Soil Quality Checkup

Well managed vineyard

WHEN we think about the important role that soil plays in the production of food and fiber for the world’s population, we realize that it is an irreplaceable resource that must be protected. Soil is the medium that supports plant growth and the source of most plant nutrients. Soil water and the soil atmosphere bathe the roots and keep the above-ground plant healthy and growing. A healthy soil environment is in everyone’s interest.

Definition of Soil Quality

Many people have attempted to define soil quality by measuring various soil characteristics and relating these to different management practices, productivity, environmental quality, or plant disease. But soil quality means different things to different people, depending on its intended use. For example, farmers generally want a soil that supports ideal crop growth year after year with a minimum of inputs. A highway builder is looking for very different soil proper- ties for a high-quality soil.

It has long been known that a major benefit of balanced crop fertilization — aside from increasing crop yields and farm profitability — is its effect on enhancing crop productivity and increasing the amount of organic matter that can be returned to the soil. Organic matter can positively influence soil properties such as structure, tilth, bulk density, and increased infiltration rates.

New Research—Mostly Good News for Soil Quality in California

A published article from the University of California reported on changes in soil quality that have occurred in the last 45 to 55 years (DeClerck, Singer, and Lindert. 2003.... click here). Soil samples collected primarily in 1945 were compared with samples collected at the same locations in 2001. These 125 sampling locations represented four major land uses throughout the state: tree crops (25 sites), row crops (44 sites), rangeland (48 sites), and vineyards (eight sites). Although these sites represent only a proportion of California agriculture, analysis of these historic samples provides an insight into changes in soil quality that have occurred throughout the state.

 Soil pH:  The average soil pH in 1945 was 6.9 compared with a value of 7.1 in 2001. This slight increase in pH is well within the acceptable range for plant growth and indicates no extreme changes towards acidification or alkalinization as a result of production practices.

Soil Salinity: The average soil salinity at the 125 sites significantly decreased during the 56-year period from 0.85 deciSiemens/meter (dS/m) in 1945 to 0.44 dS/m in 2001. The largest decrease in salinity occurred in soil used for row crops. This 48% average decrease in soil salinity likely reflects an improvement in management practices and in soil quality.

 

Soil Phosphorus (P): Concentrations of plant-available P (sodium-bicarbonate extractable) increased approximately 20% during this period, with significant increases occurring in land used for tree crops, row crops, and vineyards. The average P concentration in 1945 was 72 parts per million (ppm) and is now 85 ppm. The improved fertility status that has occurred will enhance the inherent productivity of the soil and increase the amount of crop residue that can subsequently be returned to improve the soil.

Soil Nitrogen (N) and Carbon (C):  The amount of total N and C significantly increased between 1945 and 2001— reflecting an accumulation of soil organic matter. Average soil N concentrations increased from 0.09% to 0.29% and soil C increased from 1.06 to 1.34% between 1945 and 2001. These changes in soil organic matter are typically reflected in better aggregate stability and water infiltration.

Check the soil all through the profile

Soil Texture:  The clay content of the samples consistently increased from an average of 10% to 13% for the period between 1945 and 2001. This increase in clay content may be a sign of accelerated soil erosion…which would have a negative impact on soil quality. While this increase in clay content is not great, erosion of topsoil can have very negative effects on crop production and water quality. Efforts to minimize soil loss should always be part of a farm management plan.

So What?

These results indicate that soil quality has generally been maintained or improved over the last 50 to 60 years of intensive management and cropping.  Does that mean that the status quo is fine? No, continued efforts must be made — especially to minimize soil erosion. The documented improvements in soil chemical properties and fertility reflect hard work over many years and we can’t afford to lose the advances that have been made.

However, for some soil properties, we are still losing ground. For example, a survey of soil test results for California in the 1980s revealed that between 20% and 40% of the samples were rated as medium or lower in potassium (K). This number has increased to 44 to 48% in the most recent surveys. We know that soils cannot be continually cropped and nutrients removed without depleting their native fertility and quality.

A soil scientist can evaluate suitability

Efforts to maintain high yields and soil quality are essential for long-term sustainability.  Careful management and utilization of modern technology accomplish this. The technology available in 2003 is beyond the wildest dreams of the farmers in 1945. For instance, the use of satellite-aided precision agricultural tools, computer-controlled water management, improved soil-testing techniques, rapid assessment of plant tissue samples… can all aid in protecting the quality of the precious soil resource and the environment. Let’s continue the progress that has been made and sustain our efforts to protect the soil.

Urea-Ammonium Nitrate Fertilizer

Liquid fertilizer solutions or fluid fertilizers are popular in many areas because they are safe to handle, convenient to mix with other nutrients and chemicals, and are easily applied. A solution of urea [CO(NH2)2] and ammonium nitrate [NH4NO3] containing between 28 and 32% N is the most popular N fluid fertilizer.

Production

Liquid urea-ammonium nitrate (UAN) fertilizer is relatively simple to produce. A heated solution containing dissolved urea is mixed with a heated solution of ammonium nitrate to make a clear liquid fertilizer. Half of the total N comes from the urea solution and half from the ammonium nitrate solution. UAN is made in batches in some facilities or in a continual process in others. No emissions or waste products occur during mixing. 

