Soil Management - Tillage & Soil Improvement Measures
Potato crops require an uninterrupted oxygen supply for healthy root and tuber development. It is necessary to prepare and maintain friable soil for the entire growing season. Primary tillage, seedbed preparation, and between-row cultivation are all important components of good soil management that affect production. As compaction and deterioration of soil structure are detrimental to potato production, it is important to conduct tillage operations only under ideal soil moisture conditions. Traffic over the field while the soil is too wet should be avoided, since it leads to soil compaction and water ponding, which increases the risk of erosion.
The selection of suitable equipment, the timing (spring or fall), and the frequency, depth, and orientation of tillage operations deserve serious consideration.
All tillage should be conducted across the prevalent slope where there is a serious risk of soil erosion. Fall primary tillage should be avoided where the risk of erosion is high, and the soil surface should be left rough with the greatest amount of crop residue possible left on the soil surface. Chisel plows have proven beneficial in this respect.
For rapid soil warming in the spring, a rough surface left after fall tillage is advantageous.
Seedbed preparation and cultivation should be accomplished with the least amount of traffic possible, since excessive tillage breaks down soil structure and organic matter. Spring tillage should be no deeper than necessary in order to hasten soil warming and hinder excessive drying.
Spring plowing with a mouldboard plow should be avoided, since turning the soil in the spring inhibits soil warming.
Every attempt should be made to avoid tilling when the field will be left bare and vulnerable to soil erosion. Whenever possible, a winter cover crop (such as winter rye) should be seeded.
Four commonly-used tillage systems are described below in terms of their effectiveness under various conditions, residue management, soil conservation, and relative cost of operation:
- Fall mouldboard plow, spring disk, spring harrow is a conventional system that is well suited for turning sod under. It is the most expensive, however, of all the systems described here. It buries residue and inhibits rapid decay; it provides neither surface mulch nor ridging as protection against erosion; and plowing repeatedly to the same depth can induce the formation of plow-pans.
- Fall chisel or coulter chisel plow, spring vibrashank1 is a system suited to till row crop fields or grain stubble where straw has been removed or chopped and scattered. It is less expensive than other tillage practices, incorporates residue at a shallow depth for rapid decay, leaves the soil surface somewhat protected by residue mulch and ridging, and causes minimal soil compaction.
- Fall vibrashank, spring vibrashank1 is similar to the system just described
- Fall disking, spring disking can be used on all fields but sod. It is less costly than mouldboard plowing, results in the shallow incorporation of crop residue for rapid decay, and leaves the soil surface protected by crop residue and some ridging effect. It may induce plow pans.
When tillage operations cannot be accomplished in the fall, they may be carried out in the spring.
NOTE: The brand names of tillage equipment have been used only to describe them in terms of common usage. No endorsement of any particular brand is intended.
1 Spring tillage may also be accomplished by a single pass of a soil finisher, a cultipacker, or a vibrashank equipped with sweep teeth.
Potato crops require well-drained soil, because roots and tubers need a continuous supply of oxygen. Poor drainage not only limits soil aeration, but also hinders the soil's capacity to supply nutrients to the crop, delays soil warming and drying in the spring, reduces soil strength, inhibits the use of machinery, and sets the stage for soil degradation. Poorly drained soil is susceptible to structural damage and compaction, which, in turn, increases the potential for runoff and erosion.
The field that is too wet to till within two days following heavy rain needs to be examined for signs of impeded drainage and/or compaction. A soil specialist or drainage engineer can assess the soil limitations by studying soil profiles in the field. If the main problem is poor drainage and removing water from the field is necessary, installation of a drainage system is usually the best remedy. If the problem occurs over a large area, a systematic layout is most effective.
Excessive water runoff that contributes to the poor drainage of a particular field should be directed from the field by diversion ditches or waterways.
If soil compaction is found to be the main reason for water ponding and slow water penetration into the subsoil, deep tillage may improve the situation. If the compacted subsoil is shallow (plow pans, for example), a chisel plow can be used to a depth of 25 or 30 cm. For loosening deeper compacted layers, subsoiling can be used to a depth of 40 or 50 cm.
In conditions where poor drainage is aggravated by the presence of compacted subsoil, subsoiling after installing drainage tile was found to be beneficial.
In order for drainage improvement to be efficient and durable, soil improvement operations should be conducted under suitable soil moisture conditions and augmented with good soil management practices. Traffic over the field should be eliminated while the soil is too wet, and practices which increase organic matter and improve the structure of both topsoil and subsoil should be implemented. These include following crop rotation, growing deep rooted crops (sweet clover, alfalfa, and brome grass, for example), using barn and green manures, and practicing conservation tillage.
