Management of organic soils

Introduction

Organic soils, commonly called peat or muck, have developed from plant residues and been preserved by a high water table. Many generations of plants, growing for decades have fallen in the water in which they were growing and been preserved because of the lack of oxygen. It takes nature about 500 years to accumulate 30 cm of organic soil.

Organic soils are used extensively for vegetable production in Ontario and with proper management will produce excellent crops year after year.

Subsidence

Organic soils have a major chronic problem - they subside at a steady rate. The term "subsidence" is used to describe the permanent lowering of the surface elevation of the soil. Several factors are responsible for subsidence. The most important are: oxidation of the soil organic matter, soil shrinkage, wind erosion, water erosion, and height of the water table. The rate of subsidence varies, depending on the frequency of wind erosion, the organic-matter content of the soil, the degree of water-level control, and the methods of cultivation. 

Research has shown that a muck soil, intensively cropped, subsides at a rate of 30 cm of soil every 10 years. This process can be slowed by the application of copper, a well designed water-control program, a wind abatement and cover crop program and minimum cultivation. These steps are essential for long-term continued use of organic soils for agriculture. With good water table control and soil management practices, the rate of subsidence can be reduced to 4.7 cm every 10 years.

Copper

Copper (Cu) applied as fertilizer slows down the activity of enzymes which cause subsidence by about 50%. It is recommended that 14 kg of Cu/ha be applied for the first three years of initial cultivation followed by 5 kg Cu/ha every second year, particularly when onions, carrots or lettuce are grown.

Water Control

The level of the water table influences crop production and has a major affect on the rate of subsidence. therefore, the water table should be maintained at a level which will keep subsidence to a minimum and at the same time produce optimum crop yields. 

Maintenance of an ideal water table level in organic soils is affected by a number of factors: 

Amount of Available Water 

Generally, in Ontario there is not sufficient groundwater available to replenish water lost through evaporation and that used by plants. Therefore, groundwater cannot be relied upon to maintain an ideal water table level in Ontario's organic soils.  

There are two methods by which the water table may be maintained. 

(1) Overhead sprinkler irrigation This method is reasonably economical as long as there is a large water supply nearby. The capital cost is high and there is the added cost of moving pipe, as well as maintenance costs.  

(2)Tile drainsAn ideal water table is more easily maintained when pipelines leading from main sources of water (e.g. canals, rivers, ponds) are connected to the tile-drain system via a series of valves and water-control gates. Although the initial capital expense may be high, the subsequent costs of operating this system are negligible.

Tolerance of Various Crops

The level of the water table is an important consideration in growing different crops. The following table shows the levels required.

Permeability 

The ease by which water moves in a particular organic soil affects the level that can be safely maintained. Reed and sedge peats are more permeable than the more compact sedimentary, woody mucks. After many years of subsidence, the density of the soil increases and permeability decreases. The vertical movement of water in an organic soil may vary from 0.5 to 34 cm per hour. The rate of movement of water is an important factor in drainage design and in water-control installations.

Rainfall 

The amount of rainfall during the season plays an important part in the behaviour of a crop relative to the position of the groundwater level. In seasons of high rainfall the water table should be maintained at a lower level than during periods of low rainfall. 

Flotation of Equipment 

At water levels above 60 cm, tractors and loaded wagons easily bog down, weed control becomes more difficult, and spraying and harvesting operations are hampered. 

Plant Nutrient Availability 

High water tables affect the availability of nitrogen, phosphate, and potash because of restricted root development. Nitrogen can be applied during the growing season if and when required. The ideal situation is a water-control system that permits a progressive lowering and maintenance of the water table to a level optimum for the crop as the growing season progresses (Figure 1).

Figure 1. Actual and ideal water levels for an organic soil

Wind-Erosion Control

Severe losses can result from wind damage. Many crops are either destroyed or seriously injured during the early part of the growing season. Losses of 1 to 3 cm of soil and filling in of ditches are the usual results of wind storms. 

Several methods of wind-erosion control have been used with varying success. They are: windbreaks; maintenance of a moist soil surface, either by sub-irrigation through the tile drains or overhead irrigation; interplanting of grain between crop rows or broadcast among the crop; winter cover crops; use of minimum-tillage methods to leave rough field surfaces and planting on raised beds or ridges.

Fall planted winter cover crops can provide protection from wind erosion and early spring runoff. Cover crops can be planted after early onions and carrots, lettuce, celery, radish or greens. If vegetables are to be planted the following spring, growers prefer to plant winter cover crops which die off during the winter and that do not interfere with seeding. Spring grains such as barley at 84 kg/ha or oilseed radish at 11-22 kg/ha are suitable (See Figure 2 - picture of winter cover crop). In the Bradford area, best results are obtained when fall planted cover crops are seeded before September 15. However, this date varies from season to season and in different regions of the province.

Figure 2. Oil seed radish winter covercrop - 2 seeding dates

To improve soil structure on marginal muck soils, perennial rye grass, winter rye or sudax (Sorghum-sudan) are suitable as a summer cover crop. Perennial rye grass needs warm weather to get established. 

Sorghum-Sudan has been shown to successfully improve the health and drainage of marginal muck soils. The following practices are recommended when using Sorghum-Sudan on muck soils: a pre-plant herbicide is helpful to get the crop established; only plant Sorghum-Sudan after all threat of frost has passed; broadcast seed at about 50 kg/ha and apply 45 - 56 kg of N per hectare prior to planting. The Sorghum-Sudan must be mowed down once or twice to about 6" high after it has grown to about 3 feet. This promotes tillering and deep root growth and prevents the development of woody stalks. The crop must be chopped and/or disked in the fall while it is still green. Do not let the crop head out.

