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NPK Management in Soils. Introduction Soil Nitrogen (N) Dynamics Soil Phosphorus (P) Dynamics Soil Potassium (K) Dynamics. Introduction. NPK as the three principal fertilizer elements have the most widespread influence on both natural and agricultural ecosystems.
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NPK Management in Soils Introduction Soil Nitrogen (N) Dynamics Soil Phosphorus (P) Dynamics Soil Potassium (K) Dynamics
Introduction • NPK as the three principal fertilizer elements have the most widespread influence on both natural and agricultural ecosystems. • Plants that are deficient in any of NPK are often doomed because the elements are components of essential plant compounds which deficiency cannot be easily overcome. • The importance of NPK in crop production and environmental management depends on the ecosystem in question –natural, agricultural, or polluted ecosystems.
1. Nitrogen Dynamics of Soils • Nitrogen is an essential plant nutrient • It is mainly in organic forms in soil • In it’s ionic form (NO3-, NH4+), it is very mobile in soils and plants • It is also responsible for some environmental problems. 1.1 Introduction
1.2. Role of nitrogen in plants Component of essential plant compounds • Amino acids ~ building blocks of proteins, enzymes • nucleic acid ~ hereditary control • Chlorophyll ~ photosynthesis Plants respond to good available N by having • deep green color of leaves • Increased protein content • increased plumpness of grains • Increased plant productivity in general
1.3. Deficiency of nitrogen in plants • Nitrogen is quite mobile (easily translocated) within plants, so available N is sent to newest foliage first • Deficiency exhibits the following: • Pale yellowish green color (chlorosis) in older leaves • Have stunted growth and thin stems (low shoot:root ratio) • Plants mature more quickly than healthy plants • Protein content is low and sugar content is high • There is reduced productivity in general
1.4. Oversupply of nitrogen in plants When too much N is applied, the following may occur: • Excessive vegetative growth (but weak plant stems) that lead to lodging with rain or wind • Delays maturity and cause plants to be prone to diseases • General decline of plant product quality • Environmental aspects (build up of nitrates)
1.6. Management of Soil Nitrogen • Objectives of good N management • Maintain adequate N supply • Regulate the soluble forms of N to ensure enough is readily available • Minimizing leakage from soil-plant system Strategies for Achieving the Objectives: • Taking into account N contribution from other sources so as not to oversupply N • Improving efficiency with which fertilizer is applied • Improving crop response knowledge • Avoiding overly optimistic goals of meeting crop needs that are higher than possible
2. Soil Phosphorus 2.1. Phosphorus and the environment • Total concentration of phosphorus in soils is usually low (compared to nitrogen and potassium). • Compounds in soil that are the primary source of phosphorus –phosphates are not very soluble, resulting in phosphorus being unavailable enough for plants. • When available forms of the element is applied to soil in forms of fertilizer or manure, they can be easily fixed by soil colloids. • Too little or too much phosphorus can have negative impacts on the environment. • When there is too little P –land degradation P poor soils provide little vegetative cover to prevent land surface wash by erosion, and eroded materials sediment down stream. • When there is too much P –eutrophication P enters streams from point and nonpoint sources. Timber harvest, livestock grazing, fertilizers and application of animal wastes, and soil tillage.
2.2. Phosphorus and Plant Growth • Adequate P enhances fundamental processes in plants such as photosynthesis, nitrogen fixation, flowering, fruiting, and maturation. • P is very important for the overall improvement of crop quality whether in forages or vegetables • It Stimulates root growth, particularly development of lateral roots and fibrous rootlets. • P is a component of the energy transformation compounds (adenosine diphosphate, ADP; adenosine triphosphate, ATP; nicotinamide adenine dinucleotide phosphate, NADP+).
2.3. Deficiency of Phosphorus in Plants • A P deficient plant is usually stunted and thin stemmed, but retains a dark color. • Not as simple to detect as is the case of N deficiency. • When severe cases of deficiency occur, there is yellowing and senescence of leaves. • But these occurs first in older leaves.
2.5. Managing Soil Phosphorus • Proper management of soil pH (liming or acidification) to a pH between 6-7 • Since P is not very mobile, little is leached and therefore little reaches subsurface soil. This may require some incorporation • Banding application is generally better than broadcast. Significant amounts of P can be transported with sediment to surface waters were it can cause eutrophication. • Therefore, to minimize P contamination, every effort to minimize erosion should be made.
3.1. Potassium Dynamics in Soils • It is present in soil solution as positively charged cation, K+ • It does not form any gases that is lost to air • Its behavior in soil is influenced more by cation exchange reactions than by microbiological processes • It does not cause off site environmental problems • It is not toxic and does not cause eutrophication problems
3.3. Potassium in Plant and Animal Nutrition • Potassium activates many enzymes in plants and animals that are responsible for energy metabolism, photosynthesis and other processes • As a component of the cytoplasm of plant cells, it helps to lower cellular osmotic water potential thereby increasing the ability of root cells to take up water. • K is important for nitrogen fixation in legumes • Good K nutrition has been shown to help plants adapt to environmental stress • In animals including humans, K is important in regulating the nervous system and maintenance of good blood vessels.
3.4. Deficiency Symptoms of Potassium • Reduced ability to adapt to environmental stress, e.g., drought, lodging, etc • Tips and edges of leaves become yellow (chlorosis) and then die (necrosis). • In some crops, K deficiency produces white necrotic spots that looks like insect damage.
3.7. K–Fixation by Soil Colloids Nonexchangeable-K Exchangeable-K Soil Solution-K
3.8. Potassium Management • The main problem with managing soil K is that of converting the unavailable forms of the element to available forms • Available K is usually supplemented by fertilization • Another problem is removing a lot of crops from the soil without returning the crop residues • Attempts should be made to return as much residue as possible for the natural plant-soil cycling of K to continue
Potassium Management contd • Growing high K content plants places demand on the soil supply of potassium • To have high yields of such crops, e.g., alfalfa, it needs to be planned in advance to supply soil with enough K to last the cropping season –fertilization • Light and frequent application of fertilizer is better than heavy and infrequent application to reduce luxury consumption • Treating soils with lime have been found to increase K retention in soils.
