Vanilla farmer in Kwale. PHOTO|FILE|NATION 

My grandmother used to tell me that I am not supposed to plant the same crop on the same land consecutively.
And when I asked her why not? Her reply would be it was not right. Well, that is where her understanding ended, but all she was telling me was to rotate crops to fight pests for more yields. Now, let’s explore the benefits, rules and the science behind crop rotation.
The first rule is that crops of the same family should never be planted following each other because they share the same pests and diseases.
At the time of growing the first crop, there will be pest and diseases that will build up. Therefore, by the time you plant the second crop of the same family, the diseases and pest will be ready to attack it.
Losses from such an attack can be up to 100 per cent. For example in an area where you have planted tomatoes, you cannot rotate with a crop in the Solanaceae family such as the nightshade, potatoes, eggplant, peppers (capsicum) or tobacco. Some weeds such as datura are also in this family.
Weeds in most cases are not beneficial to the crop since they act as alternate hosts of both pests and diseases. This is apart from competing with crops for nutrients, space, sunlight and water.
The second rule is that in the sequence, there needs to be a legume. Leguminous crops include the common bean, lentils, garden peas, soya beans, peanut/groundnuts, chickpeas, pigeon peas, cowpea and green grams and pastures such as alfafa, clover and lupin, among others.
The advantage of having a legume in the sequence is that it fixes the atmospheric nitrogen into the soil through a process known as biological nitrogen fixation with the help of micro-organisms found in their roots known as rhizobia. This means the atmospheric nitrogen, which is in its molecular form N2 is converted to ammonia (NH3).
The ammonia is then converted into ammonium (NH4+) which is the form that is utilised by the plant. After the legumes are harvested, the amino acids in the crop are realised back into the soil where they are converted into nitrate (NO3-), which is a form that is utilised by plants. You can rotate a cereal such as maize with soya bean or potato with common bean.
Furthermore, legumes are also called green manure. This is because of their ability to biologically fix nitrogen into the soil. For you to get the maximum out of green manure, you need to incorporate the legume into the soil. Green manure also has the advantage of improving the soil structure and the water-holding capacity of the soil.
The third rule of crop rotation is to rotate crops with different rooting depth. The deep-rooted crop should be rotated with the shallow-rooted. For example, sunflower (deep-rooted) should rotate with wheat (shallow-rooted).
The benefits of crop rotation
Crop rotation keeps the farmer in production throughout the year. Although the planting seasons are determined by the amount of rainfall, a farmer should take advantage of this using a rotational sequence
During the long rains, you can plant the high rainfall crops in this sequence:
Wheat >> Beans >> Tomatoes
During the dry season with the intermediate rains:
On the farm where the wheat (cereal) was, plant chickpea (legume), where beans were plant finger millet (cereal) and where tomatoes (solanaceae) were, plant pigeon pea (legume). All these crops are drought-tolerant, thus, there is no need to skip the season.
Chickpea >> finger millet >> pigeon pea
During the short rains, plant:
Maize (the variety will depend on where you are) soya beans and butternuts.
Crop rotation further helps to reduce the amount of fertiliser used, especially if you incorporate legumes, thus cutting the cost of production and increasing the profit margins.
Though not proven, there is an increase in yields in a rotation sequence, which some agronomists term as the rotational effect. The growing of the different crops on the same piece of land in the same season also cushions the farmer from losses.
Just like intercropping, crop rotation sequence can cushion the farmer against adverse weather condition especially if dry land crops are incorporated.
Crop rotation helps in the management of some pests such as root-knot nematode and diseases that have established in the soil over time.
Soil erosion still remains one of the greatest threats to land degradation. Crop rotation is a strategy that can be used to curb this scourge.
Different crops such as beans and other wide leaf plants can be used to reduce the raindrop impact, crops such as grasses can be used to reduce sediment detachment, transport, surface runoff and eventually soil loss.
Crop rotation encourages the utilisation of various crop species, it assists in the build-up of soil organic matter, improves soil structure and the chemical and biological soil environment.
Soil organic matter has additional advantage of improving water infiltration and retention, increased drought-tolerance and decreased soil erosion.
The main disadvantage of crop rotation is that the types of the crops chosen depends on the farmer’s preference, which is influenced by the environment especially water availability and temperatures.
The writer is based at the Department of Crops, Horticulture and Soils, Egerton University.