Nutrients

I know about crop rotation but dont understand the science behind it. If crops are rotated does that mean different plants take out different nutrients and how do the nutrients get replaced?

 

Herb - I do not know too much myself but my understanding is that different crops do have different requirements of nutrients. I think the problem is really only with food crops. If you think of a forest or a wild grass meadow, you have the same crop quite happily for 100 years or more, and it does not suffer. The reason is that the trees and grass take nutrients from the ground, but as long as man is not around all that nutrient is returned to the soil when the grass dies back, the leaves fall or the tree dies and rots. So there is no long term depletion.

But when man cuts the crop and takes it away to eat, he is permanently removing the nutrients. And he tends to leave his own manure, which would be putting some of it back, in a different place.

Some plants like legumes fix nitrogen in the soil, but use up other nutrients. If you then follow this by another crop that uses the nitrogen, but doesn't need so much of the other nutrients, you get better crops than you would if you did not rotate them. I think you will still get some crop if you do not rotate. I suspect that nature heals and rebalances itself slowly by microbal action. After all there is no shortage of nitrogen (80% of air), it is just the speed of fixing it in the soil in a form that plants can use. The same with minerals, there is a huge amount of most mineral elements in the soil, but I assume that it is not all readily available to the plant.

Nowadays crop rotation is not practiced much as we can use artificial fertilisers to supply everything that the crop needs. But there is still the problem of pests building up in the soil if one crop is grown for too long on the same spot.

 

for vegetables for example there is also the liming isuue, farmers lime every 3 to 5 years except where the soil is eg chalk
soil tends to gradually increase in acidity and lime will neutralise acidity
some plants eg cabbages and turnips need to grow in neutral soil not acid
potatoes and other root crop tend to be ok with acidity
farmers and growers test soil annually and get recommendations for next crop, the main fertiliser needs are for nitrogen, phosphates and potassium
... NPK
micro nutrients or trace elements are more complex in behaviour, and are not tested for routinely, mono culture will tend to lead to trace element efficiency, gardeners can use things like seaweed meal to ensure all miicronutrients are available to plants

 

Add to what has been said before, rotation was mainly as said for plant nutrients but also stopped the accumulation of plant problems

 

Yeah, like wire worms and such! Ugh!