Conservation agriculture: Do cover crops dry out soil?

Don’t let the supposed fear of moisture loss keep you from cultivating cover crops, says Gabe Brown, cover crop pioneer from the US state of North Dakota.

He plants all his cash crops in grazed cover crops. Brown says the last season was the driest in decades in the region. However, it is only the second year in 22 that he lost his maize harvest – but this was because there was no rain.

“I am definitely going to plant cover crops next year.”

Just like in South Africa, fears about the availability of moisture for cash crops are one of the main reasons why crop farmers are not trying out cover crops.

Research results on the topic and facts about the water holding capacity of soil however, shows the opposite: cover crops can indeed release more soil moisture for subsequent cash crops.

The American seed company Green Cover Seed explains it as follows in its latest manual.

“Let’s assume you receive 560 mm rain on average per annum. About 180 mm of that falls in May and June. If you harvest your wheat on July 1, and you plant maize the following May, you are supposed to measure 380 mm of rain between July and May, according to your long-term average.

“Most soils contain about 50 mm of moisture per 30 cm layer. A root zone that is about 90 cm deep is supposed to contain 150 mm of moisture.

“What happens with the remaining 230 mm of moisture that can’t be stored by the soil? It runs down (causing erosion), leaches out of the soil (and takes valuable nitrogen with) or evaporates. Why then don’t we use this extra moisture to grow cover crops to enhance and protect your soil?”

Living plants use more water compared to fallow land covered in crop residue, but the difference is not as significant as was thought previously, the manual states.

Recent research from the Kansas State University shows that a piece of land planted with cover crops stores 40 mm less soil moisture compared to fallow land covered with crop residue where weed was treated chemically.

However, during the next spring and summer, the moisture content in the soil planted with cover crops was higher compared to the fallow land.

The reason is that cover crops can increase the availability of soil moisture because of the following five reasons:

  • It increases water penetration dramatically. Water penetration in bare soil, even soil that is not tilled, is sometimes less than 13 mm per hour, while the same soil with cover crops can absorb 80 mm per hour.
  • Cover crops decrease evaporation dramatically. Research shows that soil covered with plant residue loses 80 mm less water over the growth period.
  • Cover crops enhance the root depth of subsequent crops. Crops with deeper roots like radishes, sunflowers, sorghum, rye-grass and rye can break down compacted soil. If the canals of these roots are not destroyed by tilling, the roots of follow-up cash crops will follow the canals and will penetrate deeper into the soil than usual.
  • Cover crops increase the amount of organic material in the soil. For every percentage point increase of organic material in the soil, soil can store an extra 25 mm of water.
  • Cover crops enhance the growth of mycorrhiza fungi in soil. The fungi colonises the roots of plants and forms a fine network of thin threads (hyphae) that moves much deeper and wider into the soil than the normal root system, to provide extra water and nutrients to the plant. This increases drought resistance.

Source: 2017 Soil Health Education and Reference Guide, Green Cover Seed.

South African agricultural journalist Marleen Smith, from the magazine Landbouweekblad, attended a tour to the US thanks to sponsorships from VKB and Pannar.

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