How Trees Help Crops

Backgrounder

Perhaps you or other farmers in your region practice a type of farming called slash and burn (shifting cultivation).  This is done by cutting down the forest, burning the trees, and planting crops on the land the trees were on.  After a few growing seasons, shrubs and trees are allowed to grow again while another part of the forest is cleared for cropping.  Then many years later that same land can be cleared again for growing crops … and so on.

Farmers who practice this type of farming notice that when the forest is newly cleared, the soil is very fertile, and they can grow plenty of healthy crops.  Fertilizers are not necessary.  After a few growing seasons, however, the soil becomes less and less fertile, and less food is harvested from the same plot of land.

These days this method of farming is less common because so many forests have been cut down and we have to use most of the land to grow crops.  But we should think carefully about the forest and the soil.  Why do crops grow so well on newly cleared land?  Why do forests grow so well without any fertilizer?  What is the connection between the good soil and the forest?

In the forest, the roots of trees and shrubs take nutrients from all levels of the soil, from near the surface, from the subsoil, and from down deep in the ground.  Then the sap of the tree takes the nutrients from the roots up to the leaves through tiny tubes in the living wood just inside the bark.  Because of this, all of the leaves and branches then have these nutrients in them. When whole trees are burned, many of the nutrients can be found in the ashes that are left on the ground.  Those ashes then become the fertilizer that makes crops grow so well for the first two or three seasons after the forest has been burned.  The time soon comes however, when most of the nutrients are used up by crops.  That is why, for such a long time, farmers have had to cut and burn another part of the forest so that they could once again use that natural fertilizer to grow good crops again.

If trees are not burned and the forest grows naturally, the tree leaves continually fall to the ground and rot, providing organic matter and some nutrients for the soil.

If we think about this cycle, we can understand the value of growing trees together with farm crops.  Because when trees bring nutrients back up into the topsoil – that is good for crops!

One method of planting trees with crops is called alley cropping.  In this system farmers grow crops between rows of low growing shrub-like trees.  They cut off the new growth regularly; they then put the leaves and stems they have cut, on the soil, in between the rows of trees where they grow their crops.  The plants are fertilized by the nutrients in the leaves and branches.  The tree leaves also form a mulch over the soil to keep it from drying out, to help prevent weeds from growing, and to reduce soil erosion due to heavy rainfall.  The trees also protect the soil by holding it together with their roots.  This helps to prevent erosion from wind and water.  Trees also act as windbreaks.

For alley cropping you need trees which do not shade the crops too much and which have deep roots so they don’t compete with crop roots for soil moisture.  Local trees that grow naturally in your area are best.

Legume trees grow well with crops.  Most legume trees not only have deep roots, but also the bacteria (Rhizobium) that grow on their roots add nitrogen to the soil.  This is important because crops really need nitrogen.  Some legume trees have small leaves, so do not cause too much shading.  Also you can use legume tree leaves as feed for farm animals.

There are ways of mixing trees with crops other than alley cropping.  For example, you might plant trees along the border of your fields.  You can leave a few trees in your fields, or plant some new trees in the fields.  Or you might have fruit trees mixed with other crops in your home garden.  Choose trees which do not shade the crops too much.

So, once again, why are trees good for your crops?  Trees bring nutrients from deep down in the soil and return them to the soil surface where crops can use them.  Leaves on the soil form a useful mulch to keep weeds down, hold water in the soil, and keep soil from washing away on sloping land.

If you grow trees on your land you will also have the benefit of the wood for fuel and building materials.  You may be able to use some of the leaves for animal feed, and if you are growing fruit trees, you will enjoy the benefits of good fresh fruit.  And if you have some wood or fruit left over you can even sell it to earn extra money.