The Mobility Concept
Plant nutrient management is strongly dependent on nutrient mobility in the soil.  Nutrient mobility in the soil is different among the essential plant nutrients and nutrient management in the field needs to take this into account.

In 1954, Dr. Roger H. Bray at the University of Illinois proposed a nutrient mobility concept that has proven to be very important in the management of nutrients for optimum efficiency (agronomically, economically, and environmentally).  Bray essentially simplified all soil nutrient chemistry to the fact that some plant nutrients are mobile in the soil and some are not. 

A great deal of research has been conducted in the past 70years supporting Bray’s nutrient mobility concept and it is now considered a fundamental feature of soil fertility and soil health management (Raun, 2017; Warren et al., 2017, Havlin et al. 2014; Troeh and Thompson, 2005).

 

Mobile Nutrients
Mobile plant nutrients in the soil move with the soil water.  Thus, plants can extract mobile nutrients from a large volume of soil beyond the direct root system.  Accordingly, plants take up mobile nutrients from a “root system sorption zone”, Figure 1.  This gives plants the capacity to utilize most of the mobile nutrients in the root system sorption zone as those nutrients will move to the plant roots with the soil water as it is taken up by the plant.

We consider the mobile plant nutrients to be nitrogen (N), sulfur (S), boron (B), and chlorine (Cl).  These mobile plant nutrients are taken up by the plant in the following forms: nitrate-nitrogen (NO₃^--N), sulfate-sulfur (SO₄^2- - S), boric acid (H₃BO₃) and borate ions (BO₃^3- - B), and chlorine is taken up as the chloride ion (Cl^-).

Figure 1.  The root system sorption zone and an illustration of the large volume of soil
from which plants extract mobile nutrients.

In a crop field where many plants are growing together, there are root system sorption zones commonly overlap.  Therefore, the root system sorption zones for adjoining plants are competing for water and mobile nutrients, Figure 2.  This is one of the main reasons that appropriate plant populations are important for optimum yield.

Due to this overlapping and potential competition among plants in the root system sorption zone, we need to fertilize and manage nutrients in direct proportion to the number plants or the potential yield of the crop.  We can make an estimate of the mobile nutrient requirements by first calculating the amount of mobile nutrient that will be taken up by the crop.

Knowing the average concentration of the nutrient in the crop and the yield goal for the crop can provide a good estimate of total mobile nutrient demand by the crop.  This is important for estimating the crop requirements for nutrients like nitrogen(N) for a crop.

Taking a soil test for plant available N, which is nitrate-nitrogen (NO₃^--N), will only provide a snapshot of the plant-available N over the season due to myriad N transformations that are constantly taking place in the soil.  Thus, it is better to work with a yield goal approach for mobile nutrients such as N.

Figure 2. Competition among plants brought about by increasing yield goal.

Yield goal example: Consider a lettuce yield goal of 30 Tons (fresh weight)/acre.  Nitrogen uptake studies on lettuce have shown that for iceberg lettuce a general average of 2.6 lbs. N/Ton is taken up by the crop and 3.62 lbs. N/T for romaine. Thus, using 3.0 lbs. N/Ton of fresh lettuce with a projected yield of 30Tons/acre indicates a total N demand by the crop of 90 lbs. N/acre (Bottomset al., 2012; USDA-ERS, 2013; Doerge et al., 1991). 

We can subtract residual nitrate-nitrogen (NO₃^--N) found in a pre-season soil test from the total N crop demand estimate and identify a target N fertilization rate. For example, 10 parts per million (ppm) residual nitrate-nitrogen (NO₃^--N) as a preseason level is very common in agricultural soils. 

Using an estimate of 2 million lbs. of soil per acre-furrow slice (6-inch-deep layer of soil in an acre area), 10 ppm X 2 = 20 lbs. nitrate-nitrogen (NO₃^--N) residual in the soil.

90 lbs. N projected requirements – 20 lbs. residual N = 70lbs. N/acre fertilizer requirement.

This would be fine if everything in the field was 100%efficient.  Due to inefficiencies that are inherent in a field production system, higher rates of fertilizer N can be required in some cases.

Our management goal is to achieve the highest levels of efficiency (agronomically, economically, and environmentally) in the field as possible (Bottoms et al., 2012 and Doerge et al. 1991).  Using the nutrient mobility concept is good to incorporate into our crop management strategy.

References:
Bottoms, T.G., Smith, R.F., Cahn, M.D., Hartz, T.K.  2012.Nitrogen requirements and N status determination of lettuce.  Hort Science 47, 1768-1774.

Doerge, T. A., R. L. Roth, and B. R. Gardner.  1991. Nitrogen Fertilizer Management in Arizona. College of Agriculture Doc. 19102.  University of Arizona.

