Potato Agronomic Principles
Potatoes produce a fibrous root system. These roots are at best no more than 60 cm long. Thus potatoes are shallow rooted compared to cereals for example, which can root to at least 120 cm depth. As a result, potatoes are often unable to exploit nutrients and soil moisture at depth within a soil profile.
While root growth occurs when soil temperatures are between 10 to 35˚C, best, most active root development is at soil temperatures of between 15 and 20˚C.
Leaf (haulm) growth occurs at temperatures of between 7 to 30˚C, but optimal growth is at around 20 to 25˚C. Optimum temperatures for stolon growth are similar.
The potato tuber is an enlarged portion of the stolon. The initiation of this tuber is triggered by short day lengths (photoperiods), and involves growth hormones. The colder the soil temperature, the more rapid the initiation of tubers and the greater the number of tubers formed. The optimum soil temperature for tuber initiation is 15 to 20˚C.
Under these conditions, the potato plant will have short stolons and shoots. Longer day lengths delay tuber initiation and favor the growth of the stolon and shoot.
Low nitrogen and high sucrose levels in the plant favor the formation of more tubers. Once formed, tubers grow rapidly, reaching a maximum rate of up to 1,400 kg/ha/day in temperate climates. Late varieties seem to be more sensitive to long day lengths or high temperature conditions.
By planting sprouted seed, crop growth can be advanced. The magnitude of this response and its effect on increasing crop yield is related to the physiological age of the seed at planting.
Seed storage temperature is the key to controlling physiological aging. Raising storage temperature above 4˚C promotes the break in dormancy and the growth of sprouts.
The accumulation of the number of day degrees from this break of dormancy governs the physiological age of the tuber at planting.
Different varieties vary in the number of day degrees needed to age to a desired level prior to planting. Old aged tubers are advantageous when planting early varieties or when the growing season is short.
Tubers that have been minimally aged are suited to long growing seasons where it is desirable to keep the potato growing to achieve maximum yields. When planting sprouted seed it is necessary to control sprout numbers and length (maximum 2 cm) to ensure optimum growth according to plant spacing, and to ensure minimal sprout damage when planting.
Soil type and management
Potatoes are grown on a range of soils varying from sands to clay loams, all with different water holding capacities. An ideal potato soil is well structured, with good drainage to allow proper root aeration, tuber development with minimal root disease infestation.
Potatoes prefer soils with a pH of 5.5 to 7.0 and low salinity. However, in practice potatoes are grown in soil pH's from 4.5 to 8.5 and this has a distinct impact on the availability of certain nutrients. Extreme soil pHs should be adjusted where it is practical to do so.
At lower pH values potatoes can suffer from aluminum and other heavy metal ion toxicity, as well as restricted P or Mo availability.
At pH values above 7.5, nutrient availability, in particular of phosphorus and the micronutrients, can be reduced, even though high total amounts of these elements may be present in the soil. Liming can ameliorate undesirable, low pH values although care must be taken to ensure that the lime is applied at least 6 months before the potatoes are to be planted. Potatoes are more prone to common scab when grown in high pH soils.
Ridging and hilling
Potatoes are often planted in ridges or hills because this ensures a well-drained, well-aerated environment for strong crop growth.
In colder soils, ridging raises the soil temperature allowing quicker germination and early growth.
When side dressing with fertilizer, the remaking of the ridge allows the incorporation of the fertilizer into the soil around the tuber.
Ridging also maximizes coverage of the developing tuber, so as to prevent greening and ensure tubers are well shaped, more evenly sized and at lower risk of damage.
Potatoes have a high water requirement – roughly 25 mm/week during bulking. Thus, for high yields irrigation is usually beneficial.
Water management is essential to minimize tuber problems. Maintaining a moist soil in the ridge at tuber initiation can minimize common scab development. Later in the season, excessive water around the tuber encourages powdery scab and lenticel growth.
Fluctuations in soil moisture status within the ridge will lead to uneven tuber bulking, malformed tubers and growth cracks. Even a 10% variation in soil moisture status can be critical. For this reason, when using drip irrigation systems, the tape should be placed in the top of the ridge.
Canopy management is also critical to maximize efficient use of water. Growers in high temperature environments need to ensure quick canopy closure to minimize water loss due to evaporation from the soil surface.
Early blight (Alternaria solani) and Late blight (Phythophthora infestans) are major diseases that can have a devastating effect on the crop.
Early blight is a problem, particularly in early varieties, spreading from the leaf to young tubers. It can result in severe defoliation, but, if managed correctly, plants can grow away from infestations.
Late blight occurs under cool moist conditions and if left uncontrolled, quickly spreads to the tuber resulting in significant tuber browning and rot.
A range of mosaic viruses also affect potato leaf growth, leading to a drop in yield. Control of the aphid or other carrying vectors, minimizes damage.
In addition, various free-living nematodes and/or potato cyst nematodes can cause significant damage. Wide rotations are needed in some countries to minimize crop loss.