External factors - crop management

2.3.1 Planting density
Potatoes grown as a raw material for French fries should be large, while those for crisps should be of medium size. The plant density affects both the total yield and the tuber size. In general, 12 -15 stems/metre2 are sufficient to get a yield of 40 - 60 tonnes/ha, made up of a high proportion of large tubers. 30,000 - 50,000 plants/ha are required to produce a sufficiently high density of stems. 

The exact number of plants per hectare is determined by variety, seed size, seed age, soil type and the end use of the crop. Larger seeds produce more stems, and well-sprouted tubers produce more stems than seed that is too young or too old.

Yield is determined by the amount of solar radiation the crop intercepts, so an early canopy closure increases production. Factors leading to early canopy closure include: higher plant densities, well pre-sprouted seed, planting depths that are not too deep, soil that is moist but not too cold, and a planting pattern where the distances between the rows are not too wide.

But while they increase yield, some of these factors may reduce quality due to the fact that high planting densities increase tuber numbers per square meter and reduce tuber size. Narrow distances between rows only allow smaller hills (ridges) and increase the risk of green tubers. Testing varieties and gaining experience with both varieties and management practices is of key importance in determining the best planting policy.

Trends in response to stem density, yield and tuber size are shown in the following graph. Lower plant densities increase tuber size (hence percentage recovery in the factory) but they may negatively influence tuber yield per hectare. 






Relationship between number of main stems per m2 and tuber yield in the various grades
(Source: H.P. Beukema/ D.E. van der Zaag).














All management by the grower should be aimed at reducing variability between tubers. Ideally, a potato crop for processing has tubers all of the same size and shape and with the same dry matter content between and within tubers. Reduction of variability in tubers is best achieved when variability in growing conditions is reduced by planting seed tubers of the same size at the same depth, with the same distances between tubers within the row, and providing an even distribution of fertilisers and moisture. Adequate hilling (ridging) with the aid of a rotary row-crop cultivator in heavy soils helps to ensure homogeneous conditions in the soil. 

2.3.2 Fertiliser
The establishment of good fertility and proper fertilisation with nitrogen, phosphate and potassium will have a positive effect on both the yield and the quality required. Nitrogen is mobile in the soil and moves in the soil water. Therefore it can be applied throughout the season. Phosphorus and potassium are hardly mobile, being bound to the soil particles, and therefore have to be applied prior to planting and have to be well mixed through the soil. By fertiliser application, the grower can influence the tuber dry matter concentration and the susceptibility to black spot. The processing industry demands a high dry matter content. However, a high dry matter content increases susceptibility to black spot and internal bruising.

When both nitrogen and potassium are applied in large quantities, they tend to reduce the tuber dry matter concentration and, consequently, the susceptibility to black spot. In this respect, the application of potassium chloride is beneficial in reducing black spot. However, under dry conditions this may lead to a somewhat lower yield. When using potassium chloride, this should be done well in advance of planting in order to avoid salinity problems. If black spot is a serious problem one might consider a crop rotation application on non-leaching soils. This is a method whereby all of the potassium that is required in a certain crop rotation period of e.g. three or four years is applied prior to planting the potato crop. This should be done well in advance of planting, during autumn or winter.

An excessive nitrogen application rate may adversely affect the colour and the non-enzymic blackening of the tuber. Moreover, a large quantity of nitrogen delays maturing of the crop resulting in a relatively low dry matter concentration and a relatively high nitrate content in the tuber. The dry matter concentration may drop so low, that the tubers cannot be used by the processing industry.


Graph 1: Effect of potassium fertilisation with K60 (kg K₂0/ha) on the susceptibility to black spot after autumn and winter fertilisation on clay soils in the Northeast Polder (1 resp. 2) and S.W. Clay area (3 resp. 4) in the Netherlands














The quantities of nitrogen, phosphate and potassium fertiliser to be applied to the potato crop should preferably be based on a mineral analysis of the soil to determine the actual soil fertility prior to planting. Other issues to take into consideration are the soil type (risk of leaching or fixation), the previous crop and the lateness of the variety. Nitrogen is preferably given in more than one application. About half of the crop’s needs are given at planting and the rest in one or more subsequent dressings. Laboratories that sample soils and crops advise how much should be applied during crop growth. The advantages of such monitoring are that yields and quality are manipulated continuously while the risks of leaching nitrogen are reduced.

As a guideline, the basic nutrient requirements of a potato crop yielding 30 tonnes/ha during growth are:

(Source: H.P. Beukema / D.E. van der Zaag)





2.3.3 Irrigation
Water is of vital importance to crops. It is indispensable for the most elementary processes such as photosynthesis and transport of minerals in plant and soil. However only about 2% of the water needed by a crop is used for plant metabolic processes. The rest is transpired. The yield of a crop is determined to a large extent by the total amount of water that is available for the crop. With each litre of water transpired, the crop produces about 6 grams of dry matter. Good moisture supply during the various growth stages is essential for the production of quality tubers. The moisture supply just before or during tuber initiation (that is, approximately three weeks after emergence) is of vital importance. More water increases the number of tubers and their size. More soil moisture at tuber initiation also reduces the occurrence of common scab.

Moist conditions during tuber initiation always result in a larger number of tubers, although dry conditions thereafter may lead to re-absorption of the smallest ones. Varieties which normally form many tubers may form too many in moist conditions. During tuber growth (bulking), a regular water supply prevents a decline in quality caused by growth cracks, secondary growth symptoms such as tuber deformation and glassiness and stem-end browning. If the conditions are dry during harvesting, particularly on heavier soils, irrigation can help prevent injuries caused by lifting the potatoes. Irregular water supply has a negative effect on the colour of the potatoes. For proper irrigation, scheduling information is needed on the water holding capacity of the soil, which is lowest in coarse sand and fine clays.Water needs are greater when daily evapotranspiration (as measured and made available by meteostations) increases when the proportion of the soil covered with green leaves is higher.

2.3.4. Pest and disease control
Only a well-matured crop will finally meet the quality standards demanded by the processing industry. Diseases may lead to lower yield due to later canopy closure or to earlier dying off of the foliage. The quality may also be reduced as early crop death is associated with lower dry matter concentrations. Direct effects are caused by e.g. common scab or root knot nematodes that affect the skin and make thicker peeling necessary. Storage diseases such as silver scurf and Pythium may cause storage losses.

The two main measures to prevent diseases and pests are: a) the use of healthy and certified seed and b) hygienic measures such as crop rotation. Healthy seed reduces the risk of bacterial and virus diseases. A crop rotation, in which potatoes are not grown more frequently than once every four to six or more years, significantly reduces the risk of soil borne pests (nematodes) and fungi (Rhizoctonia and Verticillium). Once foliar pests (aphids) or diseases (Phytophthora infestans) are present, curative measures consist of regular spraying of the crops with appropriate chemicals. Weeds should be controlled throughout the season, preferably mechanically while hilling (ridging), but the use of herbicides may be necessary to avoid yield reduction and the build up of weed seeds in the soil. When the crop nears maturity, haulm killing is often required to set the skin prior to harvest and to prevent diseases doing further harm.