In potatoes, the arrival of harvest time is recognised by the leaves and stems turning yellow and drying off, by the tubers having reached their normal size and separating easily from the plant, and by the skin thickening and hardening. The skin no longer peels off easily with a fingernail. The cut surface of the tuber is not wet but dark in appearance.
Great care must be taken when harvesting potatoes. The tubers must not be cut or bruised, and no tubers should be left in the soil. During lifting the soil must not be wet; it should be at its proper working moisture.
HARVESTING TECHNIQUES USED
1. Harvesting with a spade, digging fork or hoe
2. Machine harvesting
1. Harvesting with a spade, digging fork or hoe
Using a simple spade or digging fork, the potatoes planted along the row are brought up to the soil surface by hand labour, and the lifted potatoes are separated from the stems and gathered; it is very tiring and time-consuming work.
Harvesting with a hoe is carried out in the same way as harvesting with a spade. The only difference is that it is done with a hoe.
2. Machine harvesting
Today, various types of machine have been developed for harvesting potatoes. These can be gathered into 2 groups, semi-automatic and fully automatic machines. Semi-automatic machines mechanise only the lifting of the potatoes. They are built in two types, spinner-wheel machines and machines with a sieving unit. Fully automatic harvesters, by contrast, clean the potatoes they lift out of the ground of the various admixtures such as soil, clods, stones and haulm, and then collect them in hoppers of different shapes and sizes. Fully automatic harvesters are manufactured as tractor-drawn and self-propelled types, in single-row or multi-row versions.
Recent developments in potato harvesting mechanisation have brought a significant reduction in tuber losses and damage at harvest. The high losses and severe mechanical damage that occur with semi-automatic harvesters have, with the development of the new fully automatic machines, been reduced to 1 % and 2 % by weight respectively. (Vent, W., 1981). 1. Spinner-Wheel Semi-Automatic Harvesters These machines have active working elements such as the digging share, the spinner wheel and the retaining screen. In work, the digging share cuts the soil ridges from below, while the fingers of the spinner wheel strike the cut ridge and spread it out in a band 1-1.5 m wide. The retaining screen of the machine prevents the tubers from being thrown too far.
To prevent tuber losses and damage, it is appropriate to run the spinner wheel at a peripheral speed of 3-3.5 km/h. A higher peripheral speed increases tuber damage. When working with this machine, the soil thrown out during the harvesting of the next row covers the tubers and causes excessive tuber loss. 2. Semi-Automatic Harvesters with a Sieving Unit These are harvesters that take in 1 or 2 potato rows, sieve them and leave them on the field surface in a lengthwise windrow. In these machines a chain web, a shaker or a sieving wheel is used as the sieving unit.
The sieving units are driven from the tractor power take-off and run evenly at a constant speed of rotation. They fall into three types. Sieving-Wheel Type These have a sieving element in the form of a rotating wheel. In these machines the soil ridges cut by the digging share pass onto the sieving wheel and are turned and thrown towards the fingered retainer at the side. Sieving-wheel diggers can work better under heavy soil conditions.
Tuber losses when harvesting with this machine are between 5-7 %. Sieving Chain Web Type This has a digging share and, adjacent to it, a continuously revolving web system. The web system is formed by fixing the ends of iron rods to a pair of chains or belts, and takes its drive from the power take-off. When harvesting with this machine, the forward speed of the machine, the web speed and the amount of agitation must be kept within suitable limits if tuber loss and damage are to be reduced. Normally, at a forward speed of 2.5-3 km/h, a power take-off speed not exceeding 300 rpm is sufficient.
In addition, covering the web rods and the side strips with rubber considerably reduces tuber damage. Under suitable lifting conditions, tuber losses when harvesting with this machine are between 2-5 %. Shaker Type This has a trough-shaped 2-piece share at the front and, adjacent to it, shakers that move sideways. The shaker-sieving unit takes its drive from the power take-off through special linkages.
In work, the soil ridge cut by the digging share passes to the shaker section and is sieved there, and the potatoes and other admixtures that are not sieved out are left to one side in a windrow. When working with this machine the tractor forward speed is 2-3 km/h and the power take-off speed between 400-550 rpm. Support wheels are used to set the lifting depth of the machine. When harvesting with semi-automatic machines, hand picking should be carried out at least 1.5-2 hours after lifting, so that the tubers can dry and damage is reduced.
In this way, as the tubers dry, damage during picking and transport can be reduced. 3- Automatic Harvesters Fully automatic harvesters, which perform the tasks of lifting the potato tubers, cleaning them of admixtures, collecting them in a hopper and transferring them to a transport vehicle, are of more complex construction than semi-automatic machines.
On these machines the active working elements that come into contact with the potato tubers are the ridge roller, the digging share, the soil-sieving web and the haulm-holding web, the elevating cylinder, the mechanical sorting and cleaning units and the hopper section: tuber losses and damage arise with these working elements during the taking up of the soil, the sieving, the separation of the haulm, the removal of admixtures and the collection of the tubers in the hopper. The sieving web sieves out the soil and other fine material within the harvested ridges. The sieving web consists of iron rods riveted at set intervals to a rubber belt. As soil moisture increases, the working performance of the sieving web falls slightly on light soils and more markedly on heavy soils.
Working performance can be raised with the help of shaking and agitating devices. The haulm-holding web lies alongside the sieving web and separates out the coarse potato haulm. The elevating cylinder delivers the tubers and tuber-like admixtures to the mechanical separating and cleaning units. On a fully covered elevating wheel the drop height should be less than 25 cm. 3 types of separating unit are prominent on fully automatic harvesters. These are the counter-flow web, the rubber-fingered web with a stripper unit and the rubber-fingered web with a rotating brush. On the counter-flow web the relative movement of the tubers causes damage; on the strippers the damage is mostly scratching, while on the stone separators comparatively less tuber damage occurs.
To reduce losses;
1. Preparing potato harvest ridges of defined dimensions by means of a suitable growing technique,
2. Taking up the harvest ridges in the optimum way by using a ridge roller on the machine and setting the lifting depth,
3. Ensuring continuity in the intake of the harvest ridges with the help of the active elements,
To reduce damage;
1. Covering the sieving and haulm-separating webs with rubber and rubber-like materials,
2. Closing in the sides of the sieving web
3. Arranging the loading web appropriately,
4. Covering the sieving and haulm-separating webs with rubber and rubber-like materials,
When these measures are taken into account, it is possible to keep tuber losses and damage in machine harvesting to a minimum (the proportion of lost tubers below 1 % by weight, and severe mechanical damage below 2 %).