Using heavy equipment, tilling while the soil is too wet, removing too many stones, depleting organic matter, and erosion are factors that contribute to soil structure degradation and compaction. The consequence is reduced water infiltration, ponding, increased runoff, and further erosion. Soil structure degradation and compaction also reduce soil aeration and the soil's capacity to hold water. This results in low yields and reduced quality of tubers.
Subsoiling, designed to loosen compacted subsoils, was found to eliminate compaction in some instances. An important rule to follow when subsoiling, is to conduct the operation while the soil is relatively dry. In potato production, this would usually be on a rotational crop which precedes potatoes. Subsequent measures, such as growing deep rooted crops and employing good agronomical practices, are important if subsoiling is to have a lasting effect.
Reducing Soil Erosion
Soil erosion has adversely affected potato production in the Atlantic Provinces. Annual soil loss from poorly managed fields has exceeded ten times the rate at which the soil can replenish itself. In some areas of the region (upper part of the St. John River Valley), devastating erosion has resulted in the removal of some fields from commercial agricultural production.
Erosion not only removes fine mineral particles from the soil; it also depletes the soil of organic matter and crop nutrients. Some results of erosion, like rills and gullies, are visible and therefore alarming. Sheet erosion, which affects the entire field surface, can be more dangerous because its effects are not immediately visible. For this reason, it is often neglected for long periods of time.
The major factors that contribute to erosion are: continuous potato production; the tilling and cropping of steep fields, especially up and down the slope; the absence of winter soil cover by crops or crop residues; excessive tillage; excessive stone removal; and traffic over the soil while it is too wet.
Extension services are available to assist producers in developing soil management and conservation plans that would reduce and maintain soil loss at a sustainable level. Contact your crop, soil or drainage specialist for information on crop rotations, cover crops, subsoiling, drainage, contour and conservational tillage, strip-cropping, and the construction of terrace systems and waterways.
Stones 7.5 cm or less in diameter should not be removed from fields in New Brunswick. These stones reduce compaction, increase infiltration, and protect soil particles from the high-energy impact of driving rain.
An irrigation system permits the controlled management of water available to the crop in dry periods. High temperatures and low soil moisture content during the early stages of plant development can result in deformed tubers, gemmation (secondary tubers or heat sprouts), and small plants. Hot, dry weather during tuber initiation promotes a more serious scab problem in susceptible varieties than high soil pH. An irrigation system can substantially reduce the effects of moisture stress during dry periods.
To minimize the effects of soil degradation, potatoes should be grown in rotation with other crops. Crop rotations help maintain soil structure and organic matter levels while reducing crop and quality losses from insect and disease damage. Winter cereals, forages, or stubble from a previous grain crop help to minimize erosion in late fall, winter, and early spring. Higher yields and better quality can be expected from potatoes planted once every three years in the same field, especially when potatoes follow cereal, green manure, or forage crops in a rotation sequence.
Rotation crops must be chosen carefully. Buckwheat and red clover predispose the subsequent potato crop to Rhizoctonia, while sweet clover may predispose the next crop to clover cyst nematode infection. These infections can reduce the export potential of the potato crop.
Consult your crop or potato specialist for advice on rotational practices.
The Use of Manures
Organic matter is required to maintain good soil structure and improve the soil's capacity to retain water and fertilizers.
Potato soils are generally low in organic matter. A soil management objective is to add about five tonnes of dry matter per hectare per year to maintain a satisfactory organic matter level. This may be accomplished in several ways:
- Manures, if available, are valuable additions to the soil. Fresh manure, especially poultry manure, applied in large quantities to the potato crop, however, may increase the incidence and severity of scab while delaying crop maturity. Therefore, manures should be applied to the forage crop in the potato rotation. One tonne of solid cattle and hog manure contains approximately one-quarter tonne of dry matter; one tonne of solid hog manure contains about one-quarter tonne of dry matter; one tonne of solid poultry manure contains about one-third tonne of dry matter; and one tonne of liquid hog manure contains about one-tenth tonne of dry matter.
- Potato vines, if incorporated into the soil, will add up to two tonnes of dry matter per crop per hectare C depending on the variety.
- Cereal straw contributes two to five tonnes of dry matter per hectare when incorporated. Decomposition is relatively slow, but a desirable physical effect is achieved. Pea stubble, aftermath clover, and weeds also provide valuable organic matter.
- Green manure crops provide an excellent source or organic material. Oats and winter cereals, when ploughed down prior to heading, could contribute between one and two tonnes of dry matter per hectare. Green manure crops adapted to potato soils include: red clover, sudan grass, oats, winter cereals, and buckwheat. Clover provides up to seven tonnes of dry matter per hectare. Consult your district agriculturist or field crops specialist for further information.