More about the meaning “reaping what you have sown” as karma's basic law :
Karmic law is the law of cause and effect – not of merit and punishment. The idea of merit for good deeds, and punishment for evil or bad deeds is a moralistic and therefore relative idea. It has nothing to do with the truth. It has only to do with relative truth, what's considered good or bad in certain societies, eras, religious or philosophical systems of beliefs, and so on. Karmic law, on the other hand, deals only with the truth – the actual nature and quality of the energies (psychical and physical) consciousness is linked to, individually and as a group.
“Reaping what you have sown” is a very adequate illustration of karmic law. We must ever remember that karma isn't strictly speaking a linear thing. A strict linear understanding would be similar to something like merit and punishment, more like an “an eye for an eye” kind of law. Karma being the law of causes and effects, we, therefore, remember that causation is a multi-directional (not linear) thing. Multiple causes are at the origin of any effect. All causes are themselves effects of previous causes, and so on.
In the context of the illustration of “reaping what you have sown”, if one plants a poisonous seed, one will get a poisonous plant, then poisonous fruits, themselves producing more seeds, and so on. That would be a linear understanding. We must not forget that in this illustration, the context is also an integral part of the whole cycle of causation. The environmental conditions in which a particular seed is planted and will grow will also be an undeniably important part of the resulting karma. If the soil quality, the air, sunlight, water are not so adequate for any specific seed, it cannot grow very well, and cannot result in very potent fruits. The same applies to the planting of the most beautiful, good-willed type of seeds.
This “reaping what you have sown” illustration happens within oneself. Causes and effects are passing things, like the cycles we can all observe in nature: periods on sowing, then according to all kinds of specific cycles, periods of blooming, reaping, etc. Cycles repeat themselves, unless they are broken, or not fed enough to sustain themselves, in which case they eventually cease to be. Our inner soil, water, air, and sunlight more or less allow, or don't allow certain seeds of karma to grow and bear effects or fruits.
Any person who wishes to change or better your life's circumstances has to become as conscious as possible of the quality of their inner soil, water, air, and sunlight, and then strive to increase its quality. Due attention to right physical care, and more importantly, right psychical hygiene (resulting in good mental health) will make a difference, for karma is a law.
There must come a time when consciousness must elevate itself above the endless, cyclic, and samsaric (psychical and physical) aspects of karma. One has to dry the garden of samsaric attachments to become free of it, for the nature of consciousness is free of it. Once that is fully realized, it becomes possible to sow exceptional seeds in this world, bearing exceptionally good fruits, and yet remain free, unbounded, unlimited by even such excellent fruits.