Cultivation

Cultivation is any process that involves turning over or tilling the soil. It can include:

• pasture renewal
• cropping (such as growing maize)
• intensive cropping (such as commercial vegetable growing).

Cultivation provides us with food crops and new pastures. But it also reduces the organic matter in soil, and can lead to increased erosion. Waikato Regional Council recommends growing annual cover crops, reducing cultivation and adding organic matter (compost, manure) to soil to help restore soil organic matter.

Soil organic matter is an indicator of soil quality. We estimate it by measuring the organic carbon in soils. Regular cultivation exposes more soil to the air, which mineralises soil organic matter. This means that the soil’s organic carbon is lost as the gas carbon dioxide (CO₂).

Waikato dairy pasture soils still have good amounts of soil organic matter – but some cropping soils have lost up to 60 percent.

Organic carbon levels in cropped soils, such as those in Patumahoe, have reduced to amongst the lowest in the Waikato Region. Six years of maize cropping in Horotiu and Puniu soils have led to organic carbon decreases of 40 and 53 percent respectively.

Cultivation also leaves soils vulnerable to erosion. The compacted soils left in wheel tracks often act as channels for water. When it rains, water can then undercut and remove the surrounding soil.

Long term protection and improvement of soil organic matter is needed to maintain:

• soil structure
• microbial activity in the soil
• soil buffering (the ability of a soil to stay at the same acidity)
• nutrient storage
• water retention.

Soils with low organic matter have low microbial activity. The soil microbes break down harmful chemicals such as pesticides. When soils have low microbial activity and low water holding ability they are more likely to leach pesticides. This means that cropping soils leak pesticides more readily than other soils.

Soils that have lost a lot of their organic matter need more and more fertiliser and water to maintain productivity. To find out more about preventing fertiliser runoff read the NZ Fertiliser Manufacturer’s Research Association Code of Practice for Fertiliser Use.

Reducing cultivation and soil disturbance minimises the loss of organic carbon. By implementing reduced tillage or no-tillage, where possible, soil structure and soil organic matter will be preserved. Erosion risks will be reduced as well.

Check out the NZ Fertiliser Manufacturers' Research Association Code of Practice for Fertiliser Use.

To increase the organic matter in the soil you can:

• grow annual cover crops between productive crops – these cover crops can then be mulched into the ground
• apply compost, organic manure or effluent to the soil.

With more organic matter, soil is better able to store nutrients and water, and has improved soil structure. This makes soil more resistant to erosion by wind and water.

Cultivation causes peat to shrink twice as fast as it would under pasture, and should be avoided where possible. If cultivation is done correctly during pasture renewal, it won’t need to be done as often. Managing your pasture well also reduces the need for pasture renewal.

Find out more about managing peat.  Place an order if you would like a published paper copy of For Peat’s Sake – good management practices for Waikato peat farmers (containing information on peat land management).

Find out more about cultivation’s impacts on soil management in the Franklin District.

Footnotes

1. Gradwell, M.W. and Arlidge, E.Z. 1971: Deterioration of Soil Structure in the Market Gardens of the Pukekohe District, New Zealand. New Zealand Journal of Agricultural Research 14: 288-306.