Field conditions for sowing and planting
i. Soil moisture: Generally, seeds may be sown when the soil has the optimum moisture content for crop germination and establishment. Such soil moisture conditions can be achieved after rainfall, irrigation and drainage or from conserved water. Crop seeds such as wheat, mustard, castor, barley, rice, potato and jute may be sown under such soil moisture conditions. Seeds may be sown when the seeding zone is in wet (saturated) conditions. Pre-germinated or pre-sprouted seeds or seed materials are sown under such soil moisture, conditions. Crops such as rice, sugarcane, berseem, grasspea and black gram are sown in such conditions.
Under rainfed condition, seeds may be sown when the soil is dry; when the soil moisture around the seeding zone is so low that hydration of seeds will not occur. Seeds such as sesame, jute, jowar and bajra may be sown under such soil moisture conditions awaiting rainfall which induce germination and the establishment of crop plants. Under irrigated condition, seeds may be sown in dry or semi-dry soil conditions and immediately after sowing the seeding zone is irrigated. Crops such as onion, garlic, sugarcane, napier, para grass, sweet potato and guinea grass are sown or planted following this method.
During seeding the field should be free from established weeds, readily decomposable organic matter and toxic substances. During the process of imbibition, the seeds get wetter and the soil gets drier if additional water is not available. Thus water content in the soil, across the soil-seed interface and in the seed is important. A newly sown seed rests within an air-filled pore space surrounded by soil. Al least a few points of the seed come in contact with the soil at which water moving in the liquid phase, passes into the seed. Rough seed-coats of many seeds decrease the efficiency of physical contact between seed and soil.
ii. Soil crusting:Crusting (capping), which often occurs when flooded soil surfaces dry rapidly, results in more of a problem for the reliable production of stands than an excess or deficiency of water in many soils. Due to surface crusting the gas-regime, in the soil changes, oxygen content decreases and carbon dioxide concentration increases. Crusting changes the nature of the conversion of nutritional matter. Crusting compacts the soil and restricts the infiltration of water thus inducing a run off. Firming induces greater water stability.
Unfirmed soil has large enough pores to permit unstable water-borne particles to re-sort themselves to form a crust when the water evaporates after irrigation or rain. Slight compacting of the seed bed improves seed-soil contact, availability of moisture and imbibition and hastens germination, but may restrict subsequent root and shoot elongation and decrease field emergence.
Keeping the soil surface continuously moist even by very fine spray irrigation until the seedlings emerge or by the application of anticrusting materials including mulches, reduces the deleterious effect of capping. The longer a seed spends in the ground, the longer it is at risk. Staggered germination has low uniformity of seedlings. Earlier emerging seedlings will progressively restrict the supply of light, water and nutrients to the later emerging ones.
iii. Soil air: The seeds respire during and after imbibition. Soil microbes and other soil-inhabiting living entities compete with seeds for required ventilation in soil. The quantity and freshness of decomposable organic matter, the water content of the soil, degree of soil capping and the prevailing soil temperature determine the ambient soil respiration rate. When seeds are sown in wet soil or the soil becomes saturated immediately after sowing, seeds suffer from oxygen deficiency. Rapid and efficient drainage to the desirable extent can save the life of the seeds in the soil.
iv. Soil structure:If structural collapse is complete, diffusion within the seed bed may be within the liquid phase and seedlings may subsequently have great difficulty in emerging through the resultant cap or crust when the soil surface dries before field emergence.
v. Soil temperature:The temperature of the soil is determined by the balance between incoming and outgoing radiation at the soil surface and by the way that this energy is redistributed throughout the soil as heat. By day heat is gained, by night it is lost. There is also an annual fluctuation of soil heat. In winter the net daily gain is negative, while in summer it is positive. Where cold soil is a problem, soil temperature can be increased by the use of organic mulches and irrigation (which reduce soil temperature by around 10°C) can reduce the adverse effect of high soil temperature. Again, draining of the field or maintaining a slow flow of water reduces the hot water injury to emerging seedlings.
vi. Light: Some seeds especially those that contain the phytochrome system, germinate in response to light stimulus. The phytochrome system acts as an environment detecting device as an informer to seeds to break the dormancy and thus causes germination under favourable conditions. In the field, light does not penetrate more than 1 mm except down drying tracks, therefore, exposing seeds to bright sunshine for at least half an hour before sowing stimulates seed germination along with the other positive effects of solar radiation on seeds. Positive photoblastic seeds should be exposed to light after imbibition but before sowing.
vii. Chemicals:When the seeds are sown in the field, endogenous and exogenous inhibitory substances and other phytotoxins diffuse away from the seeds or detoxify for successful germination and establishment of crop plants.
viii. Predatory animals and birds: Several frugivorous and granivorous animals and birds feed on many seeds after they are sown in the field. Sufficient good seeds should be sown to ensure a desirable crop stand. If required, excess seedlings are removed leaving the strongest ones but if gap filling is intended, seedling vigour, age and stage of crop plants in a field will differ, which will affect in the scheduling of field operations and harvesting. The harvestable parts of crop plants in a field may ripen at different times. Gap filling with seeds staggers the period of germination and emergence. Thus ripening periods which extend over time affect the harvesting which is scheduled once for most crops, and this seriously impairs the quality of produce. There is an old axiom regarding seed rate “one for the rook, one for the crow, one to rot and one to grow” which is modifiable according to the growers’ own experience and judgment, with possible measures to save the loss of seeds in these ways.
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