Soil management consists of annual and long-term measures that are applied in order to sustain and improve soil productivity. These are interrelated among themselves and also with crop management. Well-planned and conscientious application of soil management practices will ensure the high yield and quality of tubers while diminishing the negative environmental impact.
For further information, seek the assistance of a soil and crop specialist or agricultural engineer.
Growing Potatoes Exerts High Demands On Soil
The uptake of soil nutrients by potatoes is high. Tillage and traffic over the field is frequent. The amount of crop residue that remains in the field after harvest is low. Therefore the soil surface has little protection from erosion. THIS IS WHY POTATOES ARE REFERRED TO AS A SOIL- DEPLETING CROP. The usage of heavy machinery (frequently not by choice) while the soil is too wet, also contributes to soil degradation.
Not paying full attention to soil improvement measures has led to excessive water erosion, depletion of the soil's organic matter, compaction, and soil structure and drainage deterioration.
Four SOIL MANAGEMENT ELEMENTS must be considered, planned, and consciously executed if high production of quality tubers is to be achieved and sustained:
- Field selection.
- Fertility practices and liming.
- Soil improvement measures.
Soils differ in physical and chemical properties that affect crop productivity. Soil chemical properties and soil reaction (acidity) can be managed relatively easily by adding fertilizers and lime. Changing the physical characteristics of the field, however, can be more difficult, sometimes impossible. This is why field selection (matching the physical characteristics of the field with crop requirements) is so important. Soil survey reports, maps, on-site soil profile investigations, and producers's experiences are valuable managerial tools in this respect. (Consult a soil specialist or agronomist to assist you with information on the physical characteristics of your land).
Potato crops require porous, non-compacted soil that ensures optimum water, nutrient, and oxygen supply. The ideal soil depth above the bedrock or compacted subsoil is 90 cm. Although this is seldom found in this region, one must nevertheless select fields with deep soils for potato production since the deeper the soil, the greater its capacity to retain water. The shallower the soil, the higher the risk of drought damage. The risk of drought damage is especially high for tubers during their differentiation and early development stages in June and July when the water deficit is most acute.
Soil texture refers to the proportion of clay, silt and sand particles present in the soil. Sandy loams and loams are most suitable for potato production. These soils have a potentially well-balanced capacity to retain water, form a stable structure, provide adequate aeration, and possess a suitable thermal regime. More sandy or gravelly soils present an increased risk of drought, while more silty and clayey soils have a tendency to compact and crust, thus presenting a higher risk of both drainage impedance and water erosion.
Practising potato production on steep slopes is risky and prohibitively expensive. The steeper the field's grade, the less suitable the field is for potato production. Although soil has some ability to replenish itself, runoff from a steep slope will carry away more fertile soil than the field can possibly replace. The risk of soil loss is proportional to the length of the slope and the severity of the grade. The following table lists the maximum length of fields at various grades. If the length of a slope at a given grade is greater than that listed in the table, soil loss will exceed the soil's natural ability to rejuvenate unless specific agronomical and engineering conservation measures are applied.
|Grade (%)||Slope length (m)|
Impeded drainage is a severe limitation to potato production. Excessive water content in soil limits the free movement of oxygen that is necessary for healthy root and tuber development. Excessive water in soil also decreases the efficiency of nutrient uptake, increases the incidence of fungal diseases, delays spring tillage and planting, and increases the risk of soil compaction.
The majority of soil in this region has naturally compacted subsoil. The depth of soil above the compacted layer will determine the crop's potential rooting depth, the soil's capacity to store and move water, and the likelihood of soil degradation due to compaction, impeded drainage, and erosion.
Because large stones hinder soil preparation and interfere with harvest, they must be removed from fields. Soils with up to 20 percent small stones and gravel (7.5 cm in diameter or less) were found to have good water-retaining capacity, water infiltration, and thermal regime.
Organic matter content of soil is an important characteristic that affects the soil's physical, chemical, and biological activity. In general, the higher the organic matter content, the better the production capacity of the soil. The effort should be made to maintain organic matter content above three percent. The presence of organic matter enhances the soil's structure; improves its moisture, air, thermal, and nutritional regimes; and decreases the potential of soil erosion. Organic matter is an important reservoir of soil nutrients, especially micronutrients.
FERTILITY PRACTICES AND LIMING
In order to produce high yield and quality tubers, soil reaction (pH) and nutrients must be present in sufficient amounts, and in balance. Soil analysis and assessment of the field's history will enable a soil specialist or agronomist to provide recommendations on the amount of nitrogen (N), phosphorus (P), and potassium (K) that should be applied as fertilizer, as well as the amount of calcium (Ca) and magnesium (Mg) that should be applied as lime. Other nutrients (sulphur, iron, manganese, boron, copper, zinc, molybdenum, and chlorine) are usually present in sufficient amounts, particularly if the soil's organic content is adequate.