Havlin, J.L.,Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 2014.  Soil Fertility and Fertilizers; An Introduction to Nutrient Management.  6thEdition, Prentice Hall, Upper Saddle River, NJ.

Raun, W.R. 2017.  In: Warren et al.  2017. Oklahoma Soil Fertility Handbook, Id:E-1039

Troeh, F.R. and Thompson, L.M. (2005) Soils and Soil Fertility. Sixth Edition, Blackwell, Ames, Iowa, 489.

USDA-ERS. 2013. U.S. lettuce statistics 2011. http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentID= 1576 (accessed 21 Feb. 2013).

Warren, J., H. Zhang, B. Arnall, J. Bushong, B. Raun, C. Penn, and J. Abit.  2017. Oklahoma Soil Fertility Handbook.  Id: E-1039

Vegetable season is well underway and early planted vegetable crops are already about one month old. Many fields have or soon will need to be cultivated. As such, I thought it would be timely to “repost” this video on “new” technologies for cultivating weeds - 1) a camera-guided side-shift hitch and 2) finger weeders, an in-row weeding tool (Fig. 1). The video shows the device operating in seedling corn, but it will work similarly in other crops such as broccoli or cauliflower.

We have found the devices work well. Trials conducted in cotton over 3 years showed that use of camera-guidance improved weed control by more than 30% and finger weeders removed about 45% of the in-row weeds. Weed control using the two technologies together simultaneously was roughly 90% for broadleaf weeds and about 85% for all weed species including grasses.

If you are interested in trying these technologies, please contact me. We still have the equipment and I’d be happy to work with you.

Fig. 1. Technologies for precision cultivation and in-row weeding include a) a
camera-guided side-shift hitch attached to a cultivator and b) in-row weeding
tools (finger weeders).



Watch the Video Below

Why is it that the summer months we spray herbicides, and they don’t work for some weed species. 

For example, during the hot time of the day and when desert weeds are stressed for moisture even the highest rate of Glyphosate sometimes is not effective.
Some desert weeds develop physical characteristics and growth habits that equip them to survive hot and dry conditions. We have seen that some species have different response to herbicides in cool areas than our low desert. 

Weeds in the desert have a thicker skin to preserve water. The stomates, which are the pores found in the epidermis of leaves, stems, and other organs, controlling the rate of gas exchange between the internal air spaces of the leaf and the atmosphere are closed¹. Plant transpiration is reduced, and this interferes with the translocation of herbicides in the weeds.

Take as an example Purslane, which has very succulent leaves and stems and can survive incredibly high hot and dry conditions. Then when weather is more favorable it comes out of survival mode to start growth again at the presence of moisture.

Other perennial species have underground structures that help them survive such as Nutsedge and bermudagrass have tubers, and rhizomes.
Lundkvist² stated:” The conclusion is that environmental conditions that promote growth rate of the weed stand will also increase the herbicide effect. The results were based on data from two field experimental series performed at six sites in southern Sweden during 1991–1994”.



 References:
1. https://simple.wikipedia.org/wiki/Stomata
2. A. Lundkvist,
Predicting optimal application time for herbicides from estimated growth rate of weeds,
Agricultural Systems,
Volume 54, Issue 2, 1997,

Researchers at the University of Wisconsin-Madison have developed a device called “Insect Eavesdropper” that allows real-time detection of insect pests attacking crops. The Insect Eavesdropper is equipped with highly sensitive contact microphones which allow it to capture the sounds of insects eating and moving inside plants. The researchers claimed that by eavesdropping on the sounds within the plants, one can gain insights into insect presence, insect density, insect interaction with the crops, insect feeding behaviors, etc. The device works by clipping a sensor onto plants in the field while the data is monitored remotely to determine when pests are active and where they are concentrated. According to the inventors, by using advanced algorithms, the device can accurately identify the type of pest present, even before visual signs of infestation become apparent. 

If this device works as described, it would be a very useful tool for pest monitoring. I you would like to read more about the Insect Eavesdropper technology click on this link.

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth counts decreased considerably y in the past 2 weeks and about average for late summer
 
Beet armyworm:         
Trap counts stay active in some locations (Yuma Dome Valley and Wellton) particularly near alfalfa
 
Cabbage looper:         
Cabbage looper numbers remain low consistent previous years.
 
Diamondback moth:      
DBM moths were not caught in any trap since June; average for this time of the year.
 
Whitefly:                      
Adult movement decreased in past 2 weeks consistent with previous years
 
Thrips:                         
Thrips adults decreasing in most locations, and trending comparable to previous years.
 
Aphids:                       
Aphid movement is absent and consistent with what we typically see during the summer.
 
Leafminers:                  
Adult activity has highest in Wellton and Dome Valley, and slightly above average for early August