Since UAN is a concentrated N solution, its solubility increases as the temperature rises. To prevent the N components from precipitating as crystals, UAN solutions are made more dilute in regions with cold winter temperatures. Therefore, the N concentration in commercial UAN fertilizers will vary from 28% N to 32% N depending on geography. A corrosion inhibitor is usually added to the final solution to protect the steel in storage tanks.

UAN fluid fertilizer

 Chemical Properties

Properties:                              28% N   30% N   32% N

Composition (% by weight)

Ammonium Nitrate                40            42            44

Urea                                        30            33            35 

Water                                      30             25           20

Salt-out temp (ºC)                 -18           -10           -2

(Crystal precipitation temperature)

Solution pH                        - - - - approximately 7 - - - -

 Agricultural Use

Solutions of UAN are widely used as a source of N for plant nutrition. The NO₃^- portion (25% of the total N) is immediately available for plant uptake. The NH₄⁺ fraction (25% of the total N) can also be assimilated directly by most plants, but is rapidly oxidized by soil bacteria to form NO₃^-. The remaining urea portion (50% of the total N) is hydrolyzed by soil enzymes to form NH₄⁺, which is subsequently transformed to NO₃^- in most soil conditions. 

Fluid fertilizer application

Solutions of UAN are extremely versatile as a source of plant nutrition. Due to its chemical properties, UAN is compatible with many other nutrients and agricultural chemicals, and is frequently mixed with solutions containing P, K, and other essen­tial plant nutrients. Fluid fertilizers can be blended to precisely meet the specific needs of a soil or crop. 

     

UAN solutions are commonly injected into the soil beneath the surface, sprayed onto the soil surface, dribbled as a band onto the surface, added to irrigation water, or sprayed onto plant leaves as a source of foliar nutrition. However, UAN may dam­age foliage if sprayed directly on some plants, so dilution with water may be needed.

UAN applied with fertigation

Management Practices

UAN makes an excellent source of N nutrition for plants. However, since half of the total N is present as urea, extra manage­ment may be required to avoid volatile losses. When UAN remains on the surface of the soil for extended periods (a few days), soil enzymes will convert the urea to NH4+, a portion of which can be lost as ammonia gas. Therefore, UAN should not remain on the soil surface for more than a few days in order to avoid significant loss. Inhibitors that slow these N transformations are sometimes added. When UAN is first applied to soil, the urea and the NO3- molecules will move freely with water in the soil. The NH4+ will be retained in the soil where it first contacts cation exchange sites on clay or organic matter. Within 2 to 10 days, most of the urea will be converted to NH4+ and no longer be mobile. The originally added NH4+ plus the NH4+ coming from urea will eventually be converted to NO3- by soil microorganisms.

Closing Yield Gaps?

What is a yield gap?  A recent article from the journal Nature states that closing the yield gap provides hope for feeding the global population in the future.

Maize yields- with and without fertilizer

A yield gap is used to describe the huge gap between crop yields obtained by successful farmers and those achieved by the average farmers.

Another term frequently used is ecological intensification.  This is the concept of closing this yield gap while minimizing the impacts of food production on soil degradation, water pollution, and wasting resources.  This concepts emphasizes intelligent food production that saves inputs and protects the environment (sustainable intensification).
  
A recent Nature article discusses the promise of this concept:

 Closing yield gaps through nutrient and water management

Nathaniel D. Mueller, J. S. Gerber, M. J., D. K. Ray, N. R. & J. A. Foley (2012)

The authors report that it is possible to feed the whole world by 2050, even as population jumps by 2 billion to reach 9 billion and food demand will double from the present because of better living standards.

Preparing field for rice planting

The researchers from McGill University in Montreal, Canada, and the University of Minnesota, US, gathered data from 157 countries and found that most of them suffered from a serious 'yield gap'. 

They found that global yield variability is heavily controlled by fertilizer use, irrigation and climate. Large production increases (45% to 70% for most crops) are possible from closing yield gaps to 100% of attainable yields, and the changes to management practices that are needed to close yield gaps vary considerably by region and current intensity. 

They conclude that meeting the food security and sustainability challenges of the coming decades is possible, but will require considerable changes in nutrient and water management.

Their focus on yield gaps helps us identify what plant nutrition and water availability factors are limiting global food production, but the contribution of improved genetics will also play a role.  Providing the necessary fertilizer to the areas where it is needed has proven to be a challenge.  There are considerable opportunities to improve irrigation efficiency, but there are physical limitations to global water availability.  

Closing the yield gap requires localized information on what is holding back productivity.  Gathering the necessary information is the first step.  Implementing that information through appropriate technology has generally proven to be a formidable barrier.

Soil productivity must be maintained

The International Plant Nutrition Institute has been active in identifying how soil fertility holds back the yields of major food crops (Agri-stats).  Another international group has launched the Global Yield Gap and Water Productivity Atlas to identify areas where yield gaps can be narrowed (www.yieldgap.org)