Seeds are unique to the type of plant from which they come. You cannot expect to obtain passion fruit from a grapefruit seed, nor tomatoes from a grape seed. To expect something other than what you plant is silly. Maybe this is why scripture tells us it is reasonable to expect to harvest what we plant, and we must plant in order to harvest. The one planting seeds is as significant as the seeds being planted - not to mention "where" the seeds are planted.
Light-seeds are planted in the souls of God’s people, Joy-seeds are planted in good heart-soil. (Psalm 97:11 MSG)
If God is the planter of the seeds, the harvest is more likely to produce the type of growth which will edify and build us up. He takes great care in planting just the right seeds which will grow into exactly what we need in order to dispel darkness and to increase our joy. These two "seeds" referred to in our passage really produce fruit beyond what we may realize at the first blush - probably because of where they are planted. It is in looking at where God does the planting that we find a little insight into the growth which is produced.
Light-seeds are planted in the souls of God's people. Now, thinking of the soul of man, we might think of the darkness which dwells there when God's light has not invaded the space within. Our souls are where we focus on the connection existing between God and man - the eternal part of our being which will dwell forever either in the presence of God or the fires of hell. The soul is often thought of as where we form our "morals" which guide our every decision. If our soul is shrouded with darkness, the choices we make may be less than "upright". The "morals" we form may be a little "tainted" by our viewpoint - dark places in our soul tend to yield dark actions. Our view (our morals) are made lighter by the seed planted by God's hand.
Joy-seeds are planted in good heart-soil. Knowing the heart is our seat of emotion, isn't it awesome God plants "joy-seeds" there? He plants seeds of joy - which will bring something to our hearts which all other "additions" of seed will never quite be able to produce. Joy differs from happiness in that joy is lasting, where happiness is dependent upon the circumstances and the outcomes we experience in life. Joy is totally deep-seated, or shall I say "deep-seeded", buried deep within the heart. Joy is a result of "connection" - the connection of the soil of our hearts with the roots of the seeds of JOY God plants deep within the heart. The heart holds both joy and grief. Isn't it truly a work of God to turn places of grief into places of the deepest growth - the seeds of joy producing great growth from within the soil of the grief we experience.
The absence of joy is grief. The opposite of light is darkness. The seeds God plants are exactly the opposite of the condition of the soil in which they are planted. The seeds are planted in the very type of soil where their growth will have the greatest impact. If you have ever seen a plant grow so big as to take over the area in which it was planted, I think this is probably what God has in mind when he plants the seeds of joy in the grief of our soul, or the seed of light into the darkest places of our emotions.
Getting back to the "planting" process, we see scripture advising to not expect a harvest if there is never any planting going on. So, in giving God access to our souls, we are encouraging seeds of light to be planted. In giving him access to our emotions, we are encouraging seeds of joy to be planted. In turn, we can see the harvest they will produce. These are seeds which we cannot plant ourselves, but we can open up to the planting of our heavenly Father. We attempt to plant all kinds of seeds ourselves, but they fall far short of the harvest God intends for our lives. Wouldn't it be wise to allow God to do the planting, in order to have the optimum harvest produced? Just sayin!