Gaining Ground - Excerpt from Part 2: Managing the Transition in the Fields
6 Soil Fertility and Nutrients
2. MINIMIZE NUTRIENT LOSSES FROM YOUR FARM
Just as conventional farmers do not throw away bags of fertilizers, organic farmers do not let their nutrients go to waste. They work hard to prevent nutrient losses. This makes sense from an ecological and economic point of view.
Nutrient loss takes many forms. Nutrients can literally blow or wash away. Wind erosion can cause the loss of nutrients when the topsoil blows away. Water erosion occurs when water running over the soil surface washes away the topsoil. In both cases, nutrients are lost because the top layer of soil is rich in nutrients and soil life. Both wind and water erosion are more likely to occur and are more damaging when the soil is bare.
Nutrient leaching occurs in wet conditions when the soil is saturated with water, often during the spring or fall. When the water percolates through the soil, it picks up soluble nutrients and carries these away. The nutrients end up either deeper in the soil, or more likely, in ground or surface water.
Nutrients can also be lost by exporting the nutrients off the farm. Every time a crop is harvested, or a pasture is grazed, nutrients are removed. This removal can be balanced by adding organic matter as described earlier, but farmers can also focus on reducing the losses. For example, keep in mind that by selling hay, you are selling nutrients and organic matter. If you need to sell hay, try to compensate for the losses by buying back manure to compost and apply to the soil.
Ways to minimize nutrient losses include: Keep the soil covered as much as possible, preferably with living plants;
- Prevent leaching;
- Use catch crops;
- Reduce tillage;
- Protect manure from nutrient loss during storage and handling;
- Minimize the export of nutrients;
- Match nutrient supply to nutrient demand.
2.1 Keep your soil covered
Soil that is bare, or has very little cover, is susceptible to wind and water erosion. You can protect the soil from erosion by leaving residue on the surface. The goal is to try to reduce the speed of wind and water as they travel over the soil surface. The faster wind and water travel, the more soil they can take away. Leaving crop stubble in the fields slows down the wind and water, and holds the soil in place.
The way residue is incorporated can affect its vulnerability to erosion. For example, a larger amount of sweetclover residue is left by mow-mulching or Noble-blading than by discing.
When there is no crop on the soil, use cover crops and crop residue to stabilize the soil. As much as possible, try to avoid black fallow. Other techniques to prevent erosion include strip cropping, contour farming, shelterbelts and windbreaks.
“A good organic farm is green in November. We need to protect the soil, keep it covered with green.” – Pieter Biemond, ON
“We leave windrows of high stubble on our fields, and thatch on the hilltops to help build up eroded knolls.” – Robert Tomlin, SK
“We keep our sandy loam in a longer grass rotation, and the sandiest fields in permanent pasture to eliminate soil erosion.” – Alex Scott, MB
2.2 Prevent leaching
Nutrients can be lost by simply flowing through the soil with rain or irrigation water – this is called leaching. It’s more common in areas that get lots of rain and in sandy soils. It is particularly important to keep the soil covered when water from spring or fall rains, or snowmelt, might flow over the soil surface.
When leaching potential is high, soil should be covered with living plants as much as possible. Extensive use of perennial forage crops and cover crops in the rotation can prevent nutrient loss. During the annual crop cycle, cover crops should be used throughout the winter and other non-crop seasons.
“Our cereal crops are always underseeded with ryegrass so the land stays covered over winter.” – Terrance Boyle, NS
2.3 Use catch crops
The best way to prevent nitrogen leaching is by planting a crop that will use it up before it can leach below the rooting zone. Cover crops can be used and are sometimes called ‘catch crops’ in this context, because they catch the nutrients that would otherwise leach away. Effective catch crops include most fast-growing non-legumes, particularly rye, oats, ryegrass, oilradish or mustard. After a legume plowdown or even the harvest of a heavy-feeding crop such as potatoes, the catch crop can be planted. For example, when planted in September, fall rye can take up 79 kg/ha (70 lb/a) of nitrogen by December – nitrogen that would otherwise probably leach away in spring or fall rains. The nutrients tied up in the catch crops will be returned to the soil in organic form when the plant material is worked in, usually in the spring.
Leaching can occur while a crop is still in the field. Underseeding of cover crops into the cash crop (creating a living mulch) can intercept nutrients not used by the cash crop, or can trap any nitrogen released by the roots of pulses. Intercropping also reduces the potential for erosion by reducing the amount of bare soil.
Intercropping can involve overseeding one crop into another, planting the seeds together or strip cropping. Strip cropping using alternating strips of cash crops and green manures can provide nutrients to the crops. In one study, wheat and red clover were strip-cropped to enhance the baking quality of wheat. The strips were 2.5 m (8 ft) wide. The legume mix was cut and blown onto the wheat strips. The mulch increased the protein content and baking quality of the wheat. It did not affect yield even though there was more weed pressure in the strip-cropped fields. In another study, underseeding faba beans with mustard helped prevent leaching losses because it increased the total root mass in the topsoil.
2.5 Reduce tillage
When soil is tilled, carbon (organic matter) and nitrogen are lost from the soil in a gaseous state. Repeated tillage, as in black fallow, should be avoided for this reason. When compost is applied to the soil or when a legume green manure is killed, some of the nutrients may be lost if they are left on the soil surface. Even just partially incorporating the organic residues will help to reduce losses of nitrogen to the air.
Tillage also warms up the soil, and increases its oxygen content, stimulating the activity of microorganisms. Plants that are tilled in, and organic matter released from the broken aggregates, provide the soil life with an increased food supply. The result is an accelerated breakdown of plant residue, increased nutrient cycling, and a better nutrient supply to crops in the short term.
But tillage has drawbacks: it damages soil life and soil structure. This causes losses of valuable soil nutrients and organic matter, and thus, soil fertility. Also, the short-term flush of nutrients usually occurs just before seeding, long before the crops can take up the nutrients. As a result, the nutrients might end up being leached from the soil or being used by weeds. Excessive tillage is the opposite of soil building, and because of this black fallow should be avoided.
No-till systems that involve cover crops seem ideal for organic farming, and are currently being researched. But so far there are still problems: there can be reduced nutrient supply (at least initially), lowered soil temperature, increased weed pressure, and competition from cover crops for nutrients and water.
Blackshaw, R.E. et al, 2001, Suitability of undersown sweetclover as a fallow replacement in semiarid cropping systems, Agron. J. (93): 863-868 (2001)
Schulz-Marquardt, J. et al, 1995, Strip cropping of spring wheat and forage legumes for production of quality baking wheat in organic agriculture, Institut f. oekol. Landbau
Berg, M. et al, 2003, Nitrogen management on organic farms: reducing N losses, Inst. Org Ag, Uni Bonn Website: www.iol.uni-bonn.de
For more from Part 2: Managing the Transition in the Fields and more technical information about farming during transition, order Gaining Ground: Making a Successful Transition to Organic Farming from the publications page