Early in the growing season cereals can be planted with onions and carrots to prevent wind erosion and wind damage. Such cereal windbreaks (usually barley) can be seeded between the rows of onions and carrots at a rate of about 60 seeds per metre or broadcast at a rate of 200-225 seeds per square metre (1-1/2 bu/acre). Once the barley windbreaks are 10-15 cm high they should be treated with one of the registered selective grass herbicides so they do not compete with the vegetable crop. Check the label of these products carefully for specific recommendations.

Water-Erosion Control 

Winter cover crops provide some limited control of water erosion during spring thaws, however heavy rainstorms can cause serious erosion in low areas and along ditchbanks. Recent research indicates that a proper mixture of perennial rye, fescues, timothy, clover and Kentucky blue grass can be used as a soil stabilizer along ditchbanks and headlands.

Soil Reaction and Liming 

The range of pH of most organic soils in Ontario is between 4.0 and 7.5. The acidity of organic soils is caused by the presence of organic compounds, exchangeable hydrogen, and iron sulfide. Alkaline conditions are often caused by burning (the ash from burning 30 cm of organic soil may raise the pH to 1 to 1-1/2 units); the presence of limestone, marl or shells or over-liming. 

When the pH is 5.1 or lower, an application of lime is generally recommended, particularly when a chemical analysis indicates a lower percentage of calcium in the soil.

Soluble Salts

High concentrations of soluble salts in soils can prevent or delay germination of seeds and can seriously damage established plants. Salt problems have become commonplace in some muck soils especially during rought periods. Salt levels can rise to damaging levels from excessive applications of fertilizers, runoff of salts applied to roads and chemical spills. Fertilizers such as ammonium nitrate, potassium chloride, sodium nitrate and ammonium sulfate can increase salt levels. If a salt problem develops, overhead irrigation with a low salt water can reduce the damaging effects.

Fertilizing Organic Soils 

Nitrogen, phosphate and potash are applied in various amounts depending on the specific crop requirements, the number of years the soil has been under cultivation and the soil reaction. Refer to the latest edition of Vegetable Production Recommendations, OMAFRA Publication 363 for specific requirements. 

Nitrogen exists largely as a constituent of the organic matter and requires the action of soil microorganisms to change to forms available to plants. The rate of release of nitrogen decreases as the soils are cultivated for a period of years. Strongly acid soils are usually low in nitrogen. 

Phosphorus is present largely in organic forms. Organic phosphorus has to be mineralized before it can be utilized by plants. Crops growing in most organic soils usually respond to an application of phosphate fertilizer, provided an adequate amount of potash is included in the mixture. Soils which have been under production for many years may show a high level of phosphorus. In such cases only 20 kg of phosphate are required as a starter fertilizer for most crops (40 kg/ha for celery). 

Potassium Potash applied to organic soils increases yields of crops, increases the sugar and starch content, reduces frost hazards within a narrow range (1-2EC), improves crop quality, and possibly increases disease resistance. 

Micro elements Micro-element requirements of organic soils in Ontario generally include the elements copper, boron, manganese, magnesium and zinc. 

Copper sulfate at 50 kg/ha and borax at 20 kg/ha, mixed with the commercial fertilizers, should be applied to all newly developed organic soil areas. Manganese, magnesium, and zinc, if required, are usually applied as foliar sprays during the growing season. 

The latter should only be applied if there are indications (visual symptoms or through tissue analysis) that a deficiency exists. Refer to the latest edition of Vegetable Production Recommendations, OMAF Publication 363 for micronutrient recommendations for specific crops.

Soil Testing 

Applying fertilizer according to recommendations based upon soil tests is the only sure means of maintaining a balance of nutrients in the soil. 

Virgin organic soils are usually low in available phosphorus and potassium. Changes in fertility levels brought about by application of fertilizer can be measured by soil tests. Soil testing is a valuable aid in determining the nutrient requirements of crops. 

Recommendations for nitrogen are made by extension horticulturists who take into consideration the crop requirements as well as the type of organic soils, the length of time it has been under cultivation and its decomposition rate. 

Adapted Crops 

Some vegetables that do well on properly-managed organic soils are potato, onion, carrot, parsnip, lettuce, celery, cabbage, cauliflower, table beet, sweet corn, radish, yow choy, choy sum, gai lan, tung choy and spinach. Some field crops adapted to organic soils are corn, sugar beet, mint, peas, grasses, and small grains.

Frost-susceptible crops such as sweet potato, pepper, eggplant, melon, and tomato are not well adapted to organic soils. 

Organic soils are one of our many natural resources. People are becoming increasingly aware of the need to manage and preserve these limited resources. Prevention of fire and erosion are important conservation objectives.

Wise development of organic soils will include uses for agriculture as well as for wildlife and recreation.

Acknowledgements 

Lucas, R.E. 1982. Organic Soils (Histosols), Formation, distribution, physical and chemical properties and management for crop production. Michigan State University research report 435. 77 pgs. 

Marza, C. and Irwin, R.W. 1964. Determination of Subsidence of an Organic Soil. Can. Intl. Soil Sci. 44: 243-253. 

Mathur, S.P. 1981. The Inhibitory Role of Copper in Enzyme Degradation of Organic Soils. Proc. Intl. Peat Symposium Bemidji, MN. pg. 191-219. 

Valk, M. 1976. Management of Organic Soils. Original Manuscript. Agdex 512, 76-094.