Tuesday, December 18, 2012

Don’t Forget to Keep Your Alfalfa in Top Shape with Phosphorus!

High-yielding alfalfa in beds (Arizona)
Many factors are involved in producing a top-quality alfalfa crop.  Although many factors (like weather) cannot be controlled, many other critical components need to be carefully managed.  As the demand for high-quality hay increases, a closer look at the role of proper nutrition is needed.

There is no substitute for maintaining an adequate plant nutrient supply for production of high-yielding and high-quality alfalfa.  Alfalfa production removes large amounts of nutrients from the soil that must eventually be replaced to remain sustainable (about 15 pounds of P2O5 removed in each ton of hay).  Since phosphorus (P) has many essential roles in alfalfa, both yield and quality are reduced when this nutrient is limited.

Most P in the plant is rapidly converted into organic compounds involved in a variety of essential reactions. For example, P in alfalfa is essential for formation nucleic acids, phospholipids and ATP- associated with things like photosynthesis, protein formation and nitrogen fixation.
Root nodules -site of nitrogen fixation

In addition to direct plant growth benefits, P fertilization has also been shown to increase nitrogen fixation, nodule number and nodule size.  There are frequent reports that P or K nutrition have been found to improve disease tolerance or resistance.

Low P (L) and adequate P (R) alfalfa
Soils vary in their ability to supply P and nutrient deficiency symptoms in alfalfa are hard to detect before the deficiency becomes quite severe.  Therefore, soil testing is best way of predicting the potentially available nutrient supply.  It is generally best that P be applied prior to establishing the crop, since an adequate supply of P is critical for rapid stand development and a strong root system.  For established stands, surface applications are a good way to meet plant needs. 
Polyphosphate fluid fertilizer
Granular triple-super phosphate
Many sources of fertilizer P are successfully used for alfalfa production- including both solid and liquid forms.  A number of comparisons have demonstrated that most P fertilizer sources are equivalent, when properly used.  The selection of a specific P fertilizer is generally based on local availability, ease of application, and the cost per unit of nutrient.

Phosphorus fertilization is an essential component of alfalfa production.  High-yielding alfalfa removes large amounts of P which must be replaced when the soil P supply can not meet the plant demand.  Soil and tissue tests are useful for determining the appropriate amount of P to apply.  Failure to monitor and replace the nutrients removed in harvested hay will lead to losses of yield, plant stand, and profit.
Alfalfa is "ice cream in the making"

Thursday, December 13, 2012

What Happened to Last Year’s Fertilizer?

Taking care of the soil is essential

Everyone is looking to get the most value from everything they do. With the ever-tightening squeeze between farm inputs and crop prices, it just makes good sense to reevaluate where the added fertilizer is going each year. 

The purpose of adding nutrients to soil is to create an environment for healthy and profitable plant growth.  It has long been known that well-nourished plants are better able to resist disease and insects, use water more efficiently, produce more abundant and higher quality yields, and offer a better economic return.  It remains the goal to get as much of the applied nutrients into the plant where they can be productive.

Healthy crop of mandarins
Nutrients become depleted when they removed from the soil in every harvested crop- but there are also other things happening that cause nutrients to become less available to plants.  An overview is presented here, but more details are available from your local crop advisor.

Almonds in stoney ground
When potash fertilizers are added to a field, they quickly dissolve in soil water and the potassium cation (K+) becomes held on the negatively charged sites on clays and organic matter.  Potassium is retained on these cation exchange sites until another cation comes along to replace it.  Since K is not held as strongly as some other cations (such as calcium or magnesium), it can slowly move down in the soil- especially in very sandy soils and in high rainfall areas.  In most soils however, added K remains close to where it is applied and stays available for plant uptake year after year.  

Phosphorus (P) fertilizer generally has the greatest solubility and availability for plant uptake immediately after being added to soil.  Soon after application, P fertilizers begin to react with the soil solution and soil minerals to form less soluble compounds.  The chemistry of P in the soil governs these reactions and over time, less soluble and less available compounds are formed.  All added P, whether commercial fertilizer or organic sources such as manure, eventually undergo these soil reactions. 

Onions have a shallow root system
Soil testing is the best technique for determining the nutrient value of these older P compounds and for predicting the need for any additional fertilizer required to meet the plants nutritional needs.

Application of P fertilizer in a concentrated zone or band is one simple technique that is useful in delaying these soil reactions and improving nutrient availability.  New additives are being developed that may also prove beneficial.  Special attention should be taken to avoid conditions where P can be lost in surface runoff.       

