SOIL FERTILITY AND PLANT NUTRITION: YIELD DECLINE AND OVERUSE OF N, P AND K FERTILISERS
Many times the overuse of N, P and K fertilisers can be attributed to a lack of understanding soil productivity, fertility and plant nutrition. This problem is further exacerbated by the application of all sorts of new “wonder cure” products which guarantee an increase in yields or quality of up to 20% in one season.
Most crops use approximately sixteen elements to meet their nutritional requirements. Some of these elements are needed in large quantities while others are needed in very small quantities. All these elements occur in the soil in varying amounts and a different chemical compounds. The plant can only take it up these compounds if the conditions in the plant and the soil are optimal and conducive for growth. The less optimal soil or growing conditions are the more management and input costs are necessary to farm sustainable.
With mono cropping and overuse of land, it has become necessary to resort to more drastic measures to maintain the yields of the past. This comes at a time when researchers have made technological breakthroughs in various fields of research and progress with high yielding varieties that also handle stress conditions better.
It is becoming a common practise especially in the irrigated areas to apply higher levels of N, P and K fertilisers at ever increasing costs, because the perception is that the higher the fertiliser levels the higher the yield and/or the better the fruit quality.
To address this misplaced perception it is necessary to understand the difference between soil fertility/productivity and plant nutrition.
Soil fertility / Productivity
Fertility refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions. Productivity is related to the ability of a soil to yield crops. Productivity is the broader term since fertility is only one of a number of factors that determine the magnitude of crop yields.
What makes a soil productive and fertile, and what influences soil productivity?
Active and healthy soil microbiology is able to:
- Mineralise nitrogen and sulphur
- Suppress nematodes, bacterial and fungal diseases
- Actively decompose organic material
- Improve root development
- Recycle and keep nutrients available for plants
- Improve soil physical and chemical conditions and has a high humus content
Soil physical conditions should have a
- Sandy clay loam texture
- Granular structure which improves aeration
- High waterholding capacity which is also well drained
- Infiltration rate (high)
- Effective rooting depth
- Moderate soil consistency which is able to withstand compaction
The soil chemical conditions should have a:
- pH (5,5 -6,5)
- High cation exchange capacity (CEC)
- Base saturation (high buffer capacity)
- All macro and micro elements readily available in a balanced amount
- Absence of detrimental or toxic elements (Al, Na)
- Low salinity / sodicity
All these factors are interlinked and no single soil type has all these factors in the ideal combination. It is further complicated by other factors like parent material, climate, slope, water quality (irrigated areas) and agronomic management practises.
The manipulation of the nutrient status of the soil by adding organic and inorganic fertilisers according to the crop’s need.
It is a well-known fact that plants generally need 16 elements in varying quantities and different stages of growth. These elements are:
- Carbon (C), Hydrogen (H), Oxygen (O) = organic material (humus)
- Nitrogen (N), Phosphorous (P), Potassium (K), Calcium (Ca), Magnesium (Mg) and Sulphur (S), (macro elements)
- Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo), Boron (B), Silicon (Si), (micro elements)
There is a perception in the irrigated areas that a trend of a yield plateau or yield decline exists. The concern is that higher levels of N, P and K fertilisers and other supplements are being used at increasing costs to rectify this perceived trend. In most cases the reduction in yields is not a nutritional problem but a soil fertility problem eg. mono-cropping, compaction, sodicity/salinity, increasing weed pressure and poor irrigation practices.
Agronomic practises that contribute to the yield decline / plateau
- Soils are nutritionally stripped and low microbial activity occur.
- Inefficient or no fallowing which result in increased levels of diseases and pests.
- Burning of trash and organic matter results in the gradual decline in microbial activity and N- mineralization in the soil.
- Increasing pressure from weeds.
- Increasing compaction with in-field harvesting practises causing poor infiltration and soil aeration.
- Poor irrigation practises. Over irrigation causes leaching of nutrients.
- Decline in water quality in major river systems is causing a gradual build-up of salinity and sodicity.
It is very important that you ask yourself the following:
- Is the present nutritional program effective?
- Is there a direct link between achieved yields and the higher N, P and K applications?
- Are optimal yields being achieved according to the soil potential or is a yield being bought at a higher input cost?
- Do you know what soil characteristics are required for irrigation purposes e.g. Water holding capacity, available water, depletion factor for the crop, effective rooting depth?
- When last did you take subsoil samples to monitor subsoil salinity, sodicity or acidity?
- Are all fields efficiently drained?
- How efficient are your irrigation systems in terms of water delivery to satisfy crop requirements? Are you scheduling irrigation according to the crop’s needs?
- When last did you do an identification of the weed species and the pressure they exert on crop yield for each field?
- Is your present weed control program effective? When last did you change the combination of herbicides being used?
- Are you still applying sound fallow principles from a disease perspective and sustainability point of view?
- How efficient or detrimental is your land preparation and in-field harvesting practises?
In most cases yield losses occur because of a combination of these factors mentioned. The solution to low yields/yield plateau requires a “back to basics” approach where sound agronomic management practises are adopted.
Article written by F. Botha (B.Sc Hon Soil Science)