Why vetch is a great cover crop

Ihave been producing vegetables for 17 years using no-till principles. I plant as much vacant ground as possible in April for a grazing vetch cover crop, with the remainder getting some compost before planting. Two rows of seed are planted on each bed at a rate of 5kg/ha. Waist-high vetch eventually covers the entire area. Before it sets seed, I rake all of it to the middle of the beds from either side, then use a spade to sever the rolled-over vetch at ground level.

This takes relatively little effort and leaves a 50cm-wide mulch layer. I plant on the edge of this layer.

This year, when the grazing vetch was fully dried out, I rolled up a measured section, weighed it, then extrapolated the figure to obtain the total aboveground plant material yield. I calculated this to be 9t/ha. I then contacted Cedara agronomist Guy Thibaud, who informed me that the weight of the roots roughly equals the weight of the above-ground material (leaves and stalks), giving a total of 18t/ha organic material. I could then make the following calculations:

• The 18t dry weight organic material contains 7,2t carbon (C), or 400kg C/t crop residue.

• Of this, 140kg can be converted to humus by the respiration of active microbes, releasing the balance of the carbon to the atmosphere as carbon dioxide. • As humus comprises 50% C, 280kg of humus forms per ton of crop material. Put another way, 7,2t/ha C gives about 2t of humus. This figure depends on sufficient nitrogen (N) being present. Humus contains about 10% N. In crop residue with a low N content, for example oats, this amount of dry material forms 700kg/ha humus. Grazing vetch, however, contains more than enough N.

• The 2t of humus formed would contain 10% N, a total of 200kg N/ha locked up for slow release for the following crops.

The mulch will block weed development for several months

humus-rich soil

The above enables a farmer, using the cost of chemical fertiliser as a guide, to calculate the economic benefit of this approach.

However, much of its value lies in the benefits of humus-rich soil: improved water percolation and retention, eelworm suppression, a healthy earthworm population, the presence of beneficial organisms that convert unavailable minerals into a form plants can use, and better plant health in general.

As the mulch in contact with the moist soil starts to decay through the action of various soil microbes, earthworms and arthropod shredders such as wood lice consume the rotting vegetation, transforming it into a highly fertile deposit containing growth stimulation substances.

Earthworm tunnels form pathways for rapid water penetration and the correct degree of aeration for a healthy, balanced soil food web. The mulch will block any weed development for months.

RAKED UP

I made these calculations on a per-hectare basis. But as the material is raked in from the walking space between the beds, the 2t/ha humus is in fact concentrated onto half the total area, giving an effective humus equivalent of 4t/ha in the beds where the plant roots are concentrated.

• Bill Kerr is a vegetable specialist and breeder. Email him at farmersweekly@caxton.co.za. Subject line: Vegetable Production.

bill kerr ABOVE RIGHT:A view of the soil under the dry vetch mulch showing earthworms, decaying roots and a good crumb structure. In short, this is fast-tracked soil improvement.

ABOVE:The mulched area ready for planting. Humus-rich soil means better water percolation and retention, more earthworms and tons of beneficial organisms.

bi ll kerr