Nitrogen (N) is the most difficult to manage of the primary plant nutrients.  It undergoes many complicated biological and chemical transformations that make it a challenge to keep it in the rootzone where is needed.  The primary goal is to get the added N into the plant, where it is essential for many metabolic processes, such as chlorophyll formation and protein synthesis.  

Soil processes change in wet conditions
Unfortunately, a significant portion of the added N can be lost as a gas through ammonia volatilization from surface-applied fertilizer or through denitrification from wet soils.  Nitrate is mobile in the soil solution and may be carried beyond the rootzone in water passing through the soil.  A significant amount of added N fertilizer is used by the soil microbes in building soil organic matter.  Special attention is needed for N fertilizer management because of its numerous pathways of loss, and the high plant requirement for vigorous growth.  Plant tissue testing can provide an indication of the nutritional status and the need for mid-season additions.
Nitrogen-deficient lettuce

Each of these nutrients is frequently viewed as a separate management issue- due to their diverse behavior and unique soil reactions- but it is essential that all three are managed as a package in order to provide balanced nutrition for the plant.  When any one of these nutrients is in short supply, the other two will not be used efficiently and plant performance will suffer.

One view of the complex processes in soil

Thursday, December 6, 2012

Magnesium: A Forgotten Element in Crop Production

This post is reprinted from the IPNI publication "Better Crops".  It was originally published in 2010.  Due to the interest and questions I receive about magnesium nutrition in plants, I am reproducing it here.  The original pdf version of the article can be downloaded here.

By Ismail Cakmak and Atilla M. Yazici

Dr. Cakmak (cakmak@sabanciuniv.edu) and Dr. Yazici are with the Faculty of Engineering and Natural Sciences at Sabanci University. Istanbul, Turkey.
Magnesium deficiency symptoms on bean leaves
 Magnesium nutrition of plants is frequently overlooked and shortages will adversely impact plant growth. Many essential plant functions require adequate Mg supplies, the most visible being its role in root formation, chlorophyll, and photosynthesis. Many less visible reactions are also dependent on an adequate supply of Mg. This review briefly summarizes some of the essential roles of Mg for plants.

 Magnesium has a number of key functions in plants. Particular metabolic processes and reactions that are influenced by Mg include: 1) photophosphorylation (such as ATP formation in chloroplasts), 2) photosynthetic carbon dioxide (CO2) fixation, 3) protein synthesis, 4) chlorophyll formation, 5) phloem loading, 6) partitioning and utilization of photoassimilates, 7) generation of reactive oxygen species, and 8) photooxidation in leaf tissues. Consequently, many critical physiological and biochemical processes in plants are adversely affected by Mg deficiency, leading to impairments in growth and yield. In most cases, the involvement of Mg in metabolic processes relies on Mg activating numerous enzymes. An important Mg-activated enzyme is the ribulose- 1,5-bisphosphate (RuBP) carboxylase, which is a key enzyme in the photosynthesis process and the most abundant enzyme on earth. 
Leaf yellowing in the form of interveinal chlorosis on older leaves is one of the typical symptoms of Mg deficiency stress (Figure 1). It is reported that up to 35% of the total Mg in plants is bound in chloroplasts (Figure 2).  
Chloroplasts are the organelles that host thylakoids, the Mg-containing compartments where light energy is converted to chemical energy through the process of photosynthesis
However, the appearance of Mg deficiency symptoms is highly dependent on light intensity. High light intensity increases the development of interveinal chlorosis, together with some reddish spots on the leaf blade (Figure 3). Therefore, the well-documented differences between plant species in the expression of visual Mg deficiency symptoms and also in critical deficiency concentrations of Mg in the leaf tissue may be related to the light intensity in a particular growth environment.
Symptoms of leaf chlorosis in Mg-deficient bean plants grown at high light intensity.  The green portion of the leaves was  partially shaded with filter  paper.  With an adequate Mg supply, high light did not cause any leaf chlorosis (Cakmak & Kirkby, 2008).
The leaf damage that occurs in Mg-deficient plants exposed to high light intensity has been ascribed to enhanced generation of damaging highly reactive oxygen species in chloroplasts at the expense of inhibited photosynthetic CO2 fixation. Plants growing under conditions of high light intensity appear to have a higher requirement for Mg than the plants grown under lower light intensity.

Magnesium Deficiency Is a Growing Problem
Despite the well-known role of Mg for various critical functions, there is surprisingly little research activity on the role of Mg nutrition in crop production and quality. Hence, Mg is often considered a “forgotten element”. However, Mg deficiency is increasingly becoming an important limiting factor in intensive crop production  systems, especially in soils fertilized only with N, P, and K. In particular, Mg depletion in soils is a growing concern for high-productivity agriculture.

