilitation and maintenance programme, which is cost-effective and will ultimately lead to an increased return from the soil.”
Cover crops increasing in popularity
The terms “cover crops” and “green manure crops” have become interchangeable. “They’re crops grown primarily to add nutrients and organic matter to the soil,” Simon explains. “They’re also sustainable tools to manage any number of conditions in an agro-ecosystem, like soil fertility, soil quality, water, weeds, pests, diseases, diversity and wildlife.” Simon cites sales figures as evidence that the use of cover crops is rising. “Sales of green manure crop seeds have risen from about 5t in 2003, when I started with KZN Hybrid Seeds, to 150t six years later. Cover crops fit just about any kind of farming, but they’re currently mainly used by farmers growing cane, bananas, macadamias, pineapples and vegetables.”
Cane farmers are the largest users of green manure seeds bought from KwaZulu Hybrid Seeds, directly as a result of research by the South African Sugar Research Institute (Sasri). Customers include major sugar companies Tongaat Hulett, Illovo and Umhlatuzi Sugar as well as about 150 individual growers. While many farmers express an interest in growing green manure crops, they’re put off by the idea of taking land out of production. “There are ways to get around this,” says Simon, who farmed tobacco for 18 years in Zimbabwe until he moved to Cato Ridge in 2001.
In the sugar industry, the main cover crop timings are related to the “plough-out” fields, which are traditionally allowed to lie fallow. The timing of the plough-out dictates what cover crop is planted and when. At the start of the harvesting season when the mills open, usually in March or April, the cover crop is planted after the first plough-outs. This is popularly a temperate grain variety like oats (Avena sativa) and black oats (Avena strigosa). Another option is rye (Secale cereale). In spring and summer, legume options include sunn hemp, cow peas or dolichos beans, and non-legume cover crops like forage sorghum, babala or teff.
“Cane farmers had a good season last year, but those who grew green manure increased their yield by 12% to 15% in the first ratoon. This can be attributed to improved soil health and a higher population of soil microbes like fungi and bacteria. The more food you supply to the microbes at the base of the food chain, the quicker populations explode,” Simon explains.
Vegetable farmers’ needs
But vegetable farmers are in a very different situation. Due to the speed of rotation in most enterprises and the value of land left fallow, they want to keep the whole farm in production all the time. Simon has devised a rotation schedule to minimise the financial impact of introducing a cover crop into a system. If the rotation is followed, it will take four years to complete a full green manure cycle over a whole farm unit. It involves dividing the productive area into four equal units. Each is further divided into quarters. In year one, a cover crop is grown for three months on one quarter of a unit, which is rotated every three months until the entire unit has been under a cover crop for three months.
In the second year, the next unit is given this three-month cover crop rotation. “This ensures a farmer loses only 6,25% of their income-generating land to green manure crops at any one time,” says Simon. “They can continue producing vegetables, but can rehabilitate the soil at the same time.” Farmers can also introduce harvesting in the form of forage or grazing into a mixed enterprise.
For example, a KZN cane and beef farmer grows oats as his cover crop and lets cattle graze it, usually after 42 days. After grazing, the oats is left to grow out for a few weeks, then incorporated into the soil to feed the microbes. The cover crop therefore helps the farmer generate an income through his beef.
Green manure savings
While farmers should look at the bigger picture of sustainable farming and healthy soil, it helps to know that a saving can be made. Sunn hemp seeds cost R775/ha but this is offset by the equivalent value of chemical fertilisers.In a recent Sasri trial to determine the nutrient values of sunn hemp grown as a cover crop, it was mowed at the early flowering stage and worked into the soil.
Some 28% of the dry matter yielded 2,5% nitrogen, 0,2% phosphorus and 1,7% potassium. The sunn hemp had an equivalent value of R1 300 nitrogen, R360 phosphorus and R1 000 potassium. (According to fertiliser prices as at
2 December 2008). “That’s what the microbes put back into the soil,” says Simon. “Cover crops don’t yield a response as quickly as fertiliser out the bag, because they’re worked into the soil after about 90 days and then have to be processed by the microbes. So it requires a change of mindset and the patience to wait for nature to do its thing.”
Simon explains that sowing seeds need not require buying expensive implements. “The simplest way is to use a ‘spinning disc’ implement like a Spandicar. On steep inclines where tractor-drawn implements can’t be used, broadcast seed by hand and cover by hand to ensure good seed-to-soil contact.”
Simon concludes that while growing a cover crop in rotation has many benefits, it’s not a wonder crop and won’t fix problems overnight. But with persistence and planning, it’ll enhance the soil.Contact KZN Hybrid Seeds on 082 908 4757 or 031 785 1581. |fw
Cover crops will enhance the viability of your farm and will improve your bottom line as the soil-improving effects accumulate. Other benefits won’t appear directly in the financial accounts and may be difficult to quantify. They are detailed below and vary by location and from season to season.
1. Reduced need for herbicides:
Cover crops reduce weed pressure, disease, insects and nematodes. Acting as a smother crop on weeds, some crops like oats and saia oats exude allelopathic chemicals from the roots which have a natural herbicide effect on the Cynodon species. A balanced population of soil organisms creates a holistic environment unsuitable for pest and disease organisms, and encourages the reappearance of beneficial insects like ladybirds.
2. Improved yield by enhanced soil health:
Yields are improved by better surface water infiltration, compaction relief and aggregate stability brought about by reduced tillage. Increased organic matter promotes microbial activity, leading to better nutrient cycling and uptake by plants. Soil-building crops include rye (Secale cereale) to add organic matter and sorghum/Sudan grass hybrids, which have deep penetrating roots to break compaction.
3. Improved water uptake and quality:
Cover crops reduce the striking force of rain drops on the soil and slow the flow of water through and over the soil. They slow soil erosion, water runoff and nutrient leaching into the groundwater system.
4. Increased organic matter:
Organic matter improves soil structure, infiltration and water-holding capacity and increases the cation exchange capacity.
5. Conservation of soil moisture:
If a sunn hemp crop is mowed, the resulting mulch left on the surface will protect the soil and reduce evaporation.
6. Nitrogen equivalent:
Legume green manure crops fix atmospheric nitrogen in nodules on the roots for plants to use. The nitrogen value is reason enough to justify planting a cover crop. Mixing a temperate grain crop like oats with a legume like red clover will give benefits of green manuring, as well as cover cropping.
