Cover Cropping & Intercropping

Much of what we have done in modern agriculture contradicts natural laws and principles. In our arrogance we have focused upon either changing nature or depreciating natural capital with an extractive, symptom-treating approach. There is a growing realisation that this is a bankrupt philosophy and now it is time to change. If we observe Nature, we see very rapidly that she abhors a vacuum. If land is cleared she will cover that bare soil as rapidly as possible. There will not be one weed, but a diversity of pioneer plant life that fills the gap. The reasons for this rapid response are multiple. These volunteers feed the soil life with their sugar exudates, they provide a living root mat to stabilise the soil to prevent leaching, they protect the soil from the direct impact of the sun and, in some cases, the weed roots mine minerals from deeper down in the soil profile. We can mirror this regenerative approach, but we can also choose our cover crops and rotate those most suited to the following crops. In some cases there is great gain in interplanting a legume with cereal crops.

Cover Crops – Competitors or Supporters

We can choose our cover crop based upon our goal for that field. If we are seeking to build organic matter, we might choose something like Jumbo sorghum to produce huge amounts of biomass that can be slashed, turned in and composted in-field. Alternatively, we might be using a legume before planting a cereal crop. The legume provides significant amounts of nitrogen for the following crop, but it also provides two equally-important free gifts to the subsequent crop. The roots of legumes release mild organic acids into the soil to release calcium and phosphorus in a plant-available form. These are the two most important minerals for photosynthesis, the most important process on the planet.

One of the biggest misunderstandings about cover crops, particularly in dryland farming, is that they will rob precious reserve moisture from the cash crop. This is not the case. When these cover crops are returned to the soil they increase organic matter (humus), which holds its own weight in water. More importantly, these crops feed and stimulate bacterial populations and these organisms constantly release a sticky substance that works just like water crystals in your soil. You have very often improved moisture management with a cover crop instead of stealing from the coming crop.

The Potential of Intercropping

There is also a widespread belief that there will be competition for moisture and nutrients when a legume is interplanted with a cereal crop. This legume is not intended for harvest, it is planted with the cash crop to deliver a constant supplementary supply of nitrogen, phosphorous and calcium – and this is exactly what happens. On our research farm, we planted a sweet corn crop interplanted with 25 kg of soya bean. We monitored both this crop and a control crop with regular leaf tests. There was no nutrient or moisture competition evident in the intercropped section, but there was an impressive delivery of nitrogen, phosphorus and calcium throughout the crop cycle. This was most apparent from flowering onwards, where there was an obvious phosphorus drawdown in the control that was not present in the intercropped section. There was a substantial yield increase in the intercropped area and the crop quality was also improved. Most of the large-scale biological growers in the US and elsewhere will always plant white clover beneath their cereal crops because they understand the benefits.

What to Embrace and What to Avoid

Here are three pointers for those considering cover cropping:

1. Avoid the common practice of mixing a brassica with legumes and cereals when attempting to create more biodiversity in your cover crop. The biodiversity concept is fine and a multiple species mix is great, but the brassicas can be counterproductive in the mix. Here’s why: brassicas release biochemicals to dissuade certain root-dwelling organisms. That is good news if you have a problem with root knot nematodes, but it is bad news if you want your cover crop to host all-important mycorrhizal fungi. If the brassicas are peppered throughout your cover crop, the plants that surround each brassica will struggle to achieve mycorrhizal colonisation due to the toxic root exudates nearby.

2. If you are an orchardist, dairy farmer or viticulturist and you live in a frost-free zone, you need to know about Pinto Peanut. This is a truly amazing cover crop. It produces a dense, yellow-flowered ground cover that only grows a few inches tall. It easily outcompetes weeds and requires no maintenance. Research at the Alstonville Tropical Fruit Research centre in Northern NSW several years back showed that there is no competition with the tree crop for moisture. Like all legumes, the Pinto Peanut delivers a significant supply of nitrogen, calcium and phosphorus, but this legume is unique in that it also delivers potassium. It appears that the deep-rooted legume mines potassium and delivers it to the feeder roots of the tree (in the top six inches). This was not just a token supply. One of the soils tested revealed a threefold increase in potassium just two years after the legume had been introduced. Potassium is the most expensive major mineral, so this is a huge cost-saving benefit. The legume is grown from seed, but once you have it established you just take cuttings and root them in a bucket to spread this little beauty everywhere (and believe me you will want to do this). During a seminar tour of Hawaii, I visited an iconic, mixed-species orchard where the botanist in control had Pinto Peanut on every available square metre of land. He even had several buckets full of cuttings rooting in water so he could replace his front lawn with this beautiful ground cover.

3. Oats are unique in that they produce a legume-like exudate that also breaks the bond between locked up phosphorus and calcium. Oats are a favourite of those utilising a highly productive interplanting practice called pasture cropping. Here, rows of oats are planted 60 cm apart into a winter dormant pasture, to provide supplementary feed over winter. If you treat the oat seed with mycorrhizal fungi (NTS Nutri-Life Platform®) at planting, the associated living root extension will spread to your pasture crop and this becomes a cheap technique to colonise the entire paddock.

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