Due to its potential for leaching in highly weathered soils and the interaction with Al, Mg deficiency  is a critical concern in acid soils. One of the well-documented plant adaptation mechanisms to acid soils is the release of organic acid anions from roots. Organic acid anions released from roots will chelate toxic Al ions and form Al-organic acid complexes that are no longer phytotoxic. It is well-documented that Mg is required for effective release of organic acid anions from roots to modify an Al-toxic rhizosphere (Yang et al., 2007). Like Mg, Ca is also important in alleviating Al toxicity in acid soils. However, Mg can be protective against Al toxicity when added in micromolar levels, while Ca exerts its protective role in millimolar concentrations (Silva et al., 2001). This result indicates Mg has very specific benefits in protecting against Al toxicity.
Growth of common bean (left) and wheat (right) plants with low and adequate magnesium nutrition
 Early Reaction to Mg Deficiency
In view of diverse functions of Mg in plants, a question arises as to which function or structure is affected first under Mg deficiency. The most common answer was chlorophyll level, or photosynthesis, or protein synthesis. There are a few studies published previously by Cakmak et al. (1994) in common bean, Hermans et al. (2004) in sugar beet, and Hermans and Verbruggen (2005) in Arabidopsis that provide a clear and convincing answer to that question, as discussed below in this short review paper.

Hermans et al. (2004) grew sugarbeets with either a low or an adequate Mg supply and analyzed 1) plant growth, 2) photosynthetic CO2 fixation, 3) chlorophyll concentrations, 4) photosynthetic electron transport and 5) leaf concentration of sucrose. The results obtained were clear: before any noticeable or significant change occurred in the first four measurements, there was a large accumulation of sucrose in the fully expanded leaves of the Mg-deficient plants. Magnesium-deficient leaves accumulated up to 4-fold more sucrose when compared to the Mg-adequate leaves, indicating a severe inhibition in sucrose transport out of the Mg-deficient leaves.

Cakmak (1994 a,b) studied the role of Mg nutrition in 1) shoot and root growth, 2) concentration and distribution of carbohydrates among root and shoot organs, and 3) phloem export of sucrose in bean plants. Results showed pronounced inhibition of root growth before any noticeable change in shoot growth and chlorophyll concentration. Consequently, the shoot: root ratio for both bean and wheat plants increased in Mg-deficient plants (Figure 4). This early negative effect of Mg deficiency on root growth before the development of visible leaf chlorosis is a critical issue for growers because of the importance of a good root system for plant production. Therefore, special attention should be given to the Mg nutritional status of plants before the development of any visible deficiency symptoms.

Accumulation of carbohydrates in fully-expanded leaves is a common phenomenon with Mg deficient  plants. At the beginning of Mg deficiency and under severe Mg deficiency, Cakmak (1994 a, b) found that older leaves contained 3.5-fold and 9-fold more sucrose,
respectively, compared to the Mg-adequate plants. Magnesium-deficient leaves also contained elevated amounts of starch and reducing sugars. In bean plants grown with a low Mg supply for 12 days, only 1% of the total plant carbohydrates were found in roots, whereas in the Mg-adequate plants, this value was 16%. All these results clearly indicate a severe inhibition in phloem export of sugars out of Mg-deficient leaves.

Phloem exudates were collected from bean plants with low and adequate Mg supply to study the role of Mg nutrition on the movement of sucrose out of the leaf. Magnesium deficiency resulted in severe and very early inhibition of the phloem transport of sucrose (Figure 5). There was an inverse relationship between sucrose concentration in leaf tissues and the sucrose export rate in phloem during the 12 days of Mg deficiency treatment. The inhibitory effect of Mg deficiency on sucrose transport via phloem occurred before any adverse effect on shoot growth. Re-supplying Mg to the deficient plants restored the phloem export of sucrose within 12 hours.

Leaf sucrose concentration (A) and sucrose export rate (B) in bean plants grown with adequate magnesium (control) or deficient magnesium for 12 days (Cakmak et al., 1994b)
 These results strongly suggest that the effect of Mg on phloem loading of sucrose is specific and not related to any secondary effect. The mechanism by which Mg deficiency affects phloem loading of sucrose is still not fully understood, but it appears to be related to the low concentrations of the Mg-ATP complex at the phloem loading sites. It is widely believed that Mg-ATP is required for a proper function of H+-ATPase, an enzyme that provides energy for the phloem loading process and maintains sucrose transport into phloem cells.

