This section describes crop preparation for harvesting including cultural practices, diseases and pests associated to soybean cultivation. Among the cultural practices in soybean cultivation are soil preparation and fertilization, planting, irrigation, weeds, diseases and pests control.

Preparation of the land and fertilization.

It is very important to plough and level the soil before planting soybeans (Figure 2). These labours acquire a major importance when soybean crop is mechanized. When the surface is in good condition it is much easier to make downer cuts and harvest more seeds. On the contrary, on uneven soils, not levelled, with hillocks, etc, losses due to excessive height cuts may be of concern because of head machine (combine) swinging, which requires the combine operator’s constant care to avoid uneven cuts and platform sticking. It is recommended to use planks to flatten the soil before planting. Flat and not deeper than 10 cm scratches are convenient cultural practices in soybean cropping.

 Figure 2. Soil ploughing and levelling.

When soybean is grown after other crop, it often can be planted without further preparation of the soil, once the preceding crop has been harvested or the soil has been ploughed two or three times until free from grass and other plants. Some farmers in the northwestern part of Mexico use to crop soybean after wheat harvesting. Wheat stubbles are burned and the furrows previously used are immediately irrigated without further land preparation. This practice reduces soybean production costs.

Fertilizing soybeans is not a common practice, unless the soil is known to be deficient in phosphorus and potassium. Phosphate and potash granules are spread over the fields before planting begins. Nitrogen is important for soybean cultivation. But soybeans, like other legume crops, have the capacity to "fix" nitrogen in the soil, thanks to certain bacteria contained in the nodules that form their roots. These bacteria convert nitrogen from the air and convert it to metabolized ammonium N, thus reducing soy’s need for a nitrogen fertilizer. Soybeans perform nitrogen fixation by establishing a symbiotic relationship with the bacteria, Rhizobium japonicum, in root nodules. This symbiotic nitrogen fixation system results from nodule formation known as nodulation (Liu, 1997). To ensure this process takes place, soybean seeds are inoculated with R. japonicum before planting. But as with any bean, adding inoculants to your soy crop will both allow plants to better fix nitrogen in the soil and improve the yield. Inexpensive inoculants may be purchased from seed catalogues and in hardware stores. Besides the inoculant, the soybean seed is sometimes added with a fungicide.

Some farmers in rural areas cannot afford the use of fertilizers even their lands have nutrient deficiencies, consequently, soybean yields are very low. Soybean, yielding 3 000 kg/ha, is able to extract 205 kg of nitrogen, 55 kg of phosphorus and 135 kg of potassium. Taking into accounts these numbers and the previous chemical analyses of the soil, the fertilizer needs for this crop can be calculated. In deficient soils in phosphorus and potassium, it is recommended to apply at least 300 kg/ha of a formula 5-20-20. When the soils are only deficient in phosphorus, it can be applied from 100 to 200 kg/ha of a fertilizer composed of 10-30-10 (Sánches-Potes, 1982).

Planting

The seeds are planted 1 1/2" deep, unless they are being planted late. (In that case, since the ground will be much drier, push them to twice that depth). This may sound a bit close, as the soybean plants are quite large; but this close spacing allows the floppy plants to hold each other up and the beans will produce excellently. In wetted soils, soybean is less deep planted than in dried soils. In light soils, it is deeper planted.

Seed planting density is between 60 to 80 kg/hectare, although there is a tendency to sow 80 kg/ha that is from 32 to 38 plants per linear metre to produce plants with thinner stems and pods at a higher height, which facilitate cutting. When soybean is going to be planted in soils never cultivated with soybean, it is highly recommended to inoculate soybean with the specific strain of R. japonicum. Distance between furrows depends on soybean varieties, plant height and its ability to grow. For high plant and late maturity varieties, it is recommended plant densities between 27 and 30 plants per square metre in rows spaced 60 cm. For short and early maturity varieties, a plant density between 40 and 60 plants per square metre in rows spaced 30 or 45 cm is recommended (Sánchez-Potes, 1982).

The beans can also be planted broadcast-style, then worked into the ground with a tiller. But this stand will be much weedier, as you will be unable to till it. If, however, the last crop you grew on the same spot was weed-free, weed control may not be such a problem.

Hand planting a hectare of land with soybeans often takes two workdays (Thuy, 1998). Farmers plant soybeans by inserting 7 or 8 seeds in each hole.

Irrigation or rainfed

Some moisture is necessary for germination, of course and during early development. Indeed, it is most important that the plants receive rainfall (or artificial watering) at the time of their seed-filling period. However, soybeans can withstand some drought once they are well established.

Weeds and their control

Soybean plants face a challenge from both broadleaf and grass weeds, particularly early in the growing season. If left after mid-season, weeds will seriously affect yields. In fact, weeds constitute the greatest hazard to soybean production in terms of the magnitude of losses they can cause. Weeds compete with plants for moisture, nutrients and sunlight. They also interfere with harvesting machinery and its presence in the harvest significantly reduces the trading value of the crop.

During the first days, soybean growth is slow and the crop must be kept weeds free. Some weeds may invade it, even though at the later stages of development. In each region specific weeds are predominant. It is necessary to recognize and identify them for a better control, especially if herbicides are used. Several systems must be integrated for getting weed control. An adequate land preparation allows eliminating a lot of weeds. The use of improved aggressive varieties and good coating, optimum plant spacing and an adequate fertilization, will provide advantages to soybean plants over weeds. Once the soybean seed is germinated, weeds can be eliminated by hand (Figure 3), commonly used by small-scale growers, or mechanically (Figure 4) as done by large-scale growers. The chemical control by means of herbicides is carried out before planting, as a pre-planting application, or after planting but before germination, as a pre-emergence application. Pre-planting herbicides are incorporated to the soil during the second tillage. There are many pre-emergence herbicides. These are applied by means of a sprayer machine (Figure 5) following the manufacturer’s instructions. Most large-scale growers are forced to use herbicides for weed control. Herbicides most currently used to control weeds in pre- or postemergence treatments are imazaquin, imazethapyr, chlorimuron-ethyl, flumetsulam and fomesafen. Glyphosate is the world’s most popular herbicide due to its excellent weed control capabilities and environmental safety. This herbicide cannot be applied over the top of the crop because it causes severe plant injury. Since 1996, a new weed control system for soybeans crop has been commercially available (Monsanto, Co.) through the glyphosate-tolerant soybeans called "Roundup", a transgenic plant developed in the United States (Duke, 1996). In the last years, the "Round-up" soybean has been tested worldwide. In Italy, the efficacy and total cost of weed control based on glyphosate (Round-up) were quite similar to those of good traditional control techniques (Sartoroto and Zanin, 1999).