Soil Reaction and Liming
Soils in the Atlantic Region are naturally acidic. Further acidification is caused by the addition of nitrogen and phosphorus fertilizers. Periodic applications of agricultural limestone help to keep the soil's pH level within an acceptable range. Soil with a pH of less than 5 may contain high levels of soluble aluminum and manganese, which are harmful to potato crops. In general, a pH level of about 5.5 is best for scab-susceptible round white potato varieties, while a pH between 5.5 and 6.0 is recommended for Russet-type varieties. Even though scab on potatoes is caused by the organism Streptomyces scabies and not by liming, lime contributes to factors that provide a suitable environment for this organism to multiply rapidly. In order to diminish the incidence of scab, one should avoid growing potatoes in drought-prone fields, cropping potatoes continuously, applying large quantities of manure, and planting non-certified seed.
If a pH of above 5.7 is recommended for rotational crops, lime should be applied and harrowed-in immediately after the potato harvest in the fall, provided that fall tillage would not increase the risk of erosion.
There is no adequate method for testing the availability of nitrogen in Atlantic soils. Nitrogen application rates are based on a given crop's response to applied rates of nitrogen. These must be adjusted for organic matter content, crop variety, previous field history, growing time of the crop, yield potential, and the possibility of leaching. Because nitrogen is relatively inexpensive in terms of the production cost of potatoes, the potential for overfertilization is always present. Nitrogen applied in excess of the crop's nutritional requirements can delay the maturity and make topkill and harvest difficult. With delayed maturity, the tuber's skin set is poorer and the tuber is more susceptible to bruising during the harvesting operation. In addition, specific gravity may be reduced. Excess nitrogen is easily leached from the soil. Because potatoes are inefficient consumers of soil nitrogen, a fall cover crop can help to trap nitrogen organically.
Because nitrogen can move with soil moisture, its placement is not critical; but it is critical to have nitrogen available to the crop when it is required. In this region, most nitrogen applied at planting is "banded". Topdressing during first cultivation can be an effective way to meet the crop's nitrogen requirements. For topdressing to be effective, enough nitrogen must be applied at planting to sustain the crop until the topdressing is applied. Topdressing must be uniform; there must be enough moisture to move the nitrogen to the root zone; and care must be taken not to exceed the crop's nitrogen requirements.
Phosphorus is essential for root development, therefore, an adequate amount must be in the soil during early crop development. During later stages of development, the demand for phosphorus drops. As phosphorus can, in acid soil, become unavailable to the crop, full attention to soil analysis and fertility recommendations is needed so that its level in the soil does not become inadequate or excessive. As phosphorus levels become high, yearly applications can be reduced.
Fertilizer requirements for potassium on potatoes are easily identified by soil tests. Since excessive applications of potassium will reduce the specific gravity of tubers, just enough potassium should be applied to satisfy crop needs.
Calcium and Magnesium
The calcium requirements of potato crops are usually satisfied as long as the proper pH level is maintained by applying limestone periodically. If soil analysis shows a calcium deficiency and it is not advisable to increase the pH, gypsum can be used (see Gypsum, below). When magnesium levels are low, dolomitic lime should be applied. If the pH is too high to apply lime, fertilizer enriched with magnesium should be used.
Gypsum is an excellent source of calcium and sulphur. As a source of calcium, gypsum is 100 times more soluble than lime, and does not affect pH. When applied either prior to or immediately after planting (one or two tonnes per hectare with subsequent shallow incorporation), or in the fall (two to four tonnes per hectare with subsequent deep incorporation), gypsum was found to increase the yield of tubers while reducing the incidence of internal brown spot, scab, and hollowheart. N-Hib Calcium, Cal-U-Sol, and Can 17 are calcium-rich compounds marketed to prevent internal brown spot. While effective, they are more expensive than locally-available gypsum.
Of the micronutrients (sulphur, boron, copper, zinc, manganese, molybdenum, iron, and chlorine), only boron has a positive effect on potato crops grown on soils that are low in organic matter content, or that are affected by drought. Leaflets which curl forward with marginal scorching are symptoms of boron deficiency. The symptoms are usually found on young leaves. The foliar application of two kg per hectare of boron has reduced brown heart and water core symptoms. Application of boron during early crop development was found to be more effective than late application.
Although several liquid fertilizer products have been promoted in the region, none have been proven positively effective as yet.