Practical Importance of Early Mg Deficiency
 High carbohydrate accumulation coupled with inhibited phloem export of sucrose from Mg-deficient leaves show the importance of maintaining adequate Mg nutrition of plants during periods of intensive carbohydrate transport from leaves to the growing cells. Sufficient Mg is required for maximizing the carbohydrate transport into sink organs (such as roots and seeds) to promote high yields. Maintenance of adequate Mg nutrition at the late growth stages is also essential for minimizing generation of harmful reactive oxygen species and photooxidative damage in chloroplasts. The application of late-season Mg through fertilization or foliar sprays may be useful in some circumstances. The impairment in root growth due to Mg deficiency may have also serious impacts on uptake of mineral nutrients and water, especially under marginal soil conditions.

Producing plant-based biomass as a renewable energy source is a growing and promising alternative to fossil fuel. But the productivity of these systems is directly dependent on 1) the capacity of plants to fix CO2 into organic carbon (C) through photosynthesis, 2) translocation of the assimilated C from source into sink organs, and 3) utilization of assimilated C in the sink organs for growth. All of these steps are specifically controlled by Mg. Therefore, attention must be directed to the Mg nutritional status of biofuel plants in order to achieve high biomass production and partitioning of the assimilated C in the desired plants organs (such as grains or roots).

Magnesium has long been noted for its essential role in chlorophyll formation and photosynthesis. However, growing evidence shows that sink organs (such as growing roots and developing seeds) are also severely affected by Mg deficiency.
For too long, Mg has been a forgotten element for crop production, but its vital role is increasingly being recognized in plant nutrition.

Cakmak, I., C. Hengeler, and H. Marschner. 1994a. J. Exp. Bot. 45:1245–1250.
Cakmak, I., C. Hengeler, and H. Marschner. 1994b. J. Exp. Bot. 45:1251–1257.
Cakmak, I. and E.A. Kirkby. 2008. Physiol. Plant. 133:692-704.
Hermans, C. and N. Verbruggen. 2005. J. Exp. Bot. 56:2153–2161.
Hermans, C., G.N. Johnson, R.J. Strasser, and N. Verbruggen. 2004. Planta 220:344–355.
Silva, I.R., T.J. Smyth, D.W. Israel, C.D. Raper, and T.W. Rufty. 2001. Plant Cell Physiol. 42:538-545.
Yang, J.L., J.F. You, Y.Y. Li, P. Wu, and S.J. Zheng. 2007. Plant and Cell Physiol. 48: 66–74.

Sunday, December 2, 2012

Re-evaluate your potash application- getting enough?

Nutrient removal without replacement
In many Western soils, the importance of maintaining adequate levels of potassium is frequently overlooked.  The rocks and minerals that formed many of the soils in this region were naturally high in potassium.  When these soils were first farmed, there was little response to added potassium in many cases.   
Continuous cultivation requires nutrient replacement

Many years of high-yielding crop production has resulted in a mining of this resource.  For example, an alfalfa hay yield of 8 tons/A will remove about 500 lbs K2O each year.  A 400 cwt/A yield of potatoes accumulates over 400 lb K2O/A in the plant!  When high-yielding crops are continually harvested and removed from the field, the native potassium resource finally becomes depleted and exhausted.  Even where some potassium-rich minerals remain in the soil, they frequently cannot release their nutrients at a rate to meet the peak demand periods of a rapidly growing plant.
All plants require potassium

A recent study examined the extent of potassium depletion in the Western U.S. (the balance between soil K removal by crops and replacement with potash fertilizer).

Arizona  4.4 times more removal than replacement
California 2.0 more removal
Idaho 4.3 times more removal     Montana 6.8 times more removal
Oregon 2.2 times more removal   Utah 5.2 more removal
  Washington  2.6 times more removal   Wyoming 9.2 times more removal

It is clear that on average in every Western state, we are rapidly depleting soil potassium reserves.  While these averages do not represent every specific field, the overall trend simply cannot continue indefinitely if we want to maintain our current yields.  

Potassium-deficient lettuce
Inside the plant, potassium is vitally important for many enzymes involved in photosynthesis, organic compound synthesis, translocation of important plant materials, and maintaining proper water balance.  Since potassium is mobile in the plant, deficiency symptoms appear first on the oldest leaves as yellowing around the leaf margin or specks between the leaf veins.  However, once deficiency symptoms are visible, plant growth has already declined and the crop continues to lose yield each day. 
High-yielding Idaho potatoes
Regular soil testing is the best way to predict the amount of potassium available for next year’s crop and decide on appropriate nutrient replacement rates.  If this is not possible, keep in mind the amount of potassium removed in past crops and in the coming year.  Don’t wait until deficiencies occur before replenishing the supply of this essential plant nutrient with potassium fertilizer.