Soil nutrients, soil properties and landscape characteristics vary considerably on land used to grow crops across the globe, as do climatic conditions. As a result, the crop mix and specific crop production practices (tillage, soil nutrient applications, pesticide applications, and irrigation practices) differ substantially from one part of the country to another.
If appropriate management activities and conservation practices are not used, the interaction between wind and water, soil and landscape characteristics, and crop production practices results in the loss of soil, soil nutrients, and pesticides from farm fields, contributing to water quality degradation in some watersheds.
Moreover, onsite soil erosion and soil quality degradation, if not addressed, can jeopardize prospects for sustaining future crop production.
1. Water and Wind Erosion
Water and wind erosion are major factors contributing to the loss of nutrients. Recent studies indicate that annual erosion losses in low-input production systems are about 10 kg N/ha, 2 kg P/ha and 6 kg K/ha.
Losses may be greater in high-input systems, or where rainfall is very high. Water barriers, such as grass strips and stone rows, are effective options to reduce erosion and to keep applied fertilizer and manure in place.
Erosion and runoff can also be reduced by covering the soil with a mulch layer of living or dead biomass. Soil mulch reduces water speed, avoids crust formation and improves soil porosity and infiltration rates.
Even a relatively thin layer of mulch provides a significant increase in water infiltration. Indeed, studies have shown that the application of 2 t/ha of straw led to a 60% reduction in runoff and a 90% reduction in erosion.
With 6 t/ha of straw mulch, runoff was reduced by 90% and erosion levels were reduced to zero. Leaving straw in the field leads also to significant reduction in soil losses due to wind erosion.
In Niger, 1.4 t/ha millet straw cover reduced wind erosion losses by 63%. The problem faced by most farmers is that their priority is to use organic materials for livestock feed.
Soil preparation methods may also be efficient in increasing infiltration and reducing runoff. The so-called ‘Zaï’ technique is an effective technique to deal with surface crusting: small pits are dug in the soil and small amounts of mineral and/or organic fertilizers are added.
Improving SOM content will generally reduce the susceptibility of the soil to form surface crusts and improves soil structure and water holding capacity.
2. Leaching
3 Leaching of nutrients occurs if water carrying nutrients percolates beyond the reach of crop roots in the soil profile and the nutrients are, therefore, lost to the crop.
Leaching is a particular problem in areas with high rainfall intensity (>30 mm/day) and coarse-textured sandy soils (>35% sand).
Leaching concerns mainly mineral N (principally nitrate, NO –) and exchangeable bases (K and Mg) which are often leached together with NO –. Phosphorus is generally not susceptible to leaching except in very coarse-textured sandy soils.
Some studies suggest that 50–60% of K fertilizer applied in banana plantations in Côte d’Ivoire are lost through leaching. Reducing losses due to deep drainage is difficult, but two approaches can be considered:
Promoting root development by applying nutrients and improving soil structure. This will allow the crop to better profit from water that has infiltrated into the soil below the present depth of root penetration, and therefore reduce the loss of nutrients.
Association of annual crops and trees – trees can ‘pump’ water and nutrients from depths below the rooting depth of annual crops, leading to better overall water and nutrient use.
3. Gaseous losses through denitrification and volatilization
Under anaerobic conditions (e.g. poorly drained field or paddy rice field), nitrate is reduced to N2O and N2 (denitrification). Denitrification also occurs in aerobic soils because of the presence of anaerobic microsites that are created following the application of decomposable organic resources.
The best way to reduce denitrification in upland fields is to improve soil drainage and maintain a good soil structure to avoid anaerobic growing conditions.
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Nitrogen can also be lost by volatilisation as NH3-N losses through volatilization are important in alkaline soils (high soil pH). As much as 60% of N applied as urea on paddy (i.e. flooded rice fields) may be lost due to volatilization.
Losses can be reduced by deep placement of N fertilizers, by manual incorporation.
Nitrogen is lost by NH3 volatilization during the storage and handling of manure. Losses can be reduced by using anaerobic storage pits with or without the addition of crop residues.
4. Loss of Phosphorus
Generally, the factors that cause phosphorus movement are similar as those that cause nitrogen movement. Transport mechanisms are erosion, surface water runoff from rainfall and irrigation, and leaching.
Factors that influence the source and amount of phosphorus available to be transported are soil properties, and the rate, form, timing, and method of phosphorus applied. The phosphate ion attaches strongly to soil particles and makes up a part of soil organic particles.
Any erosion of these particles will transport phosphorus from the site. Phosphorus can also be transported as soluble material in runoff and leaching water. When water moves over the soil surface, as it does in runoff events, or passes through the soil profile during leaching, soluble phosphorus will be transported with the water.
Applying phosphorus fertilizer or manures on the soil surface will subject them to both runoff and erosion, particularly if the application takes place just before a rainfall, irrigation, or wind event that can carry the phosphorus material off site.
If, however, the fertilizer or manure material is incorporated into the soil profile, it becomes protected from the transport mechanisms of wind and water. Leaching of phosphorus is at a higher risk through coarse textured soils or organic soils that have low clay content.
Phosphorus is primarily lost from farm fields through three processes:
Attached to the sediment that erodes from the field,
Dissolved in the surface water runoff, or dissolved in leachate and carried through the soil profile.
On cultivated fields, most is lost through erosion, whereas on non-tilled fields most phosphorus losses are dissolved in surface water runoff or in leachate. Cultivated acres with phosphorus-rich soils, however, can also lose significant amounts of phosphorus dissolved in the runoff or the leachate.
5. Minimizing losses of added nutrients
An important objective in fertilizer management is to implement management practices that minimize the loss of nutrients added to the farming system.
With good management practices, a significant proportion of nutrients added to the farming system in the form of mineral fertilizers or crop residues and manure can be recycled many times through crops and livestock.
Some nutrients taken up by the crop are exported in crop products (grain, tubers) that are exported from the farm but a large part of nutrients taken up by crop plants can be recycled back to the soil in the form of crop residues.
Alternatively, crop residues may be used as fodder for livestock and the manure they produce can be recycled to the field. With proper management, nutrients applied to the field build up the nutrient stocks or capital in the farm and add value to the land.
Nutrients added as mineral fertilizer, recycled in crop residues and manure as well as soil nutrient stocks may be lost from the farming system or the farm plot through water or wind erosion, leaching or gaseous losses.
Nitrogen is the most susceptible to losses because it is very mobile and can be lost due to leaching as well as volatilization. There are three main forms of N ‘capital’ in the soil:
Mineral N (ammonium NH4+ and nitrate NO3–);
N in soil organic matter; and
N in a more stable form of soil organic matter.
NH4-N can be held as an exchangeable cation or trapped in the layers within some 2:1 clay minerals, such as montmorillonite, vermiculite and illite. Under aerobic conditions (i.e. well-drained soils) nitrifying bacteria quickly transform NH4-N into NO3-N (nitrification).
Nitrate is highly mobile and easily lost by leaching or by denitrification (NO3– is transformed into the gases NO, N2O and N2). Substantial losses of NH4-N can also occur through volatilization (gaseous losses as NH3), especially in alkaline soils and where urea is applied to the soil surface.
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