Nutrient Uptake by the Potato Crop
Nutrient uptake by the potato crop per hectare, approximated in the table below, is based on a 36 tonne per hectare yield of tubers with 20 percent dry matter.
|Plant Part||Kilograms per hectare used by potato crop|
|Tops (returned to soil)||58||8||112||40||22||6|
|Tubers (removed from the soil)||153||32||209||5||16||10|
Plants are not 100 percent efficient in utilizing nutrients present in soils. Nutrient uptake will vary with the depth of the soil, the extent of the rooting system, the availability of water, the amount of exchangeable nutrients, the rate of leaching, soil structure, soil pH, and other factors. However, because the soil can release large amounts of nitrogen and potassium to the potato crop, fertilizer nitrogen and potassium recommendations are less than what is removed by the crop in the table above. On the other hand, more phosphorus must be applied than is absorbed by the crop because phosphorus is readily fixed by iron, manganese, and aluminum at low pH, and is therefore unavailable to the crop. Similarly, more calcium in the form of limestone is applied to the crop than is removed because limestone not only supplies calcium, but also affects pH and microbial activity.
Fertility recommendations are based on the results of soil tests, information provided on past soil and crop management, and the soil's ability to release soil nutrients.
Soil testing is the first major step toward successful crop production. Steps to obtain a good soil sample are:
- Obtain soil sample boxes and information sheets from your provincial soil testing laboratory or nearest agricultural office (soil bags are supplied in Newfoundland and Labrador).
- Divide, map, and number your fields (not greater than four hectares) for sampling. Each area should appear uniform and have a similar cropping history. Keep a record of this information.
- Collect sub-samples from at least 20 sites within each area to be tested. Sample each site by taking a uniform slice of soil to a normal plowing or tillage depth. Avoid old manure piles, burnt areas, spilled limestone and fertilizer, swampy areas, and any other spots which are not representative of the area.
- Mix the soil collected from each area thoroughly in a clean bucket, breaking up clods. Fill and number the soil sample box.
- Fill out the information sheet and send it with the soil sample boxes to the soil testing laboratory or nearest agricultural office.
Recommendations can be useful only if the sample sent for analysis truly represents the field.
Send soil samples for analysis to:
NB Agriculture, Fisheries and Aquaculture
P.O. Box 6000
NS Dept. of Agriculture & Marketing
P.O. Box 550
|Soil & Feed Laboratory
PEI Dept. of Agriculture
P.O. Box 1600
|Soils & Plant Lab
Provincial Agriculture Bldg
St. John's, NF
|The following general recommendations are suggested when a soil test report is not available:|
|NOTE:||A seed crop topkilled two or three weeks before the full season or an early tablestock crop requires less nitrogen (100-120 kg N/ha) than the full season crop. Light textured soils may require higher levels of nutrients, but confirmation by soil testing would be the proper approach under these conditions.|
Concentrated commercial fertilizers can be purchased containing various nutrient ratios (analyses) which are guaranteed by the manufacturer. High-analysis fertilizers are more cost-effective due to lower transportation and handling costs. For example, 1000 kg of 15-15-15 fertilizer is equal to 1500 kg of 10-10-10 fertilizer. Both supply 150 kg of nitrogen, phosphorus, and potassium; the difference in weight is due to the different amounts of nutrient carriers and fillers that these fertilizers contain.
The correct rate of fertilizer application is always important, but particularly so with concentrated fertilizers. As an example, applying an additional 100 kg/ha of a low analysis fertilizer such as 10-10-10 adds an additional 10 kg of N, 10 kg of P2O5, and 10 kg of K2O. In comparison, an additional 100 kg/ha of a high analysis fertilizer such as 17-17-17 adds an additional 17 kg N, 17 kg P2O5 and 17 kg K2O, or 70 percent more. The fertilizer applicator should be calibrated and checked frequently. Adjusting the planter or fertilizer distributor may be necessary when shifting from one grade of fertilizer to another, or from one brand to another, or when weather changes and soil conditions are appreciably different from those at the time of the original adjustment.
Fertilizer is usually placed in bands 5 cm below and 5 cm on each side of the seed piece. If high rates are needed, there may be advantages in broadcasting half or more of the potash required the previous fall. This permits the efficient use of potassium, saves time, and helps offset the decrease in tuber dry matter caused by chloride in the muriate of potash. (Also, broadcasting most of the recommended fertilizer nitrogen and potash in the spring, just prior to planting, or immediately after planting, and banding the remainder at planting, can be considered a feasible alternative). For detailed advice on the proportions to be broadcast and banded, your potato, crop, or soil specialist should be consulted.
Nitrogen and potassium are sometimes sidedressed during the season. Calibration for the rate of delivery and uniformity of spread are important. If urea is used as a source of nitrogen, it should be incorporated immediately after spreading.
Foliar sprays are sometimes applied, but selection of the product, along with the rate and timing of application, are important considerations. Not all products are suitable for foliar application, and not all products are effectively taken up by the plant. Some products can cause burning of the foliage if the rate is too high. Crop response will depend on weather and crop conditions.
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.